Control of Coherently Coupled Exciton Polaritons in Monolayer Tungsten Disulphide

Monolayer transition metal dichalcogenides (TMD) with confined 2D Wannier-Mott excitons are intriguing for the fundamental study of strong light-matter interactions and the exploration of exciton polaritons at high temperatures. However, the research of 2D exciton polaritons has been hindered because the polaritons in these atomically thin semiconductors discovered so far can hardly support strong nonlinear interactions and quantum coherence due to uncontrollable polariton dynamics and weakened coherent coupling. In this work, we demonstrate, for the first time, a precisely controlled hybrid composition with angular dependence and dispersion-correlated polariton emission by tuning the polariton dispersion in TMD over a broad temperature range of 110–230 K in a single cavity. This tamed polariton emission is achieved by the realization of robust coherent exciton-photon coupling in monolayer tungsten disulphide (${\mathrm{WS}}_2$) with large splitting-to-linewidth ratios ($>3.3$). The unprecedented ability to manipulate the dispersion and correlated properties of TMD exciton polaritons at will offers new possibilities to explore important quantum phenomena such as inversionless lasing, Bose-Einstein condensation, and superfluidity.

Figure 1

Schematic of TMD microcavity and improved ${\mathrm{WS}}_2$ exciton emission. (a) Schematic of the microcavity (MC) structure. It consists of 12.5 periods of a ${\mathrm{SiO}}_2/{\mathrm{Si}}_3{\mathrm{N}}_4$ bottom distributed DBR and 7.5 periods of a top DBR. The cavity layer structure includes an exfoliated monolayer ${\mathrm{WS}}_2$ flake sandwiched by two HSQ layers. There is a thin ${\mathrm{Al}}_2{\mathrm{O}}_3$ film atop of the top HSQ layer to protect the ${\mathrm{WS}}_2$ during the top DBR growth. (b) The ${\mathrm{WS}}_2$ photoluminescence spectra of exfoliated ${\mathrm{WS}}_2$ on ${\mathrm{SiO}}_2/\mathrm{DBR}$ and $\mathrm{HSQ}/\mathrm{DBR}$ substrates, and ${\mathrm{WS}}_2$ on $\mathrm{HSQ}/\mathrm{DBR}$ capped with HSQ and ${\mathrm{Al}}_2{\mathrm{O}}_3$ layer under the same pump and collection conditions. Besides the intensity enhancement, the PL peak position and FWHM are all consistent. The inset shows the optical image of the monolayer ${\mathrm{WS}}_2$ flake (the light-green trapezoid shape) with a laser beam at the center (the white circular shape) before depositing the top DBR, the scale bar corresponds to $10\mu \mathrm{m}$.

Figure 2

$k$-space reflectivity and photoluminescence at 110 K. (a) The reflectivity map at 110 K obtained by $k$-space spectroscopy, where the horizontal axis represents the sine function of the incidence angles ($\theta$) and the vertical axis is the photon energy. The gray color scale represents the reflectivity with darker areas corresponding to lower reflectivity. Two reflectivity modes are identified as the two anti-crossing polariton branches with dispersion fitted by a coupled oscillator model [25] with a 40 meV Rabi splitting. The dispersion plot is overlaid with the reflectivity map, with the dashed red line representing the exciton energy (2.078 eV), the dashed blue curve as the cavity photon dispersion, and the two solid magenta curves are the fitted polariton dispersion. This coupled oscillator model shows excellent agreement with the experimental reflectivity. (b) The nonresonantly pumped PL map obtained by $k$-space spectroscopy at 110 K. The orange color scale represents the PL intensity. The intensity of the upper polariton PL is magnified by $10\times$ due to its weak emission. The reflectivity dispersion is directly translated here and shows excellent agreement with the PL dispersion.

Figure 3

Temperature-dependent reflectivity and PL. (a) The $k$-space reflectivity map at temperatures of 130, 210, and 230 K. As the temperature increases, the exciton (the red dashed line) red shifts from 2.072 to 2.042 eV, while the cavity photon dispersion (the blue dashed curve) does not change, creating various cavity detunings over this temperature range. The coupled oscillator model is calculated to fit the dispersions (magenta solid curves) showing that the strong coupling regime holds at all these temperatures with large splitting-to-linewidth ratio. (b) The $k$-space PL map at temperatures of 130, 210, and 230 K. The reflectivity dispersions are directly translated here showing the agreement between the reflectivity and PL dispersions. Since the UP PL is much weaker, the intensity is still magnified by $10\times$, $5\times$, and $3\times$ at the upper panels at all these temperatures, respectively. Note here that the UP PL intensity becomes comparable with LP intensity as the UP becomes more photonlike at higher temperatures.

Figure 4

Hopfield coefficients of lower polariton states at three temperatures. Based on the coupled oscillator model, the contributions from exciton and photon components on the polariton states can be characterized by Hopfield coefficients [25]. The Hopfield coefficients given as a function of the incidence angle indicate how the polaritons are hybridized by the excitons and photons. As temperature increases, the cavity detuning changes from negative to positive, leading to a transition from a more photonlike to a more excitonlike state. This transition also alters the emission dynamics of the polaritons and thus the PL intensity distribution as seen in Fig. 3.