Electrically-injected bottom-up III-V micro-cavity lasers

Bottom-up fabrication of lasers via epitaxial growth is emerging as a promising alternative to conventional top-down methods, offering the potential to realize micro-lasers with ultra-low optical losses. In this project, we aim to demonstrate electrically injected lasing in InP/InAsP multi-quantum well micro-ring cavities, grown using the selective area epitaxy technique. These on-chip microcavity lasers hold significant promise for a wide range of applications, including telecommunications, sensing, and as light sources in next-generation photonic integrated circuits.

III-V semiconductor nanosheets as polarisation-sensitive photodetectors

Using the selective area epitaxy technique, III-V semiconductors can be grown in the form of pseudo-two dimensional (2D) nanosheet structures. The unique morphology of the nanosheets results in light absorption that is highly sensitive to the light polarisation. In this project, we aim to demonstrate polarisation-sensitive photodetector devices based on III-V nanosheet arrays, with potential applications in optical computing and advanced imaging.

Nano-scale III-V light emitters on Si

Monolithic integration of III-V semiconductor lasers on silicon platforms has been the "holy grail" in Si photonics, as it could resolve the long-standing light source issue in the technology. While planar growth of III-V materials on Si substrates has been widely demonstrated, growing III-V nanostructures directly on Si remains a significant challenge. This project aims to demonstrate the direct growth of InP/InAsP light-emitting nanostructures on Si substrates by engineering the III-V/Si interfacial energy, paving the way for efficient monolithic integration of active photonic components on silicon.

Bottom-up, parity-time (PT) symmetric micro-cavity lasers

Parity-time (PT) symmetry is an intriguing phenomenon in quantum mechanics, whereby a non-Hermitian Hamiltonian that obeys PT symmetry can exhibit completely real eigenvalues. In recent years, the extension of PT symmetry into the field of photonics has led to a plethora of new class of photonics devices that operate based on PT-symmetric exceptional points (EPs), such as PT-symmetric lasers, EP-enhanced sensors, and coherent perfect absorbing (CPA) anti-lasers.

Among these novel devices, PT-symmetric lasers are especially attractive from the perspective of practical applications, as the optical coupling in the system can be engineered to achieve enhanced side mode suppression without compromising the lasing threshold and the fabrication complexity. However, single-mode lasing in PT-symmetric micro-cavity lasers has so far only been demonstrated in systems fabricated by conventional top-down approaches. Meanwhile, bottom-up growth of III-V laser cavities has recently emerged as a promising alternative to the conventional top-down fabrication method, as it can potentially realise micro-cavity lasers with superior sidewall qualities, which is crucial to laser performance especially at the submicron dimensions. 

In this project, we aim to explore PT-symmetric lasing in III-V semiconductor micro-cavity lasers that are epitaxially grown on their substrates, free from any etching-induced damage. In particular, we aim to demonstrate performance improvements by exploiting some of the unique features of bottom-up grown laser cavities, such as coupling strength tuning by engineering the cavity sidewall facets, and enhanced lasing efficiencies due to the superior sidewall facet quality. 

Bottom-up, quasi-bound states in the continuum (quasi-BIC) metasurface lasers

Metasurface lasers based on bound states in the continuum (BICs) have recently attracted significant research interest. In theory, a true photonic BIC is characterized by complete decoupling from the radiation continuum, resulting in an infinite radiative lifetime and Q-factor. Remarkably, a slight perturbation can cause a true BIC to collapse into a Fano-like resonance, forming what is known as a quasi-BIC—a mode with a very high and tunable Q-factor. Metasurfaces composed of large ensembles of unit cells supporting such quasi-BIC modes are, therefore, excellent candidates for low-threshold laser cavities.

To date, all demonstrated quasi-BIC metasurface lasers have been fabricated using conventional top-down techniques. In this project, we aim to demonstrate lasing in a bottom-up metasurface device that supports a perturbed symmetry-protected quasi-BIC mode. The unit cell of the metasurface consists of a pair of InP nanosheet structures grown via selective area epitaxy. The high sidewall facet quality of these nanosheets is expected to minimize scattering losses within the cavity, enabling ultra-low threshold lasing at room temperature.

Nanowire photonic crystal surface-emitting lasers

Over the past decade, photonic crystal surface-emitting lasers (PCSELs) have emerged as a compelling alternative to conventional vertical cavity surface-emitting lasers (VCSELs), owing to their superior beam quality and scalable output power. Watt-class PCSELs have already been demonstrated and are now commercially available for applications in LiDAR and telecommunications. However, all existing PCSELs are fabricated using conventional top-down approaches, where structural damage from the dry etching process is unavoidable.

In this project, we aim to demonstrate PCSELs constructed from vertically aligned III-V semiconductor nanowires as the fundamental building blocks. Unlike traditional PCSELs that depend on top-down etching to define the laser cavity, nanowires naturally form atomically smooth sidewall facets during the epitaxial growth process, effectively eliminating scattering losses due to surface roughness. Furthermore, each nanowire simultaneously functions as both the optical cavity and gain medium, enabling optimal coupling between light emission and the lasing cavity. We will also explore more advanced nanowire-based PCSEL designs, including hetero-lattice PCSELs with enhanced in-plane optical feedback and topological PCSELs.

Directional metasurface lasers based on coupled nanowire pairs

Nanolasers with directional emission is of significant research interest, offering potential as compact, coherent on-chip light sources for applications spanning from LiDAR to beam-steering devices. In this project, we demonstrate spatial control of the far-field emission of III-V semiconductor nanowire lasers by engineering the waveguide mode in pairs of optically-coupled nanowires. Importantly, arranging such coupled nanowire pairs into an array can potentially enhance the far-field directionality through non-local resonance effects, highlighting the feasibility of metasurface lasers with highly directional beam emission.