Giant Optomechanical Coupling in the Charge Density Wave State of Tantalum Disulfide

We study the coupling of light and the structural order parameter in the charge density wave (CDW) state of the layered transition-metal dichalcogenide, Tantalum Disulfide (1T-TaS_2). Using time-dependent density functional theory calculations of the dielectric properties along the distortions coordinates, we show that 1T-TaS_2 displays very large change in its dielectric function along the amplitude (Higgs) mode due to the coupling of the periodic lattice distortion with an in-plane metal-insulator transition, leading to optomechanical coupling coefficients two orders of magnitude larger than the ones of diamond and ErFeO_3. In addition, we derive an effective model of the light-induced dynamics, which is in quantitative agreement with experimental observations in 1T-TaS_2. We show that light-induced dynamics of the structural order parameter in 1T-TaS_2 can be deterministically controlled to engineer large third-order non-linear optical susceptibilities. Our findings suggest that CDW materials are promising active materials for non-linear optics.