  Two-dimensional semiconductors host excitons with very large oscillator
strengths and binding energies due to significantly reduced carrier screening.
Two-dimensional semiconductors integrated with optical cavities are emerging as
a promising platform for studying strong light-matter interactions as a route
to explore a variety of exotic many-body effects. Here, in few-layered WS2
coupled with plasmonic nanoparticle lattices, we observe the formation of a
collective polaritonic mode near the exciton energy and the formation of a
complete polariton band gap with energy scale comparable to the exciton-plasmon
coupling strength. A coupled oscillator model reveals that the collective mode
arises from the cooperative coupling of the excitons to the plasmonic lattice
diffraction orders via exciton-exciton interactions. The emergence of the
collective mode is accompanied by a superlinear increase of the polariton mode
splitting as a function of the square root of the exciton oscillator strength.
The presence of these many body effects, which are enhanced in systems which
lack bulk polarization, not only allows the formation of a collective mode with
periodically varying field profiles, but also further enhances the
exciton-plasmon coupling. By integrating the hybrid WS2-plasmonic lattice
device with a field-effect transistor, we demonstrate active tuning of the
collective mode and the polariton band gap. These systems provide new
opportunities for obtaining a deeper and systematic understanding of many body
cooperative phenomena in periodic photonic systems and for designing more
complex and actively controllable polaritonic devices including switchable
polariton lasers, waveguides, and optical logical elements.
