Experimental evidence of second harmonic photoacoustic signal generation in metals
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The capability of photoacoustic (PA) spectroscopy for the detection of the second harmonic thermal waves on metal surfaces is investigated. Expressions for the first and second harmonic amplitude and phase PA spectra are derived from the one-dimensional nonlinear heat conduction equation using stepwise successive approximation method. First and second harmonic amplitude and phase PA spectra of nickel sample are measured in a heat reflection PA configuration. PA amplitude intensity dependence on excitation intensity exhibit characteristic behavior, it has linear dependence in the case of first harmonic and square dependence in the case of second. Experimental data were in excellent agreement with theoretical calculations. Modulation frequency dependence of amplitude and phase provided sufficient data for the thermal properties evaluation from PA measurements.