Nonlinear transmission measurements of silicon are used to determine the two- and three-photon absorption coefficients for photon energies above the indirect band gap of Si in the range of $1.14–2.07\mathrm{eV}$. By careful characterization of the temporal and spatial profiles of the ultrashort optical pulses, the nonlinear transmission as a function of peak optical intensity can be determined. Two- and three-photon absorption coefficients are acquired by fitting both an analytical and numerical model to the transmission traces, from which two distinct spectral regimes emerge with a clear threshold at $1.7\mathrm{eV}$—this corresponds to half the direct band gap in Si. At photon energies above the threshold, the nonlinear transmission traces are dominated by two-photon absorption, whereas below the threshold three-photon absorption is dominant with a nondetectable contribution from indirect two-photon absorption. This result is at variance with the current understanding of above-band-gap nonlinear absorption in Si, which is exclusively attributed to two-photon absorption processes.

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• Light propagation, transmission & absorption
• Nonlinear optics
• Elemental semiconductors
• Thin films
• Femtosecond laser spectroscopy
• Optical techniques
• Reflectivity