Ultrafast dynamics of coherent exciton-polaritons in van der Waals semiconductor metasurfaces

Abstract

Enabling coherent light-matter interactions is a critical step toward next-generation quantum technologies. However, achieving this under ambient temperature conditions remains challenging due to rapid dephasing in optically excited systems. Optical metasurfaces based on quasi-bound states in the continuum have recently emerged as a powerful platform for reaching the strong light-matter coupling regime in flat, subwavelength thickness devices. Here, we investigate ultrafast exciton-polariton dynamics in self-hybridized WS$_2$ thin-film metasurfaces. Using hyperspectral momentum-resolved imaging, we reconstruct the highly anisotropic exciton-polariton dispersion, with a transition from positive to negative effective mass along orthogonal symmetry axes. Femtosecond pump-probe and multidimensional spectroscopy reveal detuning-dependent polariton dynamics with a coherence time up to ~110 fs, and allow direct observation of the coherent dynamics through ultrafast Rabi oscillations with ~45 fs period. We describe this behaviour with a three-eigenstate model that couples the photonic resonance with both bright and dark excitons, extending the conventional two-state picture of strong coupling. Our results establish van der Waals metasurfaces as a promising platform for next-generation polaritonic devices, enabling coherent quantum transfer of matter excitations at room temperature.

Figures (4)

• Figure 1: Photonic band structure of exciton-polaritons in TMDC qBIC metasurfaces (a) Schematic of the pump-probe spectroscopy experiment on strongly coupled qBIC metasurfaces, where $\Delta t$ denotes the time delay between pump and probe laser pulses. Bottom inset: Illustration of a single metasurface unit cell and its key design parameters, defined, for a scaling factor $S=1$, by a periodicity of $P_{0} = 340\sp{\mathrm{\mathrm}}{nm}$, a base rod length of $L_{0} = 266\sp{\mathrm{\mathrm}}{nm}$, and a rod width of w$_{0} = 90\sp{\mathrm{\mathrm}}{nm}$. Top inset: Energy level diagram showing the strong coupling between excitons (X) and the qBIC mode, and the upper (UP) and lower (LP) polariton branches separated by the Rabi frequency ($\Omega_R$). (b) Crystal structure of a 2H-phase TMDC. In yellow the chalcogen atoms (S), in blue the transition metal ones (W). (c) Real and imaginary parts of the dielectric function of WS$_2$. Adapted from Ref.munkhbat2023nanostructured. (d) Scanning electron micrograph of a WS$_2$ metasurface fabricated on a glass substrate. (e) Numerically calculated qBIC resonance energy as a function of the scaling factor $S$ ($\Delta L = 25$ nm). The material is modelled as homogeneous with refractive index $n = 4$, approximating WS$_2$ while neglecting excitonic effects. (f) Normal-incidence reflectance spectra for WS$_2$ metasurfaces with different $S$ values. The shaded area indicates the spectral position of the exciton. Dashed lines depict the polariton dispersion. (g) Rigorous Coupled-Wave Analysis (RCWA) simulation of momentum-resolved normalized reflectance for a strongly coupled WS$_2$ metasurface ($\Delta L_0 = 75$ nm), in vacuum. The data are plotted along the $\Gamma \rightarrow X$ direction ($\theta_x$) for positive incidence angles and along $\Gamma \rightarrow Y$ ($\theta_y$) for negative angles. Inset: Brillouin zone of the qBIC metasurface. (h) Experimental hyperspectral angle-resolved reflectance imaging of fabricated WS$_2$ metasurfaces on glass, shown for different $S$ values.
• Figure 2: Ultrafast dynamics of exciton-polaritons in qBIC metasurfaces (a-b) Comparison of angle-resolved reflectance (a) and the zero-time delay transient reflectance spectra (b) for WS$_2$ metasurfaces with different detuning values. In the plot corresponding to a detuning of 232 meV, the black line represents the bulk reference WS$_2$ sample. (c) Transient reflectivity map as a function of probe photon energy and pump-probe delay time for the sample with positive detuning (149 meV), showing a strong lower polariton (LP) resonance and a weak exciton (X) resonance. (d) Corresponding map for negative detuning (-97 meV), where the upper polariton (UP) is nearly degenerate with the exciton. (e) LP dynamics extracted from pump-probe measurements for samples with different detunings. Inset: fast and slow decay components obtained from fitting the LP dynamics as a function of qBIC detuning. (f-g) Schematic illustration of LP relaxation mechanisms following high-energy excitation for positive (f) and negative (g) detuning, highlighting in the latter case the enhanced radiative decay, speeding up $\tau_1$, and the suppressed scattering from the exciton reservoir (R), slowing down $\tau_2$.
• Figure 3: 1-quantum and 2-quantum MDCS of WS$_2$ qBIC metasurfaces (a-c) Amplitude maps of the 1-quantum box-CARS MDCS third-order nonlinear signal from WS$_2$ qBIC metasurfaces with different scaling factors ($S$), corresponding to increasing positive detunings for decreasing $S$. (d) Fits of the diagonal peak for qBIC metasurfaces with varying $S$. Dashed lines represent Lorentzian fits to the data. (e) Coherence times extracted from the full width at half maximum (FWHM) of the fits shown in panel (f). (f) Two-dimensional 2-quantum spectrum of a WS$_2$ qBIC metasurface ($S = 1.05$) measured at 5 K. Peaks along the diagonal correspond to 2-quantum coherences involving identical states (either excitons or lower polaritons), while cross-peaks represent coherences involving two different transitions with distinct energies.
• Figure 4: Ultrafast coherent exciton-photon coupling (a) High temporal resolution ultrafast spectroscopy of a maximally coupled WS$_2$ metasurface ($S = 1.11$). Left panel: Differential transmission map. The dashed lines indicate the spectral positions of the lower polariton (LP) and the exciton (X). Right panel: Normalized transient transmission profiles extracted at the LP and X energies, respectively. (b) Numerical modelling of the ultrafast response, as described in Supplementary Note 10. Right panel: Simulated differential transmission map. Left panel: Normalized differential transmission time traces at the LP and X energies, showing approximately three oscillation periods within the LP coherence time. (c) Absorptive 2D maps of the WS$_2$ metasurface measured at different pump-probe delay times (t$_2$ = 0, 20, 40, and 150 fs). (d) Ultrafast dynamics extracted from the 2D maps for the (LP, LP) diagonal peak and the (LP, X) cross-peak. The solid line indicates the positive signal from the transient response, the dashed lines the negative signal contribution, as depicted in the top left panel of (c) for square and circle marker, respectively. Inset: Energy levels of the strongly coupled system, with a coherent oscillations between the LP and X states (magenta arrows).