POSTPONED - Debabrata Goswami
Femtosecond Photothermal Spectroscopy for Molecular Recognition
Photothermal effects are omnipresent in most light-matter interactions with either continuous-wave (CW) or pulsed lasers. However, the heat dissipation dynamics of systems interacting with femtosecond infra-red lasers can differ significantly from their CW counterparts. Though the heat load of individual femtosecond pulses may be trivial, a typical high-repetition-rate femtosecond laser can generate a cumulative heat load that is significantly higher in a shorter period. For liquids, this fact has been quantified from the measure of divergence of the propagating laser beam through a nearly transparent sample due to the thermal lensing effect of the photothermal process. The divergence arises from the decrease in the refractive index of the sample due to heat deposition, which is the principle behind femtosecond laser-induced thermal lens spectroscopy. The thermal lens effect is governed by an interplay of thermal load versus thermal dissipation, which needs to be described in terms of convective and conductive processes. Conduction effects dominate in the solids and are sufficient to describe the thermal dissipation dynamics. However, in the case of liquids, at least for increased thermal load, convection cannot be ignored and has led to fundamental breakthroughs that have resulted from our work in terms of broadening the horizons of spectroscopy with sensitive molecular analyses and technological breakthroughs for nanoscale thermometers and sensors. We have also developed the theory behind such thermal processes.