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 βTPA 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 βTPA varies as follows: 15 cm/GW < βTPA < 36 cm/GW ± 6 cm/GW over the two-photon energy range 4.0 eV < 2hν < 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/cm2, just below the single-shot melting threshold. At fluences above this level a much stronger nonlinearity dominates.
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