Phase manipulation of an exciton-polariton condensate
Exciton-polaritons are intriguing quasiparticles emerging from the interaction in a microcavity between the cavity photons and bound electron-hole pairs, the excitons. The result is a quantum superposition of light and matter with unique behavior. On the one hand polaritons can escape the cavity as light, but on the other hand interact with each other through their excitonic constituent. In addition, the mixed nature of polaritons facilitates the formation of a polariton Bose-Einstein condensate (BEC), the macroscopic quantum state of stimulated accumulation of boson particles at the ground energy state. Fascinating BEC-related phenomena have been observed in polaritons such as superfluidity and solitons.
We induce an ultrafast and reversible modulation of the energy of polariton BECs. Such modulation has potential applications in optical switches and quantum gates. The modulation of the energy is also accompanied by a modulation of the phase of the BEC, which is key to the investigation of the BEC-related phenomena. Our goal is to show the phase shift in a polariton BEC.
In our lab, the polaritons in the microcavity sample are excited with an ultrafast (femtosecond scale) pulsed resonant beam. Above a threshold intensity of the beam, a polariton BEC will form. The modulation is obtained through the so-called ac Stark effect, by sending an additional laser beam towards the sample at energy below polariton resonance. The Stark-induced phase shift will be manifested as a spatial shift in interference fringes of the light component of the polaritons, sent towards a Michelson interferometer.
The project consists in demonstrating BEC in the experimental setup. We will start by learning to build a Michelson interferometer with a variable optical path length (OPL) difference between the interferometer branches. By observing the resulting interference fringes pattern on a camera, we should obtain a decay of the visibility of the fringes as the OPL increases. This decay is reduced as the temporal coherence of the light increases. When a BEC is formed, it is accompanied by a buildup of temporal coherence of the polariton BEC, which is transmitted to the light component of the polaritons. The result will be a sharp decrease in the decay of the visibility of the fringes, which will be the signature of the formation of the BEC.
——————————————————————————