Coffee and Connection with Sushanta Mitra with guest speaker: Hassan Alnatah

Thursday, August 17, 2023 3:30 pm - 3:30 pm EDT (GMT -04:00)

Bose-Einstein Condensation of Microcavity Exciton-Polaritons

The Waterloo Institute for Nanotechnology (WIN) is pleased to invite you to Coffee and Connection with our executive director Sushanta Mitra with a guest speaker Hassan Alnatah a PhD candidate at the University of Pittsburgh.

This seminar is titled "Bose-Einstein Condensation of Microcavity Exciton-Polaritons" and will be held on Thursday, August 17, 2023 at 3:30 PM in QNC 1501. Registration is not required!


Exciton-polaritons are mixed light-matter particles emerging from the strong coupling between cavity photons and quantum-well excitons. As interacting bosons, polaritons demonstrate quantum phenomena such as Bose-Einstein condensation (BEC) and superfluidity at temperatures from tens of Kelvin up to room temperatures. Imaging of the leaked photons from the cavity give direct experimental accessibility to the quantum statistics of the polaritons gas, making polaritons a unique testbed for the study of macroscopic quantum effects.
I will first give a general introduction to exciton-polaritons. In the first part of my talk, I will discuss our results on the coherent fraction of a homogenous polariton gas in thermal equilibrium. We have measured the distribution function of a homogenous polariton gas near resonance and shown that it is well described by a Bose-Einstein distribution, indicating that the polariton gas is in thermal equilibrium (Fig. 1). Although condensate fraction of a strongly interacting Bose gas case has been studied extensively in helium-4 via superfluidity measurements, this has been much harder to do in the weakly interacting Bose gas (WIBG) limit with cold atoms. Therefore, a verification of WIBG theory would be of real interest. Our system allows us to directly measure the coherence of the polariton gas by creating the interference pattern of the light emitted from the gas overlapped with its mirror image. Figure 2 shows the coherent fraction of the polaritons as a function of their density. This allows comparison of our results with theoretical predictions of the coherent fraction of an interacting 2D Bose gas.
In the second part of my talk, I will discuss our results on the persistent circulation of a polariton fluid. Persistent circulation is a canonical effect of superfluidity. We directly observe quantized circulation of a polariton condensate in a well-controlled steady state. We can cause the condensate to circulate in either direction on demand using a short laser pulse. Figure 3 shows the interference images of the condensate in a Mexican-hat trap with its mirror image. The different number of fringes on the top and the bottom implies the circulation of the condensate. Surprisingly, we find that the time scale of the persistence is extremely long compared to the pulse that stimulates the circulation, suggesting that the condensate is robust to many-body interactions.


Hassan Alnatah wearing a yellow shirt
Following his BS from Oregon State University, Hassan joined Dr. David Snoke’s group as a PhD candidate at the University of Pittsburgh. Currently, he is studying the properties of Bose-Einstein condensation of polaritons created inside a III-V semiconductor microcavity. Polaritons are essentially photons dressed with an effective mass and strong interactions. Hassan’s research is focused on how a system of interacting photons can reach thermal equilibrium and how the degree of their interactions can affect their thermodynamic properties. These interacting photons can undergo Bose-Einstein condensation, which is a state of matter with spontaneous coherence. His work connects to several fundamental questions in quantum mechanics, such as how coherence can occur spontaneously in Bose-Einstein condensates and how coherence is lost in standard quantum systems.