Chris M. Lawson

Optical limiting performances of asymmetric pentaazadentate porphyrin-like cadmium complexes

The optical limiting performances of seven asymmetric pentaazadentate porphyrin-like cadmium complexes have been measured at 532 nm with nanosecond pulses. In a f/38 geometry, with sample transmission of 0.51–0.80 in a 2 mm cell, the limiting thresholds for these complexes were 1.4–3.0u2009mJ/cm2. The throughputs of these complexes were limited to 0.31–1.13u2009J/cm2 for incident fluences as high as 3.5u2009J/cm2. The limiting throughput was strongly influenced by the nature of the ligand. Lower bounds for the ratio of triplet excited-state to ground-state absorption cross sections have been estimated at 3.4–5.7. The lower limiting thresholds, lower limiting throughputs, as well as the ease of modification of the ligands, make these complexes promising candidates for optical power limiters.

Excited state lifetime and intersystem crossing rate of asymmetric pentaazadentate porphyrin-like metal complexes

The first singlet excited state lifetimes (τs) of asymmetric pentaazadentate porphyrin-like complexes, [(R-APPC)M]Cln, were measured by the time-correlated single photon counting method. Fluorescent decays of the first singlet excited state for all [(R-APPC)M]Cln were found to be shorter than 0.5ns. The intersystem crossing rates (kisc) were calculated from the τs values and previously measured triplet quantum yields (ϕT). Values for kisc for the complexes ranged from 0.68 to 3.04×109s−1 and are significantly greater than those of silicon naphthalocyanine (SiNc) and lead phthalocyanine (PbPc(CP)4). The values of τs and kisc of the [(R-APPC)M]Cln complexes are sensitive to variations in both the bridging R group and the metal center.

Degenerate four-wave mixing and Z-scan measurements of stilbazolium derivatives

Stilbazolium derivatives are very attractive optical materials for photonics and biophotonics applications because a large number of these derivatives are available and they can exhibit large third-order nonlinear coefficients and rapid responses. To quantitatively characterize their third-order nonlinear coefficients and the temporal responses of their third-order nonlinear process, we have performed degenerate four-wave mixing and Z-scan experiments on four stilbazolium derivatives with different electron donors using 40 ps laser pulses at 532 nm. The γ values for three of the compounds are on the order of 10−31u200aesu. These high values arise from electronic processes and are associated with both nonlinear absorption and nonlinear refraction. The relative contributions from nonlinear absorption and nonlinear refraction are dependent on the chemical structure and linear absorption of the stilbazolium derivative.

Nonlinear optical properties of transition metal-phosphine complexes

Abstract Nonlinear optical properties of transition metal-phosphine complexes have been measured at 532 nm by degenerate four-wave mixing. Large nonresonant second-order molecular hyperpolarizabilities, γ, have been found for complexes containing two phosphine ligands. The measured γ values are closely related to the type and coordination geometry of the phosphine ligands. A numerical fitting of γ values versus the number of substituents with π-electrons on the ligand gives a result similar to what is observed for linear conjugated oligomers.

Third-order nonlinear optical properties of an expanded porphyrin cadmium complex

The third-order nonlinear optical properties of a pentaazadentate porphyrin-like cadmium complex, [(DBP–APPC)Cd]Cl, have been investigated using 40-ps and 5-ns laser pulses at 532 nm. The molecular second-order hyperpolarizability of this complex is 9.7×10−31u200aesu, and the nonlinearity is predominantly electronic in origin with picosecond laser pulses. Temporal response of the nonlinear process is composed of at least two components with different time responses and shows a strong fluence dependence that indicates the participation of excited states in the nonlinear process. Nonlinear absorption measurements demonstrate that this complex shows strong reverse saturable absorption for nanosecond laser pulses, but exhibits a transformation from reverse saturable absorption to saturable absorption at high fluence for picosecond pulses. These results suggest that [(DBP–APPC)Cd]Cl is a very promising candidate for optical limiting of nanosecond laser pulses.

Third-order susceptibilities of asymmetric pentaazadentate porphyrin-like metal complexes

The third-order nonlinear optical properties of a series of asymmetric pentaazadentate porphyrin-like metal complexes ([(R–APPC)M]Cln) with different conjugated R groups and metal centers have been investigated by degenerate four wave mixing experiments using 40 ps laser pulses at 532 nm. The molecular second-order hyperpolarizabilities (γ) for these complexes range from 1.1×10−31 to 1.2×10−30 esu and depend strongly on both the nature of the metal and the R group. The γ values of the [(R–APPC)Cd]Cl complexes are 2–7 times larger than those of PbPc(CP)4 and SiNc, two of the most promising nonlinear optical materials for optical limiting applications. Measurements of the individual components of the χ(3) tensor indicate that the nonlinearity is predominantly electronic in origin when 40 ps laser pulses are used.

Third-order nonlinearity and optical limiting of dichloromethane solutions of cis-Mo(CO)4(PPh3)2

Abstract Oxidized dichloromethane solutions of cis -Mo(CO) 4 (PPh 3 ) 2 stabilized by the addition of excess PPh 3 exhibit unusual large molecular second-order hyperpolarizabilities with minimal linear absorption. To better understand the mechanism by which this occurs, DFWM, Z-scan and optical limiting experiments have been carried out on oxidized dichloromethane solutions of cis -Mo(CO) 4 (PPh 3 ) 2 using 5.8 ns laser pulses at 532 nm. Our results demonstrate that the mechanism by which the oxidized species enhance the third-order nonlinear response is not dominated by thermal effects, but is instead due to the strong nonlinear absorption and nonlinear refraction of the oxidized species. Moreover, the oxidized species shows very strong optical limiting properties. Such complexes would be very interesting third-order nonlinear optical materials for optical limiting applications.

Higher-order triplet interaction in energy-level modeling of excited-state absorption for an expanded porphyrin cadmium complex

Recent measurements of transmission versus fluence for a methanol-solvated asymmetric pentaazadentate porphyrin-like (APPC) cadmium complex, [(C6H4-APPC)Cd]Cl, showed the limitations of current energy-level models in predicting the transmission behavior of organic reverse saturable absorbers at fluences greater than 1 J/cm². A new model has been developed that incorporates higher-order triplet processes and accurately fits both nanosecond and picosecond transmission-versus-fluence data. This model has provided the first known determination of a higher triplet excited-state absorption cross section and lifetime for an APPC complex and also described a previously unreported feature in the transmission-versus-fluence data. The intersystem crossing rate and the previously neglected higher triplet excited-state absorption cross section are shown to govern the excited-state population dynamics of methanol-solvated [(C6H4-APPC)Cd]Cl most strongly at more-practical device energies.

Nonlinear-optical studies of molybdenum metal organics.

We have measured the third-order susceptibility, X((3)), versus concentration for a number of molybdenum-based metal-organic complexes in solution, using independent degenerate four-wave mixing and Z-scan techniques. Good agreement was obtained between the degenerate four-wave mixing and Z-scan measurements. The variation of X((3)) with concentration yielded the second-order hyperpolarizability gamma. A close correlation was observed between the number of delocalized pi electrons and the magnitude of gamma.

Nonlinear reflection at a dielectric‐carbon suspension interface: Macroscopic theory and experiment

We report on a theoretical investigation of the nonlinear reflection that has been experimentally observed when Nd:YAG laser pulses interact with a dielectric‐carbon particle suspension interface. This interaction leads to plasma formation, laser induced cavitation, and the formation of a vapor layer at the dielectric interface. This vapor layer results in total internal reflection. The theoretical fit to the experimental data shows that the calculated fluence threshold of the plasma formation process is 1 J/cm2. The theory also shows that the formation time of the vapor interface is on the order of a few nanoseconds and depends on the concentration of the carbon particles in the suspension.