Methods of designing multilayers are considered--using both computer optimization and analytical techniques. Procedures are reviewed that produce a starting design for computer optimization.

The needle optimization technique is being further developed to provide a synthesis of optical coatings with layers of more than two materials. In the new version of OptiLayer, optimal layer materials are found automatically. Designing of coatings intended for wide angle of incidence ranges is possible. Several examples demonstrate an application of the needle optimization technique to complicated design problems

A stack-by-stack synthesis method for antireflection coatings at any light incidence is presented. It is based on the idea that the original multilayer system can be partitioned into some cascaded few-layer stacks. Each stack is designed by solving a simpler auxiliary antireflection design problem, in which the indices of incident medium and substrate are defined appropriately. Since the reflectances of the designed stacks will be small but not null, the reflectance of the original system will be low but not optimal. However, it can be considered a good starting point for the final refining. The basic idea is exploited in a stochastic environment in order to overcome the drawback due to the presence of many local minima.

When a dielectric multilayer is restricted in refractiveindex to the classic Hi/Lo practicable stack,extraction of edge filter design becomes a choice between these:- i) sufficient Herpin equivalent-layer "periods" matched to the substrate, as taught by Thelen, ii) exact prototypes due to Riblet, Young (1962) and Levy (1965), modified to make their inherent index "steps" more practicable, iii) explicit near-exact formulas, deriving from the equiripple polynomial, and particularly:for index steps (Rhodes (1976)), for Fresnel reflectance (Seeley (1961)), for layer thickness (Seeley (1965)). None are exact when compared with the Chebyshev equiripple polynomial (used as spectral target). Accuracy can be improved in iii) however by including the adjacent p and r in a better formula for thickness (q), thus:( (tp+tr) ) Sin (q) = ( Imped(q) g(q) I)ISQR((1-LofHi.tp"2).(1-LoMi.tr"2)) ( Admit(p,r) ) where tØ is the adjacent half-tangent. Motivation for revisiting this problem remains a need to know "optimum" performance" for a Chebyshev Stack and how it may be designed explicitly without computer refinement. The improved stack is presumed matched to air with exact antirefiection (extrapolated from Levy) to be certain.

Progress is reported in the development of a new synthesis method for the design of filters and coatings for use in spaceborne infrared optics. This method uses the Golden Section optimization routine to make a search, using designated dielectric thin film combinations, for the coating design which fulfills the required spectral requirements. The final design is that which uses the least number of layers for the given thin film materials in the starting design. This synthesis method has successfully been used to design broadband anti-reflection coatings on infrared substrates. The 6 micrometers to 18 micrometers anti-reflection coating for the germanium optics of the HIRDLS instrument, to be flown on the NASA EOS-Chem satellite, is given as an example. By correctly defining the target function to describe any specific type of filter in the optimization part of the method, this synthesis method may be used to design general filters for use in spaceborne infrared optics.

It has been shown empirically that the lack of very low index of refraction materials is the major limitation to achieving a very low reflectance coating over a very broad spectral band. It has been shown in the same work that additional coating thickness can be employed to make up for this deficiency to a certain extent. Thicknesses which are an order of magnitude thicker than the minimum necessary for a reasonable very broad band antireflection (AR) coating can reduce the reflection to about one half that of the minimum thickness case. This result is empirically predictable to a satisfactory degree, but the underlying reasons for this have not been clear. This paper explores the principles which contribute to the understanding of this effect of additional thickness by empirical and comparative means. The Fourier viewpoint adds to the understanding of variations from the ideal design. We have previously shown that the ideal AR coating, when any and all indices are available, would be a smooth inhomogeneous `step-down' in index from the substrate to the medium. The form of the index profile is approximately a Gaussian decay from the substrate to the medium or similarly described by Southwell as a `quintic function'. When the medium is a vacuum or air and the lowest available index is represented by a real material such as MgF₂ at index 1.38 rather than values very close to that of the medium, the discontinuity from the smooth step-down profile causes a reflection residual that cannot be overcome by adjustments in the rest of the smooth profile. Additional thickness and the appropriate index profile can be used to reduce, but no eliminate entirely, this residual reflection. We discuss our observations and findings on these effects in more detail in the body of this paper.

Rugate filters have raised interest for a decade as examples of simple periodic continuous structures. As continuous, they are supposed to offer a much wider set of spectral responses than discrete structures. As periodic they typically exhibit a spectrum with high reflectivity bands in much the same way their discrete counterparts do. These properties are of a great interest, for it allows the possibility of making high reflectivity mirrors with very narrow bandwidth. The purpose of this paper is an attempt to study such filters in the particular case of sinusoidal index profiles, by solving rigorously the wave equation. This approach is compared with more conventional approximation methods such as the coupled mode theory or numerical methods.

This paper presents a stochastic algorithm which is commonly used in signal and image processing. We show that simulated annealing is a powerful minimization method to avoid local minima and find the global minimum of a function. We control three critical parameters to obtain the algorithm convergence: an initial temperature, a mathematical equilibrium criterion and a cooling factor. We illustrate these results with a synthesis of a pass-band filter. We emphasize the dependence of the algorithm performance on the cooling factor.

A method for the design of optical coatings is proposed in which the starting multilayers are made of very thin high and low index layers as the starting design of the flip-flop method. Many sequences of high and low index layers are generated randomly and a number of them are selected before starting the convergence process that is based on the times that each layer shows a high or low index within those sequences. Owing to the initial random choice, the dependence of the final result on the starting coating structure is avoided. Some typical problems are solved to test this method.

Optical thin film multilayer systems at oblique angles of incidence exhibit polarization effects because of the differences between the reflectances, transmittances, and phase shifts of the p and s states of polarization. It is shown that a thin film system is equivalent to a combination of two polarization elements, a dichroic linear polarizer and a retarder; thus, it offers the possibility of controlling the polarization states in different optical systems. The representation of thin film polarizers in the Jones calculus is given and three applications are introduced and discussed. Two of the applications are for conserving the polarization state in corner cube retroreflectors for high-power laser cavity at 1065 nm and in a penta-roof prism for long-range high-quality telescope. The last example is implemented for a divider-combiner element in bidirectional fringe-counting interferometry where the phase shifts and intensities for both states of polarization (p and s) are controlled.

In the present work it is reported a study on ZnO:Al deposited by RF magnetron sputtering of Zn and Al cathodes, or ZnO and Al cathodes in Ar+O₂ and Ar atmosphere respectively. In order to obtain high band-gap films transparent in the 300 - 1000 nm wavelength region and highly electrical conductive films, an optimization of the deposition parameters has been performed by the Robust Design Method. ZnO:Al films, with an average transmittance above 85% for about 5000 angstroms thickness and a resistivity of 2 10^-3 (Omega) cm have been grown.

The results of synthesis of the anti-reflection coatings for widely separated wavelengths based on the use of unequal layer thickness symmetric periods are discussed. The possibility of realization of the anti-reflection coatings at two or more widely separated wavelengths is shown.

The paper reports the design of two dichroic mirrors with a broadband high reflectance in the region 700 - 800 nm, and a very good transmission at 532 nm. These mirrors are designed by the admittance matching method, using dielectric materials ZrO₂ - SiO₂ and TiO₂ - SiO₂.

This paper presents a general design method for graded reflectivity mirrors, which uses a rotating mask and works even if the distance mask-substrate is no more negligible. The method was tested with a model for the deposition process which considers the mask aperture as an extended plane source and for several superGaussian orders.

Chirped dielectric rugate mirrors have been constructed for broadband dispersion control in femtosecond laser oscillators by the use of Fourier transform. Dispersive properties of the mirrors are tailored and explained on the basis of the time shifting theorem of Fourier analysis. Depending on their construction parameters, these chirped gradient-index structures may exhibit high reflectivity and nearly constant negative, i.e. anomalous group delay dispersion over frequency ranges well beyond the fluorescence bandwidths of broadband laser- active materials available to date. As a consequence, practical implementation of these novel dispersive devices would permit the full utilization of the bandwidth offered by these broadband gain media, and allow the construction of small size, compact femtosecond solid state laser oscillators generating optical pulses shorter than could have been achieved so far, directly from the laser oscillator.

The optical performance of interference filters is subject to inevitable variations of the thicknesses and indices of refraction of the layers during or even after deposition. Assuming their statistical distributions and optimizing the expected performance as a function of the nominal layer thicknesses, robust filter designs can be identified which are insensitive to these variations and are therefore easier to manufacture and more robust to use. The underlying stochastic optimization problem and practical methods for its solution using deterministic and random search procedures based on evolutionary strategies are discussed. Finally a number of examples are presented.

Conventional vacuum deposition had gradually been improved by a lot of special processes. We summarize the most important ones and give an evaluation of a few process parameters. The discussion is limited to optical coating on glass.

Dielectric coatings of ultra-low optical loss has been improved by using ion beam sputtering for more than three orders of magnitude within the last two decades. Its fabrication method and kinetic mechanisms are reviewed, and a physical model for the sputtering deposition is described. The manufacture trends and application areas in the 1990s are analyzed and some basic questions are answered.

A wide variety of thin-film materials (TiO₂, SiO₂, Al₂O₃, HfO₂, Ta₂O₅, ITO, Y₂O₃, CeO₂, Si₃N₄, ZrO₂, Au, Ag, and MgF₂) were deposited using conventional electron beam and thermal evaporation techniques with a concomitant bombardment of energetic oxygen ions from a gridless end-Hall ion source. The oxygen-ion-assisted deposition was performed with an ion-beam current of approximately one Ampere over a range of 40 to 120 eV, providing a uniform ion current density (.3 to .5 mA/cm²) impinging on a large substrate area (5024 cm²). Compared to film deposited using standard gridded ion sources this process produces optically equivalent materials with a broad ion beam which is well suited for volume manufacturing.

Ion assisted deposition (IAD) is a well known technique to improve the properties of thermally evaporated thin films. A wide range of materials and completed layer systems have already been investigated. Because of the low total ion current and the small beam size of the commercial available ion sources, the useful substrate area is strongly limited. With a newly developed advanced plasma source (APS) we have overcome these problems. A total ion current of up to 5 A with excellent uniformity over a large area substrate holder (approximately equals 1 m²) has been achieved. The plasma source is installed in conventional box coating system. Besides plasma-IAD the APS is also useful for plasma-CVD processes like plasma polymerization. The principle of operation of the plasma assisted processes with the APS is described. Results of dielectric materials and completed layer systems like shift free edge filters and AR-coatings are presented. In particular, in case of organic substrate materials, the advantages of the APS are outstanding. Scratch resistant layers in combination with AR- coatings and hydrophobic surface layers onto organic substrates are successfully introduced in production.

This paper presents the properties of silicon oxynitrides obtained by reactive ion beam sputter deposition: Dual Ion Beam System. Control of refractive index was achieved by adjusting the process parameters as ion beam current, ion beam energy and reactive gas partial pressure of oxygen and nitrogen. The main difficulty was to achieve stoichiometric nitride, it has been shown that energetic ionized nitrogen was needed to obtain silicon nitride. The major parameter, to obtain variable compositions between silica and silicon nitride, was the oxygen partial pressure with a fixed nitrogen partial pressure. Optical constants in the visible range, refractive index and extinction coefficient, have been measured by spectrophotometry and spectroscopic ellipsometry. Stoichiometry, contamination and packing density have been measured by Rutherford Backscattering and Nuclear Reaction Analysis. The correlation between the film composition and optical constants is shown. Various test results indicate that silicon oxynitrides obtained by reactive assisted ion beam sputtering are high quality optical materials. These films are homogeneous isotropic, with a high packing density. The extinction coefficient is in the order of 10^-4 after 300 degree(s)C annealing. All values of refractive index between 1.49 and 2.1 can be chosen.

An instrument for direct optical monitoring of film grown during deposition was developed. The transmittances of the original substrates, rotating for homogeneity reasons, are measured. The instrument is equipped with a double detection system including a linear photodiode array and a photomultiplier tube, working at different variable wavelengths. The design of the instrument, the main features and first produced optical devices are described.

We have realized an in-situ multichannel spectrometer system for on line optical thin film controlling during deposition. The system is working as a thin film optical monitor as well as an universal process control system. The measurements are made on the original optics during rotation, to avoid tooling factor errors and to take advantage of error compensation of successive layers. The typical thickness accuracy is 0.25% of the design wavelength.

We have implemented a process control system, based on in-situ spectral photometry, for the manufacturing of advanced optical coatings. The coating process parameters as well as the theoretical design values, like refractive indices and thicknesses, are stored in a database for documentation. Based on the high accuracy of the spectral optical thickness control, several examples for advanced coatings with enhanced optical performances are given.

We present a new tool for coating technology. Based on the known saddle field source we developed an improved neutral particle source for particle energies equivalent up to a few hundred eV. The desired energy can be selected by the operation mode. The energetic particles are neutrals. Therefore no problems arise with electric charging or with electrically caused absorption. The principal construction of the source is described. The main advantages are: no filament, no magnetic field, only dc high voltage, resistant to reactive gases.

Absorption losses in ion beam sputtered films of SiO₂ and TiO₂ were examined. Two different processes contribute to the absorption of the films. The first process is the incorporation of impurities from the target itself, the ion source and the target surroundings into the films. The second is the damage of the films if a too large amount of energy is transferred to them by the sputtered and reflected particles. Impurity incorporation was minimized by a special coating chamber configuration and the use of electron cyclotron resonance ion and electron sources. The contaminations are examined by Secondary Ion Mass Spectrometry, using ion implantations for quantification. The influence of the main impurity parts on absorption is determined by controlled addition of impurities to the films. Experimental results show a large influence of the sputter gas, the reactive gas flow and the kind of target on absorption. Numerical sputter simulations revealed the correlation of these results with energy transport to the films. Reducing energy deposition in the films reduces their absorption. Low absorption in ion beam sputtering can only be achieved using metal targets.

A new technique for the fabrication of optical interference layers is being introduced. MgF₂ and NdF₃ have been evaporated by molecular beam deposition (MBD) in an ultrahigh vacuum system. The fluoride films are used in optical interference coatings for optical components of high power excimer lasers. The growth conditions were varied by changing the remaining gas composition, variation of the substrate temperature from 50 degree(s)C to 300 degree(s)C, and interruption of grain growth. The optical losses of the films were measured by transmission spectroscopy and laser calorimetry, and the film morphology was investigated by transmission electron microscopy. It was shown, that a stratification of very thin (1 nm - 10 nm) layers of two different fluorides can avoid the growth of large grains and wide columns, and gives rise to smoother surfaces. The packing density of MBD-films is higher in comparison with fluoride films deposited under conventional high vacuum conditions.

Electron cyclotron resonance plasma enhanced chemical vapor deposition was used to deposit thin films of SiN_x of different composition from mixtures of N₂ and 30% SiH₄ in Ar onto different substrates. Index of refraction ranging from 2.05 to 3.8 (measured at 632.8 nm) was obtained by simply altering the gas flow ratio. A growth rate of 2 - 4 nm/min was obtained depending on experimental conditions. The data was used in the design and computer-controlled fabrication of one- and two-band rugate optical interference filters. The excursion of the continuously varying refractive index was chosen to be 0.8 - 1.2, and window functions and matching layers at both sides were employed for sidelobe suppression. Measured reflection patterns of filters show good agreement with theoretically simulated ones. Filters designed for a center wavelength of 1.0 micrometers , with a total thickness of 2.1 micrometers , achieved maximal reflectance of 97% in the stopband. Typical bandwidth was 240 nm. The two-band filter was designed for stopbands at 1.0 micrometers and 0.77 micrometers with reflectance of 98% and 96% and bandwidths of 150 nm and 120 nm, respectively. Total thickness was 3.33 micrometers . Deposition of filters on optical fibers was also successfully undertaken using one of the fibers for in-situ monitoring of the deposition process.

In recent years there has been increasing interest in helmet or head mounted displays in military aircraft as a method of supplying information to the pilot additional to that supplied by the Head-up Display. Although a number of these are already in service, primarily as night vision systems, prototype demonstrators are currently being developed where information from more than one display must be combined with the view of the outside world. The paper describes the design and manufacturing techniques used to create a thin film rugate filter for the combining surface of a head mounted display operating close to the pilot's eyes. As the weight of head mounted devices is of paramount importance, both lightweight glass and plastic elements have been used as substrates. The problems of coating plastic optics are discussed and methods to overcome some of these difficulties particularly when extrapolated for rugate filters are described. The properties of manufactured coatings on each substrate type are compared and the results presented.

The demand for lighter and more flexible laser protection devices is constantly increasing. Absorbing plastics have a very poor laser damage resistance which can be increased by coating the plastics with a multilayer stack. We have developed a laser protection device on the basis of multilayer coatings on polycarbonate substrates that fulfill all the requirements of cw- and pulse lasers with concern to different standards, e.g. DIN, EN, MIL, etc. This could be done by using appropriate technologies. Both the mechanical and the environmental properties, and the laser thresholds were optimized.

PICVD has been proven as a versatile tool for preparation of optical fibers and planar wave guides as well as for deposition of IR-transparent mirrors, which are implemented in miniature incandescent light sources. These PICVD coatings provide a high potential of applications due to their unconditionally high resistivity to heat and stability to environmental attack. From its basic principles, the PICVD process is well suited for the treatment of thermally labile substrates as for instance plastic lenses. As it is well known, plasma treatment acts as adhesion promoter (at low temperatures), so that excellent interfacial adhesion (between bulk material and coating) even on polymer substrates (low specific surface energy) is one of the most striking features of PICVD. Pulsed plasma mode provides an almost ideal monolayer-by- monolayer growth control in conjunction with high deposition rates. Suitable choice of precursor materials (e.g. Ti-, Si-containing compounds) allows multilayer and/or tailored gradient designs in order to obtain anti-scratch or antireflective properties of the coatings on polymer substrates, which are used for ophthalmic purposes. Up-scaling problems are avoided by using a single working station arrangement. Production proofed microwave technique (2.45 GHz) is implemented for plasma generation, so that substrate temperature can be held at about 70 degree(s)C. Large scale field tests on the durability of the coating, according to DIN prescriptions, demonstrate an excellent endurance of PICVD-coatings. Full computer control, low production costs as well as high throughput guarantees a customer tailored solution.

Single-component films of ceramic materials (Al₂O₃, ZrO₂, BN) as well as multi- component films and films with controlled inhomogeneity (ZrO₂, Ti) were deposited at different laser parameters (wavelength, fluence, mode of operation) and processing variables (processing gas pressure and composition, rf bias, target-substrate distance). The investigations include microbalance measurements of the weight loss of the target as a measure of mass removal as well as high-speed photography and emission spectroscopy as a measure of geometry, dynamics and excitation of the vapor/plasma state generated. The morphology, structure and composition of the films were investigated by SEM, XPS, AES and SAM, revealing a broad variety of different film properties useful for applications. Al₂O₃ films deposited with pulsed CO₂ and excimer laser radiation are dense and glassy. ZrO₂ films deposited with CO₂ laser radiation show columnar structures. BN films deposited with excimer laser radiation consist of hexagonal BN crystallites embedded in an amorphous matrix. Thin films produced with excimer laser radiation show a nearly flat and defect free surface allowing the construction of multilayer systems and graded or homogeneous doped layers.

Laser assisted electron beam evaporation is a new deposition technique to improve the microstructure of HfO₂ and Y₂O₃ films for optical applications. Major advantages of this method are an increased refractive index, a higher laser damage threshold at 248 nm and reduced absorption compared with films deposited by electron beam evaporation. It is possible to influence the refractive index changing by the laser power and to prepare multilayer like systems containing only one coating material.

Anti-reflective (AR) and high reflector (HR) optical coatings were made by physical vapor deposition (PVD) of Teflon AF2400, a perfluorinated amorphous polymer. The AR had the highest laser damage thresholds recorded for PVD coatings at the Lawrence Livermore National Laboratory damage facility. The HR was a multilayer of ZnS and AF2400. The bandwidth was 550 nm, centered at 1064 nm. Single layers of Teflon AF2400 deposited by PVD were characterized optically. The refractive index could be intentionally reduced below the bulk value by varying either deposition rate or substrate temperature. Scanning electron microscopy and nuclear magnetic resonance observations indicated that morphological changes caused the variations in the refractive index rather than compositional changes.

In this study the growth, structure and physical, particularly optical properties of composite thin films (copper-phthalocyanine-SiO₂) with different dye contents are investigated by means of optical spectroscopy (UV-IR), electron probe micro analysis, atomic force microscopy and scanning electron microscopy measurements and compared with the properties of pure dye thin films of different thicknesses and dye/metal oxide multilayer structures, respectively. The composite thin films show spectral shifts and changes in the extension and the intensity of the typical absorption bands in the visible spectral range depending on the dye concentration in the composites. This behavior is accompanied by pronounced color changes, e.g. from blue-green to green in the CuPc-SiO₂ system. The results show, that the CuPc- SiO₂ composite properties are mainly influenced by the size and kind of dye aggregates in the films (monomer, dimer) and not by interaction of dye molecules with the metal oxide matrix.

ZnSe-MgF₂ thin films of different compositions were deposited by coevaporation on unheated substrates. Their optical properties were measured by spectrophotometry, their composition and density by Rutherford back scattering analysis, their structure by X-ray diffraction, and their residual stresses by interferometry. In the composition range under 60% of MgF₂, the materials condensed well, and the thin films compositions were almost as deduced from the crystal rate controllers. The refractive indexes obtained vary from 1.7 to 2.6, following a Lorentz-Lorenz's law. The thin films are homogeneous and weakly absorbing (k equals 10^-3) for wavelengths above 700 nm. The stresses are compressive for ZnSe, highly tensile for MgF₂, and low compressive stresses were measured for compositions between 20 and 57% of MgF₂. This behavior is correlated with a variation of the film structure from a polycrystalline to a quasi amorphous nature between 0 and 60% of MgF₂. The residual stresses strongly depend on the film structure, while density and optical properties are statistical parameters that mainly depend on the ZnSe/MgF₂ ratio.

The optical and structural properties of a-SiC:H thin films, deposited by PECVD by using different carbon sources: SiH₄ + CH₄ and SiH₄ + C₂H₂ gas mixtures, have been characterized by transmittance-reflectance spectroscopy in the range of wavelength 300 - 2000 nm and by IR spectroscopy in the range 400 - 4000 cm^-1. By knowing the elemental composition, the results on the physical properties of the films are examined and compared.

Yttrium fluoride has been proposed as a substitute for thorium fluoride in anti-reflection coatings for the infrared range. We have studied the ion assisted deposition (IAD) of YF₃ in order to obtain dense and low absorbency layers in the 8 to 12 mm spectral window. Refractive index and extinction coefficient of this fluoride were determined from spectrophotometry measurements. We have then associated the YF₃ with ZnS and Ge layers so as to obtain four layer anti-reflection coatings on germanium. The stress induced by each layer in the coating was measured and the sum was shown to be equal to the stress of the total coating. Eventually, an industrial, high efficiency, both side anti-reflection coating on germanium was developed using IAD YF₃ film.

Optical, electrical, and microstructural properties of thermally evaporated Cr thin films assisted by the low-energy Ar ion beam were investigated. The result shows that the optical and electrical properties are close to those of the corresponding bulk Cr: both refractive index and extinction coefficient increase, reflectance increases, and electrical resistivity decreases. The tensile stress decreases while the grain size changes slightly. From this experimental study, it is found that the low-energy Ar ion beam bombardment on growing Cr films modifies the microstructure of Cr films to improve the optical and electrical properties.

When a transparent plane parallel plate is coated by a transparent film, the interference pattern of light in the plate is changed. This change has been exploited to measure the growth rate, the optical thickness, and the refractive index of the film. Using CCD, this technique enables to measure these quantities at each point on the plate, therefore, it provides a good mean for studying the homogeneity of overgrowth distribution and the change of refractive index in growth process. By this technique a quarter-wave thickness can be monitored very precisely, and also film thickness of many wavelengths can be measured. The technique easily can be applied, there is no need for calibration, and precision is quite good.

For building gravitational wave detectors large interferometer mirrors with extreme low total losses, especially low absorption losses are required. With Ion Beam Sputter Technology total losses of a few ten ppm are standard on small substrates e.g. 1 inch diameter. For the optimization of thickness uniformity on large substrates, a special double masking technique has been introduced into the coating process. Recent results for the losses of large optics at 512 nm are presented. The thickness uniformity on a 240 mm diameter substrate including the mirror center is better than +/- 0.25%. The contribution of the HR coatings to the spherical aberration of the mirrors is of the order of (lambda) /100. The absorption losses are typically below 2 ppm. The total losses are typically below 25 ppm.

Thin films play an important role in electron microscopy as they are used to improve the electron optical contrast and the structural stability of specimens, and to increase the electrical conductivity of specimen surfaces. They are, however, also the object of structural and microstructural investigations. In order to avoid overlapping of specimen and coating structures, it is necessary to understand how thin films are formed in the various coating technologies, and how to create them reproducibly as part of the different EM-preparation techniques. Electron microscopy is applied to learn more about the structural details of thin films used for instance in the optical coating industry. Heat shock fracturing and Pt-C surface replication of the cross-sections resulted in reliable TEM micrographs of the coating microstructure. These studies demonstrate that, under optimal conditions, it is possible to find a correlation between measured film properties and the microstructure of the coatings. TEM replica investigations reveal single events, so they can be useful if discrepancies in the (statistical) physical data have to be investigated.

The microstructure of optical coatings strongly influence their resistance to high fluence laser, scatter properties, as well as their mechanical and environmental stability. The relative merits of nonoptical techniques such as scanning electron microscopy, transmission electron microscopy, atomic force microscopy, and focused ion beam are discussed as they apply to optical multilayer coatings. The combination of these techniques provides a unique method to analyze defects in coatings. the long-term objective of this work is to understand the initiation and growth mechanisms of defects in optical coatings, investigate failure mechanisms of laser coatings, and suggest methods for reducing the number of defects during the deposition process. \to date, our defect analysis using nonoptical techniques has focused on hafnia/silica multilayers for high-power lasers. In summary, the information we have compiled about the defect seeds indicates that (1) seed size has an influence upon the mechanical stability of the whole defect, (2) seed shape and chemical composition reveal potential seed sources in the coating systems, and (3) defects can be initiated either as a single event or continuously during the deposition process. Also, it is shown that different vendors have characteristic defects and seeds.

Electron-gas SNMS offers favorable properties, often useful or even necessary for the depth profiling of optical coatings. A short introduction into this method, examples of SNMS depth profile analysis in the fields of composite oxide layers, control of complex thin film components, damage of coatings and characterization of multilayers for soft X-ray optics, as well as a report of ongoing developments are given, demonstrating the performance, the limits and the future potential of the SNMS depth profile analysis.

Light scattering and Atomic Force Microscopy (AFM) are used together to analyze surface roughness in a very wide frequency bandwidth, extending from macroscopic (optical) to microscopic (AFM) scales. The two techniques are shown to be in large agreement since the roughness spectra overlap at intersection of bandwidths. A particular behavior of roughness is emphasized that permits to predict scattering at very short wavelengths. Thin film materials obtained by different techniques (IAD, Ion Plating, EB) are also investigated via a comparison of roughness spectra measured before and after coating in all bandwidths.

A classic method for studying surface roughness of thin films that uses a microdensitometer analysis of electron micrographies of surface replica is revised and some defects are revealed. Two new methods are proposed. The first is based on multiresolution analysis, the second implements Wiener filtering by 2D Fourier Transform. The results are used to check the surface normality by means of statistical tests.

The mass density values of optical thin films have been determined by means of a modified floatation method. This method bases on a linear density gradient, which can be produced by stacking mixtures (CH₃OH/CHBr₃ or C₄H₁₀O/CH₂J₂) with different densities in a vertical glass tube. Due to diffusion processes an exactly linear density gradient is obtained in the tube. The thin layers are now removed from the substrates, crushed and put into the liquid together with calibration substances. After a certain period, the layer particles remain at a certain height in the tube, where the actual density of the liquid is equal to that of the particles. The density of the thin films may then be calculated by a linear regression using the well-known densities of the calibration substances, such as NaCl, KCl, crystalline Si, glass and other. In the present contribution the determination of the mass density of different optical materials is demonstrated. As examples data on so-called `diamondlike' amorphous carbon layers as an IR optical interference coating, CVD diamond layers as an IR window material and amorphous silicon layers as a material for solar cells are presented. The macroscopic mass density values are compared with those obtained from other standard methods, like the normal floatation method, the separate determination of mass and volume of the layer and microscopic densities from EELS measurements. In all cases, a good agreement could be established. Furthermore, the value of the obtained data has been verified by examination of their correlation with other relevant physical properties of optical coatings, such as elementary composition and refractive index. The different effects of microvoids and macrovoids on the refractive index are discussed.

Grazing X-ray reflectometry allows the analysis of thin layer stacks. The fitting of the reflectivity curve by a trial and error method can be used in order to determine the parameters of the films. In order to facilitate this trial and error method, the Fourier transforms of the Grazing X-ray reflectivity curves have been investigated. After the appropriate transformation of the original reflectivity curve in order to make the signal periodic, rough values of the thicknesses can be found. In first approximation, the Fourier transform leads to the auto correlation function of the derivative of the index profile of the stack. The spectrum can give also rough information about roughness by the widening of the peaks, and the height of the peaks are related to the contrast of the indices. The number of peaks in the spectrum is a function of the number of interfaces in the stack. It is shown how one can use the Fourier transform results to make a preliminary stack model before fitting the experimental data.

The possibilities of X-ray scattering methods are demonstrated for the quantitative study of supersmooth surface microgeometry. It is shown that the model of a surface which takes into account layered inhomogeneities near a surface enables one to obtain better agreement between theory and experiment in comparison with the step-like model of a surface which widely used elsewhere.

The influence of smooth (not step-like) variation of the dielectric function near a surface on reflectivity and scattering of X-rays is investigated theoretically. It is shown that the presence of the transition layer can essentially change the shape of differential scattering intensity diagram, especially when the incidence angle of X-ray beam is more than critical angle of the total external reflection. The application of model involved allows one to describe the Yoneda effect quantitatively (whereas it is impossible in the frames of a step-like model of the dielectric function).

Hardness of TiO₂-MgF₂ and TiO₂-SiO₂ composite films prepared by reactive ion-assisted coevaporation were strongly dependent on the microstructures and evaporation rates of each component. The softening of the composite films at low TiO₂ composition is caused by the dispersion of amorphous TiO₂ particles in the sort amorphous SiO₂ or soft crystalline MgF₂ matrix, which is obtained at the high evaporation rate of SiO₂ or MgF₂. In contrast, the hardening of the composite films at high TiO₂ composition is produced by the dispersion of hard amorphous SiO₂particles or hard crystalline MgF₂ grains with the size <20 nm within the amorphous TiO₂matrix, which is obtained at the low evaporation rate of SiO₂ or MgF₂. In these cased, the evaporation rate of TiO₂ is always maintained at 0.2 nm/s.

It is shown that rough surfaces of silver, magnesium and iron thin deposits may be accurately described by an autoregressive process. The autoregressive parameters are determined by using either the Yule-Walker or the Burg techniques and the advantages of describing statistically rough surfaces of thin deposits by linear models instead of the traditional autocovariance function, or the spectrum, is discussed.

Thermal conductivity of dielectric films can be measured by determining the travelling time of a thermal pulse propagating through the film. In the approach to thermal conductivity measurements described here, the energy of a laser-pulse is deposited into the test sample consisting of a totally absorbing substrate and a thin transparent test layer. As a consequence, a temperature profile builds up at the substrate-film interface. The time delay of the temperature rise at the surface of the test layer is determined by the propagation time of the thermal pulse through the layer, and is directly related to the thermal diffusivity and the thickness of the layer. Measurements were carried out with a preliminary setup and evaluated by calculations on the basis of the finite differences method. Thermal conductivity was determined for single layers of Al₂O₃, SiO₂, and Ta₂O₅ films. The measurements indicate that the thermal diffusivity is dependent on the film thickness and are essentially smaller than the corresponding bulk values.

Today there is hardly any product in the optical industry without the implication of optical coatings. Mainly in the UV spectral region, laser-induced damage thresholds (LIDT) of these coatings are a limiting factor for the development of a cost-effective resonator, beam relay, and mask imaging optics. Based on state-of-the-art LIDT knowledge, high-reflective and antireflective coatings for high-power excimer lasers were investigated. Some rules for preparing interference coatings with very high UV-LIDT will be given.

Damage threshold and absorptivity measurements on dielectric coatings for high power excimer lasers are presented in this paper. The experimental techniques applied are in accordance with respective ISO standards. In particular, we give a detailed description of a newly developed UV laser calorimeter which allows high precision determination of absorptivities for transmissive and reflective UV optical components. The measured dependence of the damage data on the laser pulse length and shape as well as the correlation between damage thresholds and absorptivities observed for high reflection coatings indicate that the thermal model for laser induced breakdown is still valid for 248 nm radiation in the nanosecond pulse length regime.

Atomic force microscopy (AFM) was used as a diagnostic for laser damage testing of micrometers -scale nodule defects in e-beam deposited HfO₂/SiO₂ multilayer mirror coatings. Earlier experiments in our laboratory showed a significant increase in damage susceptibility for defects with heights greater than 0.6 micrometers . In the present study, e-bean HfO₂/SiO₂R coatings from two additional vendors were also studied. Atomic force microscopy characterization of the defects showed that the defect shape became less 'classic' as the defect dimensions, especially height, increased. The characteristic shapes (spatter, multilobe, etc.) of the larger defects also varied between vendors. Using the AFM and optical microscopy, laser damage experiments (1064-nm, 3-ns pulses) were performed on various defects found in the coatings of the two new vendors. The data showed once again that the damage susceptibility increased as the height of the defect increased. There was also some difference between vendors in the damage susceptibility of the defects of a given size. Spatter defects, found predominantly in the coatings of one vendor, showed the highest susceptibility to damage.

In this paper, the destruction of the mirrors under continuous wave (cw) laser irradiation is studied. The existence of a critical temperature, at which the mirrors are destroyed, has been already established. But two damage processes are associated with this temperature: the threshold and the ageing. The threshold occurs when the mirror reaches this critical temperature at the steady-state. The threshold essentially depends on the thermal conductivity of the substrate. The ageing processes begin after the thermal equilibrium time and depend essentially on the physical properties of the thin films. The ageing procedure proposed is used in order to compare the mirrors lifetime.

We investigate an innovative sol-gel method that allows coating of mineral or plastic substrates with colloidal oxide-based materials and/or inorganic-organic composites and/or hybrids. To date, we have developed narrow and broadband antireflective coatings, multilayered dielectric mirrors, and a few polarizing thin films. These sol-gel optical coatings have demonstrated adequate performance for use in our proposed 2MJ/600TW (351 nm_ Nd:glass laser. To prepare such optical thin films, sol-gel technology offers outstanding technical and economic advantages over physical vapor deposition. Sol-gel technology opens new prospects in a variety of coating applications, and few areas offer greater payoff for interaction across disciplines than that of sol-gel-derived thin films.

Variable reflectivity mirrors (VRM) are dielectric thin film devices having a reflectivity profile due to a thickness variation of one or more layers. VRM for laser resonators operating at wavelengths 1.06 micrometers and 2.94 micrometers were fabricated by electron beam evaporation. A fixed-aperture-method was developed. With thin-films designs of the etalon-layer of single- layer type smooth supergaussian reflectivity profiles were obtained. Central reflectivities of up to 94% and apodising apertures were achieved. Laser damage measurements according to ISO 11254 were performed with four different thin film systems for VRM designed for 15-ns Q- switched pulses, which is comparable to results obtained from commercial uniform coatings. a less-sophisticated testing procedure was used for the 2940-nm VRM, resulting in a damage threshold)free-running) of 0.35 MW/cm².

Optical coatings with very low absorption losses have been produced on silicon and fused silica in the 1 to 3 micron spectral region. Absorption values below 30 ppm have been achieved at oxygen-iodine and hydrogen fluoride laser wavelengths with fluence levels as high as 40 kW/cm². The optical performance of these coatings in producing ultra high reflectance mirrors (R equals 99.97%) and predicted performance, extrapolated from this data, for the deuterium fluoride laser wavelength region are presented. These results are critical in the production of optical components for application in high power, high fluence mid-infrared cw laser systems.

The problems of laser damage in thin film laser mirrors is widely reported and techniques to increase laser induced damage threshold are continually being sought. This paper describes the design and manufacture of laser mirrors for high power lasers using ion beam sputter deposition techniques in a production sized box coater. Substrate preparation is described, the use of evaluation techniques to assess quality are discussed and the problems to be solved with ion beam processes are highlighted. Laser mirrors with extremely low losses (a few parts per million) have been fabricated and the laser induced damage threshold measured using high power ruby and YAG lasers. This is compared with similarly designed laser mirrors fabricated by conventional electron beam evaporation and ion assisted deposition, and the results are presented.

We have studied the changes of dielectric optical coatings after laser conditioning using atomic force microscopy (AFM). Laser conditioning is the process which consists of illuminating a film by a laser below damage threshold in order to increase this threshold value. This method is implemented for optics used in high fluence laser beam such as Nd-glass lasers (1064 nm, 3 ns pulse length). We have studied e-beam evaporation deposited highly reflective multilayer ZrO₂/SiO₂ mirrors, and also single layers of SiO₂, HfO₂ and ZrO₂. We have observed the modification of the surface after laser conditioning using the AFM. We find that this process results in localized minimization or elimination of defects (nodules, craters, etc...). Furthermore, the origin of the increase in the size of the hillocks observed after laser conditioning and previously reported by other groups, has been experimentally identified as an instrumental artifact.

As a consequence of optical absorption, the temperature of an optical component submitted to high power laser flux increases. The temperature field is known to be non uniform, this leads to a distribution of stresses which may damage the system for temperatures well below the melting point. Assuming linear, isotropic and homogeneous materials, thermal strains and stresses have been calculated in a semi infinite medium under an axisymmetric steady state distribution of temperature. Taking into account interfacial stresses, analytical expressions for the stress fields have been developed for a film bonded to a substrate and submitted to a long time laser irradiation. Last, the results are discussed and compared to observed degradation steps.

Rare earth fluorides have been investigated as possible coating materials for carbon dioxide laser optic components. AR and PR coatings on ZnSe substrates were produced by means of ion assisted deposition (IAD) of YF₃ and YbF₃. Optics with total absorption of about 0.1% and with damage thresholds of up to 30 J/cm² were produced. Effects of IAD on stoichiometry and chemical state of the rare earth ions have been examined by x-ray photoelectron spectroscopy (XPS). In-situ XPS revealed that ion bombardment, as used in IAD, and sputter cleaning result in fluorine deficient films. Oxygen incorporation during preparation could be kept below 1 - 5at%.

Liquid crystal light valve is composed by hybrid multilayer thin films. Its performance depends critically on both optical and electrical properties of each film consisted. The relation of optical-electrical between thin films and the device will be presented in this paper, and the experimental results of the LCLV device will also be introduced.

Porous silicon multilayer systems formed by different techniques were investigated. Type I layer systems are fabricated by changing the current density during the anodic etch process. Type II samples are formed with a constant current density but using a substrate with alternating doping levels. The superlattice structure is clearly visible in transmission electron microscope pictures. The quality of the interfaces depends on the formation technique. Porous silicon multilayer systems exhibit sharp peaks in the reflectance spectrum and can be used as filters. The line narrowing of the broad photoluminescence band of porous silicon by a Fabry- Perot filter structure is demonstrated.

Vacuum deposited anisotropic films offer unique optical properties but currently suffer from several problems, including instability and excessive haze. We discuss the use of transmission- mode perpendicular incidence ellipsometry and waveguiding for measuring the birefringent properties of tilted columnar films, and describe an in situ method for measuring anisotropic optical scatter distributions. Our results show that the properties of the films depend on deposition conditions, including substrate temperature.

Two anti-reflection coatings processes, based on RF magnetron sputtering, have been used to reduce the reflectance of Ti:LiNbO₃ optical waveguide/fiber interface. Measurements show typical reflectance values around -25 dB and -30 dB, at 1.55 micrometers , using respectively SiO₂/Y₂O₃ and SiO₂/TiO₂ bilayers.

The usefulness of multilayer optical coatings for realization of waveguides is discussed. Such guides are used to study the origin of losses. In particular it is shown that a model taking into account transition absorbing layers gives good explanation of the dispersion of the measured losses during propagation for the different guided modes. On the other hand a multilayer structure which could be efficiently coupled to a single mode optical fiber is presented.

Ion-assisted deposition (IAD) processes configured with a well-controlled plasma source at the center base of a vacuum chamber, which accommodates two independent e-gun sources, is used to deposition TiO₂MgF₂ and TiO₂-SiO₂ composite films of selected component ratios. Films prepared by this technology are found durable, uniform, and nonabsorbing in visible and near-IR regions. Single- and multilayer antireflection coatings with refractive index from 1.38 to 2.36 at (lambda) equals 550 nm are presented. Methods of enhancement in optical performance of these coatings are studied. The advantages of AR coatings formed by TiO₂-MgF₂ composite films over those similar systems consisting of TiO₂-SiO₂ composite films in both visible and near-IR regions are also presented.

A great deal of interest is currently being shown in filter structures with varying degrees of wavelength diversity or an ability to control the intensity of transmitted or reflected light. Potential applications cover a wide spectrum, from areas such as image processing and optical computing to the commercial world of smart windows and automatically dimming rear-view mirrors on vehicles. Most of the device designs exploit materials which respond to external stimulus via well-known optical effects including: Thermochromic processes (eg as in phase transition materials) Electrochromic processes (eg as a result of ion insertion) Thermo-optic processes (exploiting the temperature dependant of refractive index) Electro-optic effects (exploiting field dependant refractive index) Non-linear effects (intensity dependant refractive index) Many of the effects observed are fairly small and it is generally important to enhance the tuning range or degree of contrast change through the exploitation of multilayer coating designs. Some active devices also exploit the use of polarised light and the magnitude of optical phase change produced in birefringent media. Examples are the groups of liquid crystal materials, which can realise large changes in optical anisotropy as a result of electric field-induced molecular ordering processes. This paper presents a review of some of the technologies available for active filter application, highlighting examples of devices produced and the potential offered by emerging materials in thin film form.

At the moment WO₃ is the most suitable material as cathodic film for electrochromic devices, mainly because of its good mechanical and physical properties and efficiency in coloring and bleaching upon cation intercalation. We studied the effect of the electrochemical insertion of small cations (H⁺, Li⁺ or Na⁺) in WO₃ thin films by means of `in situ' transmittance and reflectance measurements. As a result we observed for Li⁺ and Na⁺ a behavior depending on the cation concentration: under a given limit (around 14 mC/cm² for a 100 nm thick film) the optical changes were only related to the number of electrons entering the film from the external circuit for the electric charge neutralization. Beside this limit a phase transition was observed with the formation of the so called tungsten bronze and the electrochemical process was no longer reversible. With regard to the intercalation of H⁺ beside the above mentioned limit the saturation of the optical density was observed, but no bronze formation was detected even for charge insertion as large as 50 mC/cm². As a consequence, during the insertion of Li⁺ and Na⁺ the current density must not exceed a maximum value that also depends on the diffusion of cations inside the film. For the WO₃ samples studied during this work we found the diffusion coefficient for Li⁺ and Na⁺ to be related to the cation concentration and varying in the range of 10^-9 divided by 10^-11 cm²/s.

Energy levels structure of the CdS_xSe_1-x microcrystallites in the glass matrix has been obtained from the analysis of the absorption spectra taking into account the crystallite random orientation, the confinement effect and the dispersions of both microcrystallite radius and composition. The peculiarities of the absorption saturation have been determined.

Low voltage reactive plasma assisted deposition (RPAD) is used here, to deposit the optical- electric films, such as transparent conductive thin films and Si₃N₄ films. Comparing to other thermal evaporation techniques, RPAD can produce more easily such kind of opto- electric thin films. Experimental results of the films deposited will be presented in this paper.

The integration of different infrared interference filters on a single substrate to obtain filter arrays is discussed. These arrays, when coupled in the focal plane to linear photodetector arrays, form the so-called `spectral retinae' which permit spectral and spatial resolution with no moving parts. Its application to new multigas sensors based on infrared absorption spectroscopy is also discussed. The different aspects emerging in the integration of specific interference filters are discussed in this paper. Special attention is paid to the problems of compatibility of the different processes involved in the obtention of the integrated filter. The improvement of detection limits for gases of great environmental interest as sulfur dioxide, and nitrogen dioxide is also discussed. Aspects related to the whole conception of the sensor are studied too, mainly those related to the improvement of signal to noise ratio by means of the optimization of the optical parameters. Finally, a new concept for using spectrally selective mirrors in long path optical cells, is proposed. In this way a considerable improvement in simplicity and performance of the spectral retinae can be obtained.

In the usual surface forces apparatus arrangement, the determination of the distances between interacting surfaces, as well as their mutual forces, is based on the analysis of interference fringes originated between the surfaces. To enhances the contrast and sharpness of the fringes, a thin metallic coating is applied to the mica plates that form the interaction chamber. In this paper we try to improve the performance of the basic system just described by optimizing the metallic coating and also by computer processing the images of the visible fringes.

Films of indium tin oxide (ITO) are ubiquitous in display applications as they combine optical transparency and good electrical conductivity. Thin ITO films are often difficult to characterize optically as they are often inhomogeneous and have complicated optical absorption spectra. In this work, we have used variable angle of incidence spectroscopic ellipsometry (VASE) to gauge the thickness, grading, and surface roughness of ITO films ranging from a few tens of nm to hundreds of nm thick on glass substrates. A two Lorentz oscillator model is used, with one oscillator representing the interband absorption in the UV end of the spectrum, and the second oscillator modeling the Drude-like free carrier absorption in the infrared. The Drude parameters obtained from the analysis of the films can be used to estimate the electrical (D.C.) conductivity of the film from the simple Drude model for free carriers. We present results for several films from 7 to 40 nm thick, and compare the nominal electrical resistivity of these films to the resistivity derived from the VASE measurements.

Fluorides and oxides are classical materials for optical coatings. At present fundamental improvements of film properties are necessary especially for optical components of high power excimer lasers and other applications in the UV spectral region. The optical properties and the laser damage threshold of the films are determined by the thin-film microstructure and by contaminations inside the films. Strong connections between optical losses and laser stability on the one hand and the film microstructure on the other hand have been shown. Therefore sensitive optical characterization techniques in the UV have to be used. We report on first results of deep UV spectroscopic ellipsometric measurement of fluoride and oxide films.

Photothermal Deflection (PD) is used for mapping very small absorption losses (as low as 10^-6). Measurements performed on bare substrates have shown the existence of highly non uniform absorption localized on the substrate surface. High absorption sites can be twenty times as absorbing as the mean value while we can find areas with very low absorption (approximately equals 10^-6). Typically for fused silica substrates, the mean value of surface absorption is about 8 X 10^-6. Comparisons between PD mapping, Nomarski microphotography and scattering mapping on the same area show that PD can give specific information about substrate-surface contamination. We have studied on different sets of substrates the influence of polishing and cleaning. We conclude that very smooth surfaces have a lower mean value of absorption then conventionally polished ones. For films deposited on fused silica substrates, we have observed precise correlation between absorption mappings on the same area of the bare then coated substrate.

Two-dimensional cw photothermal surface displacement (PTD) scans with high spatial resolution provide a new quality for thin-film characterization. This is demonstrated for optical single-layer films of MgF₂ and for 248 nm high-reflecting Al₂O₃/SiO₂ multilayer coatings. Inhomogeneities of the films were detected with a lateral resolution better than 2 micrometers . Photothermal images reveal that Al₂O₃/SiO₂ coatings with low damage thresholds differ above all in the amplitude of the PTD background signal whereas high-damage-threshold-coatings excel in a noticeable decrease in defect concentration. On the other hand, pulsed thermal MIRAGE technique is shown to be capable to yield complementary information with respect to the subdamage behavior as well as the onset of UV damage in Al₂O₃/SiO₂ laser mirrors. Thus, the physical origin of the UV radiation breakdown in optical thin films can be elucidated.

An apparatus based on guided waves and photothermal techniques permits to measure attenuation coefficients in multilayer waveguides produced by I.A.D. and Ion Plating. Zero- order modes are used to obtain accurate values of extinction coefficients of T_iO₂ and Ta₂O₅ thin-film materials at wavelength 514.5 nm. On the other hand, higher-order modes reveal strong loss anomalies that are analyzed via electromagnetic theories of scattering and absorption, both including surface and bulk effects.

The total losses due to absorption and scatter from the best optical coatings can be made as low as <EQ 3 ppm, at the limit of sensitivity of present optical-absorption measurement techniques. We show by measurement and calculation that a dramatic increase in the sensitivity of absorption measurements is obtained by using a supercritical fluid, instead of an ordinary (non-critical) fluid, as the sensing fluid in a collinear photothermal-deflection apparatus. The noise floor in our surface-absorption measurements using supercritical xenon, T_c equals 16.7 degree(s)C, corresponds to an absorptance A equals P_absorbed/P_incident equals 10^-10 under illumination of 1 W. Bulk absorption measurements are similarly enhanced: the noise floor corresponds to an absorption coefficient of (alpha) equals 10^-13 cm^-1 for 1 W of illumination in a sample of length 1 cm. These levels are three orders of magnitude more sensitive than any previously reported. The enhancement is brought about by the divergence in the coefficient of thermal expansion of a fluid near the critical point. In attempting to use this sensitivity to measure the absorption in transmission of low-absorbing (<EQ few ppm) anti-reflection coatings, we found that the bare superpolished fused-silica and sapphire substrates absorb at A approximately 2 X 10^-5. The low-level absorption at uncoated polished optical surfaces thus appears to be an important question.

Characterization of high reflectivity multilayers coatings at 1.064 micrometers are reported. Measurements of reflectivity were performed by a Fabry-Perot cavity decay time method and a 10^-6 precision was obtained. Measurements of absorption was done in the 10^-6 range by collinear mirage detection with a 10^-8 precision. Diffusion was investigated with a commercial scatterometer and total integrated scattering in the sub-ppm range on home made Rmax 0 degree(s) mirrors was measured.

Materials with ionic bonding exhibit high reflectance somewhere in the infrared spectral range: the reststrahlen band. This arises because of lattice vibrations that are excited by the incident light. For some materials, e.g. the alkali halides NaF, NaCl, KCl and polycrystalline samples of BeO, SiC, secondary minima in the reststrahlen band have been observed, which have previously not been fully understood. We show, theoretically and experimentally, that small inhomogeneities on the surface are sufficient to cause a measurable absorption and a substructure in the reststrahlen band. The electric field of the incident light beam excites surface mode resonances in the inhomogeneities which absorb light. Inhomogeneities with different shapes have surface modes with different resonance frequencies because of the strong dispersion in the reststrahlen band region. The position and width of the absorption band is determined by the details of the surface roughness: in particular the mixture of spherical, ellipsoidal and disk-shaped irregularities which can be used to describe the actual tomography. IR reflectance spectral can be used to monitor the departure from a perfect smooth surface and to determine the concentration density and shape of topographic defects on ceramic surfaces. Light scattering measurements over the wavelength range 5 - 20 micrometers for bulk samples of polycrystalline beryllium oxide and 5 - 20 micrometers for silicon carbide powder are reported in verify this interpretation.

Spectral diffuse reflectance and transmittance measurements have been performed on single films of tin oxide on Corning glass substrates. The measurements have been performed on a spectrophotometer equipped with an integrating sphere in the wavelength range 0.3 - 1.0 micrometers . The front surfaces of the films have been characterized by means of spectroscopic light scattering and optical profiler measurements after the deposition of a thin aluminum film. These measurements have given different results, which can be explained by the different bandwidths of the methods. The front surfaces have thus been found to have a short correlation length, indicating roughness with short spatial wavelengths. The front surface data obtained from the light scattering measurements have been used in a model to characterized both film interfaces. The calculated diffuse spectra agreed with experimental spectra both for reflectance and transmittance. The difference between the highest and the lowest scattering levels was more than an order of magnitude. The model calculations made it possible to establish an upper limit for the substrate-film interface.

A theoretical formalism and experimental methods are presented, which enable statistical fluctuations of the films bulk and interface roughness properties to be estimated from volume scattering and roughness scattering, respectively. The theoretical model is implemented in a numerical algorithm that allows one to optimize experimental strategies and to determine morphological parameters from measured scattering curves. Angle resolved scattering (ARS) measurements are performed on MgF₂ films on glass substrates while varying the illumination and observation parameters. Atomic force microscopy (AFM) provides helpful additional information on the surface morphology.

In the present work we offer two novel computational methods, defined as (T,R_f,R_m) and (T,R_b,R_m), for the simultaneous determination of the optical constants, n and k, and the thickness, d, of a thin film from three experimental photometric quantities. The basic experimental configuration is a thin film deposited onto a nonabsorbing substrate, half covered with an opaque metal film. An algebraic inversion technique is developed involving a numerical interpolation procedure in the last step. The methods give all mathematical solutions, and according to the specific case, the physical solution can be isolated by the combination of the two methods or by some estimates of the thin film thickness. When the photometric measurements are available in a spectral range, the (n,k,d) solutions, for which the thickness is one and the same, can be easily isolated as correct. The (T,R_f,R_m) and (T,R-b),R_m) methods can be applied without restrictions to a wide range of n and k values. A numerical example illustrates the applicability and the good overall accuracy of the methods.

Optical measurements provide a useful tool for indirect measurements of various parameters (thicknesses, refractive indices) of thin films. Parameters are established by fitting the performance of the thin film model to the measured values. There are several methods used for this purpose, e.g. spectrophotometric and ellipsometric measurements in various arrangements. To get sufficiently precise values of the parameters we need sufficient amount of information. More information can be acquired by varying the experimental setup, e.g. wavelengths of the incident light or angles of incidence, we can perform measurements in immersion liquids, on various substrates or in series of specially prepared samples. Many such methods have been already reported in literature but the choice of the proper method for the specific problem is done rather intuitively until now. The common feature of all these methods is a need for numerical processing of the data. Fitting is done usually by the least squares method, minimizing a merit function. This approach enables also the statistical error analysis. The precision of the computed parameters is estimated from the measurement errors. What is important is that provided we know parameters of the system and accuracy of the measurement, we can compute the resulting precision of parameters in advance. In conclusion suppose we can reasonably estimate the thicknesses and refractive indices of the layers and measurement errors before the measurement, we can test various experimental setups for the best resulting precision of parameters with minimum effort. This idea has already ben formulated e.g. for the special case of ellipsometrical analysis in . In this paper I would like to document its usefulness and power in a more general context.

Photopolymerizable acrylic coatings are widely used in present day optical devices. However they usually failed on the inadequate optical parameters and low water resistance. A novel class of the photopolymerizable coatings on the basis of multifunctional carbonate methacrylates with unique combination of properties is described. Their applications range from optical discs to different kinds of lenses and gradient elements for optical waveguides.

Variable angle of incidence spectroscopic ellipsometry (VASE) is commonly used for multilayer optical analysis, but in some cases this experiment (performed in reflection) does not provide sufficient information for the unique determination of the thicknesses and optical constants of the films in the given multilayer. We have found that augmenting the VASE data with data from other optical experiments greatly increases the amount of information which can be obtained for multilayers, particularly when they are deposited on transparent substrates. In this work, we describe a formalism which allows us to quantitatively characterize complex multilayer structures by using combined reflection and transmission ellipsometry, reflection ellipsometry with the sample flipped over, and intensity transmission measurements. To demonstrate the usefulness of this capability, the analysis of a complex graded, absorbing thin film structure (a Cr-based phase-shifting photomask blank), is presented.

The generalization of a recent numerical inversion procedure for the analysis of ellipsometric data of double layer samples is described. Within this generalization, data reduction for any double absorbing layer configuration is easily accomplished. Starting from a ((Delta) , (Psi) ) measurement of a single incidence angle and single wavelength ellipsometer, our method will allow finding any two configuration parameters, provided one of them is a thickness. This reduces the constraints of previous methods, leading also to a simple 1D root- finding numerical scheme. Useful graphical procedures for the analysis of double-layer ellipsometric results are easily devised.

An alternative method is presented that allows light scattering technique to characterize both faces of transparent substrates. Experimental results are presented and the limitation of the method is investigated. In addition, a simple formulae is given to approximate the embedded scattering in the substrate, that cannot merge in air. The amount of this embedded scattering cannot be neglected in most cases.

In this presentation the design concept of an instrument for measuring total integrated scattering with a resolution limit of 0.1 ppm will be described. Furthermore the result of a theoretical model will be given which includes the scattering contributions from the front and the back surface of a transparent substrate. With the result obtained, surface roughness and power spectral density functions of the individual substrate surfaces can be calculated from scatter measurements. The improved measurement facility of the instrument in conjunction with the developed theoretical model allows surface characterization of transparent substrates in case of negligible volume scattering of the bulk material. Measurement results will be presented verifying the results of the model and showing the instrument's ability to characterize polishing quality of transparent substrates. Scatter losses as low as 0.6 ppm corresponding to 1 angstrom rms surface roughness have been determined measuring a `superpolished' transparent fused quartz substrate.

Excitation of guided electromagnetic waves in thin films with periodic surface inhomogeneities is studied. It is theoretically shown that at certain conditions the intensity of the excited mode in the ideal thin film without absorption tends to infinity when the corrugation depth tends to zero. This effect can be interpreted as a resonance phenomena in ideal resonator which merit factor is inversely proportional to the corrugation depth. The results of the calculations based on the numerically explicit solution of the two-dimensional model problem support the above theoretical prediction. The main conclusion of the study is that even small inhomogeneities may cause an excitation of guided modes in thin films and considerable enhancement of the electric field.

The origin of light scattering is investigated for multilayers produced by Ion Assisted Deposition and Ion Plating. When the substrate roughness is greater than 0.5 nm, the angular levels clearly originate from a substrate effect, that is the replication of the substrate roughness throughout the multilayer. On the other hand, scattering from overcoated super-smooth silicon substrates may originate from surface roughness brought by materials or from bulk inhomogeneities. In order to separate surface and bulk effects, we measure the angular variation of the polarization ratio of the scattered waves. Measurements reveal unexpected results that lead us to calculate the influence of localized defects at interfaces and in the bulk. High-angle resolution measurements are performed to detect the presence of such discrete irregularities.

Microstructural and optical properties of obliquely deposited thin films are simulated in a computer 3D method. Identical hard sphere particles arrive serially as the simulation proceeds and relax into the nearest site where they touch the particle of impact and two others. Structural and optical properties are calculated from the resulting 3D array of particles. Peaks in the radial distribution function of simulated films are in agreement with particle to particle distances in small clusters and indicate that the films are amorphous. Columnar directions are identified by a maximum in the variance of the film density and results are given for several deposition geometries. Form birefringence is indicated by anisotropy in the 2D angular distribution function viewed down the columns, and values of the three principal refractive indices are computed by an iterative method.

The successful manufacture of metal/dielectric multilayer systems requires not only very close control of the thicknesses of the individual layers, but also a good knowledge of the optical constants of the materials they are made of. In the case of metal films, it is also essential to know whether any transition layers are formed at the interfaces and, if so, how their thicknesses and optical constants depend on the deposition conditions. Numerical modeling of the metal layers and their interfaces is a powerful tool for the determination of these parameters. To illustrate the method, a bandpass filer and a long-wavelength cut-off filter, both having a low reflectance for light incident on one side, were produced. Excellent agreement has been obtained between the calculated and the measured spectral transmittance and reflectance curves.

The model of Forouhi and Bloomer (F-B) for the optical properties of amorphous semiconductors is modified in order to more accurately describe the dispersion of the optical constants observed for amorphous carbon (a-C) and amorphous hydrogenated carbon (a-C:H) thin films. The F-B model represents the optical absorption as the product of a lineshape function and a joint density of states function, which is derived by assuming the conduction and valence bands to be parabolic and separated by an energy gap with in which no allowed electronic states lie. Two modifications to this model are discussed to address the cases of non-parabolic bands and/or electron energy levels in the energy gap. These parametric models are then fit to a large number of experimentally determined optical constant spectra, and results are presented which indicate that non-parabolicity of the conduction and valence bands is the most important correction to the standard F-B model required to describe a-C:H thin films. The modified model incorporating non-parabolic bands is shown to fit a broad range of both a-C and a-C:H spectra very well, and provides useful information about the optical absorption process and physical properties of the films.

A method for determining the complex index profile for an inhomogeneous thin film with weak absorption is proposed. It involves the characteristic 2 X 2 inhomogeneous matrix and the Fourier transform technique.

Consequences of the columnar microstructure of dielectric thin films deposited by conventional vacuum evaporation are discussed. A model associated with the columns is used together with in situ and in air measurements to show that the anisotropy of Ion Assisted Deposited TiO₂ films changes with ion energy. Moreover TiO₂ and Ta₂O₅ layers deposited by conventional evaporation present reversible disturbances of their optical properties when illuminated with high power flux. A model taking into account both thermal and nonlinear properties (localized optical Kerr type effect) of the prism coupler system is used to explain the measurements and to estimate the nonlinear refractive index coefficient of different thin films.

Evaporated coatings need generally high substrate temperature to obtain a more dense structure of the layers avoiding the humidity adsorption when the coating itself is exposed to the atmosphere. Moreover, the employment of fluorides as low refractive index layers can increase the stress level of the coating because of their frequent hygroscopicity. Under severe operational conditions, the adsorbed water causes an expansion of the layer with a consequent destruction of the coating if the adhesion strengths is not strong enough to counter balance the increased stress. The ion beam assistance can be a useful method to increase the density of the coating produced by the evaporation technique. In fact, the ion bombardment during the thin film growth improves the packing density and enhances thin film resistance to moisture penetration. Magnesium fluoride (MgF₂) and barium fluoride (BaF₂) are two low refractive index materials employed in the optical coatings production whereas yttria material (Y₂O₃) can be used as high refractive index layer. In this work the effect of the ion beam assistance on the optical coating capability to offer resistance to atmosphere humidity penetration was investigated. Since the presence of an absorption band at a wavelength of 3 micrometers reveals the presence of the water in the layers, some of the investigated coatings were deposited on silicon substrates to measure the transmittance in the infrared region.

The Photothermal Deflection Technique is used for mapping absorption in optical coatings. We have observed a strong influence of the nature of the substrate material on measured absorptance. Different single layer films deposited in the same conditions, at the same time, on different substrates located at the same distance from the rotating axis, have very different absorptances: the films deposited on fused silica or calcium fluoride substrates absorb weakly while those deposited on glass substrates (BK7, C2036^*, D2050^*) can absorb five to fifty times more. This effect is observed with different coating materials: TiO₂, Ta₂O₅, SiO₂ prepared by different techniques: I.A.D., I.P. and E.B.. We interpret these phenomena in terms of film contamination by the substrate: metallic ions present in glass substrates can be responsible for enhanced absorptance. A simple model taking into account thin absorbing interface layers is developed. We determine both interface and bulk absorptance of the studied thin films.

In this study, residual stresses and optical properties of SiO₂ films prepared by reactive electron gun evaporation have been investigated as a function of process parameters. The purpose of this work was to find process conditions resulting in SiO₂ stress free films or in a stress level balancing the high index material stress in a multilayer structure. We have found that stress in SiO₂ films becomes more compressive with conditions resulting in denser coatings (such as high substrate temperatures or low gas pressures). Using an in situ measurement technique, stress stabilities of these films were also studied under vacuum or heated environments. These experiences have confirmed the porous structure of evaporated coatings and have underlined the great importance of water in their final state of stress.

In this work the properties of Si₃N₄ and AIN thin films deposited onto unheated substrates by Reactive Low Voltage Ion Plating (RLVIP) and Reactive DC-Magnetron Sputtering (RDCMS) were investigated. In both experimental setups pure silicon and aluminum were used as starting materials. Working and reactive gas were argon and nitrogen respectively. All Si₃N₄ films showed amorphous structure in X-ray and electron diffraction whereas AIN films were found to be polycrystalline and could be indexed to the bulk hexagonal AIN lattice. The values of the film refractive index at 550 nm are 2.08 for RLVIP Si₃N₄, 2.12 for RLVIP AIN, 2.02 for RDCMS Si₃N₄, and 1.98 or 2.12 for AIN depending on the total pressure in the range of 8 E - 1 Pa and 1 E - 1 Pa during the process. The high optical transmission region for the Si₃N₄ films lies between 0.23 and 9.5 micrometers , and for AIN films between 0.2 and 12.5 micrometers . Purity and composition were measured by electron microprobe, infrared transmission, nuclear reactions, elastic recoil detection analysis and Rutherford backscattering spectroscopy. Transmission electron micrographs of Pt-C replicas of fracture cross sections of the films show their different microstructure and surface topography. Environmental tests proved the RLVIP Si₃N₄ films to be very hard, of high density and of strong adherence to glass.

It has been shown that a good interpretation of the absorption measurements performed from the photothermal deflection (PD) technique is given from a model including interface absorption. This model is applied to analyze the measurements of the losses on guided propagation, and a comparison with results obtained from PD technique is mae for various single-layer films. W show that the same parameters can explain satisfactorily both measurements. This study confirms the existence of absorbing transition layers close to the glass substrate and at the interfaces of the film.

The reflected and transmitted fields scattered from a layered structure, build up of N thin films with shallow rough interfaces and bounded by two semi-infinite media are determined to analyze the effects of roughness in the optical response of a given multilayer system. Each layer is assumed to be homogeneous, isotropic, local, linear and characterized by a frequency dependent complex dielectric function. All rough interfaces are modeled by using a stationary random process, and the Rayleigh criteria is used in the mathematical treatment because of the shallowness of the defined roughness. Two independent integral equations relating the reflected and transmitted fields to the incident wave are found. These will allow a recursive calculation of the overall optical responses. The solution involves Fourier coefficients of functions dependent on the roughness profiles. This treatment is shown to be valid for both TM(p) or TE(s) states of polarization. Numerical results are obtained for the case of a two layer system, with two of the interfaces present Gaussian roughnesses and compared to some experimental data.

Window glass is principally produced by the float technique which causes tin diffusion into glass creating a layer on each face. The complex refractive index (N_F) of each face is slightly different from that found in the bulk of the glass (N_S) and asymptotically approaches this value. The depth profile of the complex refractive index of each layer may be determined by means of multiangle ellipsometric measurements and the reflectance formula. By assuming glass as the matrix and tin as inclusions, the effective medium theory allows to determine the refractive index of this medium as a function of tin volume fraction. Knowing this evolution and the one of the layer refractive index as a function of depth, the tin volume fraction profile in the glass substrate is obtained.

An approach of ellipsometric measurements both from ambient media and from transparent or weakly absorbing substrate has been proposed to determine optical constants and thickness of anisotropic films. According to this approach the main refractive indices are calculated by an iterative procedure while an optimization is used to average the values obtained over the parameters measured at one or several incidence angles. The approach proposed has been used for a determination of optical constants and thickness of multimolecular Langmuir-Blodgett films based on the derivative of 5-[4-(stearoylamino)phenyl] isophthtalic acid.

Dielectric films exhibit very different and complex behaviors during thickness growth, leading generally to a variation of the refractive index in the depth of the film. Several causes of index inhomogeneity are examined: chemical interaction between film and substrate, transition layer, surface layer, etc. Their detection from in situ ellipsometric measurements is presented in the case of CaF₂, SiO₂ and TiO₂ films. Complementary techniques (XPS, HRTEM and AFM) have been used for assessment of realistic optical models.

Thin films of Si₃N₄ with thicknesses between 50 nm and 600 nm were deposited on different substrates by reactive d.c.-magnetron sputtering. All films proved to be stoichiometric by Rutherford Back Scattering and only small amounts of oxygen and water in the film's bulk were detected by IR absorption measurements. The refractive index of the films was calculated from optical transmission data. Increasing both the working gas (Argon) flow and the reactive gas (Nitrogen) flow resulted in a decrease of the refractive index of the thin films. This is due to the lower density of the thin films prepared at either higher working gas flow or higher total gas pressure. At a wavelength of 633 nm the refractive index ranged between 1.86 and 2.01. All films shows an amorphous microstructure. The Si₃N₄ deposition rate on the substrates was found to be linear depend on the d.c. sputtering power from 0.6 kW up to 2.2 kW. The fundamental absorption edge shifted to higher energy values by increasing the reactive gas flow. The 10% transmission value was found at a wavelength of 233 nm (5.33 eV) with the N₂ flow at 50 sccm and at 213 nm (5.83 eV) with the N₂ flow at 200 sccm.

Multilayers can play a role in any situation where flux, rather than resolution, is desired. They are used in various classical optical arrangements ranging from double monochromators to grazing incidence mirrors. Beam focusing ca also be obtained by multilayer coating on a substrate that is either polished to a given shape, or dynamically bent, or grooved as a Bragg- Fresnel lens. With hard x rays, absorption plays a lesser role, allowing production of a very large bandpass through an aperiodic multilayer design, However, the sputtered, nanometer- thick individual layers are often amorphous, a cause of structural instability in case of a temperature rise. When the beam is intense, which is often the case with a synchrotron x-ray source, a number of solutions are envisaged, including efficient cooling, use of compound materials, and modification of the electric field distribution by nonperiodic arrangement. Results on the performance of multilayers and examples of applications are presented.

Periodic Mo/Si multilayers designed for X-UV mirrors were made by ion beam sputtering deposition on float-glass, polished borosilicate glass and superpolished silicon substrates. Glancing incidence X-ray scattering from these multilayers was measured at first by fixing the incidence angle and varying the detection angle, and then by simultaneously varying the incidence and detection angles in such a way that the phase difference between the beams scattered by successive layers was kept constant. Based on the polarization distribution theory, a calculation of X-ray scattering from interface roughness accounts for the typical features observed on the experimental curves.

Pulsed laser deposition is described as a technique for the synthesis of multilayers showing X- ray optical quality. The state of the art is characterized by results that demonstrate a development of the instrument basis superior to that of conventional PLD systems. Multilayers of the Ni/C, Mo/Si- and W/C-types prove the versatility of the method and the output of layer stack characterization by HREM, SPM, XD, AES, XPS, ellipsometry and image processing ensures a high quality with regard to stack regularity, layer homogeneity and interface smoothness.

Distributed electron cyclotron resonance (DECR) plasma sputtering was used for depositing W/Si multilayers for x-rya optics. The argon plasma used in the sputtering process was excited by the DECR method. The DECR argon plasma diffusing the middle of the deposition chamber was characterized with a Langmuir probe. The ionization rate was found to be 7 X 10^-4. This allowed us to sputter W and Si with a large range of target bias values at low pressures. Deposited single layers and W/Si multilayers were characterized with grazing incidence x-ray reflectometry. As an illustration of the capabilities, data is shown for a W/Si multilayer with period d equals 3.0 nm and an interface roughness (sigma) < 0.47 nm.

A laterally graded (gamma) multilayer interferential mirror (MIM), (where the division parameter (gamma) is the ratio between the thickness of the high refractive index material and the MIM's period), has been manufactured by means of a sputtering technique. The multilayer design is carefully described. This special MIM has been characterized by specular reflectivity X-ray measurements using a diffractometer equipped with a diffracted beam monochromator and Cu-K_(alpha ) (1.5418 angstroms) radiation. It exhibits the properties expected by the so-called X-ray diffraction dynamical theory. We give an experimental law of variation of (gamma) which has been used as a calibration curve. Moreover, results are shown to be in good agreement with computer simulations and transmission electron microscopy study. Absolute reflectivity measurements at 930 eV confirmed these properties. The rocking curves for various (gamma) values of the sample have been measured by means of an original reflectometer. The results presented in the paper show the feasibility of a laterally graded (gamma) MIM. Practical applications in X-ray spectroscopy are obvious and this enhances the potentialities of the MIMs compared with the familiar crystals used in X-ray spectroscopy.

Supermirrors are multilayer structures where the thickness of the layers down through the structure changes so that wide-band reflection occurs. The principles were developed in the mid-70's and have been used extensively for neutron optics. Absorption in the upper layers limits the attainable reflectivity for X-rays. For hard X-rays (>= 15 keV), the absorption, however, is low enough that it is possible to design supermirrors with 10 - 70% reflectivity in a band approximately equals 3 times the width of the total reflection regime. Supermirrors of W/Si and Ni/C have been successfully fabricated and characterized. The measured X-ray reflectivities are well accounted for by the standard dynamical theories of multilayer reflection. Hard X-ray applications that could benefit from X-ray supermirror coatings include focusing and imaging instrumentation for astrophysics, and collimating and focusing device for synchrotron radiation.

Stresses of single and multilayer films in the range of 5 to 40 micrometers are discussed together with their spectral performance. The flatness of a substrate after coating can be less than 4 fringes (2 (lambda) ), even 1 fringe at 0.5875 micrometers . The thermal evaporated thin film materials are II_B - VI compounds (II_B being Zn etc. and VI being S and Se etc. in the periodic table), and PbTe or Ge.

Multilayer reflectors are necessary for spectroscopic and imaging systems, working with soft X-rays ((lambda) equals 1..20 nm), for instance for X-ray lithography, microscopy and holography. We have tested a simple method for deposition of multilayer X-ray reflectors by dc-plasmatron sputtering. W-Si multilayers with up to 80 layer pairs, prepared by this method, were characterized by X-ray reflection, AES, RBS, SNMS and AFM. Optical constants, individual layer thicknesses (1..3 nm) and interface roughness were determined by fit of the X-ray reflection curves.

This paper presents a new type of X-ray reflectometer that measures the grazing incidence reflectivity at an energy of 8 keV for angles from 0 - 2.5 degree(s) simultaneously. With an acquisition time of 5 s this allows to analyze the layer growth during a deposition process with a thickness accuracy in the sub-nm range. As a second independent method we are using kinetic and spectroscopic ellipsometry to follow the same deposition process.

Four types of single-cavity thin-film narrow bandpass filters whose FWHM range from 0.5 to 1.1 nm are produced by ion-assisted deposition of alternating TiO2/SiO2 or Ta2O5/SiO2 layers onto eight substrates having differing coefficients of linear expansion, and the temperature stability of their center wavelengths are examined in the 1540-nm wavelength region. For the eight substrates, whose coefficients of linear expansion range from 0 to 142 X 10^-7/ degree(s)C, the temperature stability of the filters ranges from +0.018 to -0.005 nm/ degree(s)C. Calculations based on a newly developed elastic strain model reveal the main reason temperature stability of the center wavelengths exhibits substrate dependency is due to a reduction in film packing density brought about by volumetric distortion of the film, which is caused by stress induced from the substrate.

Optical components for space optics--especially coated optical elements which represent the external surfaces of optical space instrumentation--have to work under harsh operation conditions like thermal loads, irradiation by photons, electrons and protons, as well as in atomic oxygen environments at low earth orbits. Additionally they have to withstand other cross contamination coming from the spacecraft. Therefore, the stability against these influences is a decisive factor for the application performance of optical coatings in space- borne devices. Some very recent results, based on the Surface Effects Sample Monitor flight experiment carried out aboard the ORFEUS-Shuttle Pallet Satellite, STS-51, Discovery, are presented here along with laboratory experiments in an UHV-surface analysis system. The topics include ground simulation of selective and complex particle bombardment of optical coatings analyzed by XPS as well as the verification of these results by flight experiments in combination with optical measurements (transmission, scattering).

Low earth orbit exposes space materials simultaneously to atomic oxygen and ultraviolet light. Numerous materials were sputtered, e-beam evaporated, and CVD deposited. Experiments with these samples aboard the NASA (USA) 1992 and 1993 space flights STS-46 and STS-51 are discussed, and comparisons made with laboratory studies in an oxygen plasma environment. Multiple samples of thin films of Al, diamondlike carbon, diamond, silicon nitride, silicon carbide, and solar concentrator multilayer stacks were prepared. These were characterized both before and after flight by spectroscopic ellipsometry, spectrophotometry, interferometry, Auger spectroscopy, Raman spectroscopy, and atomic force microscopy.

The properties of materials in thin films are strongly dependent on the coating techniques and on the technological parameters. We have investigated about some optical and structural properties of YF₃ thin films prepared using different energetic techniques: ion assisted deposition (IAD) and ion beam sputtering (IBS). The properties of the thin films obtained by these energetic processes are compared to the properties obtained by classical electron beam evaporation. In classical evaporation, the optical properties in the visible range depend on the temperature of the deposition and on the incidence of the vapor flux. The optical properties are correlated with the density of the films measured by Rutherford backscattering. In the case of IAD, the influence on optical properties, both in the visible and in the infrared range, of some technological parameters (pressure, ion energy and ion density) are illustrated. The refractive index and the extinction coefficient have been obtained by spectrophotometry. Furthermore, we show that IBS may lead to YF₃ layers of high density.

A series of nitrogenated and nitrogen-free carbon films was prepared under various deposition conditions (substrate temperatures between 40 - 535 degree(s)C and working gas pressures between 0.09 - 5.0 Pa). Rutherford backscattering, Raman scattering, surface profilometry, ellipsometry, hardness and adhesion measurements were performed and results were related to both nitrogen and oxygen content in the layers.

In space optics, optical components are required more and more insensitive to extremely severe environmental conditions, with guaranted time of life and, concerning coatings, very severe spectral specifications. For most applications, coatings made with oxyde layers are not anymore convenient, because of their low compactness. Absorption and desorption of water in atmosphenc or vacuum conditions induce stress in the stack which can involve incertainty in the stability in life. Indexes of materials are higher in atmospheric pressure than in vacuum, which makes shift the spectral curve. High and low index material which are quarter wave during evaporation in vacuum are not anymore quarterwave after beeing put in atmospheric conditions, which can completely affect the shape of the spectral curve. This phenomenon is particularely sensitive for bandpass interference fifters which cannot be manufactured by clasic thermal evaporation of oxyde materials. Less hard coatings can be used if protected by bonding but these kinds of components are sensitive to climatic conditions, involving changes in wavefront. Moreover, the resistivity of the cement to irradiations is not fully convenient. So it was necessary to develop new kinds of hard coatings with very high compactness, deposited on external faces of single substrates.

Antireflection coated ZnS and Ge substrates erode under severe operational environmental conditions. High velocity water drop impact and high velocity sand particle impact are primarily military concerns that originated with the advent of faster aircraft. High speed flight through rain and sand storms seriously erodes forward facing components such as infrared transmitting windows and/or domes. This erosion of windows and/or domes causes reduction in transmission, resulting in the reduction of detection and recognition sensitivity of the electro-optical sensor. A single film of one quarterwave thick hard-carbon coating has been used on germanium to increase optical transmission (reducing Fresnel's reflection losses on Ge surface) as well as to reduce rain and sand impact damage to some extent, at a lower speed. At high speed, the damage becomes more severe, resulting in unacceptable large transmission losses. Recently, new hard carbon coatings have been developed for Ge which have substantially increased the damage threshold of the coated substrates. The rain erosion test was performed at Wright-Patterson AFB facility in Dayton, Ohio, and the sand tests were performed at PDA Engineering in Santa Ana, California. In addition, a multilayer AR coating utilizing hard carbon film as one of the low index films has also been developed at Hughes for ZnS substrates. The optical properties, rain erosion, sand erosion, and sand abrasion test results of these coatings are also presented in this paper.

Storage ring Free Electron Lasers (FELs) are very promising tunable and powerful laser sources for the UV range. However, the relatively small gain obtained in the UV, requires the use of high reflectivity multidielectric mirrors. In addition, all the synchrotron radiation (not only the first harmonic where the laser operates) emitted by the undulator, consisting of a wide spectrum of the harmonics of the fundamental wavelength extending towards the X rays is received by the front mirror of the laser cavity. These higher harmonics are responsible for the mirror degradation. The reflectivity of the multidielectric mirrors has to remain at a high level even in the presence of synchrotron radiation covering a wide spectral range, leading to specific requirements for FEL optics. The FEL at 350 nm was obtained with Ion Beam Sputtering of Ta₂O5/SiO₂, with super polished substrates. Extension towards shorter wavelength requires to use another type of material such as HfO₂/SiO₂. Substrates and mirrors are characterized in roughness, transmission and total losses. The evolution of the mirror degradation is studied versus the deposition technology. The realization of high reflectivity mirrors submitted to the radiation from the undulator remains a challenging issue for the development of tunable short wavelength FELs.

A new, very simple AR coating synthesis method is further developed and compared to a recent `quasi-optimal' technique. A remarkable resemblance is found in the results. The reason for this similarity is given and the performance of both approaches is discussed.

The ablation damage thresholds for electron beam evaporated HfO₂/SiO₂ single-, double-, and triple-layer systems of (lambda) /4 optical thickness ((lambda) equals 248 nm) are measured by the pulsed photoacoustic mirage technique. It is shown that the apparent threshold depends on the specific system design and results are interpreted in terms of the electric field distribution in the respective configuration. For one system it is demonstrated that the threshold for the onset of damage measured by the mirage technique can be enhanced by a factor of two when the layers are deposited by laser assisted evaporation technique.

Characterization of rough single and double layers is performed by interpreting spectral dependences of the coherent reflectance measured for these layers. Possibilities and limitations of the method are illustrated by means of several concrete samples of both the rough single and double layers represented by models corresponding to fully correlated (identical), partially correlated and fully uncorrelated boundaries.

An original way of synthesizing optical multilayer systems is presented. Based on a genetic algorithm (GA), this method searches for the optimal sequence of materials and the thicknesses of each layer. The basic theory of GA's as well as their specific application to the design of optical coatings is described. Different examples are shown and the interest of the method is discussed.

A recent program at Pilkington Optronics (Barr & Stroud) has resulted in the identification of a nonradioactive thorium fluoride replacement film material with reduced toxicity. As described in this paper, the materials has been incorporated into high-efficiency single and dual-band AR coatings for germanium, zinc selenide, and zinc sulphide, without compromising optical and durability performance.

Ion beam sputter deposition (IBS) and dual ion beam sputter deposition (DIBS) of tantalum oxide films was investigated at room temperature and compared with similar films prepared by e-gun deposition. Optical properties ie refractive index and extinction coefficient of IBS films were determined in the 250 - 1100 nm range by transmission spectrophotometry and at (lambda) equals 632.8 nm by ellipsometry. They were found to be mainly sensitive to the partial pressure of oxygen used as a reactive gas in the deposition process. The maximum value of the refractive index of IBS deposited tantalum oxide films was n equals 2.15 at (lambda) equals 550 nm and the extinction coefficient of order k equals 2 X 10^-4. Films deposited by e-gun deposition had refractive index n equals 2.06 at (lambda) equals 550 nm. Films deposited using DIBS ie deposition assisted by low energy Ar and O₂ ions (E_a equals 0 - 300 eV) and low current density (J_i equals 0 - 40 (mu) A/cm²) showed no improvement in the optical properties of the films. Preferential sputtering occurred at E_a(Ar) equals 300 eV and J_i equals 20 (mu) A/cm² and slightly oxygen deficient films were formed. Different bonding states in the tantalum-oxide films were determined by x-ray spectroscopy while composition of the film and contaminants were determined by Rutherford scattering spectroscopy. Tantalum oxide films formed by IBS contained relatively high Ar content (approximately equals 2.5%) originating from the reflected argon neutrals from the sputtering target while assisted deposition slightly increased the Ar content. Stress in the IBS deposited films was measured by the bending technique. IBS deposited films showed compressive stress with a typical value of s equals 3.2 X 10⁹ dyn/cm². Films deposited by concurrent ion bombardment showed an increase in the stress as a function of applied current density. The maximum was s approximately equals 5.6 X 10⁹ dyn/cm² for Ea equals 300 eV and Ji equals 35 (mu) A/cm². All deposited films were amorphous as measured by the x-ray diffraction method.

A laser calorimeter, which is able to measure absorption as los as 1 - 10^-6 of incident laser power even with the use of an argon laser of less than 1 W at 514.5 nm, was developed. The required stability of the temperature background is achieved by a special design of the measurement system minimizing heat flow. Evaluation of absorption is realized by numerical simulations taking into account all relevant processes of heat generation and dissipation. Furthermore, absorption is determined according to ISO/DIS 11 551. Interference influences from the substrate are taken into account.

This paper describes the optical layout and spectral design requirements for filtering in the High-Resolution Dynamics Limb Sounder and reports progress o the manufacturing and testing of the submillimeter-sized cooled filters. We also report on the spectral and environmental performance of prototype wideband antireflection coatings that cover the entire spectral range of the focal plane array channels.

The dispersion relation of optical constants of sol-gel deposited hydrated tungsten oxide films have been measured using spectroscopic ellipsometry. The films were deposited on glass substrates from a tungsten butoxide precursor solution using a dip-coating technique and fired a t three different temperatures, 200 degree(s)C, 300 degree(s)C, and 400 degree(s)C. Ellipsometric measurements at 7 different angles of incidence and over the range 1.5-5 eV have been made for each of the films. The amorphous semiconductor dispersion relation proposed by Forouhi and Bloomer was used to model the tungsten oxide optical constants and to fit the measured ellipsometric angles to determine the parameters in the model. the refractive index and extinction coefficient have been found using this procedure. Computer simulations of transmittance of the films, based on the optical constants obtained from the ellipsometric measurements and the fitting procedure, have been compared to the measured spectra. This comparison confirms that the optical constants found from ellipsometry describe the system accurately. The effect of increasing the annealing temperature of the films on the optical parameters is also discussed. In particular, changes are observed in the optical energy gap and film thickness. The reasons for these variations is discussed in terms of the microstructure of the films.