UC San Diego

Condensates are a hallmark of emergence in quantum materials with superconductors and charge density waves as prominent examples. Excitonic insulators (EI) present an intriguing addition to this library, exhibiting spontaneous condensation of electron-hole pairs in the vicinity of an insulator-semiconductor phase transition. Across this library, canonical condensate observables can be obscured through parasitic coupling to various degrees of freedom. Broadband terahertz (THz) spectroscopy can disentangle such obscurants through temperature and fluence dependent measurements of the quantum dynamics. We target Ta_2 NiSe_5 (TNS) a putative room-temperature EI where electron-lattice coupling complicates the identification of excitonic correlations. The first three chapters cover a concise background of strongly correlated materials, the power of ultrafast spectroscopy as a tool to unveil novel physics of condensed matter systems, properties of TNS as an EI, experimental techniques built and developed by myself. In the following chapter, I report the optical signatures of exciton condensate dynamics in TNS where a pronounced increase in the THz reflectivity manifests following photoexcitation showing a BEC-like temperature dependence. This occurs well below the structural transition temperature (326K), suggesting a novel approach to monitor exciton condensate dynamics stripped of residual static interactions. Nonetheless, dynamic condensate-phonon coupling remains as evidenced by peaks in the enhanced reflectivity spectrum at select infrared active phonon frequencies. Indeed, the origin of the parametric reflectivity enhancement arises from phonon squeezing upon condensate perturbation, validated using Fresnel-Floquet theory and DFT calculations. Our results highlight that coherent order parameter dynamics can, in the presence of signal photons, drive parametric stimulated emission with concomitant possibilities that includes novel entangled photon sources at terahertz frequencies.