The second harmonic generation in the set of model lead-silicate glasses and also in lead-germanate glasses was investigated. The glass composition for effective second harmonic generation and optical waveguides has been optimized. Some features of optical waveguides formation in lead-silicate glasses were studied.

Phosphate glass doped at 0.2% was synthesized. Transmission electron microscopy and electron diffraction study demonstrated microcrystals arising in the glass matrix, their growth up to 10 nm, and gave evidence of hexagonal structure of the microcrystals. Distances between crystalline planes of the crystals are same as distances for hexagonal CdS crystals. Annealing conditions necessary for the microcrystals growth (350 to 370 degree(s)C, 15 to 75 min duration) are very soft in comparison with standard annealing conditions for silicate semiconductor doped glasses. Quantum size phenomenon (shift of absorption edge with increasing of microcrystals) was observed in the glass designed.

Nonlinear optical and carrier dynamics of CdMnSe and CdMnTe were studied using picosecond pulses from a mode-locked, Q-switched Nd:YAG laser operating at 1064 nm. The study of the effects of free carriers on the refraction and absorption of a delayed probe beam resulted in the determination of the magnitude and sign of these nonlinearities for different Mn concentrations. It was found that at high carrier concentrations the magnitude of the nonlinear refraction coefficient is not constant and decreases with an increase in the electron concentration. The carrier dynamics were studied using forward propagating pulsed probe degenerate four wave mixing. The study was done for a number of different pump beam irradiances. A saturation in the diffraction efficiency was found at higher carrier concentrations. The lifetime of the electrons in the conduction band was determined by monitoring the lifetime of the free carrier absorption.

A great deal of work has taken place over the last several years centering on the synthesis and development of organic molecules as nonlinear optical materials for various signal processing applications. Although substantial progress has been made in advancing these materials toward practical military and commercial applications various limitations have yet to be resolved. These include limited environmental, thermal, and alignment stability, narrow optical transparency ranges (especially at 830 nm), and low nonresonant nonlinearity. The nature of aromatic heterocyclic rings allows them to function as either electron rich or electron deficient centers thereby making it possible to tailor the (pi) electron distribution within individual molecules. Heterocyclic aromatic rings offer structural building blocks that can be used by the synthetic chemist to design new molecules which exhibit marked differences in the mechanism of their nonlinear response as well as increased environmental and thermal stability. In addition improvements in the transparency at shorter wavelengths and in compatibility with high performance thermoplastic polymer hosts have been realized. In this presentation, the second-order NLO properties for a number of chromophores containing various heterocyclic rings which have been synthesized in our research program will be discussed. Correlations between molecular structure, physical properties, optical properties, and NLO activity will be considered as well as comparisons made with more conventional chromophores.

The nonlinear absorption in a series of compounds has been investigated using the open aperture Z-scan technique. The compounds investigated were diphenyl acetylene, diphenyl butadiene, diphenyl butadiyne, 1,4-bis (phenylethynyl) benzene, bis (4-biphenyl) acetylene, 4,4'-bis (phenylethynyl) biphenyl. Solutions of the compounds were made in chloroform and showed no linear absorption at the laser wavelength. The Z-scan measurements were carried out using 35 ps laser pulses at 532 nm. Nonlinear absorption was observed for all of these materials, and the nonlinear absorption coefficient (beta) has been determined from this data. The nonlinear absorption process for two of the compounds, bis (4-biphenyl) acetylene, 4,4'-bis (phenylethynyl) biphenyl, is most likely caused by a two-photon absorption to a two-photon allowed state. The nonlinear absorption mechanism for the remaining compounds appears to be two-photon absorption followed by linear absorption from this excited state. The refractive nonlinearity has been more difficult to determine, and in most cases appears to be indistinguishable from the solvent contribution.

The macroscopic third order optical susceptibility, (chi) ⁽³), of a series of metal free, nickel and copper containing salicyaldehyde based metal organic complexes was measured by degenerate four wave mixing at 532 nm. Insertion of the metal ion into the ligand greatly enhanced the magnitude of (gamma) , the microscopic second order molecular hyperpolarizability. Maximum (gamma) values of 2.6 X 10(superscript -29 esu were measured for the materials studied.

We report on the correlation between features of the molecular structure of polyphenyls and the measured parameters of optical limiting: limiting threshold and clamped output. Optical limiting in these materials is mainly due to a strong light scattering (third order negative nonlinear effect). We show that in many simple cases the limiting threshold decreases with increased conjugation length, linearity, planarity and with the substitution of acceptor/donor end groups. The UV spectra and the wavelength dependence of the limiting parameters suggests that a two photon absorption mechanism is necessary for a low threshold and efficient light scattering.

Nanosecond and picosecond studies have been carried out on a platinum ethynyl compound dissolved in tetrahydrofuran. Nonlinear absorption was observed at 532 nm using nanosecond pulses and at 1064 nm using picosecond pulses. The nonlinear absorption observed at 532 nm was found to have a long time constant associated with it indicating the presence of excited state absorption. A possible route for this excited state absorption may be single photon absorption from the ground state to the T¹ excited state followed by excited state absorption from the T¹ state. Strong nonlinear scattering was observed at 532 nm using nanosecond pulses and at 1064 nm using picosecond pulses. Optical limiting studies using 532 nm, nanosecond pulses revealed that below input fluences of 6 J/cm² nonlinear absorption is the dominant limiting mechanism whereas above 6 J/cm² there is significant nonlinear scattering. Picosecond experiments indicated that the scattering may, in part, be attributed to strongly absorbing scattering centers present in the solution, possibly originating from laser induced breakdown of the platinum ethynyl material. Excite-probe studies indicated that the time constant associated with the scattering is of the order of tens of milliseconds suggesting that this platinum ethynyl compound is a good candidate for use against picosecond to microsecond pulses.

The nonlinear absorptive properties of C₆₀ are of significant interest especially in the near IR. In this paper we describe spectral emission measurements on C₆₀ solutions. In toluene solutions the laser induced emission in the fluence regime of 3 to 1500 J/cm² excited by nanosecond and picosecond pulses is measured. In C₆₀/toluene solutions a broad double feature is observed with peaks at 692 nm and 732 nm matching the published fluorescence spectra of C₆₀. No evidence for breakdown is apparent in the C₆₀ solutions though there is in the neat toluene. A survey study to determine solvent effects on the electronic states of C₆₀ is reported. Thirteen solvents were chosen from among those in which C₆₀ is soluble to give a wide range of dielectric constant and polarity. The molar extinction coefficient and fluorescence is measured in these solutions. A large solvent induced increase of the near IR molar extinction coefficient is observed in most solvents relative to toluene. Evidence for complex formation is observed. The changes of these spectra relative to that observed for toluene solutions indicate that solvent induced change of the quantum efficiency of triplet production is possible.

We examine optical limiting with C₆₀ solutions for nanosecond optical pulses at 532 nm and 694 nm and microsecond pulses at 514 nm. When the input fluence is less than 50 J/cm², optical limiting is due to a combination of reverse saturable absorption (excited state absorption) and self-defocusing. Nonlinear scattering was not observed. For a peak input fluence greater than 50 J/cm², an acoustic report and broadband emission indicate that optical breakdown occurs. At 532 nm, optical limiting for C₆₀-toluene is comparable to carbon black suspension (CBS). For use at 694 nm, C₆₀-toluene has a larger excited state absorption than at 532 nm, which partially compensates a much smaller ground state absorption. For microsecond optical pulses, C₆₀ appears to be a better optical limiter than CBS.

This paper presents the results of a comparison study among three carbon-based solutions: a carbon-black suspension, C₆₀ in toluene, and C₆₀ in chloronaphthalene. The carbon-black suspensions were used as baseline samples for comparison with the C₆₀ solutions and were fabricated with the same transmission at 694 nm as the C₆₀ samples. A ruby laser operating both with and without the Q-switch was used in this study. The limiting results of the C₆₀ materials are compared with the predicted results from a five-level model previously used to calculate the optical limiting performance of C₆₀ at 532 nm.

This paper describes the investigation of nonlinear behavior in a solution of C₆₀. A Q-switched Nd:YAG laser operating at 532 nm was used to look at the change of absorbance during the time of the pulse. The open literature has suggested that scattering may provide part of the nonlinear process found in C₆₀ solutions and this problem is addressed in this paper. Nonlinear scattering in toluene, a carbon suspension in toluene, and C₆₀ solution in toluene was investigated between 0 and 90 degrees. The C₆₀ solution produced as much nonlinear scattering as the toluene on its own.

We have investigated nonlinear reflection and transmission processes in carbon particle suspension interfaces. Laser pulses incident on these interfaces cause plasma formation, laser induced cavitation, and the formation of a vapor layer at the dielectric interface. This vapor layer leads to optical self- switching via total internal reflection, which, in combination with other nonlinear processes such as plasma scattering, leads to optical power limiting. We will present experimental power limiting results for device prototypes based on this concept in the context of realistic optical system configurations. In addition, we have developed a theoretical model of the nonlinear reflection processes and fit this model to experimental nonlinear reflection data to determine the plasma formation threshold. The vapor interface formation time, which limits the maximum single- pulse nonlinear reflectivity, is also shown to vary as the inverse of the cube root of the carbon particle concentration. This implies that this formation time is determined primarily by the time it takes the expanding microbubbles to intersect each other and the substrate.

Nonlinear absorption of yellow (587 nm) laser light by a liquid crystal material has been investigated using dual-pulse (`excite/probe') measurement techniques. Two separate absorption phenomena were observed; these were identified as two-photon absorption from the ground state, and single-photon absorption from an excited state whose lifetime exceeded 10 ns. Strong blue fluorescence was emitted by the material following excitation. Measurements of the spectrum of this fluorescence identify its source as the S₁ to S₀ transition. Lifetime and energy level evidence suggest that the S₁ state may also be responsible for the excited state absorption. Estimates of the absorption cross-sections (sigma) _ex/(sigma) _gr exceeds 3 X 10⁵, which suggests that the excited state absorption may be exploited to produce very strong nonlinear effects.

The conventional Z-scan technique using Gaussian beams for measuring intensity dependent optical nonlinearities is extended to allow the use of beams with top-hat profile. Potential applications of the top-hat beam Z-scan technique include the measurement of (chi) ⁽³) over a wide spectrum which typically requires the use of a dye laser. We present a graphical method which allows straightforward determination of both the real and imaginary components of (chi) ⁽³) from Z-scan data. Using the top-hat Z-scan scheme, we have measured (chi) ⁽³) as function of wavelength for typical liquid crystal materials, over the visible spectrum. The spectral behavior of (chi) ⁽³) gives insight into the underlying physical process on various time scales. We also present results of pump-probe measurements of the nonlinear absorption coefficient of the nematic liquid crystal 5CB. Our results indicate that the nonlinear absorption process in the nanosecond time regime is dominated by the two-photon excited-state absorption process. The time constant of this process is estimated from experimental data. A simple model is presented which agrees with our observations.

Experimental and theoretical results are given for studies of optical limiters designed to increase the functional range without incurring optical damage. In particular, we investigate a tandem geometry with two passive nonlinear elements, one placed in the focal plane of a lens and the second placed `upstream' of the focal position to protect the material at focus from damage. To provide a proof-of-principle demonstration of this geometry, simple limiters consisting of combinations of reverse saturable absorber dyes and a carbon black suspension in thin cells were tested. Our results show that a substantial increase in device performance can be achieved by use of a tandem limiter geometry. Simple modelling predicts that the dynamic range of a separate- element tandem limiter is given by the product of the dynamic ranges of the individual component limiting elements, in agreement with our experimental results. We also describe our numerical beam propagation code, which must be applied for thick limiting elements and in other cases where simple modelling is invalid.

Intensity dependent nonlinear materials placed at an intermediate focal plane in a simple two lens optical imaging system will limit radiation on a detector plane. We report the dependence of the energy and fluence limiting on aperture size, nonlinear material position, and the magnitude of the nonlinearity. The approximate performance of this same imaging system is determined for a laser source located far from the limiter. The nonlinear material requirements to limit the fluence at or below some predetermined level is determined. A dynamic range for the limiting behavior is determined which includes damage to the nonlinear material.

Factors which affect the design of passive optical power limiters utilizing nonlinear refraction and absorption are discussed. Recent theoretical modelling and experimental studies of the effects produced by varying the thickness and position of the nonlinear medium are reported. Excessive thickness can produce large insertion loss for the limiter in the event of linear absorption or scattering, and also makes retrofitting to existing equipment difficult. Insufficient thickness results in less than optimal limiting. We show that for a third-order refractive nonlinearity a suitable medium thickness is six times the Rayleigh length of the beam inside the medium. Nonlinear absorption can enhance the limiting, allowing for the use of larger apertures or no apertures, and hence greater fields of view. We also show how the optimal position of the medium depends upon the ratio of nonlinear refraction to nonlinear absorption.

The EZ-scan, an improved Z-scan technique, shows a sensitivity for measuring nonlinearly induced wavefront distortion of approximately equals (lambda) /10⁴. We show that the nonlinear refraction and nonlinear absorption coefficients can be determined separately by a single EZ-scan measurement. We describe application of this technique to several organic thin films.

Focusing and defocusing of laser light has been observed for many years. Kerr type materials exhibit this effect but only for high intensities. We show experimental evidence that photorefractive materials can also produce dramatic focusing and defocusing. Whereas Kerr materials produce this effect for high intensities, photorefractive materials produce these effects independent of intensity indicating that this effect would be ideal for an optical limiter. We compare the characteristics of Kerr and photorefractive materials, discuss the physical models for both materials and present experimental evidence for photorefractive defocusing. Self-focusing and defocusing was observed for any incident polarization although the effect was more pronounced using extraordinary polarized light. In addition, self-focusing or defocusing could be observed depending on the direction of the applied electric field. When the applied field was in the same direction as the crystal spontaneous polarization, focusing was observed. When the applied field was opposite the material spontaneous polarization, the incident laser light was dramatically defocused.

Photoelectron spectroscopy of coordinatively unsaturated organometallic anions can provide a means to probe the ground and low lying excited states of the corresponding neutral radicals. We report results for the early 3D transition metal monocarbonyls VCO and CrCO, and for the late metal complexes FeCO, CoCO and NiCO. Each spectrum displays a transition to the ground state of the neutral complex, and to an excited state whose spin multiplicity differs by two from that of the ground state. For a given complex, these states share nominally the same electron configuration but differ in the spin coupling of the metal 4s electron. There is a reversal in the state ordering as one proceeds across the transition series, from a high spin ground state for VCO (⁶(Sigma) ⁺) and CrCO (⁷(Sigma) ⁺) to a low spin ground state for FeCO (³(Sigma) ^-), CoCO (²(Delta) ) and NiCO (+1)(Sigma) )⁺). The measured state splittings and vibrational frequencies provide insight into the factors that determine the ordering and bonding properties of these states. Recent results for the linear H-M-CO isomers of Fe and Co are also reported.

In the research of self-focusing and self-defocusing phenomena, it is generally known that the used nonlinear optical material possesses the coefficient of nonlinear refractive index n2(r), which is usually uniform in spacial distribution, that is, n2(r) is all the same in the material. Therefore in order to obtain the desired self-focusing or self-defocusing, the radial distribution of incident beam intensity I(r) must be Gaussian form and uniform beam can't cause self-focusing or self-defocusing. In addition, for the material n2 < 0, self-focusing is obtainable, but self-defocusing isn't. Otherwise, for the material n2 < 0, only self-defocusing can be obtained.

This paper discusses the output property of a Fabry-Perot interferometer filled with a nonlinear medium. Because of the optical-field-induced refractive index, the transmissivity across monochromatic beam through the interferometer depends on the distribution of the beam intensity. The output beam is different from the input. As the reflectivity of two mirrors increases, the distinction changes clear, and there is a change close to discontinue.