  Layered materials can be assembled vertically to fabricate a new class of van
der Waals (VDW) heterostructures a few atomic layers thick, compatible with a
wide range of substrates and optoelectronic device geometries, enabling new
strategies for control of light-matter coupling. Here, we incorporate
molybdenum diselenide/boron nitride (MoSe$_2$/hBN) quantum wells (QWs) in a
tunable optical microcavity. Part-light-part-matter polariton eigenstates are
observed as a result of the strong coupling between MoSe$_2$ excitons and
cavity photons, evidenced from a clear anticrossing between the neutral exciton
and the cavity modes with a splitting of 20 meV for a single MoSe$_2$ monolayer
QW, enhanced to 29 meV in MoSe$_2$/hBN/MoSe$_2$ double-QWs. The splitting at
resonance provides an estimate of the exciton radiative lifetime of 0.4 ps. Our
results pave the way for room temperature polaritonic devices based on
multiple-QW VDW heterostructures, where polariton condensation and electrical
polariton injection through the incorporation of graphene contacts may be
realised.

Mesoscale and Nanoscale Physics