Note:
Photoacoustic saturation arises when the sample is optically opaque and thermally thin, i.e., when the thermal diffusion depth, \(L\), is greater than the optical absorption depth, \(\mu_{\beta}\), and less than the sample thickness. The condition may be relieved by increasing the modulation frequency sufficiently that \(L\) becomes less than \(\mu_{\beta}\), i.e., that the sample becomes thermally thick.