Christopher M. Lawson

Solvent effect on the third-order nonlinearity and optical limiting ability of a stilbazolium-like dye

Stilbazolium-like dyes, which contain different heterocyclic groups as electron donors and an N-methylpyridinium iodide group as an electron acceptor, are very interesting optical limiting materials. These materials exhibit low linear absorption, but potentially very strong nonlinear absorption at high intensities. In this paper, the results from investigations of the third-order susceptibilities and nonlinear refractive indices of a stilbazolium-like dye, trans-4-[2-(pyrryl)vinyl]-1-methylpyridinium iodide (PVPI), in different solvents using degenerate four wave mixing (DFWM) and Z-scan techniques with 6.8 ns laser pulses at 532 nm are reported. The molecular second-order hyperpolarizability of PVPI increases from 1.6x10-31 esu in methanol to 3.4x10-29 esu in chloroform, a factor of 212. The third-order nonlinearity of this compound is dominated by nonlinear absorption, which leads to strong optical limiting of nanosecond laser pulses. The strength of the optical limiting is dramatically influenced by the solvent used, with limiting decreasing in the order chloroform>dichloromethane, 1,2- dichloroethane>methanol, acetonitrile. While both nonlinear absorption and nonlinear scattering contribute to the optical limiting, nonlinear absorption is the dominant mechanism.

Characterization of the third-order nonlinearity of [(CH3-TXP)Cd]Cl

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.

Syntheses, X-ray crystal structures, and optical, fluorescence, and nonlinear optical characterizations of diphenylphosphino-substituted bithiophenes.

A series of bithiophene derivatives that are either symmetrically disubstituted with two Ph(2)(X)P groups (X = O, S, Se) or monosubstituted with one Ph(2)(X)P group (X = O, S, Se) and an organic functional group (H, CHO, CH(2)OH, CO(2)Me) have been synthesized. The X-ray crystal structures of Ph(2)(Se)P(C(4)H(2)S)(2)P(Se)Ph(2), Ph(2)(O)P(C(4)H(2)S)(2)H, Ph(2)(S)P(C(4)H(2)S)(2)H, and Ph(2)(O)P(C(4)H(2)S)(2)CH(2)OH exhibit very different solid-state structures depending on the type of intermolecular π-π interactions that occur. The compounds have been characterized by electronic absorption and fluorescence studies. Of particular interest is that the quantum yields of Ph(2)(O)P(C(4)H(2)S)(2)H, Ph(2)(O)P(C(4)H(2)S)(2)P(O)Ph(2), Ph(2)(O)P(C(4)H(2)S)(2)CO(2)Me, and Ph(2)(O)P(C(4)H(2)S)(2)CH(2)OH are significantly larger than that of bithiophene (factors of 13, 14, 14, and 22, respectively). This behavior is quite different from that of analogously substituted terthiophenes in which substitution results in only modest increases in the quantum yields over that of terthiophene (factors of 0.94, 2.7, 1.3, and 1.5, respectively). DFT studies of the emission process suggest that modifying the Ph(2)(X)P group affects both the fluorescence and nonradiative rate constants while modifications of the organic substituents primarily affect the nonradiative rate constants. The higher quantum yields of the substituted bithiophenes make them promising for application in organic light-emitting devices (OLED). The optical power limiting (OPL) performances of these Ph(2)(X)P-substituted bithiophenes were evaluated by nonlinear transmission measurements in the violet-blue spectral region (430-480 nm) with picosecond laser pulses. The OPL performances are enhanced by heavier X groups and when by higher solubilities. Saturated chloroform solutions of Ph(2)(O)P(C(4)H(2)S)(2)H and Ph(2)(S)P(C(4)H(2)S)(2)H exhibit significantly stronger nonlinear absorption than any previously reported compounds and are promising candidates for use in broadband optical power limiters.

Five-energy level computer model for fitting Z-scan measurements in disubstituted chalcogenidodiphenylphosphino bithiophenes

We explicitly describe in detail a five-energy level computer model for analysis of ps and ns z-scan measurements of novel chalcogenodiphenylphosphino- and phosphonato-substituted oligothiophenes exhibiting strong nonlinear optical absorption in the violet-blue spectrum.

Relationship between chemical structures and optical limiting properties of penta-azadentate porphyrinlike metal complexes

The optical limiting performances of nine asymmetric pentaazadentate porphyrin-like metal complexes {[(R- APPC)M]Cln} 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.

Nanosecond switching in carbon microparticle suspensions

We have investigated several nanosecond nonlinear switching mechanisms in carbon microparticle suspensions. These switching mechanisms are based on combinations of effects such as plasma scattering and cavitation-induced total internal reflection (TIR). The contributions from each of these effects is studied. The dominant nonlinear switching mechanism in the majority of these samples is laser induced cavitation which leads to TIR switching. This occurs when the incident laser energy that is absorbed by a carbon particle is sufficient to heat up and vaporize a small volume of the suspending liquid forming a microbubble. TIR switching is observed when these vapor bubbles expand to dimensions that are similar to the transverse dimensions of the incident beam and form a glass-vapor interface at the front substrate surface. Using this mechanism, nonlinear refractive index changes as large as 0.3 have been experimentally obtained on a nanosecond time scale using low power, Q-switched, frequency doubled ((lambda) equals 0.532 micrometers ), Nd:YAG laser pulses.

Structure-Property Relationships in Transition Metal-Organic Third-Order Nonlinear Optical Materials

Materials exhibiting third-order nonlinear optical (NLO) properties have applications in a number of important technologies including power limiting for sensor protection and optically addressed optical switches for photonics switching, all optical signal processing and optical computing.1–5 Because of the potential importance of these technologies, there is currently intense research interest in developing new third-order NLO materials with large effective third-order NLO susceptibilities, χ(3), and the appropriate properties for the various applications.

Four-wave-mixing measurements on metal organics

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.

Nonlinear absorption of stilbazolium derivatives

Stilbazolium derivatives are very attractive nonlinear optical (NLO) materials for photonics and biophotonics applications due to their low linear absorption at low incident intensities in most of the visible spectral range and potentially very strong nonlinear absorption at high intensities. In this work, we investigated the nonlinear absorption of five stilbazolium derivatives, trans-4-[2-(pyrryl)vinyl]-1-methylpyridinium iodide (PVPI), trans-4-[2-(1-ferrocenyl)vinyl]-1-methylpyridinium iodide (FcVPI), trans-4-[2-(1-ferrocenyl)styryl]-1-methylpyridinium iodide (FcSPI), trans-4-[4-(dimethylamino)styryl]-1-methylpyridinium iodide (DASPI) and trans-4-(4-aminostyryl)-1-methylpyridinium iodide (ASPI) using 6 ns and 40 ps laser pulses at 532 nm. These compounds exhibit different nonlinear absorption behavior for nanosecond and picosecond laser pulses. In the nanosecond time regime, they all show reverse saturable absorption, with PVPI exhibiting the best result. With 90% linear transmission in a 2-mm cell, the transmittance of a chloroform solution of PVPI drops to 5% when the incident fluence is increased to 7 J/cm2. The nonlinear absorption behavior of these compounds is influenced dramatically by the nature of the electron donating group, with reverse saturable absorption decreasing in the order of PVPI>FcVPI>FcSPI>ASPI>DASPI. In contrast, for picosecond laser pulses, only PVPI exhibit slight reverse saturable absorption, while DASPI, FcVPI and FcSPI show saturable absorption, and ASPI shows no nonlinear absorption. The different nonlinear absorption for ns and ps laser pulses may be due to the relative contributions from triplet excited state absorption and singlet excited state absorption.

Nonlinear optical properties of metal-organic complexes with phosphorous-donor ligands

Nonlinear optical materials with large intensity-induced changes in refractive index and absorption, which can be characterized by their effective third order susceptibility, are needed for numerous optical device applications. We have reported that Mo(CO)(Ph2PX) complexes exhibit strong, non-resonantly enhanced optical nonlinearities. However, the relationship between the structure of these complexes and their nonlinear optical properties is poorly understood. We have now synthesized Pd(II) and Pt(II) phosphine complexes similar to the Mo(O) complexes and have studied their nonlinear optical properties using Z-scan experiments. The Pd(II) and Pt(II) complexes have a low linear absorption at the working wavelength of 532 nm and exhibit both nonlinear optical refraction and nonlinear optical absorption effects. Our measurements indicate that the nonlinear optical properties of these complexes depend both on the nature of the phosphine ligand and on the nature of the metal center and its coordination geometry. However, this dependence is not as pronounced as we have previously observed in the Mo(CO)(Ph2PX) complexes.