Femtosecond Laser-Induced Photothermal Effect for Nanoscale Viscometer and Thermometer

Abstract

A new method of utilizing photothermal effect at nano-volume dimensions to measure viscosity is presented here that can, in turn, provide the surrounding temperature. Our measurements use high repetition rate, low average power, femtosecond laser pulses that induce photothermal effect that is highly influence by the convective mode of heat transfer. This is especially important for absorbing liquids, which is unlike the typical photothermal effects that are due to such ultrashort pulses. Typical thermal processes involve only conductive mode of heat transfer and are phenomenological in nature. Inclusion of convective mode results in additional molecular characteristics of the thermal process. We measure traditional thermal lens with femtosecond pulse train through geometric beam divergence of a collimated laser beam co-propagating with the focused heating laser beam. The refractive index gradient in the sample arising from a focused heating laser creates a thermal lens, which is measured. On the other hand, the same heat gradient from the focusing heating laser beam generates a change in local viscosity in the medium, which changes the trapped stiffness of an optically trapped microsphere in its vicinity. We use co-propagating femtosecond train of laser pulses at 1560 and 780 nm wavelengths for these experiments. We also show from the bulk thermal studies that use of water as sample has the advantage of using conductive mode of heat transfer for femtosecond pulse train excitation.