ABSTRACT
The probe laser used in the PDS experiment should have a stable power (of the order of a few milliwatts is sufficient) and small angular divergence. In addition, the probe laser must have a good pointing stability, which can be a major noise source, especially at low chopping frequencies. Few commercial laser manufactur ers specify the pointing stability of their products. Currently, we use a He-Ne laser (Uniphase, model 1103P) which is ideal for our applications. An inexpensive laser diode can also be used as a substitute, although it is more difficult to focus and align. During alignment, the probe beam is focused so that it is parallel to and above the sample surface. The distance between the probe beam and the sample surface should be minimized because the photothermal signal decays exponentially from the sample surface. To facilitate such adjustment, the sample cell can be mounted on a three-dimensional minitranslation stage (Edmund Scientific). The photothermal deflection is detected by a quadrant cell (United Detector Technology) under reverse bias. A lock-in amplifier (Standard Research Model SR830), operated under differential inputs, is used for phase-sensitive detection. The entire experi ment was fully computer controlled.
