Nonlinear near-field spectroscopy of exciton-polaritons in a van der Waals layered waveguide

Abstract

Layered van der Waals materials offer novel opportunities for on-chip waveguiding and development of integrated photonic circuits. In the strong light-matter coupling regime, their nonlinear response can be significantly enhanced, which is crucial for developing active photonic devices. However, probing the nonlinearity of waveguide modes in subwavelength-thick structures is challenging as they are not directly accessible from far-field. Here we apply a novel nonlinear near-field spectroscopic technique based on a GaP solid immersion lens and femtosecond laser excitation to study nonlinearity of guided modes in monolayer WS$_2$ encapsulated in hBN under the strong light-matter coupling regime. We reveal formation of exciton-polaritons with $\sim 50$ meV Rabi splitting and demonstrate a pump-induced transition from strong to weak coupling. Our results show that exciton resonance saturation and broadening lead to an efficient nonlinear response of guided polaritons, which can be employed for developing compact van der Waals photonic switches and modulators.

Figures (4)

• Figure 1: Strong light--matter coupling in an all-van-der-Waals hybrid waveguide. (a) Schematic illustration of a layered waveguide consisting of monolayer WS$_2$ encapsulated in two multilayer hBN slabs placed on a SiO$_2$ substrate. (b) In-plane field amplitude evaluated at the monolayer WS$_2$ position in the layered structure as a function of the bottom and top hBN layer thickness. (c) Distribution of the E-field amplitude along the out-of plane z-axis for the lowest-order TE mode in a hBN/WS$_2$/hBN/SiO$_2$ structure (black curve) and its modification in the presence of GaP solid immersion lens placed 200 nm above the structure. (d) Calculated dispersion of the lowest-order TE waveguide mode (green curves) showing Rabi splitting and formation of lower (LP) and upper (UP) polariton branches, together with light line (LL, black dashed line).
• Figure 2: Measurement of guided exciton-polaritons in an all-van-der-Waals hybrid waveguide. (a) Schematic illustration of the experimental configuration for angle-resolved measurements below the light line including a GaP solid immersion lens with tunable lens--sample gap and high-NA microscope objective. (b) Measured optical reflection as a function of photon energy and relative in-plane wavevector component k$_x$/k. The region corresponding to guided modes lies to the right of the blue dashed line indicating the refractive index of SiO$_2$. (c) Simulated optical reflection as a function of photon energy and relative in-plane wavevector component k$_x$/k. (d) Measured wavevector-resolved reflection spectra around the exciton resonance showing anti-crossing behavior and formation of lower (LP) and upper (UP) polariton branches. (e) Corresponding wavevector-resolved PL spectra.
• Figure 3: Measurement of the nonlinear response of guided exciton-polaritons. (a) Schematic of the measurement, with femtosecond laser pulses exciting exciton-polaritons in the sample through SIL (top) resonantly in both photon energy and wavevector (bottom). (b) Experimental exciton-polariton dispersions measured as wavevector-resolved optical reflection spectra at 3 selected values of pump fluence: 1 $\upmu$J/cm$^2$ (left panel), 7 $\upmu$J/cm$^2$ (middle panel), and 30 $\upmu$J/cm$^2$ (right panel). (c) Simulated exciton-polariton dispersions for 3 different values of exciton radiative decay rate: 1.8 meV (left panel), 1.3 meV (middle panel), and 0.9 meV (right panel).
• Figure 4: Power-dependent parameters of guided exciton-polaritons in hBN/WS$_2$/hBN. (a) Extracted from measurements (symbols) upper (top panel) and lower (bottom panel) polariton energies as functions of pump fluence, together with corresponding modeling results (curves). (b) Extracted from measurements light--matter coupling strength as a function of pump fluence (dots), together with corresponding modeling results (red solid curve) and effective relaxation rate for evaluating the strong light--matter coupling regime condition (blue dashed curve). (c) Extracted from measurements exciton energy as a function of pump fluence (dots), together with corresponding modeling results (curve).