The nonlinear transmission of Zn:Tetrabenzporphyrin was measured in a Z-scan setup using 532 nm wavelength laser light with a 13 ns pulse duration. The excited-state absorption cross section, the excited-state refractive index cross section and the linear and nonlinear absorption contribution to a thermal image index change are discussed. The effects of fluorescence and acoustic waves on the nonlinear response of TBP have been determined. Limiter performance was modeled in an f/14 limiter and saturation effects were identified.

We report measurements of the nonlinear transmission spectra of organic dye solutions. The reverse saturable absorption (RSA) of these organic dyes in the visible and near IR was measured using a 200 fs 425 nm excitation and a temporally delayed 200 fs white light continuum probe. These RSA materials have attracted attention due to their potential usefulness in optical limiting devices. Knowledge of the dynamics of the nonlinear response along with the spectral dependence is important, in order to determine the range of operation of a given material and to properly model the level structure and lifetimes. We developed an optical source based on an argon ion pumped, Kerr lens modelocked Ti:sapphire oscillator followed by a Cr:LiSAF regenerative amplifier producing millijoule level, 200 fs pulses (FWHM) around 850 nm. A single pulse is then split to generate a second harmonic (SH) at 425 nm and a femtosecond continuum that are used as the pump and probe respectively in a standard pump-probe geometry. The SH is produced in a thin BBO crystal and the continuum is produced by focusing the 850 nm light into a water cell. This results in up to 50 microjoules of pump and 3 microjoules of probe in the spectral range from 200 nm to 800 nm. These pulses have been used to temporally resolve the nonlinear spectra of several organic solutions including zinc tetra (p-methoxyphenyl) tetrabenzporphyrin, lead phthalocyanine, and silicon naphthalocyanine up to delays of several nanoseconds.

We have studied the third-order nonlinearities of Ni(II) and Cu(II) metal-organic complexes in solution using wavelength tunable DFWM experiments in the 550 - 600 nm spectral region associated with d-d transitions introduced by the metal atoms. A room temperature, frequency doubled LiF:F₂^- color center laser was used as the tunable laser source for these experiments. Additional resonant enhancement over thermally induced nonlinearities is observed for the Cu-based metal-organic complexes in these DFWM studies. Information about the sign of the nonlinearity and relative roles of nonlinear refraction and absorption was obtained with Z-scan experiments. Energy transmission measurements indicated that nonlinear absorption occurs in all samples. Relationships between the nonlinear response and the spectral absorption features of these metal-organics are discussed.

The nonlinear optical properties of a series of substituted thiophene oligomers have been investigated using the Z-scan technique. As the length of the thiophene increases from the monomer to an oligomer containing nine thiophene units, the absorption spectrum shifts to longer wavelength. This shift in wavelength moves the absorption band into resonance with the laser wavelength and the nonlinear optical properties of these materials change. In the samples where there is no linear absorption at the laser wavelength, the nonlinear absorption appears to originate from a two-photon absorption process. The nonlinear refraction observed is dominated by the contributions of the solvent. In the samples where there is linear absorption at the laser wavelength, several competing mechanisms appear to be responsible for the nonlinear response observed. At low laser irradiance, saturable absorption is observed. As the laser irradiance is increased, a decrease in the transmitted energy is observed. The nonlinear refraction in these materials is now large and negative.

Experimental methods based on nonlinear optical processes such as electrical field induced second harmonic generation and hyper-Rayleigh scattering are often used in measuring the second order microscopic susceptibility of optically nonlinear organic molecules in solution. One of the fundamental problems in these experimental procedures is the influence of the presence of solvent molecules on the optical nonlinearity of solute molecules. We have investigated this solvent effect experimentally and our preliminary results are presented.

Using picosecond laser operated at 532 nm, the observed time-resolved degenerate-four-wave-mixing (DFWM) spectra of nonether polyphenylquinoxaline (PPQ) solution show significant change in optical nonlinearity with changing solvents. For PPQ in cresol the DFWM spectrum obtained is composed of an instantaneous coherent optical response signal associated with the third-order susceptibility and a rapidly damped, laser-induced acoustic phonon signal. The lifetime of the generated acoustic phonon was estimated to be 1.5 ns. For PPQ in chloroform, an anomalously enhanced phonon signal overwhelms the intense coherent component. The generated acoustic phonon in PPQ-chloroform is much longer- lived. The mechanism of the observed optical response is discussed and the (chi) ³ value is calculated through resolving the coherent peak from the intense phonon signal.

Time-resolved imagery is presented showing the changes that occur in the focal volume of nonlinear liquids and suspensions as well as neat solvents, following the arrival of single Q-switched doubled Nd:YAG laser pulses. The images show that increasing input pulse energy results in partial beam deflection in silicon naphthalocyanine, scattering and absorption centers in suspensions, and plasma, shockwaves and bubbles following the breakdown of neat solvents. The technique involves converting a portion of the laser pulse to probe pulses which are then delayed through varying lengths of optical fiber. The focal volume is probed perpendicular to the incident beam at various times from 12 ns to 2.9 microseconds after the arrival of the pulse in the test cell. Several unsuspected phenomena have been found. A discussion of the type of information which may be gleaned from this data as well as several key findings are given.

Z-scan profiles very different than the typical peak-valley signatures have been observed. In the closed over open ratio data, a feature centered around z equals 0 and with normalized transmission greater than 1 has been observed in three classes of materials. Experimental artifacts are ruled out. In one class, tolane melts, the usual peak and valley signature is not observed even at low irradiances. In the other two material classes, C₆₀ fullerene solutions and benzophenone-thiophene polymer solutions, the usual signature is observed at low irradiances. A model based on the Gaussian decomposition method is used to interpret the data. This analysis leads to the interpretation of the observations as a diffractive effect arising from the distorted amplitude profile due to nonlinear absorption.

In this paper, a method is described which may be applied to an experimental system used for characterization of nonlinear properties of a liquid limiter, the pinhole is placed in front of the detector to measure nonlinear refractive effects. To measure the nonlinear absorption, the pinhole is removed. The nonlinear wave propagation equation is solved using the finite difference method, assuming the beam incident on the nonlinear medium to be that appropriate for a small f-number, apertured system. Effects of both nonlinear absorption and nonlinear refraction are included. Results are provided for the case described and only for the case in which the nonlinear absorption mechanism is two- photon absorption and the nonlinear refractive mechanism is thermal.

The numerical simulation of the nonlinear optical behavior of bacteriorhodopsin in a solution of water is described. Relationships for the intensity dependent absorption coefficient and index of refraction are developed and used in the numerical simulation of bacteriorhodopsin as an optical limiter and as defocussing element for laser pulses in the picosecond regime. The algorithm is a transient finite volume method that is coupled with a 'ray model' of the radiation which simultaneously solves the heat transfer and Maxwell's equations. The nonlinear behavior of the material is included in this analysis using a modified Euler predictor-corrector integration technique. Calculated power limiting and z-scan curves are in qualitative agreement with experiments. These results indicate that the code can be used to investigate and optimize optical systems which use the nonlinear behavior of bacteriorhodopsin.

A numerical software package for (2 plus 1)-dimensional simulation of optical limiters has been developed. The purpose of this effort is to provide the sensor protection community with a means to develop, understand, and optimize optical limiters. A graphics interface has been implemented to guide the user in both running the program and interpreting the results. Scientific visualization tools have been integrated with the package to facilitate a physical understanding of the numerical results. We have used it successfully to perform (2 plus 1)-dimensional nonlinear propagation of a focused optical beam with arbitrary intensity profile, such as Gaussian or tophat profile, in a f/5 system. The package can handle various nonlinear mechanisms, including intensity dependent refraction and absorption. Non-local nonlinearities such as thermal effects may be included as a modular subprogram. Temporal effects may also be included by simulating the integrated spatio-temporal evolution of the light and material properties, although this approach is memory- intensive and often requires the use of a super-computer.

Transient energy transfer or two-beam coupling is demonstrated in CS₂ and other transparent Kerr liquids using frequency chirped, 17 picosecond (HW1/eM) 532 nm pulses with several polarization combinations. As the temporal delay between pulses in a standard pump-probe geometry is varied within the coherence time, the first pulse always loses energy while the second pulse gains this energy. Scattering from phase gratings can lead to coherent energy coupling only if the nonlinearity has a finite relaxation time. This two-beam coupling in Kerr media such as CS₂ is associated with stimulated Rayleigh-wing scattering (SRWS). The frequency difference needed for beam coupling can be achieved with chirped pulses or with short pulses in nonlinear materials if irradiance dependent phase shifts are being developed during the laser pulse due to self and cross-phase modulation. Here we consider the interaction between linearly chirped pulses obtained from our modelocked, Q-switched Nd:YAG laser. This leads to an energy transfer linearly proportional to irradiance, so that the signal can be observed at irradiances lower than those needed for induced phased modulation. The measurements are performed on CS₂ but the results are valid for any Kerr liquid that has a nonlinear index of refraction with a relaxation time on the order of the laser pulse width. We demonstrate that the interaction follows the polarization dependence of SRWS. The only parameters needed for the theoretical fittings are the nonlinear index n₂, its relaxation time and the linear chirp of the laser pulse. The first two are well known for CS₂ and the laser chirp is independently measured using first and second order autocorrelations.

We describe a coherence filtering technique based on degenerate four wave mixing (DFWM) in a thin nonlinear optical material. In contrast to previous works which used ultra-short laser pulses, we performed low-coherence filtering techniques through scattering media with broad- spectrum nanosecond pulses. In our first 'proof of principle' experiments we used a 100 micrometer thick layer of dye solution as a nonlinear optical material and investigated a one dimensional case for depth-resolved measurements through a scattering media consisting of a highly scattering suspension of dielectric microspheres in water. We also describe a technique to obtain instantaneous cross-sectional images (which can be depth scanned to obtain the third dimension) performed with a low-coherence nanosecond laser source on a liquid crystal doped with an infrared dye. Experimental results were obtained with room temperature LiF:F₂^- and LiF:F₂⁺ color center lasers, and a Q-switched alexandrite laser. This technique can be used to provide instantaneous, single-shot, two-dimensional images of the internal structure of materials versus depth.

A nonlinear mode mixing effect was observed in capillary fiber, filled by liquid benzene both for the picosecond 2-nd harmonic Nd:YAG pump laser beam and the Stokes ones, originated from the effective multiwavelength stimulated Raman scattering due to the breathing vibration of the benzene rings. A formation of dynamic quasi-monomode channel inside the liquid fiber core was detected similar to the analogous regime in silica-based fiber. A critical test of modern theories of nonlinear mode mixing in optical fibers was fulfilled.

We describe a series of experiments on solutions of the inorganic metal cluster molecules Mo₂Ag₄S₈(PPh₃)₄ and compare them with data on a suspension of carbon particles in liquid (ink). The optical limiting behavior is measured using both single picosecond 532 nm pulses and nanosecond long trains of these picosecond pulses. Both materials show reduced transmittance for increasing fluence (energy per unit area). We also perform picosecond time-resolved pump-probe measurements. We find that the observed pump-probe data is nearly identical for the metal cluster solution and the carbon particle suspension (CBS), and we conclude that the nonlinear mechanisms are the same for the two materials. Our previous studies have shown that the nonlinear losses are due to scattering and absorption by microplasmas formed after thermionic emission from heated particles of carbon or inorganic clusters.

We study one free base and seven metallo- octabromotetraphenylporphyrins by several techniques. In a pico-second pump-probe experiment, we monitor the transient transmission of each sample up to 11 ns after it is irradiated by an intense laser pulse. Combined with the results from time-resolved fluorescence spectroscopy, we propose a simple model to interpret the transmission data. We attribute the reduction in the transmission to triplet state absorption and extract the triplet state absorption cross-sections, as well as the lifetimes from the time dependent transmission data. In a separate experiment where the transmission of a nanosecond laser pulse is measured with various input energies, our measurement in the cross- sections predicts the correct optical limiting behavior. We assess the overall optical limiting performance of all 8 samples by direct comparison with C₆₀ at the same ground state transmission.

We describe several methods for optimizing optical limiters, including a modification to existing geometries called the stepped limiter. We show that a stepped limiter may have a performance close to that of a fully optimized limiter with a graded molecular density. Given the difficulty in making a graded molecular density, the stepped limiter may be an attractive approach toward making practical devices. We also discuss the importance of damage threshold of the nonlinear limiting material on the limiter design and performance. Liquids have high damage thresholds and for this reason we may use tandem limiting geometries for devices based on nonlinear absorber molecules in liquid solutions. With currently available materials, this is still the best approach. Our experimental results on a tandem limiter based on Zn:tetra((rho) -methoxyphenyl) tetrabenzporphyrin show the best limiting performance to date.

An optical limiter based on a large birefringence which is optically induced in bacteriorhodopsin is presented. The induced birefringence is observed to be a function of incident intensity, but saturates at a value of about 0.454 W/cm². A measured value of (Delta) n of 6.6 multiplied by 10^-4 at wavelength of 514 nm is reported. The observed birefringence is found to be in good agreement with a proposed model based on vector-tensor approach.

We studied the nonlinear absorption properties of tetrabenzporphyrins (TBP). Two dimensional delocalized pi- electron systems such as the macrocyclic dyes tetrabenzporphyrins exhibit nonlinear absorption. Their nonlinear absorption and refraction are due mainly to a mechanism that involves the formation of large populations of excited states. This takes place in the spectral regions between B and Q bands. The relevant time frame in which the nonlinear absorption takes place is determined by the dipole dephasing times in the case of the excited singlet states (which is of order picosecond). TBPs have been proposed as candidates for applications in optical power limiting. We have monitored the nature of the nonlinear absorption of TBP samples with picosecond laser pulses at 532 nm wavelength. Open aperture picosecond Z-scan measurements obtained with solutions of TBP in tetrahydrofuran (THF) exhibit reverse saturable absorption (RSA). Our experiments also reveal RSA for a thin film of TBP in a polymethyl methacrylate (PMMA) matrix. On the other hand we observed that for films deposited using the Langmuir-Blodgett (LB) technique the RSA is decreased. A simple extrapolation of solution measurements to thin film samples might result in erroneous conclusions. We investigated the nonlinear absorption of TBP in THF-water mixture to elucidate the role of molecular aggregates in the nonlinear properties of TBP samples. The RSA observed in the case of THF solutions is lost in the mixture.

Two-photon absorption, excited state absorption, nonlinear refraction and scattering are the processes explored for optical power limiting applications. We report a novel approach for power limiting in polyphenylquinoxalines (PPQ) solutions based on two-photon induced cluster formation. The samples of PPQ were measured in solution in chloroform. The solutions are completely transparent at 532 nm. The contributions of the real and imaginary parts of the susceptibility were studied using the Z-scan technique. The nonlinear index of refraction of the samples is found to be positive. Open aperture Z-scan experiments indicate two photon absorption induced cluster formation and scattering. Values for the effective nonlinear absorption coefficient and the nonlinear index of refraction were obtained. Though two-photon absorption is responsible for triggering the process of cluster formation, optical power limiting in this case is due both to two-photon absorption and to scattering of light by the molecular clusters. The process of two- photon absorption and induced scattering yield a very attractive figure of merit of 10.3 for a solution sample of 2 mg/ml concentration compared to values reported in the recent literature. PPQ is an interesting candidate for applications in optical power limiting.

We have studied the nonlinear optical transmission of several isotropic liquid crystal cored fiber and fiber array. These fiber arrays are capable of good image transmission as well as optical limiting of nanosecond and picosecond laser pulses with low threshold and clamped transmission. The nonlinear optical mechanics involved are nonlinear absorptions and nonlinear scattering by thermal and density index changes.