Low Threshold Lasing in Gallium Nitride Based Microcavities

Abstract

Optical microcavities confine light to small volumes by resonant recirculation. Devices based on optical microcavities have become indispensable in modern optics, finding applications not only in laser devices, optical data recording, fiber optic data transmission, but also as invaluable tools in the realization of many optics experiments in scientific research. Semiconductor microcavities with embedded quantum dots (QDs) or quantum wells (QWs) are excellent platforms for investigating spontaneous emission and cavity lasing. With the right choice of material, microcavities containing optical emitters can offer the possibility of room temperature realization of light-matter interaction. Gallium nitide (GaN) and its alloys have several exceptional optical and electrical properties, making the material system an outstanding candidate for studying low-threshold lasing in microcavities. Some of the advantages of III-nitride based microcavities include enhanced room temperature performance due to large exciton binding energy, ability to be engineered to emit at any visible wavelength, low surface recombination velocities, and high optical robustness in the presence of defects. However, many challenges also need to be overcome for us to take advantage of this wonderful material. The main challenge in fabricating high quality GaN based cavities comes from the material’s chemical inertness which brings about great difficulties in the processing of GaN. The practical constraint of the need to grow GaN on lattice mismatched substrates gives rise to large density of strain-induced defects and threading dislocations throughout the material. The intrinsic polarization field present in III-nitrides, coupled with internal field arising from strains in the material, reduces carrier recombination efficiency and hinders efficient lasing. In spite of these difficulties, low-threshold GaN microdisk and photonic crystal lasers containing indium gallium nitride (InGaN) emitters have been realized and examined in recent years, giving us valuable initial insights into the working mechanisms behind cavity-emitter interaction. The ability to control cavity photon dynamics through efficient cavity-emitter coupling is at the heart of the research into low threshold microcavity lasing. This dissertation builds on previous work and aims to investigate the fundamental limitations to low threshold lasing in GaN/InGaN microcavities, by focusing on the respective roles InGaN quantum dots and fragmented quantum wells (fQWs) play in relation to cavity modes and the photon loss mechanism in laser operation. Microdisks, micro-rings, and photonic crystal nanobeams based on GaN/InGaN are designed and fabricated to aid our understanding of the interaction between cavity and active mediums. This dissertation is organized into five chapters. Chapter 1 gives an overview of the problem at hand and describes the general approach we take to investigate these questions. Chapter 2 discusses the relevant background knowledge that aids in the understanding of later discussions. In this chapter we give a description of the gallium nitride material, a thorough examination of optical microcavities and its emitters, as well as a review of the fundamental working mechanisms of a laser. Chapter 3 dives into the realization of low-threshold lasing in microdisk cavities containing QDs, fQWs and QWs. This chapter describes in detail the design, growth, fabrication and characterization of microdisk lasers. Additional experiments comparing InGaN QDs and fQWs as gain mediums in a microdisk laser builds the foundation for our understanding of the different roles emitters play in relation to gain, capture efficiency, and cavity mode selection. Chapter 4 explores the fundamental limitations to low-threshold lasing through studying micro-ring lasers containing heterogeneous emitters: InGaN QD and fQWs. The chapter walks readers through the process of designing, fabricating and optimizing the micro-ring cavity geometry in the eventual realization of an ultra-low-threshold micro-ring laser with a threshold of 6.2 micro-J per square centimeter, an order of magnitude lower compared to previous generation microdisk designs. The chapter highlights the mechanism of photon loss from the low-quality cavity modes, and how we can leverage this by spectrally and spatially overlapping low-quality modes with low-gain fQW emitters, so as to remove undesirable photons from the cavity and drive down the lasing threshold even further. Chapter 5 looks at lowering lasing thresholds from a different dimension – by altering the cavity design entirely. In this chapter we demonstrate a high-quality factor low-modal-volume photonic crystal nanobeam laser, with only a single mode overlapping the gain region of the emitters. A discussion on the optimal choice of gain material in relation to specific cavity design is also presented.