2D radiation of acoustic waves generated by an IR laser pulse in anisotropic semiconductors

Abstract
When a 1064 nm laser pulse is focused at the surface of silicon, it has been observed that the material contracts whereas heating should produce expansion. In addition to the usual thermal acoustic source, a non thermal sound excitation results of photo-excitation of electron-hole pairs. Therefore, to describe the optoacoustic effect in semiconductors it is necessary to account for thermoelastic and other deformation sources simultaneously. In this paper, a model is implemented that accounts for the diffusive character of charge carrier motion and heat conduction. Relevant differential equations are coupled together and with the wave motion equation. Three sets of equations are linearised in a 2D Fourier domain and solutions are found with an appeal to convenient boundary conditions. Signals are calculated for off epicentre positions at the rear side of the anisotropic semiconductor. Experiments are performed on a 5 mm thick silicon plate with a Nd:Yag laser that delivers 20 ns pulses. Signals are measured for epicentre and off epicentre positions. Very good agreement is obtained with calculated signals for both positions demonstrating that both sound generation mechanism and anisotropy are accurately taken into account.