Slow, Nanometer Light Confinement Observed in Atomically Thin TaS2

TL;DR

This study experimentally demonstrates extreme nanometer-scale light confinement and slow light behavior in atomically thin TaS2 layers, revealing potential for nanoscale optical applications and advancing understanding of plasmonic phenomena.

Contribution

First experimental observation of nanometer-scale light confinement and slow plasmonic modes in atomically thin TaS2 monolayers and bilayers using momentum-resolved EELS.

Findings

Achieved confinement ratio up to 300 at large wave vectors

Observed slow light with group velocity ~10^{-4}c

Detected 2D to 3D Coulomb interaction transition

Abstract

Extreme light confinement down to the atomic scale has been theoretically predicted for ultrathin, Ta-based transition metal dichalcogenides (TMDs). In this work, we experimentally demonstrate in 2H-TaS$_2$ monolayers and bilayers a lateral confinement ratio up to 300 at large wave vectors of $q = 0.15 \, \r{A}^{-1}$, and slow light behaviour with a group velocity $\sim 10^{-4}c$. Quantitative momentum-resolved electron energy loss spectroscopy (q-EELS) with a momentum resolution of $0.0056 \, \r{A}^{-1}$ was used as a platform for the nanoscale optical measurements. With it, momentum-dispersed, two-dimensional (2D) plasmon resonances were experimentally observed, showing a transition from 2D to 3D Coulomb interaction in the high-momentum regime, equivalent to light confinement volumes of $1\text{-}2 \, \text{nm}^3$. Remarkably, the resonant modes do not enter the electron-hole continuum, predicting even further enhanced optical field confinements for this material at cryogenic temperatures.