Charge density wave order in a transition metal dichalcogenide probed by second harmonic generation

Rotational anisotropy second harmonic generation (RA-SHG) is a nonlinear optical probe which has been used to study electronic order in solid state materials. Because the leading-order contribution to the second harmonic signal is zero in the bulk of centrosymmetric crystals, RA-SHG can be used to study the surface properties of ordered materials. We use RA-SHG to study electronic order on the surface of a transition metal dichalcogenide (TMD). We find that the symmetry-breaking character of ordered charge density wave (CDW) states in the TMD can be identified using RA-SHG, and that our technique can be used to differentiate surface CDW domains. We also posit a new framework for understanding RA-SHG data from crystal structures with incommensurate charge order.