Intracavity photothermal measurements of ultralow absorption

A new method of ultralow optical absorption measurement based on the photothermal deflection effect is reported. An increase in sensitivity up to 4 orders of magnitude in comparison to a conventional transverse photothermal deflection method is achieved by placing the sample inside a high quality laser resonator. The method is realized for a diode laser pump emitting at 687 nm and an amorphous silicon (a-Si:H) sample of 1 mu m thickness, using the air as a deflecting medium. The advantages of the method are discussed.