Mirror‑Induced Field Compression Drives Tunable Strong Exciton-Plasmon Polaritons in WS₂/h-BN/Au Heterostructures

Deep‑subwavelength confinement and strong coupling at visible frequencies are central to scalable nanophotonic and quantum technologies. While transition metal dichalcogenides are promising candidates for such confinement, achieving tunable strong coupling often relies on patterned plasmonic nanostructures or chemical modification of the excitonic material. Furthermore, in metal-coupled systems, the distinct role of metallic mirror-induced electromagnetic confinement is often experimentally obscured by exciton-plasmon energy hybridization. Here we introduce a vdW-integrated, chemically non-invasive WS₂/h-BN/Au heterostructure in which the h-BN spacer forms a controllable nanogap that tunes image-charge confinement while preserving the excitonic material. Scattering‑type near‑field microscopy directly maps propagating interference fringes and reveals a collapse of the TM‑polariton wavelength to ∼172 nm under 633‑nm excitation as the spacer is reduced to 5 nm (λ₀/λ_p≈3.7, corresponding to ∼71% wavelength reduction), which is one of the highest degrees of optical confinement reported for room-temperature TMDC polaritons. Full‑wave simulations and transfer‑matrix analysis, aided by boundary‑condition engineering from an ideal conductor to real Au, separate a purely geometric mirror mode from plasmon‑assisted hybridization. This platform establishes a deterministic route to deeply subwavelength field control, offering a scalable architecture for nonlinear optical enhancement and integrated quantum devices.