Induced transparency in polythiophene bearing azobenzene moieties

The nonlinear absorption of poly[2-[ethyl-[4-(4-nitro-phenylazo)-phenyl]-amino]-ethane(3-thienyl)ethanoate], PAzT, was investigated using single pulse and pulse train open aperture Z-scan techniques to discriminate between fast and accumulative nonlinearities. We observed an induced transparency for excitation with picosecond laser pulses at resonant conditions, which was attributed to the azobenzene moieties attached to the polymer backbone. The fast transparency is caused mainly by the build up of molecules in the first excited state that have a negligible absorption cross-section compared to the ground state. When using pulse trains, the nonlinear response is mainly due to the accumulation of molecules in the cis configuration, which presents a ground state cross-section slightly smaller than that of trans. The cumulative effect arises from a combination of the less intense excitation and longer lifetime of the cis population. We believe that the process observed here makes PAzT an attractive polymer for photonics applications, such as optical switches and storage devices.