Effect of the Thermal Properties of the Substrate on the Nonlinear Photoacoustic Signal Characteristics of Opaque Media

The effect of thermal nonlinearity (TN) owing to the temperature dependence of the thermal parameters of the substrate, gas layer, and an opaque sample, as well as its black-body coefficient, on the characteristics of the nonlinear photoacoustic (PA) signal at the fundamental (FH) and second (SH) harmonics during gas-microphone reception is studied theoretically. It is shown that the frequency dependence of the FH is ~ω –1/2 and of the SH, ~ω –3/2 . The dependence of the amplitude of the FH on the incident beam intensity is nontrivial and is expressed in terms of the dependence of the temperature increments of the irradiated Θ 0 and back W 0 sides of the sample surface on I 0 , while this dependence for the SH is ∼ l 0 2 .