Reservoir-induced linewidth broadening of exciton-polariton laser

Bianca Rae Fabricante, ANU

Microcavity exciton-polaritons (or simply polaritons) can form a Bose-Einstein condensate (BEC) in semiconductors. Decay of this macroscopic coherent state releases coherent photon emission – a polariton laser. Continuous-wave photon emission from a polariton BEC is only possible under continuous pumping due to ultrashort polariton lifetime of ~1-102 ps. Furthermore, the pump directly generates an incoherent excitonic reservoir which causes decoherence of the BEC, thereby adversely affecting its quantum application.

In order to increase coherence time and narrow the linewidth of the polariton laser, it is important to understand and control the decoherence mechanisms. In this work, we create condensates with ultra-narrow (< 0.1 GHz) Lorentzian spectral profile, corresponding to > 3.2-ns coherence time, greatly exceeding the polariton lifetime. This is achieved by confining a polariton BEC in an optically induced round trap created by a single-mode off-resonant laser to minimise the condensate-reservoir overlap and to reduce number fluctuations in the condensate.

To uncover the main driver of decoherence, we add an off-resonant beam to inject incoherent reservoir particles on top of the condensate. This results in a homogeneously broadened linewidth. Our results confirm the detrimental role of the reservoir on the polariton coherence and demonstrate an ultra-narrow linewidth polariton laser.