Fe:LiNbO₃ in a simple focal plane geometry has demonstrated efficient optical limiting through two-beam coupling. The performance is largely independent of the total Fe concentration and the oxidation state of the Fe ions, providing the linear optical transmission of uncoated crystals is between 30% and 60%. Both the maximum change in optical density ((Delta) OD) and the speed improve with increasing pumping intensity, and neither the (Delta) OD or the speed have shown any signs of saturation for local cw pumping intensities up to 10 kW/cm². Fe has been found to be the best dopant for LiNbO₃, giving the widest spectral coverage and the greatest optical limiting. Optical limiting in Fe:LiNbO₃ has been shown to be very much grater than predicted by simple diffusion theory. The reason for this is a higher optical gain than expected. It is suggested that this may be due to an enhancement of the space-charge field from a combination of hot diffusion with the photovoltaic effect. The standard two-beam coupling equations have been modified to include the effects of the dark conductivity. This has produced a theoretical intensity dependence on the (Delta) OD which closely follows the behavior observed in the laboratory. A further modification to the theory has also shown that the focusing lens f-number greatly affects the optical limiting characteristics of Fe:LiNbO₃. A lens f-number of approximately 20 gives the best results.

We describe studies of optical limiting by thermally induced nonlinear light scattering in highly porous solgel glasses. We impregnate the porous glass with a solution of reverse saturable absorber in a solvent that is index-matched to the solgel matrix at room temperature. We observe a strong enhancement of the limiting properties at high energies in the porous glass, compared to the performance of the reverse saturable absorber alone. We attribute this effect to nonlinear scattering. However, the properties of this scattering are substantially different to what was originally expected. We provide evidence that the observed effect is due to formation of gas bubbles in the focal region of the laser beam. These bubbles are trapped in the host matrix, allowing the effect to accumulate over several laser shots.

We report optical power limiting in undoped single crystals of the wide band-gap semiconductor Zinc Oxide (ZnO). The ZnO crystals were grown at the Sensors Directorate of the Air Force Research laboratory using the hydrothermal method. The crystals are of high optical quality and are transparent in the visible part of the spectrum. The contributions of the real and imaginary parts of the third-order nonlinear optical susceptibility were studied using the Z-scan technique. The Z-scan experiments were carried out using picosecond pulses at 532 nm wavelength with the electric field polarization oriented orthogonal to the crystal c- axis. The nonlinear refractive index of the crystals is found to be negative. The ZnO samples exhibit two photon absorption and high damage thresholds. Our results indicate that the ZnO crystals may be good candidates for applications in optical power limiting and pulse stabilization.

A series of azulenic compounds are synthesized and their third-order nonlinear optical properties are determined. The compounds are studied in solution using degenerate four wave mixing and z-scan at 532 nm with 30 ps pulses. Extrapolated values for the third-order nonlinear optical susceptibility (chi) ⁽³) of the compounds are in the range 10^-9 to 10^-8 esu. Z-scan results indicate reverse saturable absorption. Optical power limiting with a f/5 system is demonstrated for all of the compounds studied.

The application of a numerical algorithm to the design procedure of a multicell stepped limiter (MCSL) is discussed. This application illustrates the ability to predict thermal damage and the thermal lensing effects that occur within the stepped limiter. Both the optical limiting and the temperature profile through the MCSL are determined for three design cases. The results illustrate the potential application of a numerical simulation in designing a MCSL and of predicting thermal damage.

Nonlinear transmission and optical limiting of picosecond and nanosecond laser pulses in an organic fiber core liquid are investigated. The measured effective absorption coefficients and other limiting characteristics are analyzed using a model that accounts for linear, two-photon, intermediate, and excited-state absorptions. In the picosecond regime, two-photon absorption process is the dominant mechanism. On the other hand, in the nanosecond and longer time scale, other processes such as excited state absorption and nonlinear scattering contribute significantly, giving rise to an effective nonlinear absorption coefficient that can be two orders of magnitude larger than the picosecond counterpart.

The passive optical limiters are of significant for protecting sensors and human eyes from high intensity laser. The selection of materials is still the key point for optical power limiters. The optical limiting performances for five new pentaazadentate metal complexes with transmission of 0.45 - 0.70 in a 1 cm cell in tetrahydrofuran have been measured at 532 nm with 35 ps pulses, in which the reverse saturable absorption from the singletyieldssinglet excited state is responsible for the optical limiting. The optical limiting thresholds of pentaazadentate and benzopentaazadentate cadmium complexes at T/T_o equals 0.5 are 44 mJ/cm² and 146 mJ/cm² respectively, the lowest nonlinear transmittance for pentaazadentate complex I is 7.7%, while the incident maximum fluence reaches to 881 mJ/cm². The bounds of figure of merit-like are estimated at 1.7 - 8.7. The limiting thresholds are strongly influenced by the nature of the substituents on the benzene ring. Electron withdrawing groups of the complexes can enhance the limiting capability, while the effect of heavy atom acts opposite roles. The optical limiting abilities are compared with C₆₀, tetra- phenyl-tetrabenzoporophyrin-zinc and octanoxyl- phthalocyanine free base at same conditions. The sequence of the limiting capability is pentaazadentate cadmium complex >C₆₀ > tetra-phenyl-tetrabenzoporphyrin-zinc > octanoxyl-phthalocyanine free base.

The influence of configuration parameters in one-cell system on pulse shape of stimulated Brillouin scattering was investigated experimentally. Stokes pulse duration was easily changed from 1.5 ns to 8 ns by varying the depth of focusing. Stokes pulse shape was numerically simulated by a theory model of stimulated Brillouin scattering initiated by thermally excited acoustic waves. The theoretical calculation was well coincident with experimental results.

We present our recent advances toward the development of high-performance solid-state optical limiting devices using reverse saturable absorption (RSA) dyes doped into optical host materials. Femtosecond transient absorption spectroscopy was employed to determine both the spectral regions of strong RSA, and the singlet-triplet excited-state dynamics. The optical limiting in the visible spectrum in both metallo-phthalocyanines and metallo-porphyrins is due to a combination of singlet and triplet RSA. Optical limiting performance was studied for RSA dyes in dual tandem limiters (both in solution and solid-state). Our best results in the solid-state yielded an attenuation of 400X, and a damage threshold of up to several mJ at f/5 focusing. The optical limiting at f/5 is further enhanced, particularly in the solid-state, by self-defocusing thermal nonlinearities.

The significance of microscale heat transfer mechanisms during short pulsed laser radiation of thermally stimulated nonlinear optical material is investigated. Significant differences occur between the temperature predictions of the diffusion equation and microscale model. The temperature response effects the index of refraction gradient and results in large differences in the predicted instantaneous transmittance values of the laser beam. The results show that the percentage difference error in the calculated average transmittance value for the two models will decrease from over 30% initially, to less than eight percent after four times the material relaxation time, (tau) , has passed. The material relaxation time is related to the sonic velocity in the material. The results demonstrate that the microscale heat transfer mechanism dominates for times less than 4(tau) . The diffusion or Fourier heat transfer mechanism should be used only when the laser pulse duration is much longer than the material relaxation time.

We have developed methods of measuring absorption of transient species utilizing stepped-scan Fourier transform interferometry that allows a combination of broad spectral coverage (10,000 - 15,000 cm^-1 per spectrum), good spectral resolution, and up to ns temporal resolution with possibilities of extension to the ps domain. Nanosecond, psec or fsec laser systems, tunable from UV to IR can be used as the pump source to prepare the transient species. The absorption of that species is measured with broadband, incoherent light and can be simultaneously time and frequency resolved.

The nonlinear optical properties of four novel azulene-type donor-acceptor compounds have been measured by means of the 2D Z-scan technique. These compounds exhibit considerable nonlinear refraction and one shows strong nonlinear saturated absorption at 527 nm. The transient spectra and kinetics, after excitation of the charge-transfer absorption band, have been studied by means of a picosecond transient absorption spectroscopy, allowing us to measure absorbance changes as small as (Delta) A equals 0.005 and time resolution of 1.5 ps. Several compounds exhibited quite strong reversed saturable absorption in a wide region of the visible spectrum. The reversed saturable absorption induced an increase in the effective absorption of the material at high power, which suggests that, it may be useful as an optical limiting medium.

The third-order nonlinear optical properties of a methyl substituted Texaphyrin, [(CH₃-TXP)Cd]Cl, were studied by degenerate four wave mixing and Z-scan techniques using 40 ps laser pulses at 532 nm. The molecular second- order hyperpolarizability ((gamma) ), the excited-state absorption cross section ((sigma) _ex), the nonlinear refractive cross section ((sigma) _t), and the optical limiting performances at both nanosecond and picosecond time scales have been determined. We have also studied the third- order nonlinearity of SiNc, one of the most promising optical limiting materials in literature, for comparison. The (gamma) value for [(CH₃-TXP)Cd]Cl is 6.9 X 10^-31 esu, which is 4 times as larger as that of SiNc. The nonlinearity of [(CH₃-TXP)Cd]Cl is predominantly electronic in origin with picosecond laser pulses. The excited-state absorption cross section ((sigma) _ex) and the nonlinear refractive cross section ((sigma) _t) obtained from the theoretical simulation and calculation of Z-scan results are 7.0 X 10^-17 cm² and 1.7 X 10^-17 cm², respectively. The complex shows strong optical limiting performance via reverse saturable absorption for 5 ns laser pulses. The nonlinear absorption of this molecule for 40 ps laser pulses exhibits a transition from reverse saturable absorption to saturable absorption when the fluence is higher than 0.3 J/cm². These data suggest that this complex and related complexes are a promising class of nonlinear optical materials.

There has been considerable recent interest in the design of new organic chromophores, oligomers and polymers with potentially large two-photon cross-sections for a variety of applications that span such diverse areas as photo-dynamic therapy to optical power limiting of nanosecond and picosecond laser pulses. One particularly attractive system is based on poly[p-phenylene vinylene] (PPV) oligomers containing electron-donating substituents. We have recently designed and synthesized several PPV dimers with bis- diphenylamino) donor groups attached to the terminal phenyl rings, and have demonstrated that these materials have very large two-photon cross-sections for nanosecond pulses. It is probable that these enhanced cross-sections are due to excited state absorption following the initial two-photon absorption. We have also examined bis- (diphenylamino)diphenylpolyenes, and more recently extended our design concept to dendrimer structures based on bis- (diphenylamino)stilbene repeat units. Initial studies on the dendrimer structures and bis-(diphenylamino)-PPV dimer reveal extremely large two-photon cross-sections which we have also ascribed to probable excited-state absorption. The efficacy of this design approach will be discussed, as well as projected future design paradigms for even greater TPA enhancement.

The excited state absorption of lead phthalocyanine is investigated using ultrafast transient white light absorption spectroscopy. Singlet state absorption features are identified on the basis of time. Increases in optical density up to 1 in the near infrared region of 900 nm to 1.1 micrometers are observed by pumping at 800 nm. In addition, several artifacts of this technique are discussed, with an aim towards improvements. In particular, self-pumping of the sample via the supercontinuum probe is observed and investigated.

In response to 6-10-ns laser irradiation of nematic liquid- crystal layers, unusual approximately 0.5 - ten second build-up time, high-contrast, far-field, concentric elliptical diffraction rings in a 532-nm, linearly polarized, Gaussian beams, are observed at 2 - 10-Hz pulse- repetition rate, a phenomenon strongly dependent on the orientation of the nematic molecules, under experimental conditions excluding ordinary orientational spatial self- phase modulation, for a geometry in which incident laser- light polarization is parallel to the nematic director and propagation direction perpendicular to the planar-nematic cell. Once formed, each set of rings may, upon further irradiation, exist for up to several minutes. The number of these rings varies systematically with laser intensity (between 1 - 2 and (up to) 20 rings). Cells of nematic 5CB (or its mixture with 7 CB) with 30, 50, 105, or 125-micrometers thickness and laser-beam diameters (at 1/e² level) of approximately 35 - 50 and 160-micrometers are used. The threshold of the nonlinearity is modified either by changing the laser repetition rate or by adding a two-photon absorbing chromophore. For CW laser irradiation of twice higher power density than the average power density in the pulsed mode the effect disappears.

We have fabricated thin film of the star-like C₆₀(CH₃)_x(PAN)_x copolymer. Optical property and ultrafast nonlinear optical response of the copolymer films were measured. The absorption and photoluminescence spectra showed the intramolecular excitation transfer process existed in the copolymer. From the femtosecond optical Kerr effect measurement, third-order nonlinear optical susceptibility of the copolymer film was found bigger than that of pure PAN and fullerene molecule which result from a new lower excited state and the redistribution of the electron cloud in the copolymer.

In the present work an application of z-scan technique to the study of asphaltenes is presented in order to determine their nonlinear properties. For asphaltenes toluene solution a high value of nonlinear refraction index (n₂ equals 6.04 X 10^-12) it is obtained. Strong dependence of the two photon absorption coefficient ((beta) ) with the input intensity was observed for higher solutions concentration, which can be explained by molecular aggregation.

The spectroscopy and nonlinear absorption of bis(diphenylamino) diphenyl polyenes have been studied in octane and dichloromethane solvents. The amines exhibit high fluorescence quantum yield and two photon excited emission. Two photon absorption cross section, (sigma) ₂, was measured by Z-scan experiments. Strong two photon absorption is indicated by high values for (sigma) ₂. Solvent has strong influence in the measurement of (sigma) ₂ values.

To understand the photophysics of nonlinear absorbers, we have investigated the photophysics of a series of di(2- thienyl-3,3',4,4'-butyl)polyenes. Spectroscopic measurements, including UV/Vis, fluorescence, fluorescence lifetimes, fluorescence quantum yields, triplet state lifetime, solvent effects and two-photon absorption coefficient were obtained as a function of the number of double bonds (n equals 1 - 5). Trends in the data reflected the ordering, energy gap between and mixing of ¹B_u* and ¹A_g* excited state configurations. We investigated the solvatochromism of a series of (alpha) ,(omega) -di(2- dithienyl 3,3',4,4'-butyl) polyenes. Absorption (n equals 1 - 5 double bonds) were collected in a series of aprotic solvents. The absorption energy dispersion effect sensitivity increased smoothly with n, reaching asymptotic behavior as n approached 5. The emission energy had less solvent sensitivity, giving evidence for a polar ¹B_u* absorbing state and a nonpolar ¹A_g* emitting state. We observed sensitivity of the absorbing and emitting states to solute-solvent dipole-dipole interactions, suggesting the dithienyl polyenes had a polar syn ground state conformation.