We have theoretically studied the effect of thermal nonlinearity, due to the temperature dependence of the thermophysical and optical parameters for thermally thick opaque media, on the characteristics of the fundamental photoacoustic signal when the signal is detected by a gas microphone. We have shown that the dependence of the amplitude of the nonlinear component of the signal on the intensity of the incident radiation I 0 is expressed by means of the dependence of the temperature rise for the irradiated sample surface Θ 0 on I 0 , and the thermal nonlinearity does not affect the phase of the photoacoustic signal. We propose a theory for generation of the second harmonic of the photoacoustic signal. We have established that the phase shift of the photoacoustic signal is equal to 3π/4, while its amplitude depends on the frequency (,ω −3 ⁄ 2 ) and the intensity (,I 0 2 ).
A theory is proposed for of the generation of the first two harmonics of a nonlinear photoacoustic signal of a solids sample with an arbitrary value of thermal conductivity. For limiting cases (thermally thin and thick samples) has been obtained simple expressions for the dependence of the amplitude of the excited photoacoustic signal from the emissivity of the sample and the thermophysical parameters of the sample, gas and substrates, including their thermal coefficients. Keywords: photoacoustics, thermal nonlinearity, second harmonic.
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