Deterministic placement and effective-mass pinning of topological polariton bound states in the continuum

The exciton-polaritons derived from the light-matter interaction of an optical bound state in the continuum (BIC) with the strong excitonic resonance in a transition metal dichalcogenide (TMD) monolayer can inherit ultra-long radiative lifetimes and significant nonlinearities up to room temperature. Yet such realization can be challenging with conventional approaches to the photonic cavity design, typically due to poorly-resolved Rabi splittings at room temperature and an unstable energy positioning of the BIC state. We show and experimentally validate a strategy to dramatically improve the state-of-the-art on both points, by embedding a tungsten disulfide (WS2) monolayer deep within a Bloch-surface-wave stack, where the photonic mode is moulded by a 1D photonic crystal with a compound periodicity. In particular, we introduce a deterministic placement principle to the design of the PhC, allowing to stabilize the energy positioning of a topologically-protected BIC polariton eigenstate, with an effective mass which we can robustly pre-assign at choice as either positive or negative. This is in stark contrast to typical waveguide realizations of polariton BICs: only negative polariton effective masses can be commonly achieved, while sudden jumps to a weaker-interacting positive-effective-mass BIC are at the same time possible upon small perturbations, in fact hijacking the advantage from a topological protection when present.