Circular photogalvanic effect from third-order nonlinear effect in 1T'-MoTe₂

The two-dimensional layered material MoTe₂ has aroused extensive research interests in its rich optoelectronic properties in various phases. One property of particular interest is the circular photogalvanic effect (CPGE): a conventional second order nonlinear optical effect that is related to the chirality of materials. It has been demonstrated in T_d-MoTe₂, a type-II topological Weyl semimetal candidate, while it has been unclear so far whether it exists in the semimetallic 1T' phase, another interesting phase that hosts a quantum spin hall state. In this article, we report a clear experimental observation of in-plane CPGE in 1T'-MoTe₂. The observation is confirmed under various experimental designs with excitation by normally incident mid-infrared laser, and we find it to be related to an in-plane internal DC electric field. We attribute the circular photogalvanic response to a third-order nonlinear optical effect involving this DC electric field, which is consistent with the crystal symmetry of the lattices and present in both the 1T' and T_d phases of the material.