Investigations of third-order nonlinear optical response of poly (p-phenylenevinylene) derivatives by femtosecond optical Kerr effect

By using two-color femtosecond optically heterodyned optical Kerr effect and off-resonant single beam Z-scan methods, we investigated the ultrafast photo-induced birefringence of three poly(p-phenylenevinylene) (PPV) derivatives, MEH-PPV (poly-5-(2-ethylhexoxy)-p-phenylenevinylene)), DO-PPV (poly-(2,5-dioctyloxy-1,4-phenylenevinylene)) and DD-PPV (poly-(2,5-didodecyloxy-1,4-phenylenevinylene)). As the 400 nm laser pulse was near the absorption region of PPV derivatives, an ultrafast rise of optical Kerr signal followed by a slow decay process with lifetime about several hundred picosecond was observed. The magnitudes of the real part of χ⁽³⁾ for PPV derivatives were calculated to be in the order of 10 ^-10 esu which was one order of magnitude larger than that in 800 nm OKE case. We assign this enhancement to the populated excited state due to the near-resonant excitation in these PPV derivatives.