Rotational Anisotropy Soft X-ray Second Harmonic Generation from GaAs

Abstract

We demonstrate rotational anisotropy soft X-ray second harmonic generation (RA-SXSHG) using a free-electron laser, extending nonlinear X-ray spectroscopy into the symmetry-resolved regime with elemental specificity. By performing RA-SXSHG measurements on GaAs(001) as a function of sample azimuthal orientation, we have demonstrated that this technique can effectively probe the crystalline structure of the As ions. Our results validate that the extension of established rotational anisotropy second harmonic generation frameworks, conventionally developed in the optical wavelength regime, remains robust and predictive in the soft X-ray regime, with close correspondence between theoretical modeling and experimental data. We also find that this soft X-ray nonlinear optical method is highly sensitive to the electronic properties of the surface space-charge layer with element selection. These findings lay the theoretical foundation for RA-SXSHG and establish it as a powerful nonlinear X-ray spectroscopic technique for probing surfaces and buried interfaces with symmetry and element specificity.