Self-Hybridized Exciton-Polaritons in Sub-10-nm-Thick WS 2 Flakes: Roles of Optical Phase Shifts at WS 2 /Au Interfaces.

Exciton-polaritons (EPs) can be formed in transition metal dichalcogenide (TMD) multilayers sustaining optical resonance modes without any external cavity. The self-hybridized EP modes are expected to depend on the TMD thickness, which directly determines the resonance wavelength. Exfoliated WS 2 flakes were prepared on SiO 2 /Si substrates and template-stripped ultraflat Au layers, and the thickness dependence of their EP modes was compared. For WS 2 flakes on SiO 2 /Si, the minimum flake thickness to exhibit exciton-photon anticrossing was larger than 40 nm. However, for WS 2 flakes on Au, EP mode splitting appeared in flakes thinner than 10 nm. Analytical and numerical calculations were performed to explain the distinct thickness-dependence. The phase shifts of light at the WS 2 /Au interface, originating from the complex Fresnel coefficients, were as large as π/2 at visible wavelengths. Such exceptionally large phase shifts allowed the optical resonance and resulting EP modes in the sub-10-nm-thick WS 2 flakes. This work helps us to propose novel optoelectronic devices based on the intriguing exciton physics of TMDs.