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Dissertation Defense

Subwavelength Grating Based Microcavity and Its Applications in Many-body Polariton Systems

Zhaorong Wang


Semiconductor lasers, widely used in optical communication and information processing, Lidar and gas sensors is fundamentally changing people's lives. A new addition to the semiconductor laser family is the so-called polariton laser based on the spontaneous coherence of exciton-polaritons. Exciton-polaritons are hybrid particles in semiconductors resulting from the strong coupling between quantum well (QW) excitons and microcavity photons. Combining the strong intrinsic nonlinearities from the exciton part and the small effective mass from the photons, polaritons can undergo Bose-Einstein condensation and emit coherent light at 3 orders of magnitude lower threshold, which allows an ultralow power consumption polariton laser. This is particularly meaningful for our data-exploding society. Polariton laser is also fundamentally linked with quantum many-body processes such as superfluidity and superconductivity. It provides a controllable chip-level testbed to study many-body physics and macroscopic quantum phenomena, which may enable practical quantum computing and simulations in the near future.

In this thesis, I introduce a new cavity architecture for polariton laser based on high-index-contrast subwavelength gratings (SWG). I will demonstrate its advantages in confining and manipulating polaritons in a non-destructive fashion. In addition, thanks to the anisotropy of SWG, exciton reservoir of our system can be monitored in the orthogonal polarization, together with the polariton condensate. My data showed a coexistence of Mott-transitioned excitons and robust polariton condensate, which provides strong evidence to the long-believed photon-induced electron-hole binding and the new polariton BCS phase. These findings will help clear up the controversies in the high-density nonlinear regime of exciton-polariton lasers. In the end, I will show the engineering efforts towards SWG cavity 2.0– creating larger Rabi-splitting polaritons for room-temperature operations and dispersion-engineered polaritons to facilitate efficient lasing.

Sponsored by

Prof. Hui Deng & Prof. Duncan Steel