  Strong interactions between surface plasmons in ultra-compact nanocavities
and excitons in two dimensional materials have attracted wide interests for its
prospective realization of polariton devices at room temperature. Here, we
propose a continuous transition from weak coupling to strong coupling between
excitons in MoS2 monolayer and highly localized plasmons in ultra-compact
nanoantenna. The nanoantenna is assembled by a silver nanocube positioned over
a gold film and separated by a dielectric spacer layer. We observed a 1570-fold
enhancement in the photoluminescence at weak coupling regime in hybrid
nanocavities with thick spacer layers. The interaction between excitons and
plasmons is then directly prompted to strong coupling regime by shrinking down
the thickness of spacer layer. Room temperature formation of polaritons with
Rabi splitting up to 190 meV was observed, which is the largest plasmon-exciton
Rabi splitting reported in two dimensional materials. Numerical calculations
quantified the relation between coupling strength, local density of states and
spacer thickness, and revealed the transition between weak coupling and strong
coupling in nanocavities. The findings in this work offer a guideline for
feasible designs of plasmon-exciton interaction systems with gap plasmonic
cavities.
