"Studying and uncovering quasiparticles in 2D materials with THz light"
In condensed matter, important ground-state properties and elementary excitations of a system are often found in the low-energy terahertz (THz) region, and identifying these signatures enables us to understand the system deeply and sometimes uncover a new excitation, as exemplified by the discovery of magnons. In recent years, two- dimensional (2D) materials and their van der Waals (vdW) interfaces have emerged as a platform that allows us to design and study emergent ground states and excitations. Yet, quantifying their low-energy THz responses has been very challenging due to the small lateral size and atomically thin nature. We previously demonstrated a far-field approach on large-area, cm-scale samples, which enabled us to quantify the unconventional charge generation process in TMDCs at the monolayer limit. In this seminar, I will present our new approach for near-field THz spectroscopy applicable to µm-sized vdW structures. For this, we explored THz emission from 2D ferroelectric materials and discovered that the vdW in-plane ferroelectric NbOI 2 controllably emits very strong THz radiation under an optical pump, much stronger than the current standard THz emitter, ZnTe. We show that the ultrathin 2D THz emitter can be easily integrated with a vdW sample of interest, enabling in situ near-field THz spectroscopy of the target vdW structure. Further, through the study, we (accidentally) observed extremely narrowband THz radiation from NbOI 2 at its thinner limit, which we elucidated originates from collective excitation of ferroelectric order, ferron: a quasiparticle that has been only predicted in theory. Due to the intrinsically strong electric dipolar nature, coherent ferrons showed ultrafast propagation over macroscopic dimensions, promising new information carriers and novel quantum interconnects. In addition to its own significance awaiting further studies, the vdW ferroelectric will uniquely provide a new venue to optically probe, electrically read, and coherently manipulate low-energy quantum phenomena in various vdW structures.