a Degenerate Four-Wave Mixing Study of Third-Order Optical Nonlinearity of Transition Metal Containing Organic Complexes.

All-optical devices play a major role in acquiring, storing, transmitting, and processing information rapidly. Transition metal-based organic complexes are prospective materials for these kinds of devices. These metal-organic materials have a number of advantages over conventional nonlinear optical materials such as: ultrafast response time, the ability to engineer molecular structure, and a high laser damage threshold. A backward degenerate four-wave mixing apparatus has been built to measure the properties of metal-organic complexes. The light source is a Nd:YAG Pockels' cell Q-switched pulse laser. The output energy of the laser is approximately 7 mJ when operated at a rate of 2 pps. The laser pulse duration is 20 nS, and the fundamental lasing wavelength is 1064 nm. The output intensity variation of the laser is about of 3%. The Nd:YAG laser is frequency doubled using a KDP (potassium dihydrogen phosphate, KH _2PO_4) crystal so that 532 nm pulses are used for the degenerate four -wave mixing experiment. The third-order nonlinear optical property for samples of molybdenum (Mo), palladium (Pd), platinum (Pt), rhodium (Rh), and tungsten (W) complexes has been measured by the degenerate four-wave mixing technique. Most of the complexes have a molecular structure of Cequiv O and phosphine ligands coordinated on the metal atom. The experimentally measured second-order molecular hyperpolarizabilities range from gamma=1.9 times 10^{-31} esu for cis -Pt(CO)_2(PPh_2thiophene) _2 and gamma = 1.7 times 10^{-31} esu for cis -rm MO(CO)_4(PPh_3)_2 to gamma = 1.5times 10^ {-34} esu for Mo(CO)_5 (PPh_3) and gamma = 1.7times 10^{-34} esu for Rh(CO)_2(acac). The results indicate that the nonlinear optical mechanisms that lead to these gamma values are nonresonant. Analysis shows that the electron donor/acceptor ability of the substituents on phosphine ligands does not affect the mono(phosphine) Mo complex's third-order nonlinear optical response at all, while its effect on the bis(phosphine) complex is limited. The major factor that dramatically enhances second-order molecular hyperpolarizability is the pi electron structure of the substituents in the phosphine ligands. It has been found that the larger the number of the substituents of a pi-electron structure on phosphine ligands, the larger the observed third-order nonlinearity. For molybdenum complexes, the magnitude of the second-order molecular hyperpolarizability, gamma, increases approximately as the 5th power of the number of substituents having a pi-electron structure. It is also observed that for the complexes with the same type phosphine ligand, the cis-type complexes yield a larger gamma value than that of the complexes with the trans-type coordination geometry. The charge transfer is probably through the ligand -to-ligand mechanism rather than through the ligand-to-metal -to-ligand mechanism. The type of metal does affect the measured second-order molecular hyperpolarizability. Complexes with a metal of second row of the periodic table exhibit a larger gamma value, though the effect of the metal is not as drastic as that of the pi electron structure substituents on ligands.