This review is written by non-expert and used for personal study only.
Low-threshold nanolasers based on miniaturized bound states in the continuum
- Optics
Exploring the World of Nanoscale Light Generation
Nanoscale light generation is a cutting-edge area of research with implications for many scientific fields. Miniature lasers that use minimal power have a broad range of applications. However, there are some challenges: plasmonic cavities experience significant energy losses, and dielectric nanolasers are limited in their lasing volume and power output. Bound states in the continuum (BICs) have emerged as a promising approach to address these issues. BICs can reduce out-of-plane radiation and increase Q factors (a measure of resonance sharpness) in planar optical resonators, which leads to the development of ultrafast switchable and multiplexed nanolasers with unique properties.
Creating Miniaturized BICs for Improved Nanolasers
Researchers are striving to develop miniaturized BICs with high Q factors to create practical nanolasers that produce higher output power. They use BICs to reduce out-of-plane radiation and photonic bandgaps (PBGs) to minimize in-plane optical losses, resulting in high-Q optical resonators that occupy less space. They successfully fabricated gallium arsenide (GaAs) membranes that support mini-BICs with high Q factors, enabling low-threshold laser oscillations. This innovation could lead to further applications of BICs in cavity quantum electrodynamics (QED) and integrated nonlinear photonics, which involve the interaction of light with matter and the manipulation of light properties, respectively.
Building a Laser Cavity with Photonic Crystals
The research team designed a laser cavity using photonic crystals (PhCs), which are periodic structures that manipulate light, combined with indium arsenide (InAs) quantum dots (QDs) as the light amplification materials. They employed mini-BICs to reduce out-of-plane radiation and enhance the Q factor and mode volume V (a measure of light confinement). To create the mini-BIC patterns, they used an electron-beam resist, which was then transferred to the GaAs layer using a dry etch process. The sharp resonances measured in the cavity area matched the simulations, revealing the vectorial nature and momentum localization of the mini-BIC modes, which are essential properties for understanding their behavior. Investigating Lasing in Mini-BIC Devices The researchers measured the micro-photoluminescence (μ-PL) spectra of a cavity under different excitation power levels. They observed sharp cavity modes on a broad emission background at low excitation powers, which indicates the presence of light emission. When the excitation power was increased, the $M_{11}$ mode (a specific type of light mode) became dominant, and its linewidth (a measure of resonance width) decreased significantly, suggesting the presence of lasing oscillation, which is the process of amplifying and emitting light.
However, there is an ongoing debate about using the kink feature (a sudden change in slope) on intensity-output (IO) curves as the only criterion for determining nanoscale lasing. A non-lasing device displayed a threshold feature and linewidth narrowing behavior similar to a lasing device, but without any phase transition from thermal emission to a coherent light state. Instead, this non-lasing device operated as a nano-light-emitting diode (nano-LED), producing nonlinear output intensities as the excitation power increased. The researchers used pulsed excitation to explore the possibility of turning non-lasing behavior into lasing oscillation. The lasing device demonstrated a clear phase transition from spontaneous emission (random light emission) to stimulated emission (controlled light amplification), while the nano-LED device remained in a thermal state, indicating that it did not achieve lasing.
Required Additional Study Materials
Introductory material
- “Nanophotonics” by Arthur McGurn
- “Principles of Nano-Optics” by Lukas Novotny and Bert Hecht
Reference
SCIENCE ADVANCES 23 Dec 2022 Vol 8, Issue 51 DOI: 10.1126/sciadv.ade8817
Topological supermodes in photonic crystal fiber
- Fiber
Introduction to Metamaterial Structures and Topology
Metamaterial structures have been utilized to design topological bands in photonic and acoustic systems. These bands enable the creation of stable lasing modes and secure pathways for entangled quantum states. Topological modes are inherently protected due to a property called topological invariant. While larger structures can attain topological properties for microwaves, a challenge remains in creating waveguides for optical frequencies that combine micrometer-scale confinement with topological protection.
Advantages of Topological Fiber in Communication Networks
Topological fiber offers benefits for classical and quantum networks, such as low signal attenuation and resistance to variations caused by fabrication. Topological photonic crystal fiber (TopoPCF) allows for the control of topological modes by bending and mechanically adjusting the fiber, which is not feasible in flat waveguide structures.
Developing a Light-Guiding Topological Lattice
Scientists have designed a topological lattice capable of guiding light over long distances by carefully controlling the connection between the fiber cores. This lattice is based on the Su-Schrieffer-Heeger (SSH) chain model and features alternating small and large air holes to create a uniform shape. The fiber directs light in standard modes, and the high symmetry within each core leads to minimal mixing of polarization.
Topological Fiber: A Versatile Platform
The fiber platform enables adjustable control of topological states through bending, which is not achievable with short, rigid waveguides or solid-state devices. When the fiber is bent, changes in the propagation constant affect the system’s coupling matrix, disrupting the localizing topological protection and allowing light in the edge mode to disperse into the bulk modes. Topological protection breaks down when the bend radius, R*, reaches a point where the ratio of disorder (Δβ) to the average coupling strength (C) is near one. This mechanism offers a simple, reversible method for globally breaking and restoring topological protection. Topological fiber serves as a flexible platform for applying topological effects in scalable classical and quantum photonic networks, making it more accessible to a wide range of applications.
Required Additional Study Materials
Introductory material
- Optics, Light and Lasers: The Practical Approach to Modern Aspects of Photonics and Laser Physics by Dieter Meschede
Reference
SCIENCE ADVANCES 21 Dec 2022 Vol 8, Issue 51 DOI: 10.1126/sciadv.add3522
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