Characterization of the thermal refraction in ionic liquids induced by a train of femtosecond laser pulses

We investigate the nonlinear refraction induced by a train of ultrashort laser pulses (repetition rate 80.75MHz) in a set of ionic liquids: [EMIM][BF 4 ], [BMIM] [BF 4 ], [HMIM] [BF 4 ], [OMIM] [BF 4 ] and [DMIM] [BF 4 ]. Due to the high repetition rate, linear absorption processes are developed which cause a non local increase of temperature that generates an inhomogeneous and non local spatial variation of the refractive index. This effect is known as thermal refraction. The goal of this work is to characterize by z-scan the induced changes in the refractive index at λ=810nm and to determine the thermooptic coefficient of this liquids at this wavelength