Dependence of photoacoustic amplitude on chopping frequency for porous silicon

The variation in the photoacoustic signal produced by porous silicon on a silicon substrate with chopping frequency was investigated using excitation energies of 1.22 and 2.75 eV, below and above the band gap of porous silicon, respectively. At 1.22 eV, the porous silicon is transparent, and photoacoustic amplitude follows a −32-power dependence with chopping frequency. At 2.75 eV, at which the porous silicon is opaque, an exact −12-power dependence was observed for lower chopping frequencies in addition to the ordinary −1-power dependence at higher frequencies. The transition frequency between these two dependencies decreases with anodization time.