Absorption of free carriers in diamond determined from the visible to the mid-infrared by femtosecond two-photon absorption spectroscopy

Abstract

Femtosecond laser pulses tunable in a large spectral range from 0.41 to 5.5 μm are used together with an intense ultraviolet pulse for noncollinear two-photon absorption (TPA) in type IIa diamond. The absorption of the photogenerated free carriers is determined within the whole probing range. The absorption coefficient increases with wavelength ∝ λ 0.55 from 0.41 up to ≈1.5 μm and then ∝ λ 2.7 for wavelengths⩽5.5 μm. The TPA coefficient of diamond decreases from 2.4 cm/GW at 7.6 eV to 0 at 5 eV overall energy with a secondary maximum around 5.2 eV indicating an excitonic enhancement.