Im{χ(3)} spectra of 110-cut GaAs, GaP, and Si near the two-photon absorption band edge

Abstract

Spectra of the degenerate two-photon absorption coefficient β(ω), anisotropy parameter σ(ω), and dichroism parameter δ(ω)=[σ(ω)+2η(ω)]/2 of crystalline 110-cut GaAs, GaP, and Si, at 300 K were measured using femtosecond pump–probe modulation spectroscopy over an excitation range in the vicinity of each material’s half-bandgap Eg/2 (overall 0.62<ℏω<1.91 eV or 2000>λ>650 nm). Together, these three parameters completely characterize the three independent components of the imaginary part of the degenerate third-order nonlinear optical susceptibility tensor Im{χabcd(3)(ω)}. In direct-gap GaAs, these components peak at ℏω≈0.78Eg, which is close to the peak at ℏω=0.71Eg predicted by the Jones–Reiss phenomenological model. The dispersion is comparable with ab initio calculations. In indirect-gap GaP and Si, these components tend to increase with ℏω over our tuning range. In Si, the dispersion differs significantly from predictions of semi-empirical models, and ab initio calculations do not account for transitions below the two-photon direct bandgap, motivating further investigation. Kleinman symmetry was observed to be broken in all three materials. We also note anomalies observed and their possible origins, emphasizing the advantages of a 2-beam experiment in identifying the contribution of various nonlinear effects.