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

Semiconductor microcavity polaritons have attracted intense research in the past 20 years because of its deep connections with macroscopic quantum phenomena such as Bose-Einstein condensation (BEC), superfluidity and superconductivity. Experimental polariton systems have evolved as powerful research tools for many-body physics, and have shown promise for novel devices such as ultra-low threshold laser and polaritonic integrated circuit. A central issue to all experimental polariton systems is how to effectively confine and manipulate polaritons. Existing systems all have their limitations such as small modulation depth, destructive to the active medium, and difficult to fabricate or reproduce. In this thesis, we develop a sub-wavelength grating (SWG) based microcavity to generate and control polaritons, which overcomes the limitations of existing systems and has unique properties leading to new physics that was inaccessible before.;We demonstrated discrete polariton modes in a fully confined zero dimensional SWG cavity and lasing in the ground state via a thorough set of optical measurements. This shows that the new SWG-cavity can not only support polariton modes but also maintains low loss and allows the formation of coherent polariton condensate. This is the prerequisite of using our system to study macroscopic quantum phenomena and novel many-body physics.;Further, polariton nonlinearity was studied from a unique perspective, revealing phenomena contrary to commonly held understanding. Thanks to the polarization anisotropy of the SWG mirror, exciton reservoir of our system can be directly probed through the emission of the weakly-coupled excitons that co-exist with the strongly-coupled polaritons. We show that polariton nonlinearity originate mainly not from exciton energy renormalization, but saturation. The saturation pair density was matched to theoretical values. Reflectance measurements unambiguously show that, at high pump density, excitons already undergo Mott-transition while polariton and polariton lasing is maintained. This is in contrary to previous belief of polariton lasing is realized at far below Mott-density of excitons. Our results point to the light mediated electron-hole binding in a BCS-like state of polaritons.;Finally, we demonstrated polariton mode engineering through the design of SWG. Specifically, a SWG is optimized to reduce the mode volume of the cavity, which enhances the coupling strength between excitons and vacuum photons by up to 67% compared to conventional GaAs polariton systems. The larger coupling strength can help increase the operating temperature of polariton systems. Further, SWG cavities with engineered dispersion are demonstrated by designing the angular phase response of SWG mirror. Polariton dispersion is therefore strongly modified, which may enable different polariton dynamics and even exotic quantum orders. As an experimental effort of mode engineering, we demonstrate coupled 0D SWG cavities and quasi-1D polariton lattice. Theoretical modeling using harmonic potential traps and gaussian potential barriers matches well with experiments. The potential depth ranges from 4meV to 20meV. These engineered SWG polariton systems provide an unique venue for research on lattice physics and quantum optical circuits.