Probing structural symmetry breaking in TMDC alloy via terahertz emission

Abstract

We report on terahertz (THz) emission from bulk Mo0.5W0.5Se2 alloy surfaces, which exhibit a mechanism fundamentally distinct from that of their pristine transition metal dichalcogenide (TMDC) constituents. Systematic measurements of THz emission as a function of pump fluence, polarization angle, and incident angle reveal that the primary mechanism is a transient in-plane photocurrent, which exceeds the nonlinear contribution by over an order of magnitude. The dependence of the THz field on the azimuthal angle reveals a distinct reduction in structural symmetry. This behavior is attributed to the formation of quasi-ordered, stripe-like Mo- and W-rich domains, which effectively break the lateral symmetry of the alloy. This study establishes THz-emission spectroscopy as a powerful, noninvasive technique to probe symmetry breaking, internal electric fields, and structural anisotropy in TMDC alloys.