Enhanced Terahertz Spectroscopy of a Monolayer Transition Metal Dichalcogenide

2D materials, including transition metal dichalcogenides, are attractive for a variety of applications in electronics as well as photonics and have recently been envisioned as an appealing platform for phonon polaritonics. However, their direct characterization in the terahertz spectral region, of interest for retrieving, e.g., their phonon response, represents a major challenge, due to the limited sensitivity of typical terahertz spectroscopic tools and the weak interaction of such long-wavelength radiation with sub-nanometer systems. In this work, by exploiting an ad-hoc engineered metallic surface enabling a ten-thousand-fold local absorption boost, enhanced terahertz spectroscopy of a monolayer transition metal dichalcogenide (tungsten diselenide) is performed and its dipole-active phonon resonance features are extracted. In addition, these data are used to obtain the monolayer effective permittivity around its phonon resonance. Via the direct terahertz characterization of the phonon response of such 2D systems, this method opens the path to the rational design of phonon polariton devices exploiting monolayer transition metal dichalcogenides.