Intrinsically chiral exciton polaritons in an atomically-thin semiconductor

Abstract

Photonic bound states in the continuum (BICs) have emerged as a versatile tool for enhancing light-matter interactions by strongly confining light fields. Chiral BICs are photonic resonances with a high degree of circular polarisation, which hold great promise for spin-selective applications in quantum optics and nanophotonics. Here, we demonstrate a novel application of a chiral BIC for inducing strong coupling between the circularly polarised photons and spin-polarised (valley) excitons (bound electron-hole pairs) in atomically-thin transition metal dichalcogenide crystals (TMDCs). By placing monolayer WS$_2$ onto the BIC-hosting metasurface, we observe the formation of intrinsically chiral, valley-selective exciton polaritons, evidenced by circularly polarised photoluminescence (PL) at two distinct energy levels. The PL intensity and degree of circular polarisation of polaritons exceed those of uncoupled excitons in our structure by an order of magnitude. Our microscopic model shows that this enhancement is due to folding of the Brillouin zone creating a direct emission path for high-momenta polaritonic states far outside the light cone, thereby providing a shortcut to thermalisation (energy relaxation) and suppressing depolarisation. Moreover, while the polarisation of the upper polariton is determined by the valley excitons, the lower polariton behaves like an intrinsic chiral emitter with its polarisation fixed by the BIC. Therefore, the spin alignment of the upper and lower polaritons ($\uparrow\downarrow$ and $\uparrow \uparrow$) can be controlled by $σ^+$ and $σ^-$ polarised optical excitation, respectively. Our work introduces a new type of chiral light-matter quasi-particles in atomically-thin semiconductors and provides an insight into their energy relaxation dynamics.

Figures (4)

• Figure 1: Optical properties of the WS$_2$/BIC heterostructure.a Schematics of the spin configuration and emission properties of upper and lower polaritons under (top) $\sigma^+$ and (bottom) $\sigma^-$ polarised optical excitation. b Microscope image and (inset) schematic illustration of the heterostructure consisting of the metasurface governed by chiral BICs Chen2023 and the monolayer WS$_2$. The measured areas are marked with yellow and black circles. c Electric field distribution of the metasurface at the BIC resonance $\lambda = 597~\mathrm{nm}$ in its linear polarisation bases (left) $\left|E_y\right|$ and (right) $\left|E_x\right|$ in the yz- and xz-plane, respectively. d Angle-resolved differential reflectivity spectra of the metasurface next to the position of the monolayer (black circle in b) of (left) $\sigma^+$ and (right) $\sigma^-$ polarised white light, where the reflected light is filtered in its $\sigma^+$ and $\sigma^-$ polarisaton bases, respectively. The response of the chiral BIC is marked with a black dashed circle. e Polarisation resolved reflectivity spectra of panel d plotted along $k=0$. f Reflectivity and photoluminescence spectrum at the position of the monolayer WS$_2$ (yellow circle in b) at $T\approx4~\mathrm{K}$, measured at large momenta ($k_{||}\approx 4.5~\mu\mathrm{m^{-1}}$, see Methods) to avoid the influence of the BIC.
• Figure 2: Model of exciton-photon coupling.a Band structure of the (black) relevant photonic, (green) excitonic and (red) polaritonic modes of the sample in the strong exciton-photon coupling regime, and schematic illustration of relaxation and diffraction processes that lead to photoluminescence. b Folded Brillouin zone of the photonic components, showing that both (green) excitons and (red) polaritons coexist within the light cone. The light cone of our experimental setup (see Methods) is shaded in yellow and the location of the relevant chiral BIC ($\sigma^+$) on the photonic dispersion is marked with black dashed circles in panels a,b. c Modeled white light reflectivity in the far field, which is dominated by the polariton response (red dashed line) at the native polarisation of the chiral BIC ($\sigma^+$), and by the exciton response (X) when the polarisation of the reflected light is opposite to the polarisation of the incoming light (i.e., $\sigma^{+-/-+}$). The polariton (UP/LP) and uncoupled exciton/photon (X/P) dispersions are marked as red and black dashed lines, respectively.
• Figure 3: Reflectivity and photoluminescence spectra.a Experimental angle-resolved photoluminescence spectrum, with the fitted (LP) lower and (UP) upper polariton branches (red dashed lines), and the uncoupled (X) exciton and (P) photon dispersions (black dashed lines). The part of the upper polariton dispersion inheriting the chiral BIC properties is marked with a red dashed circle. The data in the inset are normalised to 0.5 for each $k_{||}$ value to visualise the peak positions of the low-intensity upper polariton emission. b, c Experimental and theoretical reflectivity and photoluminescence spectra at (red) $k_{||}=0$ nd (blue) $k_{||}\approx4.5~\mu\mathrm{m}^{-1}$, respectively. The arrows in panel (c) illustrate the order of magnitude enhancement of the PL intensities of the upper polariton between the lower energy state at $k_{||}\approx4.5~\mu\mathrm{m}^{-1}$ and the higher energy state at $k_{||}=0$ .
• Figure 4: Angle- and polarisation-resolved PL spectra.a-d Circularly polarised components of the angle-resolved PL spectra of the sample excited with a circularly polarised laser pump. The $\sigma^{pe}$ convention reflects the sign of the circular polarisation of the pump ($p$) and the PL emission ($e$). e Measured and modeled s$_3$ spectra with (black) $\sigma^-$ and (red) $\sigma^+$ polarised pump. The arrows ($\uparrow$,$\downarrow$) mark the spin configurations of the lower and upper polaritons, respectively, under (red) $\sigma^-$ and (black) $\sigma^+$ polarised excitation. f PL spectrum of the LP with (black) $\sigma^+$ and (red) $\sigma^-$ polarised pump, with the PL spectra of the biexciton (XX) plotted as reference. g s$_3$ spectra of the LP at (black) $T=4~\mathrm{K}$ and (red) $T=150~\mathrm{K}$, respectively.