Measurement of femtosecond single-pulse and time-resolved absorbance in an optically thin silicon film is used to extract the direct two-photon absorption (TPA) coefficient β<sub>TPA</sub> of silicon over a wide spectral range; it is also used to distinguish other nonlinear absorption channels, even with the fundamental pulse far above the indirect gap. We find that β<sub>TPA</sub> varies as follows: 15 cm/GW <β<sub>TPA</sub> <36 cm/GW ± 6 cm/GW over the two-photon energy range 4.0 eV < 2hv < 4.5 eV, consistent with available calculated values. The spectral structure of the results is related to the underlying band structure. The results show that second-order absorption nonlinearities dominate for fluences up to 0.07 J/cm<sup>2</sup>, just below the single-shot melting threshold. At fluences above this level a much stronger nonlinearity dominates.
© 1990 Optical Society of America
D. H. Reitze, T. R. Zhang, Wm. M. Wood, and M. C. Downer, "Two-photon spectroscopy of silicon using femtosecond pulses at above-gap frequencies," J. Opt. Soc. Am. B 7, 84-89 (1990)