Exciton-polaritons in van der Waals heterostructures embedded in tunable microcavities

Monolayer films of van der Waals crystals of transition metal dichalcogenides (TMDCs) are direct band gap semiconductors exhibiting excitons with very large binding energies and small Bohr radii, leading to a high oscillator strength of the exciton optical transition. Here we report fabrication of ‘van der Waals quantum wells’ (QW) made from molybdenum diselenide (MoSe2) light-emitting monolayers and hexagonal boron nitride (hBN) barriers. Such heterostructures are embedded in an optical microcavity consisting of a concave and a planar dielectric distributed Bragg reflectors (DBR) separated by a tunable micrometer sized gap. We observe the strong exciton-photon coupling regime and formation of exciton-polariton eigenstates, whose energy can be tuned by changing the gap between the DBR mirrors. Furthermore, we demonstrate that the magnitude of the characteristic anti-crossing between the cavity modes and the MoSe2 excitons (a Rabi splitting) can be enhanced by embedding a multiple-QW structure, containing two MoSe2 monolayers separated by an hBN barrier. At a temperature of 4K, for a single QW sample the vacuum Rabi splitting of 20 meV is observed for the neutral exciton state, which is increased to 29 meV for the double QW, following closely the NQW dependence. An intermediate coupling regime is observed for the charged exciton, where the polariton states are not fully resolved as the Rabi splitting is similar to the charged exciton linewidth. This work opens a new avenue in the field of polaritonics in a new material system of van der Waals crystals and heterostructures with a potential for polariton devices operating at room temperature.