A new resonant photoacoustic technique for measuring very low optical absorption in crystals and glasses

In this paper, a new photoacoustic technique is described for measuring very low optical absorption in crystals and glasses. The technique differs from the conventional photoacoustic technique in that it does not use a high-power laser pulse to excite an acoustic signal in the sample, but instead uses a continuous laser with the beam modulated at an acoustic resonance frequency of the sample. By exciting the sample at resonance, the acoustic signal is enhanced by a factor proportional to the quality factor (Q) of the resonance. Because highly transparent crystals and glasses may have a Q on the order of 104 to 106, the technique is orders of magnitude more sensitive than conventional techniques. The Q is enhanced through the use of noncontact acoustic transducers, which also eliminate the problem of background signals due to scattered light. Measurements with various quartz samples have been used to verify the technique. Using a 10-W continuous laser, the noise level of the resonant photoacoustic technique would be equivalent to an optical absorption coefficient of ∼3×10−9 cm−1.