Three photon absorption in silicon for 2300–3300 nm

We measure the spectral dependence of the degenerate three photon absorption coefficient, γ, for a Si [100] wafer using 200 fs pulses in the range 2300–3300 nm, i.e., photon energy between half and one-third the indirect band gap. For pulses linearly polarized along the [001] crystal axis γ increases from a value of near 0 cm³/GW² at 3300 nm to a peak value of 0.035 cm³/GW² at 2700 nm before decreasing with shorter wavelength; this is consistent with the dispersion expected from allowed-allowed-allowed transitions. At 2600 nm the γ value is ∼30% larger for light polarized along [011] than along [001].