Femtosecond Laser induced Photothermal Spectroscopy

Femtosecond thermal lens spectroscopy (FTLS) is a specific application of thermal lensing in photothermal spectroscopy that uses ultrashort laser pulses, typically on the order of femtoseconds, to heat a sample rapidly and create the thermal lens (TL) effect. The high repetition rate of these pulses allows for continuous measurement of the sample, and makes FTLS a highly sensitive and non-destructive technique for studying photothermal signatures in liquids and mixtures. Since high repetition rate (HRR) femtosecond lasers require around ten times smaller average powers than CW lasers, there is no excessive heating of the bulk media using this approach within appropriate experimental timeframes. The strong signal-to-noise ratio (SNR) of low-power femtosecond HRR lasers makes them ideal for studying TL. Due to the small amount of energy delivered by each femtosecond laser pulse to the sample, the high SNR is feasible. In heavily absorbing samples, localized heat accumulation owing to incident femtosecond pulses causes a superheating state, which also alters the sample's nonlinear properties leading to heat dissipation through both convective and convective heat transfer processes. The convective heat transfer makes the TL signal to be strongly dependent on the molecular characteristics and physical properties of the system, and becomes the cornerstone of the of the molecular spectroscopic advantage of the FTLS in fluids. This correlation has driven the development of ultrafast laser-induced thermal processes for sensitive spectroscopy.