Strong coupling between excitons in transition metal dichalcogenides and optical bound states in the continuum

TMDC are direct-gap semiconductors, exhibiting strong light-matter coupling. These structures support excitons characterized by both large binding energies and large oscillator strength. While the former leads to the existence... [ view full abstract ]

TMDC are direct-gap semiconductors, exhibiting strong light-matter coupling. These structures support excitons characterized by both large binding energies and large oscillator strength. While the former leads to the existence of strong excitonic response at room temperature, the latter provides substantial exciton-photon interaction. These properties allow the observation of the strong coupling regime in structures comprising TMDC monolayer and an optical cavity.

The design of structure is shown in Fig.1 - a WSe2 flake is placed on top of an one-dimensional Ta2O5 grating. The strong light-matter coupling demands high-Q photonic structures. We address this problem by tuning the PCS shape and material parameters to the regime of bound states in the continuum(BIC) providing a giant Q-factor of resonator which is limited by surface roughness and finite size of the sample only.

We begin with analysis of the eigenmode spectrum (Fig.2) of the PCS applying the guided-mode expansion (GME) method. The spectrum of energies and inverse lifetimes of upper(UP) and lower(LP) exciton-polariton branches calculated using GME is shown in Fig. 3(a,b). Figure 3(a) demonstrates strong coupling between the exciton and the off-Gamma BIC which manifests itself as an avoided resonance crossing with Rabi splitting of the order of 3 meV. The radiation losses of both exciton-polariton branches are shown in Fig. 3(b) in comparison with the losses of photonic and exciton modes. The lifetime of polariton modes can exceed the bare exciton lifetime by almost three orders of magnitude and reaches 0.66 ns. Such giant enhancement is the special effect intrinsic to BIC. The most important, Fig. 3(b) shows the maximal lifetime can be realized not at the center or the edge of the Brillouin zone, but at the point of phase space, where the group velocity of the mode is finite. For the LP branch, this practically means that polaritons can condense to a long-living state with nonzero energy flow at low temperatures, resulting in moving condensate. Our findings open new route for the realization of the moving exciton-polariton condensates with non-resonant pump and without the Bragg mirrors which is of paramount importance for polaritonic devices.