Nonlinear Optical Investigations of Novel Charge Density Waves

Charge density waves (CDWs) are formed when the conduction electrons and their underlying lattice of a crystalline material undergo a spontaneous translational symmetry breaking. For a long time in the CDW research, this aspect of broken translational symmetries is emphasized across the CDW phase transitions, whereas the aspect of broken point symmetries is much less focused. More recently, thanks to the findings of novel CDW phases, such as ferro-axial CDWs, chiral CDWs, etc., we gradually recognize that broken point symmetries not only are present, but also play a significant role in these new CDWs. My group has been making efforts in using nonlinear optics to probe broken point symmetries in novel CDW systems. In this presentation, I will focus on two representative examples, 1T TaS₂ which host a ferro-axial CDW and EuAl₄ which is considered as a chiral CDW candidate, to show how nonlinear optics can capture their broken point symmetries and hence their CDW phase transitions. I will further discuss how the nonlinear optical fields couple with the CDW order parameters and potentially their critical fluctuations. I will finally provide an outlook on how nonlinear optics can be used in detecting novel phases and phase transitions in quantum materials beyond CDWs.