Cavity Control of Phase Transitions and Nonlinearities in Quantum Materials

The possibility of engineering the light-matter coupling with optical cavities has recently emerged as a new path to manipulate the collective properties of quantum materials. In this doctoral thesis, we explore the effect of light matter coupling inside a cavity on the complex materials CuGeO3, 1T-TaS2 and SrTiO3 through terahertz spectroscopy. In order to exploit the light matter interaction in cavity confined system, we built a Terahertz Fabry-Pérot resonator that can couple with low energy excitations and subsequently affect the collective behaviours in complex materials. The capability of the setup to tune the cavity fundamental mode at the cryogenic temperatures makes it highly versatile to study a wide range of quantum materials. Firstly, we will show the signatures of vibrational strong coupling in the normal phase of a spin-peierls compound CuGeO3 and the charge density wave(CDW) material 1T-TaS2 when embedded inside a resonant terahertz cavity. Observation of Rabi splitting indicates hybridization between material excitations and fundamental mode of the cavity in the strong coupling regime. Notably, for 1T-TaS2, we reveal a multimode vibrational coupling scenario arising from the coupling between the fundamental cavity mode and multiple charge density wave phonon modes in the dielectric phase of the material. We will then investigate the first order metal-to-insulator phase transition of the CDW material 1T-TaS2 inside a cavity. We unveil that the effective phase transition temperature can be controlled by tuning the cavity frequency and mirror alignment pointing to a scenario in which the cavity can filter the heat load on sample from the electromagnetic environment in a Purcell-like mechanism. Finally, we will discuss the cavity mediated nonlinear reponses in a paraelectric material SrTiO3. The findings suggest that the nonlinearities likely associated with the soft phonon mode, are enhanced within the cavity’s electromagnetic environment hence unveiling the potential to achieve the anharmonic regime of quantum materials without the need for intense lasers.