Epitaxial growth of III-V micro-ring lasers for optoelectronic applications
Date
2023
Authors
Wong, Wei Wen
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Photonic integrated circuit (PIC), the technology that integrates multiple photonic components into a functional circuit on a single chip, is one of the key technologies for high-speed data communication and telecommunication. However, the integration density of PICs has been limited by the physical dimensions of on-chip light sources. Thus far, on-chip lasers with a micro-scale device footprint and low power consumption remain elusive due to various fabrication challenges. While micro-cavity lasers fabricated by conventional top-down etching suffer from scattering losses due to sidewall roughness, traditional bottom-up laser fabrication approach such as the vapour-liquid-solid growth of nanowire lasers faces challenges including limited scalability, poor fabrication reproducibility, and potential metallic contamination.
In this thesis, we demonstrate a bottom-up approach to fabricate uniform arrays of III-V micro-ring lasers as potential on-chip light sources. Three different aspects of the bottom-up III-V micro-ring lasers will be covered in this thesis. In the first part of the thesis, we demonstrate the growth of InP laser cavities with a uniform ring-like morphology by utilising a shape engineering technique via selective area epitaxy. The micro-ring cavities are terminated by sidewalls consisting of high-quality crystal facets, which can act as optical mirrors to form a whispering-gallery mode (WGM) cavity with a high Q-factor. With an optimised cavity and growth mask opening design, we demonstrate room-temperature lasing in the as-grown InP micro-ring laser cavities with lasing threshold as low as 50 uJ cm-2 pulse-1. Remarkably, we also establish deterministic control over the lasing modes in the micro-ring cavity by tuning the vertical ring height during the epitaxial growth process.
In the second part of the thesis, we demonstrate the direct incorporation of an InAsP/InP multi-quantum well (MQW) gain medium in the bottom-up micro-ring cavities. By optimising the adatom diffusion lengths on specific sidewall crystal facets of the InP micro-ring cavity, we demonstrate the growth of high-quality InAsP QW layers that conform to the uniform ring-like morphology of the laser cavity. Furthermore, with an atomic-resolution transmission electron microscopy analysis, we unveil the detailed growth mechanism of the MQW gain medium. The MQW micro-ring cavities show low-threshold lasing at room-temperature, with tunable emission wavelengths in the telecommunication O-band. More importantly, with a high-throughput laser characterisation technique, we record a remarkable > 80% fabrication yield in over a thousand micro-ring lasers - a chip-scale fabrication process.
Finally, in the third part of the thesis, we report on the fabrication of a directional laser based on an optically-coupled InP micro-ring/nanowire system. While conventional WGM lasers are known to have poor emission directionality, in this work we achieve vertical emission by coupling the micro-ring laser emission into a vertical nanowire grown at the ring centre, which acts as a directional antenna. Efficient optical coupling between the two cavities is facilitated by enhanced light scattering at the crystal planes that constitute the sidewalls of the micro-ring cavity, which is unique to bottom-up grown laser cavities. Vertical laser emissions with strong far-field directivities in the coupled systems are confirmed using a Fourier imaging technique.
Overall, this thesis provides an overarching solution to the on-chip light source problem by demonstrating bottom-up III-V micro-ring lasers with low lasing thresholds, tunable emission wavelengths, and controllable far-field beam profiles. Furthermore, with an excellent fabrication yield, scalability, and reproducibility, our bottom-up laser fabrication approach proves to be a promising alternative to the conventional top-down laser fabrication process.
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