Photoinduced Processes and Resonant Third-Order Nonlinearity in Poly (3-Dodecylthiophene) Studied by Femtosecond Time Resolved Degenerate Four Wave Mixing

摘要

The dynamics of optical processes in organic molecules and polymers is an activearea of experimental and theoretical investigation because it offers new challenges for both basic research and potential device applications. Especially complex is the role of electronic resonances in third-order nonlinear optical processes. Electronic resonances enhance parametric processes such as third harmonic generation which are the coherent nonlinear response of the medium. In addition, photogenerated excitations also create a change in refractive index; this intensity dependent refractive index describes a third-order nonlinear response which is determined by the population density of excitation. We call it dynamic nonlinearity or incoherent nonlinearity. In inorganic multiple quantum well semiconductors, the dynamic nonlinearity predominates. As a result, multiple quantum well semiconductors exhibit strong manifestations due to intensity dependence of refractive index but very weak coherent response such as third harmonic generation. The time evolution of the dynamic response, therefore, can be used to obtain information on the dynamics of photoexcitation.