Hyperbolic heat diffusion in photothermal experiments with solids

We will take into account the relaxation or build-up time necessary for the onset of a heat flux after a temperature gradient is imposed on a sample, to derive the hyperbolic heat diffusion equation. It is shown solving this equation that, in photothermal experiments involving intensity modulated laser light, there exists a critical modulation frequency at which the heat flux changes its diffusive character to a wave propagation process with the velocity of sound. On the other hand, in the case of time domain experiments, i.e., when a sample is exposed to an ultra-short laser pulse, our calculations show the existence of thermal waves propagating through the sample, whose behaviour will be discussed