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Lasing action from an optically pumped bound state in the continuum cavity is demonstrated, of both fundamental interest and with applications from optical trapping to biological sensing and quantum information. Light confined within the radiation continuum Light can be trapped by confining it between mirrors or in a cavity. However, a curious effect of trapping waves in an open system, a continuum, known for almost a century from quantum-mechanics theory, was recently rediscovered as a general phenomenon and applied to acoustics as well as optics. Until now, 'bound in continuum' light states have been realized in passive systems. But here, Boubacar Kanté and colleagues report the construction of nanophotonic structures in which such bound states in a continuum are used to produce laser action at room temperature. The effect could help researchers to explore novel light–matter interaction effects. In 1929, only three years after the advent of quantum mechanics, von Neumann and Wigner showed that Schrödinger’s equation can have bound states above the continuum threshold 1 . These peculiar states, called bound states in the continuum (BICs), manifest themselves as resonances that do not decay. For several decades afterwards the idea lay dormant, regarded primarily as a mathematical curiosity. In 1977, Herrick and Stillinger revived interest in BICs when they suggested that BICs could be observed in semiconductor superlattices 2 , 3 . BICs arise naturally from Feshbach’s quantum mechanical theory of resonances, as explained by Friedrich and Wintgen, and are thus more physical than initially realized 4 . Recently, it was realized that BICs are intrinsically a wave phenomenon and are thus not restricted to the realm of quantum mechanics. They have since been shown to occur in many different fields of wave physics including acoustics, microwaves and nanophotonics 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 . However, experimental observations of BICs have been limited to passive systems and the realization of BIC lasers has remained elusive. Here we report, at room temperature, lasing action from an optically pumped BIC cavity. Our results show that the lasing wavelength of the fabricated BIC cavities, each made of an array of cylindrical nanoresonators suspended in air, scales with the radii of the nanoresonators according to the theoretical prediction for the BIC mode. Moreover, lasing action from the designed BIC cavity persists even after scaling down the array to as few as 8-by-8 nanoresonators. BIC lasers open up new avenues in the study of light–matter interaction because they are intrinsically connected to topological charges 17 and represent natural vector beam sources (that is, there are several possible beam shapes) 18 , which are highly sought after in the fields of optical trapping, biological sensing and quantum information.

The quantum Hall effect involves electrons confined to a two-dimensional plane subject to a perpendicular magnetic field, but it also has a photonic analogue1–6. Using heterostructures based on structured semiconductors on a magnetic substrate, we introduce compact and integrated coherent light sources of large orbital angular momenta7 based on the photonic quantum Hall effect1–6. The photonic quantum Hall effect enables the direct and integrated generation of coherent orbital angular momenta beams of large quantum numbers from light travelling in leaky circular orbits at the interface between two topologically dissimilar photonic structures. Our work gives direct access to the infinite number of orbital angular momenta basis elements and will thus enable multiplexed quantum light sources for communication and imaging applications.A topological photonic crystal design directly generates light that carries orbital angular momentum with high quantum numbers. The beam contains several different states at the same time, promising integrated and multiplexed light sources.

Resonant cavities are essential building blocks governing many wave-based phenomena, but their geometry and reciprocity fundamentally limit the integration of optical devices. We report, at telecommunication wavelengths, geometry-independent and integrated nonreciprocal topological cavities that couple stimulated emission from one-way photonic edge states to a selected waveguide output with an isolation ratio in excess of 10 decibels. Nonreciprocity originates from unidirectional edge states at the boundary between photonic structures with distinct topological invariants. Our experimental demonstration of lasing from topological cavities provides the opportunity to develop complex topological circuitry of arbitrary geometries for the integrated and robust generation and transport of photons in classical and quantum regimes.

In the past few years, concepts from non-Hermitian (NH) physics, originally developed within the context of quantum field theories, have been successfully deployed over a wide range of physical settings where wave dynamics are known to play a key role. In optics, a special class of NH Hamiltonians – which respects parity-time symmetry – has been intensely pursued along several fronts. What makes this family of systems so intriguing is the prospect of phase transitions and NH singularities that can in turn lead to a plethora of counterintuitive phenomena. Quite recently, these ideas have permeated several other fields of science and technology in a quest to achieve new behaviors and functionalities in nonconservative environments that would have otherwise been impossible in standard Hermitian arrangements. Here, we provide an overview of recent advancements in these emerging fields, with emphasis on photonic NH platforms, exceptional point dynamics, and the very promising interplay between non-Hermiticity and topological physics.

Topological insulator lasers (TILs) are a recently introduced family of lasing arrays in which phase locking is achieved through synthetic gauge fields. These single frequency light source arrays operate in the spatially extended edge modes of topologically non-trivial optical lattices. Because of the inherent robustness of topological modes against perturbations and defects, such topological insulator lasers tend to demonstrate higher slope efficiencies as compared to their topologically trivial counterparts. So far, magnetic and non-magnetic optically pumped topological laser arrays as well as electrically pumped TILs that are operating at cryogenic temperatures have been demonstrated. Here we present the first room temperature and electrically pumped topological insulator laser. This laser array, using a structure that mimics the quantum spin Hall effect for photons, generates light at telecom wavelengths and exhibits single frequency emission. Our work is expected to lead to further developments in laser science and technology, while opening up new possibilities in topological photonics. Topological insulator lasers offer robustness and efficiency due to their unique properties but usually require cryogenic temperatures or optical pumping. Here the authors demonstrate an electrically pumped topological insulator laser operating at room temperature.

Tunable acousto-optic (AO) lenses have recently emerged as versatile tools for optical beam shaping, imaging, and particle manipulation. Conventional AO lenses rely on light propagating orthogonally to a standing ultrasonic field, producing Bessel-like beam patterns via the Raman–Nath effect that compromise focal localization and depth of field (DoF). Here, we introduce a novel class of AO lenses based on a three-dimensional, dynamically variable refractive index profile generated by a z-axis-scanning focused ultrasound transducer. By exploiting co- and counter-propagating light–sound interactions over an extended axial range, we achieve fully controllable, localized optical focusing with an instantaneous extended DoF. We demonstrate dynamic tuning of focal position, lateral resolution, and optical power throughput by adjusting ultrasound parameters. This approach offers a promising platform for applications requiring precise remote focusing, three-dimensional micromanipulation, and deep tissue imaging or therapy. Graphical Abstract Highlights • Collinear co- and counter-propagating light–sound interactions create a dynamic 3D-variable refractive index profile for tunable optical focusing. • The proposed acousto-optic lens achieves localized single-point focus with instantaneous, real-time extended depth of field, without laser–ultrasound synchronization. • Dynamic control of focal location, depth of field, lateral resolution, and optical throughput is demonstrated by adjusting ultrasound duty cycle and input voltage. • This novel light–sound interaction scheme shows promise for advanced applications in high-resolution 3D/4D imaging, adaptive microscopy, and remote photonic manipulation.

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In integrated photonics, cavity resonators play an important role. They are the basis of light sources, which are one of the fundamental building blocks of any integrated circuit. So far, cavities are designed base on their size, shape, and photon lifetime, and requiring any extra features increase their complexity. However, cavities can present some topological behaviors with peculiar characteristics that can enhance their functionalities. Two of these topological behaviors, which are the main focus of this thesis, are Topological insulators (TIs), and Bound states in the continuum (BIC). In the following thesis, we explored theoretically and experimentally the topological singularities in cavities made of periodic structures, and their applications in designing integrated light sources (i.e., lasers). Structures are constructed on a gain material of InGaAsP multiple quantum wells, which emits in the telecommunication wavelength range (λ~1.55 μm), and operates at room temperature. In the first part of the thesis, we study TIs and design topological cavities for integrated light sources using hybrid photonic crystals (PhCs) with non-zero phase transition between them. Thus the optical wave is fully confined at the boundary of PhCs, and propagates in one direction. The topological cavities can have any arbitrary geometry while preserving high functionality. Furthermore, we demonstrate that topological cavities are able to be used to generate structured lights with very large topological charges, while they maintain small foot-print and no-complexity. The second part is dedicated to the bound states, which are the type of topological singularities with positive energies in the continuum region. These topological singularities offer many unique characteristics such as tunability of their position in the reciprocal space and carrying non-zero topological charges. Furthermore, the number of singularities can be controlled by crystal symmetry. In this thesis, we present the first experimental demonstration of simultaneously generation and steering multiple vortex beams form an extended PhC cavity. Our results indicate the application of the topological behavior of cavities as an extra degree of freedom in designing integrated photonic chips with enhanced functionalities.

In this numerical study, a new compact and tunable beam-steering device composed of graded index photonic crystal lens and liquid crystal material has been presented and demonstrated. The beam-steering device can work in two modes: First, incident single wavelength beam steers by tuning the refractive index of the liquid crystal material. Second, by fixing the refractive index of the liquid crystal material, incident multi-wavelength beam steers with different off-axis angles for each wavelength. The numerical simulations with finite-difference time-domain method reveal that wide-angle, ±28.4°, beam steering can be achieved when the length of the liquid crystal layer equals 5.17 μm. Moreover, the wavelengths over λ = 1.518–1.553 μm or λ = 1.487–1.518 μm can be steered to different angles. A special characteristic of the presented structure gives an opportunity to be used as an efficient element in a high integrated optical device for miniaturisation and tuning purposes.

Topological Insulator-based devices can transport electrons/photons at the surfaces of materials without any back reflections, even in the presence of obstacles. Topological properties have recently been studied using non-reciprocal materials such as gyromagnetics or using bianisotropy. However, these effects usually saturate at optical frequencies and limit our ability to scale down devices. In order to implement topological devices that we introduce in this paper for the terahertz range, we show that semiconductors can be utilized via their cyclotron resonance in combination with small magnetic fields. We propose novel terahertz operating devices such as the topological tunable power splitter and the topological circulator. This work opens new perspectives in the design of terahertz integrated devices and circuits with high functionality.