Infrared absorption and momentum relaxation of free carriers in silicon generated by subpicosecond above band gap radiation

The absorption of infrared radiation in Si of various dopings is investigated after free carriers have been generated by absorption of a subpicosecond laser pulse of above band gap photon energy. A theoretical model is presented which predicts the transmission coefficient for an infrared light pulse through a photogenerated e-h plasma in Si of various surface free carrier densities. Experiments are performed in which the transmission coefficient is measured for a 10.6 /spl mu/m, /spl sim/20 ps laser pulse through a crystalline Si-wafer after it has been irradiated by a 490 fs, 616 nm laser pulse of intensities varying over several orders of magnitude. By fitting the experimental data to the theoretical predictions the imaginary component of the dielectric constant is accurately determined. From the results the free carrier absorption cross sections and the average momentum relaxation times are calculated. The momentum relaxation times in n-doped Si at 10.6 fs are much shorter than that of 26.5 fs observed in intrinsic and p-doped Si.<>