Nonlinear optical response in quasi-2D Charge Density Waves

Transition metal dichalcogenides (TMDs), a class of 2D materials, host a rich plethora of quantum phenomena, such as metallic (semiconducting) charge-density-wave (CDW) phases and excitonic semiconductors with indirect band gaps. Light is a powerful tool to probe the electronic properties of CDW, including phonon modes, the CDW gap, exciton, quantum confinement effect, and domain size. One type of CDW material is particularly interesting: a mixed CDW phase with domain walls, such as 1T-TaS2. At room temperature, this CDW phase comprises commensurate, incommensurate, and uncondensed metallic regions. While 1T-TaS2 has received much attention lately, we were the first to discover that it exhibits a nonlinear optical response at low-intensity visible-light illumination in our previous work. As such a phase is susceptible to perturbations, we proposed a mechanism of light-induced stacking reconfiguration along the c-axis that changes the in-plane commensuracy. In this study, we further explored the source of nonlinearity via laser-coupled STM imaging. We aim to showcase the direct evidence of the change in commensuracy as laser power is tuned. We also perform STM and optical characterizations on other CDWs like VTe2 and VSe2, which feature more complicated CDW transitions.