We report on a novel pump-probe thermal-lens method for the measurement of absorption in extended samples using nearly collimated continuous wave (CW) and low-power probe and pump beams. We report the measurement of the absorption of a column of distilled water up to 2 meters thick. The method allows the determination of the absorption with sensitivities up to 10<sup>-10</sup> W cm<sup>-1</sup> in materials with photothermal parameters similar to distilled water.
A. Marcano O., L. Rodriguez, and N. Melikechi, "Thermal Lensing in Extended Samples," Appl. Spectrosc. 56, 1504-1507 (2002)