Michael M. McKerns

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 1.4–3.0 mJ/cm2. The throughputs of these complexes were limited to 0.31–1.13 J/cm2 for incident fluences as high as 3.5 J/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.

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.

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.

Oxidative enhancement of the χ(3) values of solutions of cis-Mo(CO)4L2 (L=PPh3, AsPh3, PPh2Np) complexes as measured by DFWM

Abstract Detailed degenerate four wave mixing (DFWM) studies of four molybdenum complexes, cis-Mo(CO)4(PPh3)2, cis-Mo(CO)4(AsPh3)2, cis-Mo(CO)4(PPh2Np)2 and cis-Mo(CO)4(P(OPh)3)2 are reported in this paper. Upon exposure to air/oxygen, the χ(3) values of solutions of the first three complexes increase dramatically with time. However, when excess free ligand is added to any of the solutions, no additional increase in the χ(3) value is observed. Hence, it is possible to obtain solutions with high χ(3) values by first oxidizing the solution and then adding excess ligand to stabilize the solution. Further, the increase in the χ(3) values of the solutions parallels the increase in the linear absorption, but it is unclear whether the increased linear absorption plays any role in the increase in the χ(3) values of the solutions. This study also shows how degenerate four wave mixing can be used as a very sensitive technique to detect the chemical change in a solution when the third order optical nonlinearities of the reactant and product are very different.

Studies of Metal Organic Complexes with a powerful tunable laser

We have built a new, continuously tunable (over 0.85 - 1.05 um, and harmonics thereof) laser system utilizing LiF:F2+** color centers as the active medium to research the NLO material parameters which are wavelength-dependant. The design, construction and properties of this new laser and its preliminary utilization will be discussed here.