Demonstrating a nano viscometer using femtosecond laser induced photo-thermal effect

Abstract

We present a new method to utilize photo-thermal effect at nano-volume dimension, to measure viscosity, which could be useful to eventually correlate to in-vivo conditions. We present our measurements using high repetition rate low average power femtosecond laser induced conditions under which the photo-thermal effect is highly influence by the convective mode of heat transfer. This is especially important in absorbing liquids. This is unlike the typical photo-thermal conditions not involving such ultrashort pulses. Typical thermal processes involve only conductive mode of heat transfer and are phenomenological in nature. Inclusion of convective mode results in some additional molecular characteristics to the thermal process. We measure the traditional thermal lens studies 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 due to the 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 of wavelength 1560 nm and 780 nm for these experiments. From the bulk thermal studies, we show that use of water as sample provides us the advantage to use conductive mode of heat transfer even with femtosecond pulse train excitation.