The quasistatic process in pulsed photothermal detection

In the pulsed photothermal (PT) displacement detection, the signal displays an apparent quasistatic characteristic, i.e., the signal reaches its maximum point rapidly, and then decreases exponentially. For a quantitative numerical simulation, the quasistatic approximation, i.e., neglecting the inertia term in the thermoelastic displacement equation, is usually considered, which is different from the studies in the laser ultrasonics. In this work, we present a theoretical investigation on the availability of this approximation for a Q-switch laser pulse and a finite thickness of the sample. The results show the signals are characterized by a quasistatic process as the laser pulse rise time is on the order of 0.1 μs, but has a typical dynamic wave behavior as the rise time decreases to the order of 10 ns. The effects of the thermal diffusivity and the sample thickness on the quasistatic characteristic are also discussed in detail