Polariton hyperspectral imaging of two-dimensional semiconductor crystals

Christian Gebhardt,Michael Förg,Aditya D Mohite,Christopher Gies,Matthias Florian,Theodor W Hänsch,Malte Hartmann,Alexander Högele,David Hunger,Hisato Yamaguchi,Ismail Bilgin

doi:10.1038/s41598-019-50316-8

Abstract

Atomically thin crystals of transition metal dichalcogenides (TMDs) host excitons with strong binding energies and sizable light-matter interactions. Coupled to optical cavities, monolayer TMDs routinely reach the regime of strong light-matter coupling, where excitons and photons admix coherently to form polaritons up to room temperature. Here, we explore the two-dimensional nature of TMD polaritons with scanning-cavity hyperspectral imaging. We record a spatial map of polariton properties of extended WS2 monolayers coupled to a tunable micro cavity in the strong coupling regime, and correlate it with maps of exciton extinction and fluorescence taken from the same flake with the cavity. We find a high level of homogeneity, and show that polariton splitting variations are correlated with intrinsic exciton properties such as oscillator strength and linewidth. Moreover, we observe a deviation from thermal equilibrium in the resonant polariton population, which we ascribe to non-Markovian polariton-phonon coupling. Our measurements reveal a promisingly consistent polariton landscape, and highlight the importance of phonons for future polaritonic devices.

Highlights

Exciton polaritons can enable novel photonic elements such as ultra-low threshold lasers[1,2], Bose-Einstein condensates[3], or quantum nonlinear optical elements[4]
Our experimental platform is a fiber-based Fabry-Perot microcavity[26] consisting of a laser-machined optical fiber serving as a micromirror and a planar mirror with monolayer flakes of WS2 synthesized by chemical vapor deposition (CVD) and covered with a thin film of polymethyl methacrylate (PMMA)
Away from the flakes, the transmission of the bare cavity shown in Fig. 1b features the characteristics of a stable Fabry-Perot resonator with Hermite-Gaussian eigenmodes that exhibits a strong main resonance and a blue-detuned weak resonance stemming from higher transverse modes

Summary

Introduction

Exciton polaritons can enable novel photonic elements such as ultra-low threshold lasers[1,2], Bose-Einstein condensates[3], or quantum nonlinear optical elements[4]. Thin crystals of TMDs offer a promising platform to study and harness exciton polaritons due to a strong exciton binding energy[5,6] and a large oscillator strength[7,8,9] Both properties in concert have enabled the demonstration of exciton polaritons at room temperature[10,11,12] and under cryogenic conditions[13,14,15]. In this work we reveal spatial variations and environmental influences on polaritons by hyperspectral scanning cavity microscopy in the strong coupling regime[20] We complement this data with cavity-enhanced photoluminescence and extinction microscopy[21], which allows us to identify clear correlations of the polariton splitting with intrinsic excitonic properties such as oscillator strength and linewidth, and extrinsic effects such as Quantum Center, and Center for NanoScience (CeNS), Ludwig-Maximilians-Universität, Geschwister-SchollPlatz 1, 80539, München, Germany. Our measurement technique reveals a promisingly homogeneous two-dimensional polariton landscape with variations that we can directly trace back to intrinsic and extrinsic influences, and with strong impact from polariton-phonon coupling, providing important insight for future polaritonic devices

Methods

Results

Conclusion