Remote determination of phase separation with Femtosecond thermal lens spectroscopy for sensitive in-situ biomedical applications*

We demonstrate sensitive detection of the liquid-gas phase interface through our novel Femtosecond laser-induced Thermal Lens Spectroscopy (FTLS) experiments. FTLS is built on the principle that the high repetition rate of femtosecond lasers induces a cumulative thermal effect even in highly volatile media. The cumulative heat load is significantly higher in a shorter time, which results in the divergence of the propagating laser beam through nearly transparent samples. A decrease in the system's refractive index due to heat deposition results in laser beam divergence, also providing sensitive spectroscopic fingerprints. The thermal lens effect is governed by an interplay of thermal load and thermal dissipation, which differ for each phase. In the liquid phase, thermal dissipation needs to be described in terms of convective and conductive processes, while in the solid phase, conduction effects dominate and are sufficient to explain the dissipation dynamics. Our experiments across several sample interfaces show how sensitive our FTLS technique is to changes in phase separation interfaces, which can provide an accurate size measure of aerosols. Mapping microscopic aerosol distribution is also critical for COVID-19 transmission, which occurs through the dispersion of aerosol droplets containing the virus. FTLS thus paves the way for selective and controlled laser ablation with biomedical applications, including inhibiting cell cycle pathways or cell division for disease mitigation.