Laser-induced nonlinear absorption in silicon: Free-carrier absorption versus thermal effects

The optical-absorption coefficient of silicon has been determined under moderate (0.15 J/cm²) nanosecond laser excitation, between 2.0 and 2.6 eV. With respect to the cw low-power value, an enhancement under irradiation is observed on the whole range of fluence and wavelength. This enhancement builds up with the pump pulse and persists a long time after its end. It is shown that the features of the effect, i.e., dependences on time, fluence, and wavelength, can be interpreted by a thermally induced absorption enhancement. This mechanism is operative above 2.0 eV, whereas, under this value, the free-carrier absorption process is dominant.