Exciton-polaritons are strongly coupled excitons and photons, which can exhibit strong repulsive interactions owing to the underlying exciton component and very light effective mass due to the photon component. These particles can be realized in a semiconductor microcavity with quantum wells placed at the position of antinodes of the confined photon modes. Due to the extremely light mass, about 10,000 times smaller than the mass of an electron, we can observe a Bose-Einstein condensation of the polaritons at much higher temperatures (∼ 20 K) than what is typically required for atomic gases (∼ 200 nK). With the availability of the state-of-the-art instruments and techniques to grow nearly perfect semiconductor heterostructures, we have access to microcavities with high-quality Bragg mirrors and quantum wells with low disorder. This has significantly improved the lifetime of the polaritons from 2 ps to 200 ps in the last seven years, allowing them enough time to reach thermal equilibrium and travel macroscopic distances up to a millimeter. Using a combination of experimental and theoretical techniques my research explores these two themes of equilibration and transport to understand the collective behavior of polariton condensates. A simple yet non-trivial unit for such an architecture is a ring microcavity.

Polaritons are intrinsically driven-dissipative by nature as they only live for a short time before leaking out from the cavity. Therefore, for any practical technological applications, they need to be continuously pumped. However, the dissipative nature allows us to perform nondestructive measurements by analyzing the leaked photons which carry spectral information of the polaritons when it was inside the cavity. I study the polaritons in the ring by exciting them with a short picosecond optical pulse in a small region and directly observing them with picosecond resolution as they flow in the ring. Time-resolved imaging together with spatial and spectral resolution enabled me to study macroscopic dynamics of condensate from a quench to equilibrium. In our samples, a small wedge during the growth of the microcavity structure gives rise to an “artificial" gravity for the polaritons. Together with the ring geometry, the wedge creates a rigid rotor potential for the polaritons in which the polariton condensate showed damped oscillations like a rigid pendulum in the air under gravity. This analogy was further verified from the radius dependence of the time-period of the oscillations. From these experiments, I made a quantitative measurement of the strength of the interactions between the polaritons. The accurate measurement of the interaction strength is extremely important for designing nonlinear polaritonic devices. I also reported extremely long (∼ 150 µm) transport lengths for the exciton reservoir, which were previously known to migrate only a couple of microns from the point where they were created. Thus, opening new possibilities for controlling and manipulating polariton condensate in a channel by reservoir engineering.

Another aspect of the condensate flow in a ring geometry is connected to the spin-orbit coupling which originates from the energy splitting between the transverse electric and transverse magnetic modes of the cavity photon in a confined structure. I studied the dynamics of the spinor condensate using polarization-resolved measurements which showed connections to spin Hall physics. This extended our understanding of the thermalization in the spin sector which is related to the degree of polarization of the polariton condensate.

10/2020