One of the reasons why the current optics used in 193 nm lithography do not have sufficiently high reflectivity and have poor optical timing is the lack of a low-refractive-index material with low absorption, low scattering, low residual stress, amorphous structure, and good aging resistance. In this study, we investigated the effect of the AlF3 content in MgF2-based low-refractive-index materials on the microstructure and optical properties of Mg1-xAlxF nanocomposite coatings prepared by thermal evaporation. Based on these findings, this study successfully solved the problems of large absorption, easy crystallization tendency, and poor aging performance of thermally evaporated AlF3 films. A novel low-refractive-index nanocomposite material with improved absorption reduction, scattering suppression, and aging resistance was obtained, which can be applied to the study of reflective optics for 193 nm lithography.
Optical coatings are enabling technology for modern optical systems, they are almost applied on every surface of the optical components. However, due to the uniform structure of the optical coatings, optical coatings have limited capability manipulating electromagnetic characteristics. Optical metasurfaces can locally manipulate optical field and enhance light–matter interactions, thus offering fascinating possibilities to control various properties of light, such as amplitude, phase, and polarization. While many new physical effects and applications were demonstrated based on metasurfaces, their practical application still faces challenges of low optical efficiency. Here, we propose the quasi-three-dimensional subwavelength structures, consisting of optical coatings and metasurfaces, to promote the efficiency of metasurfaces. We will present our recent advances in high-efficiency quasi-three-dimensional subwavelength structure devices. Our results pave the way to realizing optical meta-devices facing strict efficiency requirements in realistic applications.
La1-xAlxF3 nanocomposites with different AlF3 doping ratios were synthesized using a dual-source electron beam co-evaporation technique to achieve amorphous La1-xAlxF3 coatings with low loss and tensile stress. We analyzed the evolution laws of optical constants, microstructures, film stress, and film loss of La1-xAlxF3 nanocomposites with the change of element content. When x ≥ 0.30, high-refractive-index nanocomposites La1-xAlxF3 demonstrated reduced absorption, integrated amorphous structure, and lower tensile stress. The nanocomposite exhibited superior performance with an excellent overall structure, as well as reduced tensile stress. Additionally, this material was employed to create a high-performance reflective film with a high reflectivity.
Dispersive coating is the enabling technology of state-of-the-art ultrafast laser systems. This paper presents the research progress range from the multi-objective design method, coating materials, advanced monitoring technique to improve the performance of the dispersive mirrors.
At sufficiently high intensities the electronic nonlinear behavior of optical materials dominates the classical linear phenomena. Through laser calorimetric absorption (LCA) measurements this behavior has been characterized and an increase in absorption of over one order of magnitude has been observed. Quantum nanolaminates (QNLs) are uniquely suited to investigating these phenomena as it is possible to tune the refractive index and bandgap. The absorption and electronic behavior of QNLs deposited with Titania and Niobia in conjunction with Silica were investigated using LCA. The impact of defects on the measurements are also discussed.
Nonlinear absorption is mainly governed by mechanisms involving excitation processes of electrons. Typically, two phenomena are considered when discussing nonlinear absorption; the multiphoton absorption where multiple photons interact directly with a single electron, and tunnel ionization, where the high electric field results in a shifting of the bandgap allowing an electron to tunnel into the conduction band. Electrons in the conduction band can be accelerated through the absorption of further photons until they obtain enough energy to excite further electrons to the conduction band, leading to runaway absorption and finally damage of the sample. By laser calorimetric measurement of the nonlinear absorption, it is expected that the laser damage threshold can be predicted without damaging the optic. Before accurate predictions can be made, the process must be thoroughly characterized and understood. The nonlinear behavior of the absorption was demonstrated with potential increases in absorption of an order of magnitude. Initial results show a noticeable impact of contaminants, though a nonlinear response is still observed.
A laboratory-based reflectometer designed for characterizing the reflectivity of optical coatings in 30- to 200-nm wavelength range was recently developed at IPOE. An RF-produced gas-discharge light source is applied to generate characteristic lines. The light source is mounted on a grazing incident monochromator with a 146-deg deviation angle between the incident and diffracted arms. By precisely adjusting the toroidal grating inside the monochromator chamber, monochromatic lights are acquired through the exit slit. A collimator mirror and two sets of collimation slits with 2 mm × 2 mm dimension are utilized for reducing the divergence of the beam incident on the sample. A high-precision six-axis translation stage, which allows a heavy sample with a maximum diameter of 100 mm, is used to control positions of the samples and the detector. A chopper disk used both for incident light intensity monitor and signal modulation is placed with an incidence angle of 70 deg relative to the incident light beam. The configuration, adjustment process, and test results of the reflectometer are presented in detail. The experimental reflectivity results for Al / LiF / MgF2 film obtained from our laboratory and BESSY-II Synchrotron as well as Hefei Synchrotron Light Source are given and compared for demonstrating the reliability of the system.
HfO2 prepared by ion beam sputtering (IBS) is widely used as a high-refractive index material for making low-loss laser films. The challenge of a high-performance HfO2 film is to simultaneously obtain an amorphous morphology with low scattering and a stoichiometric structure with low absorption. Furthermore, nanometer-sized voids are commonly present in IBS films due to excessive oxygen and argon adsorption during deposition, which is the primary barrier to achieve a smooth surface and low optical loss of HfO2. Thin amorphous SiO2 layers were added periodically into HfO2 coatings using the IBS process to synthesize amorphous HfO2 / SiO2 nanolaminate-based composites. The resulting composites exhibited excellent comprehensive performance with a dense amorphous microstructure and a void-free smooth surface. High-temperature annealing was performed to ensure superior stoichiometry and lower absorption. However, the crystalline states and microstructure of some composites evolved during the gradual annealing. We present a detailed study of the crystallization, surface topography, and absorption evolution in HfO2 / SiO2 nanolaminates as a function of HfO2 sublayer thickness and thermal annealing temperature. Moreover, the interplay between crystallization, surface topography, and absorption is elucidated. The HfO2 / SiO2 nanolaminate with 19 thin layers maintained a dense amorphous structure with low absorption after annealing. Finally, a 1064-nm HfO2 / SiO2 nanolaminate-SiO2 high-performance reflector was prepared and achieved lower absorption with a smooth surface after annealing, which demonstrated the great potential of the HfO2 / SiO2 nanolaminates for considerably improving optical loss.
The CAFE (Census of warm-hot intergalactic medium, Accretion, and Feedback Explorer) and LyRIC (Lyman UV Radiation from Interstellar medium and Circum-galactic medium) have been proposed to the space agencies in China respectively. CAFE was first proposed as a joint scientific CAS-ESA small space mission in 2015. LyRIC was proposed as the independent external payload operating on the Chinese Space Station (CSS) in 2019. Both missions are dedicated to mapping the Lyman UV emissions ( ionized oxygen (OVI) resonance lines at 103.2 and 103.8 nm, and Lyman series) for the diffuse sources either in our Galaxy or the circum-galactic mediums of the nearby galaxies. We present the primary science objectives, mission concepts, the enabling technologies, as well as the current status.
The 1064nm partially reflective coatings has been widely used in high-power laser optical components. The current research status on 1064nm partially reflective coatings is that it is difficult to balance the electric field and spectral tolerance. In order to satisfy both high spectral efficiency and high laser induced damage threshold, we use HfO2 and SiO2 to design and fabricate three kinds of coatings with different electric field distributions. This paper measured the actual spectra of the three coatings and obtained high spectral efficiency. The damage threshold of the coatings is measured by the damage threshold test platform built by the laboratory. The final research results show that when the matching layer is on the side of the substrate, the electric field is lower and the damage threshold of the coatings is higher.
Thin-film narrowband filter can be used as a beam combining component in a laser beam combining system, the output beam has the advantages of narrow spectral width, good wavelength stability and high brightness. However, due to the narrow cut-off band, the electric field inside the thin film structure is very large under the laser irradiation, which leads to the great absorption and scattering losses of the thin film and affects the output beam quality. In this paper, we analyze the electric field distribution and measure the absorption and scattering losses of the thin film filter working around the wavelength of 1064nm by continuous high-power laser irradiation. Moreover, the influence of annealing process as well as super polished substrate on the absorption and scattering losses and transmission spectrum of the filter is explored.
We have designed two kinds of narrow-band thin film filters with Ta2O5/SiO2. In order to use them as laser beam combining elements, the multi-field simulation of two kinds of filters under CW (continuous wave) laser oblique incidence is carried out. After comparing the electric field, thermal field and stress field of the two kinds of film profiles during transmission and reflection, the change rules of each field under transmission and reflection conditions are obtained. For different film structures, the electric field strength, temperature and stress in the film are higher than the reflection’s. After that, the multi-field results of the same design is analyzed and the internal relations among the fields are obtained. The higher the electric field strength is, the more the temperature rise is and the greater the thermal stress is. Finally, the temperature rise of the filters with different focusing effects is simulated when the CW laser is oblique incident. We found that with the enhancement of focusing effect, the overall temperature rise increases, otherwise the temperature rise decreases.
Ultralow-loss thin films have a very high prospect in applications such as frequency - stabilized lasers and optical atoms. GaAs/Al0.94Ga0.06 As distributed Bragg reflector(DBR) prepared by the metal organic vapor deposition (MOCVD). Systematically studied the GaAs/Al0.94Ga0.06As Bragg reflector scattering phenomenon and the absorption loss of DBR. The results show that the prepared DBR exhibits excellent surface morphology, RMS is about 0.2nm. The first order vector perturbation theory is used to analyze the scattering phenomenon of DBR. The ARS value of Bragg mirror is basically consistent with the experimental results under the full correlation theory, and the interface correlation coefficient is very close to 1. The total backscattering (TSb) of DBR was measured, and the results were less than 5ppm. At the same time, the absorption loss was studied, and a slightly higher absorption loss was detected, about 200ppm. Hall effect shows that the carrier concentration of DBR is as high as 1E18/cm3, and the free carrier absorption caused by P-type background doping is suspected to be the root cause of the excessive absorption.
Multilayer dielectric gratings (MDGs) have been widely used in spectral beam combining, chirped pulse amplification, and external cavity diode lasers systems. However, the one-dimensional MDGs suffer from the polarization-sensitive properties due to the modulation in only one direction. Two-dimensional (2D) MDGs have more flexible control ability to realize polarization-independent properties but are rarely reported. Here, we demonstrate the realization of polarizationindependent broadband MDGs via 2D grating array. The 2D Si:H MDG exhibits more than 97% non-polarized diffraction efficiency over 90 nm. Further, the influence of the refractive index on high-efficiency bandwidth of the MDGs was investigated via a comparative study using Si:H and TiO2, materials with a large difference in refractive indexes. The high-efficiency bandwidth of Si:H MDGs is near 1.4 times the breadth of TiO2 MDGs. Our results provide a new platform and deeper understanding to realize polarization-independent broadband MDGs.
HfO2 film prepared by ion beam sputtering (IBS) has greater potential in low loss laser films. However, it is prone to form sub-stoichiometric and polycrystalline structure resulting in high absorption and scattering loss. Besides, the adsorption of excess oxygen atoms will produce oxygen inclusions in the surface of HfO2 during fabrication, leading to the formation of surface hole defects, thereby increasing the surface roughness. Annealing is an important method to achieve better stoichiometry, but it tends to induce crystallization or recrystallization of the film. In this paper, thin amorphous SiO2 layers were periodically added into HfO2 coatings to synthesize HfO2/SiO2 nanolaminate layers, which can effectively suppress crystallization during HfO2 growth, increase the crystallization threshold temperature during thermal annealing, eliminate hole defects on the film surface and minimize the absorption and scattering loss. We present a detailed study of the crystallization, surface topography and absorption evolution in HfO2/SiO2 nanolaminate layers as a function of sub-layer thickness and thermal annealing temperature. It was found that thinner HfO2 layers show higher threshold temperature of crystallization, so it can maintain almost zero absorption and amorphous state under high temperature annealing.
Third harmonic separator has proven to be a critical component in high-power laser systems, which can be used for beam splitting and optical path change of 355 nm laser. It is desirable for the coating to have high spectral efficiency and laser induced damage threshold. Through precise fabrication, the spectrum efficiency can easily meet the requirements with high reflection at 355nm and high transmittance at 532nm and 1064nm. However, the issue is more complicated and quite challenging for obtaining high LIDT, because high refractive index materials tend to have high absorption at ultraviolet, which contribute to a decreased LIDT together with the electric field inside the film. It is significant to investigate the interplay between absorption, electric field intensity and LIDT. In this paper, materials of HfO2 and SiO2 are used to design and fabricate the two third harmonic separator with similar film structure, high spectral efficiency and different electric field distribution by electron beam evaporation. Spectral measurements results show that the prepared samples all have similar spectra with high spectral efficiency. Damage threshold and weak absorption test results show that high intra-film electric field distribution results in higher weak absorption and lower damage threshold. Our research has important reference significance for the design and preparation of high damage threshold third harmonic separator.
Laser-induced damage (LID) of optical coatings has been extensively studied since the invention of the laser. It has been found that the defects, which are unavoidable in real-world optical coatings, are the main reason for triggering laser damage of optical components at low fluences. In particular, embedded nodules in dielectric multilayer coatings are the main limiting defects found in reflective optics operating in nanosecond regimes. During laser irradiation, thermomechanical damage occurs preferentially at nodules because of enhanced energy absorption due to electric-field intensity (EFI) enhancement and the degradation of mechanical stability due to discontinuous boundaries. This report reviews the recent studies of the LID due to nodular defects in dielectric multilayer coatings. First, we present statistical studies on the geometric model and laser damage mechanism of nodules in the real world and introduce the solutions to control the formation of nodules. However, the low density and diverse properties of real nodular defects make the systematic study of LID initiating from localized defects a time-consuming and challenging task. In this regard, experimental and theoretical studies of localized defect-driven LID using artificial defects with properties that can be controlled are highlighted. We also present recent research progress on the damage mechanism of artificial nodules interpreted from aspects of mechanical properties and electric-field enhancement. In addition, approaches for modifying the deposition process or multilayer design are examined to reduce the EFI enhancement in the nodules and to improve the laser damage resistance of near-infrared high-reflective coatings based on the deeper understanding of the underlying physics of the damage process.
Defects in multilayer coatings significantly increase scattering loss and degrade the performance of the advanced laser systems like gravitational wave detection, laser gyroscope and ultraviolet laser, etc. The quantitative description of the defect-induced scattering in multilayer coatings was rarely addressed. In this work, the scattering characteristics of the nodular defects in Ta2O5/SiO2 multilayer coatings for 1.064 μm wavelength was simulated using finite-difference time-domain method. First, the simulation process was given, several important steps that include structure modeling, far field calculation and far field to angular resolved scattering were presented. The scattering distributions of the nodular defects are similar to Airy diffraction pattern that is from a circular hole. The roughness-induced scattering from the coatings was calculated using first-order perturbation theory. The characteristics of the defect-induced scattering is significantly different from the roughness-induced scattering. The total scattering loss is approximately three orders of magnitude stronger than the roughness-induced scattering.
Efficiently controlling the propagation of light is a most important issue in the optics. The traditional optical elements manipulate light by phase accumulation through light propagation. Metasurfaces, serving as nanoscale phase shift, have attracted the interest because they possess the ability offering fascinating possibilities to realize unprecedented photonic phenomena while interact with light over the scale of the wavelength. Although kinds of novel functionalities are reported, monolayer metasurfaces and hybrid metasurfaces are limited by their less geometric structure parameters. Since the hybrid multilayer metasurfaces have more adjustable parameters, expanding the capacity to control the light beam, we use the hybrid multilayer metasurfaces to realize polarization-insensitive anomalous reflection and polarization-sensitive focusing. The gradient metasurface exhibits high efficiency anomalous reflection for both x-polarized and y-polarized light. The metalens has opposite focus for right-handed circularly polarized and left-handed circularly polarized incident light. The results may offer some help to find various potential applications in nanophotonics.
Planarization of nodular defects was investigated in order to improve the laser-induced damage threshold (LIDT) of high-reflection coatings. Monodisperse SiO2 microspheres were first deposited on the substrate surface by a spin coating process. Using a dual ion beam sputtering system, these engineered seeds were used to create artificial nodules in 1064nm HfO2/SiO2 high-reflection coatings and Ta2O5/SiO2 high-reflection coatings. These SiO2 microspheres were then smoothed by a single thick SiO2 planarization layer, where the relationship between the thickness of the planarization layer and the size of the microspheres was investigated. When the planarization layer (etching layer) thickness is slightly larger than the diameter of the seeds, the seeds could be completely planarized to obtain smooth thin films. In addition, the LIDT of the high-reflection coatings with different coating materials and different planarization layer thicknesses were tested. The results showed that the nodular defects planarization could noticeably improve the damage resistance of high- reflection coatings. In addition, the surface roughness of Ta2O5/SiO2 high-reflection coatings was shown to decrease after the planarization, while the surface roughness of the 1064nm HfO2/SiO2 high-reflection coatings was shown to increase.
Optimal solution for the multilayers AR coating of 4J solar cell including design and fabrication was investigated. The real layer structure of 4J solar cell was determined first through reverse fitting, then the multilayers AR coating could be designed on the complete 4J solar cell to find the global optimization. After clarifying that the performance of multilayers AR coating is affected by oxidation of AlInP, the oxide layer was characterized and compensated to improve the spectrum. Through transmission electron microscopy (TEM), the oxidation depth was determined to be 5nm. Finally, the multilayers AR coating was re-optimized by considering oxidation layer into 4J solar cell. Using ion assisted deposition, the multilayers AR coating was fabricated with a slightly increased weight average reflectance of only 0.5%.
We have designed and experimentally demonstrated a periodic multilayer structure of SiO2 and Cr thin interlayers to achieve an ultra-broadband perfect absorber based on optical admittance matching method. The successive Nano-Cr-film make significant contribution to improving the absorption intensity of the structure. Measurements reveal high absorption over 85%, when averaged over the range 0.4–7.2 μm. Remarkably, it is the most broadband planar absorber film without involving lithography in fabrication. Incident angle and polarization dependence of the absorption spectra are also considered. The manufactured absorber also has potential applications for thermal shielding, detecting, imaging, photovoltaics (PVs), sensing, etc.
Optical metasurfaces, two-dimensional counterparts of the metamaterials, are comprised of arrays of subwavelength engineered inclusions that can locally modify optical field and light–matter interactions, thus offering fascinating possibilities to realize unprecedented photonic phenomena. Here, a micro-structured optical coating or a hybrid multilayer structure is proposed. It is easy to achieve phase control over 2π by changing the geometric parameter, which is used to design gradient metasurface exhibiting anomalous reflection for linearly polarized light (1μm). By arranging the arrays elaborately, the efficiency can reach as high as 93.6%. Besides, reflective metalens taking advantage of the Pancharatnam-Berry phase is demonstrated. The electric field distribution unambiguously indicates that the metalens can focus the plane wave with high efficiency (60.3%). Such structure may find various potential applications in nanophotonics because of their high freedom to design metasurfaces.
Low absorption, low scattering and high-density HfO2 coatings lead to an important improvement of
high power laser systems. In order to suppress the crystallization of HfO2 coatings fabricated with ion
assisted deposition (IAD), we used double electron-beam (EB) coevaporation with ion beam assisting
method to fabricate HfO2-SiO2 mixtures from two independent material sources. Crystallization
following the different mixture ratios was investigated. Several prototypes were designed, featuring
different HfO2-SiO2 ratios with similar physical thickness (500nm). The samples were deposited on
fused silica and silicon substrates. X-ray diffraction patterns show that the degree of crystallization
gradually fades away with increasing SiO2 contents and when SiO2 component reach 18%, the mixture
film becomes almost amorphous. To decrease the high absorption originating from IAD method,
thermal annealing in air at progressive temperatures was subsequently performed. It was found that
post-annealing treatment at 600°C could eliminate the absorption at 1064nm. However, high
temperature annealing would induce the crystallization of these initial amorphous coatings. In order to
suppress the crystallization further to obtain amorphous structure even after 600°C annealing, the 25%
SiO2 sample was fabricated and it successfully obtain low roughness and low absorption equivalent to
the bare substrate. Finally, a 1064 nm HR coating using SiO2 and mixed film of 25% SiO2
concentration was prepared and annealed to prove its practical application in low loss and high LIDT
optical elements.
The operating fluence of traditional zigzag slab laser amplifier is limited by the interfacial laser damage between the laser crystal and SiO2 film at the total internal reflection surface. A novel coating solution for the total internal reflection surface was proposed to increase the laser damage resistance. A HfO2 layer with optimal thickness was inserted between the laser crystal and SiO2 film to manipulate electric-field intensity at the vulnerable crystal-film interface to be zero. The laser damage resistance of this new coating for the total internal reflection surface is about two times higher than that of traditional zigzag slab laser amplifier.
For the sol-gel method, it is still challenging to achieve excellent spectral performance when preparing antireflection (AR) coating by this way. The difficulty lies in controlling the film thickness accurately. To correct the thickness error of sol-gel coating, a hybrid approach that combined conventional sol-gel process with ion-beam etching technology was proposed in this work. The etching rate was carefully adjusted and calibrated to a relatively low value for removing the redundant material. Using atomic force microscope (AFM), it has been demonstrated that film surface morphology will not be changed in this process. After correcting the thickness error, an AR coating working at 1064 nm was prepared with transmittance higher than 99.5%.
It has been verified that a broadband high-reflection (HR) film could restrain electric-field intensity (EFI) enhancement effect in the nodular defects at normal incidence. However, it’s impossible to design an omnidirectional HR coatings to avoid the light penetration from all incident angles at oblique incidence. In this paper, the EFI enhancement is simulated by using a three-dimensional finite-difference time-domain (FDTD) code. Two types of polarizers that prevent light penetration at low and high incident angular range (IAR) are proposed to explore the influence of transmission band at different angles in the case of oblique incidence. The damage morphologies of nodules initiating from different diameter silica microspheres in polarizers reproduce the simulated EFI distributions very well. These results indicate that light penetration at high IAR rather than at low IAR contributes mostly to EFI enhancement. Then, the conclusion is proved further by the films with low and high IAR at normal incidence. Controlling the angle position of transmission band at small angle can reduce the EFI enhancement in the usual case and increase the laser-induced damage threshold (LIDT) of films.
A broadband high-reflection (HR) coating was used to suppress the electric-field intensity (EFI) enhancement in artificial nodules with five different sizes. However, the finitie-difference time-domain simulations reflected that the nodules initiating from 1.0-μmSiO2 seeds showed abnormally stronger EFI enhancement, which is almost two times higher than the EFI enhancement of 1.0-μmSiO2 seeds in a quarter-wave HR coating. This was also confirmed by the laser-induced damage threshold measurement. Our previous model combining light focusing and light penetrating effects was carefully examined to check whether the hotspots in a nodule initiating from the 1.0-μmSiO2 seed were in the focal area or not. Although it was found that the focal length of the nodule decreased with reducing seed diameter, the hotspots in nodules initiating from a 1.0-μmSiO2 seed were still much shallower than the focal area. In the broadband HR coating, the standing-wave EFI profiles at different working angles were given, which showed that the standing-wave EFI at the hotspots region was not negligible. Some complex interference or diffraction may cause the light to arrive at the hotspots region in phase and result in strong EFI enhancement. More work is necessary to gain a deeper understanding of this phenomenon.
Laser-induced damage of high reflection (HR) coatings, working at near ultraviolet (NUV) and near infrared (NIR)
regions was investigated. For NIR HR coatings, the nodules still remain the most limiting defects. The E-field intensity
(EFI) enhancement in nodules plays a central role for triggering laser-induced damage. We established a simple model
for EFI enhancement in nodules using the focusing and light penetrating concept. With the help of finite-difference time
domain (FDTD) simulations, we found that refractive indices and nodular geometries affected the focal length as well as
the size of focal spots. Furthermore, the angular reflection bandwidth (ARB) of nodules determined the fraction of light
that can penetrate to the focal region. For NUV HR coatings, we explored the increase of the laser-induced damage
threshold (LIDT) by increasing the incident angle from 0 degrees to 65 degrees for S-polarization. The EFI in a 65
degree HR coating is more than 4 times lower compared to 0 degree HR coatings, which suggests that the LIDT of 65
degree HR coating is much higher compared to 0 degree HR coating. However, we found some contradictory results. For
small testing laser beam size with a diameter of 20 μm, the LIDT of 65 degree HR coating is 3.5 times higher compared
to a 0 degree HR coating. However, for a large sized testing laser beam with a diameter of 1000 μm, the LIDT of 65
degree HR coating is 2 times lower compared to a 0 degree HR coating. Possible reasons for the observed damage
phenomena are discussed.
Surface cleanness and roughness to BK-7 substrates are important factors affecting the performance of laser optics. The
conventional RCA cleaning method is widely used in removing particles from substrate surface, with high removal
efficiency but rough surface. Therefore, more precise control of the chemical cleaning performance of BK-7 substrate is
required than what is available today. In this study, four groups of BK-7 samples were dealt with different cleaning
treatments to explore the effects of chemical solutions. The influences of chemical solutions on removal efficiency,
etching depth and surface roughness were studied. An optimal cleaning method of BK-7 substrates was proposed, which
could remove contaminations completely and gets smoother surface.
By creating nodules from artificial seeds, the damage behaviors of engineered nodules were systematically studied from
experimental approaches. The seed diameters, seed absorption and film absorption were varied independently to uncover
a single factor's influence on the damage behavior of nodules. First, non-absorbing monodisperse SiO2 microspheres
with five different sizes were used to create engineered nodules in 1.053 μm HfO2/SiO2 high reflectors that were
prepared by EB process. Laser damage test results showed that the ejection fluences of nodules monotonically decreased
with the increase of silica microsphere diameters. And to our surprise, nodules initiating from 0.3 and 0.6 μm silica seeds
survived the maximum fluence of 170 J/cm2 (10 ns). Film absorption also has big influence on the damage behaviors of
nodules. Compared to the nodules in low absorbing reflectors that were prepared by EB process, the nodules in high
absorbing reflectors that were prepared by IAD process exhibited a much lower ejection fluences, although the seed
diameters for the comprising nodules were same. Moreover, aluminum seeds were also used to create engineered nodules.
Laser damage test results showed that the ejection fluences of nodules initiating from aluminum seeds were around 2
J/cm2 (10 ns), which is more than an order of magnitude less than the ejection fluences of nodules created from nonabsorbing
silica seeds. This result revealed that the seed absorption played a very important role in the laser damage of
nodules.
Nodules have been proved to play an important role in the activation of laser damage in 1.053 μm HfO2/SiO2 high
reflectors. However, some damage test results revealed that the ejection fluences of some big nodules with height around
1 μm were abnormally high. To find the correlation between the surface dimensions of nodules and their susceptibility to
nano-second pulsed laser radiation, monodisperse SiO2 microspheres with five different sizes were used to create
engineered nodules in 1.053 μm HfO2/SiO2 high reflectors. The defect density of nodules that were created from SiO2
microspheres was purposely controlled to be around 20-40 mm2 and special care was taken to minimize clusters of SiO2
microspheres as less as possible. This enabled us to take a raster scan test and to get the statistical value of ejection
fluences of these engineered nodules. The height and width dimensions of the engineered nodules, especially the
discontinuity of nodular boundary, were measured by cross-sectioning of nodular defects using a focused ion-beam
milling instrument. Based on the above information, the damage test results were interpreted from the aspects of electric
field enhancement model and mechanical stability of nodular structures.
In order to evaluate the laser induced damage threshold (LIDT) of our HfO2/SiO2 high reflectance films prepared by
reactive electron beam evaporation process at 1064nm and 532nm, the four popular test methods are performed
according to the ISO 11254 and other relevant standards. The improvement of laser conditioning effect and influence of
cumulative effect have been studied and estimated during the tests by comparing the deviations of the thresholds along
the damage probability curves and relationship between the thresholds and the pulses number. Moreover, the details are
carefully inspected during raster scan especially at 1064nm with all the >2μm nodules and damage sites marking and
recording, then the ejection probability and growth rate of nodules are given. Note that attention should be paid to the
submicron absorbing particulates at 532nm which are probable to trigger the damage.
One TiO2/SiO2 high reflector with absorption of 300 ppm and two HfO2/SiO2 high reflectors with absorption of 40 and
4.5 ppm were fabricated using electron beam evaporation method. The influence of average absorption on laser induced
damage threshold (LIDT) at 1064 nm of these coatings was studied using r-on-1 test mode. The LIDT of high absorbing
TiO2/SiO2 coating is only 6.2 J/cm2 at 3 ns, whereas, two weak absorbing HfO2/SiO2 high reflectors got almost the same
LIDT that was over 100 J/cm2 at 3 ns. Then a raster scan test method was employed to find the typical damage
morphologies of these coatings with different absorption level. For TiO2/SiO2 high reflector with high average absorption,
the representative damage morphologies are shallow pits that were probably caused by absorbing centers. However, for
HfO2/SiO2 high reflectors with low average absorption, the dominant damage morphologies are micrometer-sized
nodules ejected pits and the delamination initiating from the pits. The absorption of HfO2/SiO2 coatings is low enough to
have minor influence on the laser induced damage.
HfO2/SiO2 dichroic mirrors were prepared using reactive electron beam evaporation process. The mirrors have high
reflectance at 532 nm (S polarization) and high transmittance at 1064 nm (P polarization) for angle of incidence of 45°.
Here we report the laser damage behaviors of the dichroic mirrors that were irradiated by 532 nm and 1064 nm
nanosecond laser pulse separately. The influence of substrate polishing quality on the laser damage resistance of
HfO2/SiO2 dichroic mirrors was also discussed. At 1064 nm, the nano-sized absorbers between coatings and substrate
interface or subsurface are the precusors for creating damage. And the poor substrate polishing quality significantly
decreases the laser induced damage threshold (LIDT). At 532 nm, two distinct damage behaviors initiating from visible
nodules and nano-sized absorbers were observed. Nodules are ejected at low fluence but the ejected pits are very stable
until quite high fluence around 20J/cm2 (1 ns). The nano-sized absorbers trigger damage at medium fluence around
14J/cm2 (1 ns), and this kind of damage grows very fast during subsequent irradiations. The substrate polishing quality
has minor influence on the LIDT at 532 nm.
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