Condensates are a hallmark of emergence in quantum materials such as superconductors and charge density waves. Excitonic insulators are an intriguing addition to this library, exhibiting spontaneous condensation of electron–hole pairs. However, condensate observables can be obscured through parasitic coupling to the lattice. Here we employ nonlinear terahertz spectroscopy to disentangle such obscurants through measurement of the quantum dynamics. We target Ta2NiSe5, a putative room-temperature excitonic insulator in which electron–lattice coupling dominates the structural transition (Tc = 326 K), hindering identification of excitonic correlations. A pronounced increase in the terahertz reflectivity manifests following photoexcitation and exhibits a Bose–Einstein condensation-like temperature dependence well below the Tc, suggesting an approach to monitor the exciton condensate dynamics [1, 2]. Nonetheless, dynamic condensate–phonon coupling remains as evidenced by peaks in the enhanced reflectivity spectrum at select infrared-active phonon frequencies, indicating that parametric reflectivity enhancement arises from phonon squeezing. Our results highlight that coherent dynamics can drive parametric stimulated emission.

Sheikh Rubaiat Ul Haque did his undergraduate in Applied Physics from the University of Tokyo in 2017. He then moved to UC San Diego where he received a PhD in Physics under Professor Richard Averitt in 2023. During his PhD, he performed research on time-resolved broadband terahertz spectroscopy on excitonic insulator Ta2NiSe5 where he demonstrated that light-induced terahertz parametric amplification can monitor exciton condensate dynamics. He has also worked on optical control of plasmonic modes in semiconducting metamaterials. Currently, Rubaiat is a postdoctoral scholar at Stanford University working with Professors Tony Heinz and Aaron Lindenberg on the strong light-field dynamics of low-dimensional materials. When not doing research, Rubaiat can be found reading detective novels, listening to classical music or enjoying time with his family.
 

[1] S.R.U. Haque et al., Terahertz parametric amplification as a reporter of exciton condensate dynamics. Nature Materials (2024). DOI: https://doi.org/10.1038/s41563-023-01755-2
[2] M.H. Michael*, S.R.U. Haque* et al., Photonic time-crystalline behavior mediated by phonon squeezing in Ta2NiSe5. Nature Communications (In press). arXiv link: https://arxiv.org/abs/2207.08851

Please contact phweb@ust.hk should you have questions about the talk.