Laser-induced thermal effects on the optical properties of free-standing porous silicon films

A detailed study of the unique optical properties of free-standing oxidized porous silicon films has been performed. Under continuous-wave laser irradiation, a strongly superlinear light emission and a very large laser-induced absorption are observed. The nonlinear emission is characterized by a sharp intensity increase that is in proportion to at least the eighth power of the excitation intensity. This emission has a broad peak (600-1300 nm), slow time constant (greater than or equal to 10 ms), and very low polarization memory (less than or equal to 0.01 near the emission peak). The induced absorption increases linearly with the pump laser intensity and can be as large as several times the linear absorption. The increase in the normalized absorption coefficient is almost independent of the oxidation temperature and emission wavelength, with essentially no polarization dependence. These experimental results are discussed in terms of laser-induced thermal effects. An evaluation of the temperature rise under the laser irradiation is performed both experimentally and theoretically. A remarkably high-temperature rise of greater than or equal to 700 degrees C is estimated for a moderate excitation intensity of 20 W/cm(2). (C) 2000 American Institute of Physics. [S0021-8979(00)08104-4].