Fig. 1: vdW-HMs.
From: Atomic-layer assembly of ultrathin optical cavities in van der Waals heterostructure metasurfaces
a, Illustration of a vdW-HM, composed of a WS2 semiconductor monolayer encapsulated between two thin hBN layers. The monolithic metasurface design is patterned into the heterostructure via top-down nanofabrication processes. The asymmetric unit-cell design, composed of two nanorods with different lengths, gives rise to symmetry-protected qBIC resonances, which strongly confine light and resonantly interact with the 2D WS2 excitons. Top right: schematic of the strong-coupling regime between exciton (EX) and qBIC (EqBIC) resonances, with the formation of the upper (UP) and lower (LP) exciton–polariton branches separated by Rabi energy ΩR. b, Strong coupling between the exciton in the WS2 semiconductor (left panel) and the photonic qBIC resonance (right panel) can be reached by maximizing the coupling strength g. Numerical finite-difference time-domain simulations of the electromagnetic-field enhancement (|E|/|E0|)2 for an hBN metasurface at the qBIC resonance show strong fields inside the resonators, ideally suited for boosting light–matter coupling. c, Numerical simulations of the transmission of an hBN metasurface (ΔL = 50 nm) demonstrate the effective spectral tuning of the qBIC resonance via a lateral scaling factor S applied to the unit cell. The PL spectrum of the room-temperature emission for a monolayer WS2 encapsulated in hBN is shown in black. d, Analytical calculation of the dispersion following a coupled harmonic oscillator model (Methods) and the formation of LP and UP branches in the strong-coupling regime, as a function of detuning between the cavity (qBIC) and emitter (excitons, X). e, Momentum dispersion of the qBIC–exciton system as a function of wavevector kx, parallel to the nanorod long axis. The solid line and dot–dashed line represent the exciton and qBIC modes, respectively. The dashed lines are the polariton branches. When the qBIC mode is positively detuned from the exciton, the negative angular dispersion makes the Rabi splitting appear at larger angles, where the two resonances cross.
