Understanding optomechanical coupling and nonlinear phononics effects in materials

Controlling optomechanical effects, i.e. the coupling of atomic degrees of freedom with light, is of fundamental importance for quantum sensing, quantum transduction, and quantum machine learning applications. Of particular interest to applications is the stabilization under laser irradiation of states of matter otherwise unstable in thermodynamics equilibrium. In this talk, I will present our recent theoretical progress in understanding the atomic-scale, non-thermal response of materials to visible light. Using first-principles methods as an approximation to the time-dependent Schrödinger equation, I will show how light can result in non-thermal excitations of the atomic lattice in simple materials like III-V semiconductors, and how simple classical models of this response based on generalized Langevin equation can capture the light-induced dynamics. I will then show how non-thermal effects become prominent in more complex, broken-symmetry materials such as Charge Density Wave (CDW) materials. Using the CDW state of the layered transition-metal dichalcogenide, Tantalum Disulfide (1T−TaS2) as a case study, I will show how light can couple with structural order parameter in CDW materials, leading to optomechanical coupling coefficients two orders of magnitude larger than the ones of diamond and ErFeO3, an effect confirmed experimentally. Our findings suggest nonlinear phononics processes in CDW materials can be deterministically controlled to engineer states of matter with large non-linear optical susceptibility.