Knowledge Bank

We report here the investigation of the two-photon thermal lens effect using a synchronously pumped picosecond dye laser as the excitation source. The relatively slow response of the thermal lens effect provides an integration of the heat generated by the individual pulses of picosecond two-photon absorption. This allows the detection of a stronger thermal lensing signal induced by weak two-photon absorption, where the picosecond laser is chopped as in the conventional dual-beam thermal lens setup. In addition, the pulse-to-pulse stability of the synchronously modelocked dye laser is significantly higher than that of nanosecond pulsed lasers. Two-photon absorption spectra of naphthalene and diphenyl butadiene in $CCl4$ are measured by the thermal lens technique using the synchronously pumped dye laser. The spectra are in agreement with two-photon fluorescence excitation spectra for these compounds, demonstrating that the thermal lens can be used to measure two-photon absorption in weakly fluorescent molecules.