Resonant Nonlinear Optical Processes and Charge Carrier Dynamics in Photoresponsive Polymers

Abstract

Abstract Resonant third order optical nonlinearity, X(3), in several photoresponsive polymers is studied by picosecond and femtosecond degenerate four wave mixing to investigate the role of photoexcited charge carriers. Both the magnitude and the response time of the observed optical nonlinearities seem to vary over a wide range. In the case of the poly-N-vinyl carbazole: trinitrofluorenone polymer composite photoconductor, the observed resonant X(3), dependent on the composition of the composite, is attributed to the photoexcited correlated electron-hole pairs with a response time (decay time) in hundreds of picoseconds. In the case of a soluble polyacetylene-polymethyl methacrylate graft co-polymer, the observed resonant X(3) has an extremely fast initial decay and is consistent with what can be expected from the intrachain soliton dynamics. In the case of an electrochemically formed polymer, specifically polythiophene, we observe a relatively large X(3) with, again, a very fast initial decay in subpicoseconds consistent with the intrachain polaronic processes. An in situ study of the nonlinear optical behavior as a function electrochemical redox cycle shows a drastic reduction of the overall X(3) as the film is oxidized.