Enhancing optical confinement of GaN-based microcavity

Abstract

The performance of lasers is critically dependent on optical confinement within the optical cavity. In this thesis, several novel approaches have been proposed and successfully demonstrated to enhance the lasing performance of GaN-based microcavities. A platform that isolates high-quality GaN thin films from the substrate using an omnidirectional reflector comprising a dielectric Bragg mirror and an Al-based mirror is proposed. This high-quality GaN thin-film platform not only reduces internal absorption losses but also enhances the overlap between cavity modes and the gain region. Making use of this platform, a high-performance optically pumped microdisk laser with a threshold of approximately 46.5W/cm^2 and a Q factor of approximately 18200 has been achieved, which not only improves the laser performance but also enhances the stability of the microdisk. In the ODR-confined microdisk, competition among higher-order modes is observed. To address this issue, a special Bragg mirror for the microdisk is designed, where the periodicity and duty cycle are adjusted such that the bandgap of Bloch waves mismatches the propagation conditions of higher-order whispering-gallery modes (WGMs). This configuration provides strong optical confinement while selectively leaking higher-order WGMs, enabling single-mode operation of an 8um-diameter GaN thin-film microdisk with a threshold of approximately 60 W/cm^2 and a Q factor of around 15500. The Bragg mirror, combined with a thin indium tin oxide (ITO) layer, is embedded at the bottom of the GaN microdisk. With this device structure, electroluminescent (EL) microdisk operation with a current density threshold of approximately 0.796kA/cm^2 and a Q factor of about 4500 is demonstrated. Furthermore, a monolithic grating for vertical emission is developed for both VCSELs and microdisks. The monolithic grating designed for VCSELs serves as the top reflector and is applied to an Ag-confined GaN thin-film platform, achieving a high Q, DBR-free VCSEL with a threshold of approximately 5.5kW/cm^2 and a Q factor of 4600. For microdisk lasers, the designed grating can extract WGMs carrying orbital angular momentum (OAM) from the top of the microdisk and can also serve as a mode selector.