Imaging propagating terahertz collective modes in two-dimensional semiconductor double layers

TL;DR

This paper introduces an optical method to image and analyze propagating terahertz collective modes in two-dimensional TMD semiconductors, revealing their dynamics and magnetic field effects with high spatial resolution.

Contribution

The study presents a novel optical readout technique for real-time imaging of THz plasmons in 2D semiconductors, enabling detailed exploration of their collective excitations.

Findings

Imaged THz plasmons with micron-scale resolution.

Measured propagation velocities of THz collective modes.

Observed coherent cyclotron oscillations under magnetic fields.

Abstract

Two-dimensional transition metal dichalcogenide (TMD) semiconductors exhibit a wide range of novel phenomena at millielectronvolt (terahertz-frequency) energy scales, including superconducting and correlation-induced insulating gaps that are frequently accompanied by symmetry breaking. However, due to the subwavelength dimensions and the often low conductivities of these systems, their intrinsic THz plasmons and meV-scale excitation gaps are difficult to access experimentally. Here we report an optical readout method that can image propagating THz-frequency collective modes in real time. The method relies on a strong coupling between the optical polarons of monolayer TMD semiconductors and the local THz fields in a waveguide, which enables us to image THz plasmons with micron scale spatial resolution and determine their propagation group velocities. Moreover, at finite magnetic fields, we observe coherent cyclotron oscillations resulting from Landau level repopulation induced by the THz field. Our findings provide a new near-field platform for probing collective excitations in strongly correlated two-dimensional semiconductors and enable "all-photonic" TMD-based architectures for time-domain THz plasmonics and optoelectronics.