Measurement of small absorption coefficients using pump-probe thermal lens method with nearly collimated beams

Summary form only given. Thermal lensing is a well-known method that allows the measurement of absorption coefficients with high sensitivity and versatility. In a pump-probe version of the technique, the pump beam induces a thermal lens in the absorbing medium which is monitored by a collinearly propagating probe beam. Usually the pump and the probe beam are tightly focused onto the sample. In this configuration, increasing the path-length of the sample much over the confocal distance of the focused pump beam does not enhance the amplitude of the signal. Increasing the path-length of the sample can compensate the decrease of the signal caused by the absence of focusing. We show that a thermal lens experiment can be conducted with nearly collimated pump and probe beams for the measurement of small absorption in large pathlength samples.