Very large continuous-wave-laser-induced optical absorption in porous silicon films: Evidence for thermal effects

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Abstract
A very large optical absorption increase induced by continuous-wave laser irradiation is reported for free-standing porous silicon films oxidized at 800–950 °C. A maximum reversible optical-density change of 2.5 is obtained with a pump laser intensity of ∼20 W/cm 2 . The induced absorption is found to be strongly decreased when the sample is attached to materials with a higher thermal conductivity. The experimental results can be satisfactory fitted with an empirical formula for the thermally induced absorption increase in bulk crystalline Si. These results strongly suggest that the observed nonlinearity is originating from thermally induced band gap shrinking in Si micro/nanostructures.