Heat wave dynamics in frozen water droplets with eosin molecules under the femtosecond excitation of a supercontinuum

Abstract

In this study, we considered thermal processes in liquid and frozen water droplets with added dye molecules and metal nanoparticles at the moment of supercontinuum generation. We studied optical non-linear processes in a water droplet with a diameter of 1.92 mm, cooled (+2 °C) and frozen to -17 °C, with eosin molecules and ablative silver nanoparticles upon femtosecond laser treatment. When we exposed a cooled water droplet and a piece of ice containing eosin molecules and ablative silver nanoparticles to a femtosecond laser beam (l = 1030 nm), we recorded two-photon fluorescence, enhanced by plasmon processes. Also, supercontinuum generation took place, with a period of decay t = 0.02 s. The geometry of non-linear large -scale self-focusing (LLSS ~ 0.45–0.55 mm) was studied. The value of microscale self-focusing (LSSS ~ 0.1 mm) of SC radiation in the laser channel was determined experimentally. The study shows that the energy dissipation in the SC channel increases when the thermal non-linearity exceeds the electronic non-linearity. We modelled the thermal processes and determined the temperature gradient of the heating of the frozen droplet exposed to a femtosecond pulse. Based on the experimental data, the heat wave propagation velocity was calculated to be n = 0.11 m/s.

Keywords

• supercontinuum
• femtosecond excitation
• water
• piece of ice
• eosin fluorescence
• ablative silver nanoparticles
• surface plasmons
• two-photon excitation
• thermal optical non-linearity
• temperature gradient
• heat wave
• wave propagation velocity