Coherent Nonlinear Phenomena in Subwavelength-Grating Based Microcavities

[摘要] Bose-Einstein condensation of microcavity polaritons has shown remarkable phenomena such as coherent lasing below population inversion, quantized vortices, and Josephson effects in a semiconductor platform. With nonlinear interaction between exciton-polaritons, it has also drawn great interests for nonlinear photonic devices. To control nonlinear interaction strength, many efforts to confine polariton condensates have been made. In this work, we use a subwavelength-grating (SWG) mirror to confine exciton-polaritons in a lithographically defined area. Being a one-dimensional photonic crystal, it is possible to control emission polarization depending on the grating parameters. Moreover, SWG mirrors require only two layers which reduce the fabrication complexity compared to distributed Bragg reflectors (DBRs). In this thesis, we present two approaches to realize exciton-polariton systems with SWG. One is air-suspended and the other is fully monolithic.With SWG-DBR microcavities, we first achieve single-mode lasing in both energy and polarization. In a conventional DBR-DBR microcavity where single-mode lasing in both energy and polarization is nearly impossible, excess intensity fluctuations from polariton lasers were observed. The SWG microcavity removes any mode-competition which has adverse effects on intensity coherence and in turn phase coherence. As a result, we show shot-noise limited intensity fluctuations and phase coherence that is well explained by atom laser theory with two-body interactions. Further optimization of the phase coherence is shown by varying device sizes with different interaction strengths. We then realize coupling between single-mode condensates by spatial control of exciton-photon detuning. New frequency comb mechanism was recently proposed from two coupled polariton condensates with complex couplings. We experimentally observe multifrequency components, which are generated from limit-cycle oscillations. Numerical simulation of the coupled polariton equation shows good agreement with the experimental results. This polariton comb allows non-resonant excitation with a power input below the conventional semiconductor laser. The comb line spacing, determined by the interaction and coupling strengths, is adjustable up to multi-terahertz frequency for a chip-scale low power terahertz source. We also investigate polariton optomechanics for potential q-switched polariton lasers. Since the SWG is a thin single slab that is suspended in the air, it is a good candidate for optomechanics as shown in VCSEL. Unique to polariton system, the mechanical motion of the mirror not only changes resonance frequency but also the dissipation rate, important to realize q-switching. Here, we show bright emission of short pulses, which resembles q-switched lasers, when the mirror oscillates. Finally, we present the orbital angular momentum (OAM) and vector beam generation in SWG-based microcavities. The OAM and polarization states provide additional degrees of freedom for information transmission. Direct generation of the OAM source from micro or nanostructure has been desired but challenging. In polariton systems, a potential for the OAM laser has been shown as the quantized vortices have been observed around naturally occurring defects on the sample. The remaining challenge was to create deterministic defects that form vortices or OAM. For SWG microcavities, it is very easy to create defects because we can remove the entire top mirror by etching only a single layer. We created many different defects and show formations of quantized vortices. By breaking the symmetry of the defects, we create polariton lasers which possess nonzero OAM. Using polarization selectivity, we also create circular gratings which have radially polarized emission.

[发布机构] University of Michigan

[效力级别] Subwavelength Grating

[关键词] Microcavity Polariton;Subwavelength Grating;Physics;Science;Electrical Engineering