Picosecond and Femtosecond Time-Resolved Four - Mixing in Condensed Phase Systems.

The predominant theme of the material presented in this thesis is concerned with the resolution of microscopic dynamics that occur in condensed phase systems on picosecond and femtosecond time scales. To gain information on the dynamics, transient holographic grating and photon echo techniques are used. These are nonlinear optical techniques which are various forms of four-wave mixing. Part 1 (chapters 1-3) is concerned with the dynamics that occur both in thin-film hydrogenated amorphous silicon (a-Si:H), and in anthracene (pure and mixed crystals). In chapter 1, a general overview of the electronic band structure in organic molecular solids and inorganic semiconductors is given. How the differences in the band structure affect electronic excitations in these materials will be emphasized. In addition, how the four-wave mixing techniques provide direct observables for the dynamics, will be discussed. In chapter 2, results of a picosecond transient grating experiment in thin-film a-Si:H will be presented. Optically generated carriers are found to have a diffusion constant of 1.0 times 10^{-2 } cm^2/sec for transport in the thin-film plane. Using a model based on surface quenching a value of 0.4 times 10 ^{-2} cm^2 /sec is found for transport perpendicular to the thin-film plane. In part 1 of chapter 3, results from a picosecond transient grating experiment, used in an attempt to detect the coherent motion in anthracene, are presented. It is found that under the experimental conditions, polaritons are the predominant optical excitations. How the polaritons affect the transient grating observable will be discussed. In part 2 of chapter 3, results from a picosecond photon echo experiment on an anthracene mixed crystal will be presented. It is found that despite a large electron-acoustic phonon coupling, the predominant optical dephasing mechanism is due to the coupling of the electronic excitations to librations. The librational frequency is found to be 26 cm^ {-1}. The material in part 2 (chapters 4-6) is concerned with femtosecond time-resolved four-wave mixing experiments. In chapter 4, the development of ultrafast laser systems is briefly reviewed. In addition, advantages of femtosecond time-resolved studies are discussed with emphasis on four -wave mixing in amorphous solids. Details of a novel, tunable, amplified femtosecond dye laser system will be presented in chapter 5. Finally, in chapter 6, results of a femtosecond transient grating studies of the orientational Kerr relaxation in the liquid crystal 5CB are presented. It is found that in addition to a long time scale exponential signal decay, there exists a faster nonexponential decay. Several qualitative models are presented to explain this behavior.