Raman excitation induced thermal lensing effect in transparent molecular liquids

Nonlinear refraction of liquids C₆H₆, C₆H₅Cl, C₆H₅Br, and C₆H₅I was investigated, for the first to our knowledge, using the Z-scan technique with 82 MHz 35 fs laser pulses at 800 nm. For all samples studied a negative lensing effect was observed, opposite to that obtained with 10 Hz 19 picosecond laser pulses. The negative lensing effect was found to increase with the pulse train width and gradually turn steady on the time scale of thermal diffusivity. The sign difference is thus considered as a thermal lensing effect which sustains across femtosecond pulses, but not the picosecond pulses. In light of previous Raman induced Kerr effect (RIKE) studies on a variety of simple transparent molecular liquids, the negative nonlinear refraction observed with 35 fs pulses is attributed to electronic or resonant nuclear motions (librational and vibrational motions). In this study, results show that the shape and bond strength of molecules seems to have strong impact on the thermal lensing effects.