Nonlinear optical responses of Rashba spin-split GeTe thin films

Ferroelectric alpha-phase germanium telluride (α-GeTe) has been known to have a giant Rashba spin split band, and hence can be a promising material for the spintronic application. We prepare α-GeTe thin films on a Si substrate with a thickness variation from 2 to 100 nm, and investigate their structural and electrodynamic properties by using second harmonic generation (SHG) and terahertz emission spectroscopy. From the azimuth-dependent anisotropy observed in the SHG responses, we find that all the films have a non-centrosymmetric crystal structure identified as a 3m point group. From the THz emission results, we demonstrate that a fairly strong built-in field exists at the film interface with the Si substrate. In particular, we observe a non-negligible helicity-dependence in the THz emission which can be a possible signature of a spin-polarized photocurrent arising from the spin-split Rashba bands.