Magnified imagery of the focal volume within a nonlinear material, following the arrival of a high power laser pulse, provides insights on the response of the material. Emitted and scattered light images are presented along with time-resolved shadowgraphs (obtained through a pump-probe scheme) showing the response from various limiting materials in both liquid and solid hosts. The different types of images taken together make it possible to determine what occurs and where it occurs within the limiting material. Often unsuspected phenomena are found to be important. Conclusions from the data and the general utility of the technique are discussed.

We discuss several imaging techniques that have been studied for optical imaging through turbid media. The effectiveness of many of these techniques hinges on the nonlinear optical material used in the imaging process. We have used Rhodamine dye-doped K15 liquid crystal films in a degenerate four-wave mixing geometry in order to study the potential of this material for low-coherence imaging applications. First, we observed the dc-field-induced Freedericksz transition, which leads to a reorientational nonlinearity in our nematic liquid crystal film, and is responsible for the large optical nonlinearity that leads to phase conjugation. For our low absorbing films, this threshold voltage was on the order of 400 V/cm. Next, we show that our liquid crystal films exhibited higher phase conjugate reflectivities (0.6% maximum) for smaller beam crossing angles, which is necessary for wider field-of-views in the imaging process. Finally, we studied the time-dependent response of the phase conjugate reflectivity due to electric field assisted nonlinearities in dye-doped liquid crystal films. The motivation of these studies is to improve the imaging resolution demonstrated in our previous work.

The purpose of this paper is to discuss the general unclassified requirements for optical limiting devices and materials that might be used to protect U.S. military personnel and equipment from damaging laser radiation. Many researchers who consider working in this area are not familiar with the basic requirements that the U.S. military have for any material or device that would be fielded. This paper will attempt to set a general guideline for researchers in the field.

We have developed compact millimeter thick optical fiber arrays with nonlinear optical liquid guiding cores. These image transmitting fiber arrays are capable of passive optical limiting action against frequency agile short (picosecond- nanosecond) laser pulses in the visible spectrum. We present the results of further studies of their limiting performance. We also discuss how a recently discovered extraordinarily large nonlinear optical response of methyl-red doped nematic liquid crystal film may be employed for limiting action on longer pulse or cw lasers with nanowatt threshold power.

In this report we examine how the fluid host influences the nonlinear optical response of carbon black suspensions on both the nanosecond and microsecond timescales. It is shown that there is a strong fluid dependence on the microsecond timescale and a smaller but still significant dependence on the nanosecond timescale. The temporal dynamics are studied and it is proposed that bubble formation is responsible for the enhanced microsecond response observed in the more volatile fluids. A beam propagation scheme is introduced and it is demonstrated that by making simple assumptions about the microscopic nonlinear loss mechanism the experimental data can be simulated.

Pentaazadentate metal complexes are a macrocyclic ligand containing 22 (pi) -electrons, which coordinate with numerous metal ions to form near coplanar configurations with excellent thermal and light stabilities. A more extended optical window between Soret (420 nm) and Q (760 nm) band in visible region provides a condition for requiring high transmittances at low incident light intensity and they are of also large third- order nonlinear optical susceptibilities comparing with porphyrin's and phthalocyanine's metal complexes. The third- order optical susceptibilities (chi) ⁽³)₁₁₁₁ are determined at 647 nm in methanol by pump-probed procedure and the molecular second hyperpolarizabilities (gamma) ₁₁₁₁ which are of approximately 10^-31 esu are also obtained. Optical limiting based on reverse saturable absorption are observed for ns pulses laser. The optical limiting behaviors may be alternated by introduce different substituents onto the benzene ring or/and different metal ion into the center of macrocylic ligands The limiting threshold at T/T₀ equals 0.5 is 131 mJ/cm² for complex [(PL- NO₂)Cd]Cl, the corresponding clamped energy is 34 mJ/cm². The optical limiting performance may be influenced by solution concentrations.

A design method for reverse saturable absorbing (RSA) dye concentration gradient limiters, termed here the Absorption Diffraction Balance (ADB) design method, is used to produce designs for multiple chromophores and is extended to allow incident plane waves. The ADB design method is reviewed for Gaussian beams applied to a constant fluence design and a linear fluence design. These two designs are combined to allow different dyes to be used in different portions of the limiter. It is found that this hybrid design significantly enhances performance under some circumstances. It is also shown to reduce the probability of dye photodegradation. The ADB design method is extended to allow for incident plane waves or a top-hat beam profile. The field at the geometric shadow edge, expressed in terms of Lommel functions, is shown to closely match the Gaussian field when the incident irradiance, power, and second moments are the same. Since the irradiance distribution is not monotonically increasing in the focal region, the required concentration distribution has regions of negative concentration, i.e. gain regions. These designs are useful for initiating numerical nonlinear beam propagation studies.

Over the past several years, organic molecules exhibiting significant two-photon absorbance have been of intense interest for a wide variety of applications including high speed communications, data storage, imaging, and optical limiting. However, it has been commonly observed that the local molecular environment can significantly affect the linear and nonlinear optical properties of the chromophores. In an effort to examine these effects, the influence of the solvent environment on the linear absorbance and photoluminescence of a series of donor-acceptor heterocyclic chromophores was examined. The Stoke's shift associated with one-photon absorbance and photoluminescence was observed to increase with increasing solvent polarity. This behavior is adequately described by the Lippert equation and is related to relaxation of the solvent molecules around an excited molecule. Additionally, it was observed that the spectral shape, as well as the solvent dependence, of two-photon and one-photon pumped photoluminescence were similar, thus indicating that the longest-lived luminescing excited state is independent of the method of excitation. These results have direct implications to two-photon applications which rely on up-converted fluorescence. They also yield insight into the structure-property relationships governing their linear and multi-photon behavior including the potential contributions to the effective two-photon cross-section from excited state absorbance.

We describe two methods for the spectral measurement of nonlinear absorption and refraction in reverse-saturable absorber materials. In the first, we use a picosecond optical parametric oscillator to perform Z-scan at many different wavelengths to measure excited state refraction and absorption cross sections throughout the visible. The second methods uses a chirped-pulse amplification scheme to produce 100 fs pulses at 840 nm. Focusing these into sapphire generates a white light continuum that is used as a probe in an excite-probe experiment. The excitation beam is derived from the second harmonic of the remaining 840 nm light. By measurement of the transmission spectrum of the probe as a function of excite- probe delay time, we can determine the spectral dependence of the excited-state absorption cross section. Moreover, by use of Kramers-Kronig relations, the excited state refraction can also be extracted from this data. We describe our measurements using both methods in a Zn:tetrabenzporphyrine derivative (TBP). The fact that both methods give excellent agreement not only verifies the utility of continuum measurements, but also reveals some interesting properties of the excited states of TBP.

The nonlinear optical properties of N,N-diphenyl-7-[2-(4- pyridinyl) ethenyl]-9,9-di-n-decylfluoren-2-amine [AF- 50] have been investigated. The nonlinear absorption of a saturated solution of this material in acetone was investigated with 430 femtosecond pulses at 790 nm. From these results, the two-photon absorption cross-section was determined to be 25 X 10^-50 cm⁴sec/photon molecule. This number is in agreement (within a factor of 2) with theoretical calculations. Nonlinear absorption and optical limiting measurements were also made using a Nd:YAG pumped dye laser with 4.3 ns pulses at 694 nm. These results suggest inherent differences in the performance of two-photon absorbing materials in these two different geometries.

A two-dimensional Z-scan technique employing a CCD camera was used to study the nonlinear optical parameters of materials. Using the known beam distribution at the lens plane, measured by the CCD camera, and the split step beam propagation method, we simulate the evolution of the beam profile within the sample. This technique may be applied to any arbitrary beam distribution and sample thickness. We applied our two- dimensional Z-scan technique for the investigation of zinc tetraphenyltetrabenzoporphyrin and three azulene-containing donor-acceptor compounds. It was found that some samples exhibit strong nonlinear refraction while others show strong nonlinear absorption. The use of these materials in optical limiting devices is discussed.

Detailed degenerate four wave mixing (DFWM) studies of one molybdenum complex, cis-Mo(CO)₄(PPh₃)₂ in dichloromethane (CH₂Cl₂) and tetrahydrofuran (THF) are reported in this paper. Upon exposure to air/oxygen, the (chi) ⁽³) values of CH₂Cl₂ and THF solutions of this complex increase dramatically with time. However, when excess free ligand is added to CH₂Cl₂ solution of the complex, the increase in (chi) ⁽³) value is stopped. Hence, it is possible to obtain a CH₂Cl₂ solution with high (chi) ⁽³) value by first oxidizing the solution and then adding excess ligand to stabilize the solution. For THF solution, probably due to the different mechanism causing the increase of (chi) ⁽³) values, the increase in (chi) ⁽³) values doesn't stop even though excess free ligand is added. Further, the increase in the (chi) ⁽³) values of both the CH₂Cl₂ and THF solutions parallels the increase in the linear absorption but it is unclear whether the increased linear absorption plays any role in the increase in the (chi) ⁽³) value of the solution. This study also shows how degenerate four wave mixing can be used as a very sensitive technique to detect the chemical change in a solution when the third order optical nonlinearities of the reactant and product are very different.

One of the key issues involved in the development of passive optical power limiters is the search for appropriate materials that show effective reverse saturable absorption, Metallophthalocyanines seem to be good candidates for such applications because of their higher optical nonlinearity and the unique electronic absorption characteristics. A series of 1,4,8,11,15,18,22,25-octa-decyloxy metallophthalocyanines containing various central metal atoms such as zinc, copper, palladium, nickel, and cobalt were characterized for their nonlinear absorptive properties to evaluate their suitability to function as reverse saturable absorbers. Nonlinear transmission measurements were analyzed in terms of five-state model and magnitudes of the parameters of the model were determined.

The optical limiting performances of nine asymmetric pentaazadentate porphyrin-like metal complexes {[(R- APPC)M]Cl_n} have been measured at 532 nm with nanosecond pulses. In a fl38 geometry, with sample transmission of 0.51 approximately 0.80 in a 2 mm cell, the limiting thresholds for these complexes were 1.4 approximately 150.0 mJ/cm². The throughputs of these complexes were limited to 0.31 approximately 1.42 J/cm² for incident fluences as high as 3.5 J/cm². The limiting throughput was strongly influenced by the nature of the ligand and metal ions. Lower bounds for the ratio of triplet excited-state to ground state absorption cross sections have been estimated at 2.3 approximately 5.7. The lower limiting thresholds, lower limiting throughputs, as well as the ease of modification of the ligands and variation of the metal ions, make these complexes promising candidates for optical power limiters.

Porphyrins are attractive compounds for optical applications. We have been investigating the relationship between molecular structure and optical properties of a number of porphyrin compounds. Structural variations explored include insertion of metal ions, extension of conjugation, halogenation and alkylation either at the pyrrole position or the meso-aryl groups. The characterization of these chromophores includes measurement of UV/Vis, fluorescence and fluorescence lifetimes. Furthermore, we have investigated their nonlinear absorption, excitation dynamics. The significant factors influencing limiting behavior appear to be the heavy atom effect, electron donating and withdrawing substituents conformation distortion and changes in conjugation. Detailed understanding will be gained from measurements of photophysical parameters underlying limiting behavior.

A numerical scheme for modeling of z-scans with samples ranging from thin samples to beyond the thick sample limit was introduced in our previous work (A. Eriksson et al., J. Opt. Soc. Am. B15, pp. 810-816, 1998). The method relies on a multilayer approach, where all layers are treated as independent, and may have different linear and nonlinear optical properties. The theoretical scheme can be used for irradiance as well as fluence, dependent absorbers. It allows for an arbitrarily shaped aperture in front of the detector. Here the method is tested and compared with the results of analytical thick sample theory and previously published numerical simulations. Ways of optimizing the performance of an optical limiting device are modeled and discussed. Preliminary experimental z-scan results of both thin and thick sample chloro-aluminum phthalocyanine were analyzed.

The photophysics of a series (alpha) ,(omega) -dithienyl polyenes was studied. Both one photon excited fluorescence and fluorescence originating from two photon absorption states were investigated. There is strong evidence of two photon absorption from fluorescence studies in agreement with our earlier studies involving nonlinear absorption.

Two polymer systems including polymer elastomers and gels have been studied as host materials for optical limiting applications. Both systems have high laser damage thresholds (LDT), typically 20 to 35 times higher than commercial PMMA bulk materials. For the polymer elastomers, Epotek optical epoxy 301-2 and 310, the LDT increases with an increase of the molecular flexibility. We speculate that the thermo-mechanical fracture may be the mechanism for the laser induced damage. For the hydrogel system, the LDT increases with increasing water content. The mobility of the water plays a key role in determining the LDT by facilitating laser energy dissipation and self-healing. It appears that the polymer elastomer and hydrogel systems both have potential for high power laser applications.

We developed a set of computer codes organized in a software package that allows us to model high-energy laser pulse propagation through bulk nonlinear optical media. Nonlinearities included in the model are two-photon absorption, the electronic Kerr effect, excited-state and free-carrier absorption along with the associated electrostrictive and photo-acoustic refractive index change. The propagation of CW beams, nsec/psec pulses and picosecond pulse trains is determined for various spatial distributions of the input beam. We used a cylindrically symmetric spatial geometry typical for laser outputs to reduce the CPU and memory requirements making modeling a real-time task on PC's, even for significant propagation paths (many Rayleigh ranges). The validity of the numerical outputs was tested against known results for a large range of parameters. In particular the codes are being used to investigate and design optical limiting devices with single or multi-element geometries, as well as limiters using stepped or graded density of nonlinear material. The linear propagation module integrated with the nonlinear beam propagation method (BPM) codes allow the simulation of typical experiments such as Z-scan (sample position changes in a focused beam) and limiting experiments (different input energy values).