Dispersion engineering is a powerful and versatile tool that can vary the speed of light signals and induce negative-mass effects in the dynamics of particles and quasiparticles. Here, we show that dissipative coupling between bound electron-hole pairs (excitons) and photons in an optical microcavity can lead to the formation of exciton polaritons with an inverted dispersion of the lower polariton branch and hence a negative mass. We perform direct measurements of the anomalous dispersion in atomically thin (monolayer) WS2 crystals embedded in planar microcavities and demonstrate that the propagation direction of the negative-mass polaritons is opposite to their momentum. Our study introduces a new concept of non-Hermitian dispersion engineering for exciton polaritons and opens a pathway for realising new phases of quantum matter in a solid state.
@article{arxiv.2204.04041,
title = {Negative-mass exciton polaritons induced by dissipative light-matter coupling in an atomically thin semiconductor},
author = {M. Wurdack and T. Yun and M. Katzer and A. G. Truscott and A. Knorr and M. Selig and E. A. Ostrovskaya and E. Estrecho},
journal= {arXiv preprint arXiv:2204.04041},
year = {2023}
}