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.