High-order (N=4–6) multiphoton absorption in GaP, ZnSe, GaSe, and ZGP crystals

With the ability to generate few-cycle optical pulses from mode-locked lasers, focused power densities in excess of 100 GW/cm² are easily achievable even at full (~100 MHz) repetition rates using oscillator outputs. The development of ultrafast sources in the mid-infrared range (MIR, λ > 2 μm) made it possible to demonstrate high-field effects, such as the generation of high harmonics in solids, the generation of a multi-octave supercontinuum, and the creation of an ultra-wideband subharmonic frequency combs and single-cycle MIR transient generation via optical rectification. Therefore, it is important to know the behavior of non-linear materials in terms of high-order detrimental effects such as multiphoton absorption. Here we report on our study of high-order multiphoton absorption (MPA) and its anisotropy in four notable mid-infrared χ⁽²⁾ crystals: GaP, ZnSe, GaSe and ZGP using 2.35 µm femtosecond pulses with peak intensity up to 200 GW/cm² in combination with the Z- scan method. We found that, at the intensities used, the nonlinear absorption obeys a perturbation model with multiphoton absorption (MPA) orders from N = 4 to 6, in good agreement with the bandgaps of the crystals. A study of the role of free carrier absorption, performed by increasing the pulse duration from 30 to 70 fs while maintaining a constant peak intensity, showed that, at our intensity levels, free carriers absorb much stronger than would be expected from their linear absorption cross section. Possible mechanisms of this enhanced absorption by free-carriers include highly non-thermalized distribution of carriers within the lowest conduction band combined with field-induced intravalley scattering and direct absorption to higher conduction bands.