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Exciting high-power resonators pose challenges such as managing power reflections, which can cause energy losses and damage system components. This is crucial for applications like Lower Hybrid Current Drive (LHCD) systems in tokamaks, where plasma stability and confinement depend on efficient energy transfer. In this work, we introduce the Virtual Critical Coupling mechanism to address reflection-related challenges in S-band resonators. We theoretically designed a complex frequency excitation signal tailored to the resonator's characteristics, facilitating efficient energy storage and minimizing reflections without mechanical modifications. Using a custom low-level RF system, we conducted experiments at 32 mW and 600 kW with a 5 MW S-band klystron, demonstrating a ninefold reduction in reflection coefficients compared to traditional monochromatic excitation in high-power tests. This approach enhances the efficiency and stability of high-power resonant systems, potentially advancing nuclear fusion energy production.

Backscattering in micro-ring cavities induces mode mixing and limits device performance. Existing methods to mitigate backscattering often involve complex fabrication processes or are insufficient for complete suppression. In this work, we introduce a novel method to eliminate backscattering by operating the cavity at an exceptional point (EP). By engineering non-conservative coupling between degenerate clockwise (CW) and counter-clockwise (CCW) modes, we achieve chiral transmission that prevents degeneracy lifting and suppresses unwanted mode coupling. Unlike previous approaches that rely on precise gain-loss balance or complex structures, our method utilizes non-conservative coupling between the counterpropgating cavity modes. Using this method, we further show significant enhancement in the cavity performance in Floquet mode conversion efficiency at the EP. Our highly adaptable approach enables seamless integration into various photonic platforms with electro-optic modulators. This advancement mitigates backscattering and improves the precision of light-matter interactions, offering promising applications in quantum communication and information processing., Comment: 5 pages, 4 figures, Letter

Achieving oscillations with small dimensions, high power, high coherence, and low phase noise has been a long-standing goal in wave physics, driving innovations across classical electromagnetic theory and quantum physics. Key applications include electronic oscillators, lasers, and spin-torque oscillations. In recent decades, physicists have increasingly focused on harnessing passive oscillatory modes to manipulate these oscillations, leading to the development of diverse gain-loss coupled systems, including photon-photon, exciton-photon, photon-magnon, magnon-phonon, and magnon-magnon couplings. This review provides a comprehensive overview of these systems, exploring their fundamental physical structures, key experimental observations, and theoretical insights. By synthesizing insights from these studies, we propose future research directions to further advance the understanding and application of gain-loss coupled systems for quantum science and quantum technologies. (The field of gain-loss coupled systems is vast. The authors welcome suggestions and feedback from the community to continuously improve this review article until it is published)., Comment: 20 pages, 10 figures

Fano resonances in photonics arise from the coupling and interference between two resonant modes in structures with broken symmetry. They feature an uneven and narrow and tunable lineshape, and are ideally suited for optical spectroscopy. Many Fano resonance structures have been suggested in nanophotonics over the last ten years, but reconfigurability and tailored design remain challenging. Herein, we propose an all-optical pick-and-place approach aimed at assemble Fano metamolecules of various geometries and compositions in a reconfigurable manner. We study their coupling behavior by in-situ dark-field scattering spectroscopy. Driven by a light-directed opto-thermoelectric field, silicon nanoparticles with high quality-factor Mie resonances (discrete states) and low-loss BaTiO3 nanoparticles (continuum states) are assembled into all-dielectric heterodimers, where distinct Fano resonances are observed. The Fano parameter can be adjusted by changing the resonant frequency of the discrete states or the light polarization. We also show tunable coupling strength and multiple Fano resonances by altering the number of continuum states and discrete states in dielectric heterooligomers. Our work offers a general design rule for Fano resonance and an all-optical platform for controlling Fano coupling on demand.

Recent research in light scattering has prompted a re-evaluation of complex quantities, particularly in the context of complex frequency signals, which exhibit exponential growth or decay unlike traditional harmonic signals. We introduce a novel approach using complex frequency signals to reveal hidden or invisible poles--those with predominantly imaginary components--previously undetected in conventional scattering experiments. By employing a carefully tuned complex frequency excitation method, we demonstrate the efficient conversion of non-oscillating fields into oscillating ones. This effect is shown in both RF and optical domains, specifically within the C-band infrared spectral range, which is crucial for communications. This study enhances the theoretical framework of wave interactions in photonic systems, paving the way for innovative applications in invisibility cloaking, advanced photonic devices, and the future of optical communication and quantum computing.

In experimental physics, it is essential to understand electromagnetic (EM) wave scattering across EM spectrum, from radio waves to X-rays, and is pivotal in driving photonics innovations. Recent advancements have uncovered phenomena like bound states in the continuum (BICs) and parity-time (PT) symmetric systems, which are closely associated with the characteristics of the scattering matrix and are governed by passivity and causality. The emergence of complex frequency excitations has transcended the constraints imposed by passivity and causality in a system, revealing effects such as virtual critical coupling and virtual gain. However, applying the concepts of complex frequency excitation in more complicated systems remains challenging. In this work, we demonstrate the extension of the lumped element model of circuit theory to the analysis of anomalies in light scattering in the complex frequency domain. We demonstrate that the circuit model approach can facilitate design and analysis of effects such as virtual perfect absorption, BICs, real and virtual critical coupling, exceptional points, and anisotropic transmission resonances (ATRs). These findings broaden comprehension of EM wave phenomena and pave the way for significant advancements in photonics, offering new methods for designing and optimizing optical devices and systems with broad-ranging applications.

Twistronics, originally conceptualized within the electronics domain to modulate electronic properties through the twist angle between stacked two-dimensional (2D) materials, presents a groundbreaking approach in material science. This concept's extension to photonics, especially using metasurfaces, offers a promising avenue for manipulating light at subwavelength scales across a broad electromagnetic spectrum. Nevertheless, the possibilities that twistronics presents within the realm of photonics are still significantly untapped. In this work, we explore a photonic configuration consisting of a dual-layer wire metasurface operating within the microwave frequency spectrum, where the interlayer twist angle governs the system. Our findings reveal that such manipulation significantly alters the electromagnetic response of the structure, leading to enhanced resonances, tunability, and mode coupling. This twist-induced modulation results in a resonance shift towards the low-frequency range, effectively miniaturizing the structure's electrical dimension. Our study demonstrates the feasibility of integrating twistronics into photonic designs and opens new pathways for developing tunable and reconfigurable photonic devices.

Active tuning of the scattering of particles and metasurfaces is a highly sought-after property for a host of electromagnetic and photonic applications, but it normally requires challenging-to-control tunable (reconfigurable) or active (gain) media. Here, we introduce the concepts of anisotropic virtual gain and oblique Kerker effect, where a completely lossy anisotropic medium behaves exactly as its anisotropic gain counterpart upon excitation by a synthetic complex-frequency wave. The strategy allows one to largely tune the magnitude and angle of a particle's scattering simply by changing the shape (envelope) of the incoming radiation, rather than by an involved medium-tuning mechanism. The so-attained anisotropic virtual gain enables directional super-scattering at an oblique direction with fine-management of the scattering angle. Our study, opening a unique light-management method, is based on analytical techniques that allow multipolar decomposition of the scattered field, and is found, throughout, to be in excellent agreement with full-wave simulations., Comment: 6 pages, 4 figures

Plasma ignition is critical in various scientific and industrial applications, demanding an efficient and robust execution mechanism. In this work, we present an innovative approach to plasma ignition by incorporating the analysis of fundamental aspects of light scattering in the complex frequency plane. For the first time, we demonstrate the high-power virtual perfect absorption (VPA) regime, a groundbreaking method for perfectly capturing light within a resonator. By carefully designing the temporal profile of the incident wave, we effectively minimize reflections during the ignition stages, thereby significantly enhancing the efficiency and resilience of the process. Through comprehensive experimental investigations, we validate the viability of this approach, establishing VPA as a powerful tool for reflectionless excitation and optimal control of plasma discharge. By addressing the limitations of conventional plasma ignition methods, this research represents a pivotal step towards transformative advancements in plasma technology, with promising implications for improving the performance and sustainability of numerous applications.

Superconducting microwave cavities with ultra-high Q-factors are revolutionizing the field of quantum computing, offering long coherence times exceeding 1 ms, which is critical for realizing scalable multi-qubit quantum systems with low error rates. In this work, we provide an in-depth analysis of recent advances in ultra-high Q-factor cavities, integration of Josephson junction-based qubits, and bosonic-encoded qubits in 3D cavities. We examine the sources of quantum state dephasing caused by damping and noise mechanisms in cavities and qubits, highlighting the critical challenges that need to be addressed to achieve even higher coherence times. We critically survey the latest progress made in implementing single 3D qubits using superconducting materials, normal metals, and multi-qubit and multi-state quantum systems. Our work sheds light on the promising future of this research area, including novel materials for cavities and qubits, modes with nontrivial topological properties, error correction techniques for bosonic qubits, and new light-matter interaction effects.

Weyl semimetals hold great promise in revolutionizing nonreciprocal optical components due to their unique topological properties. By exhibiting nonreciprocal magneto-optical effects without necessitating an external magnetic field, these materials offer remarkable miniaturization opportunities and reduced energy consumption. However, their intrinsic topological robustness poses a challenge for applications demanding tunability. In this work, we introduce an innovative approach to enhance the tunability of their response, utilizing multilayered configurations of twisted anisotropic Weyl semimetals. Our design enables controlled and reversible isolation by adjusting the twist angle between the anisotropic layers. When implemented in the Faraday geometry within the mid-IR frequency range, our design delivers impressive isolation, exceeding 50 dB, while maintaining a minimal insertion loss of just 0.33 dB. Moreover, the in-plane anisotropy of Weyl semimetals eliminates one or both polarizers of a conventional isolator geometry, significantly reducing the overall dimensions. These results set the stage for creating highly adaptable, ultra-compact optical isolators that can propel the fields of integrated photonics and quantum technology applications to new heights.

The field of thermal emission engineering shows great potential for various applications, such as lighting, energy harvesting, and imaging, using natural or artificial structures. However, existing structures face challenges in fabrication or do not provide the necessary degree of control over key parameters such as emission intensity, spectral composition, and angular distribution. To address these limitations, we propose a novel approach that leverages in-plane hyperbolic response, embedded eigenstates enabled by epsilon-near-zero, and exceptional tunability through twisting in {\alpha}-MoO3 heterostructures. By adjusting the twist angle, we can manipulate the system's properties, transforming it from a near-perfect reflector to a perfect absorber. This enables us to exert control over thermal emission power, spanning an order of magnitude. Furthermore, our research has uncovered a significant angular dependence of thermal emission, which varies with relative rotation.

Magneto-optical isolators and circulators have been widely used to safeguard quantum devices from reflections and noise in the readout stage. However, these devices have limited bandwidth, low tunability, are bulky, and suffer from high losses, making them incompatible with planar technologies such as circuit QED. To address these limitations, we propose a new approach to quantum non-reciprocity that utilizes the intrinsic nonlinearity of qubits and broken spatial symmetry. We show that a circuit containing Lorentz-type qubits can be transformed into Fano-type qubits with an asymmetric spectral response, resulting in a significant improvement in isolation (up to 40 dB) and a twofold increase in spectral bandwidth (up to 200 MHz). Our analysis is based on realistic circuit parameters, validated by existing experimental results, and supported by rigorous quantum simulations. This approach could enable the development of compact, high-performance, and planar-compatible non-reciprocal quantum devices with potential applications in quantum computing, communication, and sensing.

One of the most significant breakthroughs in physics of the last decade has been the discovery that materials with non-trivial topological properties for electronic, electromagnetic, acoustic and mechanical responses can be designed and manufactured at our will through engineered metamaterials (MMs). Here, we review the foundation and the state-of-the-art advances of topological photonics, acoustics and mechanical MMs. We discuss how topological MMs enable nontrivial wave phenomena in physics, engineering, of great interest for a broad range of interdisciplinary science disciplines such as classical and quantum chemistry. We first introduce the foundations of topological materials and the main concepts behind their peculiar features, including the concepts of topological charge and geometric phase. We then discuss the topology of electronic band structures in natural topological materials, like topological insulators and gapless Dirac and Weyl semimetals. Based on these concepts, we review the concept, design and response of topologically nontrivial MMs in photonics and phononics, including topological phases in 2D MMs with and without time-reversal symmetry, Floquet TIs based on spatial and temporal modulation, topological phases in 3D MMs, higher-order topological phases in MMs, non-Hermitian and nonlinear topological MMs and the topological features of scattering anomalies. We also discuss the topological properties emerging in other related contexts, such as the topological aspects of chemical reactions and polaritons. This survey aims at connecting the recent advances in a broad range of scientific areas associated with topological concepts, and highlights opportunities offered by topological MMs for the chemistry community at large.

Light scattering is one of the most established wave phenomena in optics, lying at the heart of light-matter interactions and of crucial importance for nanophotonic applications. Passivity, causality and energy conservation imply strict bounds on the degree of control over scattering from small particles, with implications on the performance of many optical devices. Here, we demonstrate that these bounds can be surpassed by considering excitations at complex frequencies, yielding extreme scattering responses as tailored nanoparticles reach a quasi-steady-state regime. These mechanisms can be used to engineer light scattering of nanostructures beyond conventional limits for noninvasive sensing, imaging, and nanoscale light manipulation.

Leveraging topological properties in the response of electromagnetic systems can greatly enhance their potential. Although the investigation of singularity-based electromagnetics and non-Hermitian electronics has considerably increased in recent years in the context of various scattering anomalies, their topological properties have not been fully assessed. In this work, we theoretically and experimentally demonstrate that non-Hermitian perturbations around bound states in the continuum can lead to singularities of the scattering matrix, which are topologically nontrivial and comply with charge conservation. The associated scattering matrix poles, zeros and pole-zero pairs delineate extreme scattering events, including lasing, coherent perfect absorption, and absorber-lasers. The presented framework enables a recipe for generation, annihilation and addition of these singularities in electric circuits, with potential for extreme scattering engineering across a broad range of the electromagnetic spectrum for sensing, wireless power transfer, lasing and thermal emission devices.

Controlling the electromagnetic properties of materials beyond those achievable with natural substances has become a reality with the advent of metamaterials, artificially designed materials offering a wide range of unusual physical phenomena. The extreme properties that metamaterials provide can protect optical and electromagnetic systems from surrounding ordinary materials and substances - a feature never explicitly used yet. Wireless energy transfer, i.e., the transmission of electromagnetic energy without physical connectors, demands reliable and stable solutions for charging high-power systems like electric vehicles with no effect on people, animals, plants, etc. Here we tackle this challenging problem and suggest a novel approach of using metamaterials with extreme parameters to enable targeted and protected wireless energy transfer. We design and experimentally implement epsilon-near-zero (ENZ) and epsilon-and-mu-near-zero (EMNZ) metamaterials that provide an energy transmission if and only if both the transmitter and the receiver are equipped with these metamaterials. The fact of absence of materials with such extreme parameters protects the system against surrounding objects, which cause neither noticeable change in the system operation nor experience any detrimental effect. The system behind the proposed approach can be realised in virtually any frequency band by appropriate scaling and suitable choice of material. This technology will find applications in targeted wireless energy transfer systems, especially where high power is needed, including electric vehicles.

Light emitters are bound to strongly interact with light through enhanced absorption and scattering, which imposes limitations on the design and performance of photonic devices such as solar cells, nanoantennas, and (nano) lasers. Overcoming these limitations forces the use of ineffective nonreciprocity approaches or separation of radiation and scattering in the frequency or time domain. A design that combines the properties of an efficient emitter in one state and the property of being invisible in another state is vital for various applications. In this work, we propose a nanolaser design based on a semiconductor nanoparticle with gain coated by a phase transition material (Sb2S3), switchable between lasing and cloaking (nonscattering) states at the same operating frequency without change in pumping. The operation characteristics of the nanolaser are rigorously investigated. The designed nanolaser can operate with optical or electric pumping and possesses attributes of a thresholdless laser due to the high beta-factor and strong Purcell enhancement in the strongly confined Mie resonance mode. We design a reconfigurable metasurface composed of lasing-cloaking metaatoms that can switch from lasing to a nonscattering state in a reversible manner.

Structural colors generated due to light scattering from static all-dielectric metasurfaces have successfully enabled high-resolution, high-saturation, and wide-gamut color printing applications. Despite recent advances, most demonstrations of these structure-dependent colors lack post-fabrication tunability. This hinders their applicability for front-end dynamic display technologies. Phase-change materials (PCMs), with significant contrast of their optical properties between their amorphous and crystalline states, have demonstrated promising potentials in reconfigurable nanophotonics. Herein, we leverage tunable all-dielectric reflective metasurfaces made of newly emerged classes of low-loss optical PCMs, i.e., antimony trisulphide (Sb$_2$S$_3$) and antimony triselenide (Sb$_2$Se$_3$), with superb characteristics to realize switchable, high-saturation, high-efficiency and high-resolution dynamic meta-pixels. Exploiting polarization-sensitive building blocks, the presented meta-pixel can generate two different colors when illuminated by either one of two orthogonally polarized incident beams. Such degrees of freedom (i.e., material phase and polarization state) enable a single reconfigurable metasurface with fixed geometrical parameters to generate four distinct wide-gamut colors. We experimentally demonstrate, for the first time, an electrically-driven micro-scale display through the integration of phase-change metasurfaces with an on-chip heater formed by transparent conductive oxide. Our experimental findings enable a versatile platform suitable for a wide range of applications, including tunable full-color printing, enhanced dynamic displays, information encryption, and anti-counterfeiting., Comment: arXiv admin note: substantial text overlap with arXiv:2105.01313

The rapid development of chargeable devices has caused a great deal of interest in efficient and stable wireless power transfer (WPT) solutions. Most conventional WPT technologies exploit outdated electromagnetic field control methods proposed in the 20th century, wherein some essential parameters are sacrificed in favour of the other ones (efficiency vs. stability), making available WPT systems far from the optimal ones. Over the last few years, the development of novel approaches to electromagnetic field manipulation has enabled many up-and-coming technologies holding great promises for advanced WPT. Examples include coherent perfect absorption, exceptional points in non-Hermitian systems, non-radiating states and anapoles, advanced artificial materials and metastructures. This work overviews the recent achievements in novel physical effects and materials for advanced WPT. We provide a consistent analysis of existing technologies, their pros and cons, and attempt to envision possible perspectives.

Photonics and optoelectronics are at the foundations of widespread technologies, from high-speed Internet to systems for artificial intelligence, automotive LiDAR, and optical quantum computing. Light enables ultrafast speeds and low energy for all-optical information processing and transport, especially when confined at the nanoscale level, at which the interactions of light with matter unveil new phenomena, and the role of local symmetries becomes crucial. In this Perspective, we discuss how symmetry violations provide unique opportunities for nanophotonics, tailoring wave interactions in nanostructures for a wide range of functionalities. We discuss geometrical broken symmetries for localized surface polaritons, the physics of moir\'e photonics, in-plane inversion symmetry breaking for valleytronics and nonradiative state control, time-reversal symmetry breaking for optical nonreciprocity, and parity-time symmetry breaking. Overall, our Perspective aims at presenting under a unified umbrella the role of symmetry breaking in controlling nanoscale light, and its widespread applications for optical technology.

Structural colors generated due to light scattering from static all-dielectric metasurfaces have successfully enabled high-resolution, high-saturation and wide-gamut color printing applications. Despite recent advances, most demonstrations of these structure-dependent colors lack post-fabrication tunability that hinders their applicability for front-end dynamic display technologies. Phase-change materials (PCMs), with significant contrast of their optical properties between their amorphous and crystalline states, have demonstrated promising potentials in reconfigurable nanophotonics. Herein, we leverage a tunable all-dielectric reflective metasurface made of a newly emerged class of low-loss optical PCMs with superb characteristics, i.e., antimony trisulphide (Sb$_2$S$_3$), antimony triselenide (Sb$_2$Se$_3$), and binary germanium-doped selenide (GeSe$_3$), to realize switchable, high-saturation, high-efficiency and high-resolution structural colors. Having polarization sensitive building blocks, the presented metasurface can generate two different colors when illuminated by two orthogonally polarized incident beams. Such degrees of freedom (i.e., structural state and polarization) enable a single reconfigurable metasurface with fixed geometrical parameters to generate four distinct wide-gamut colors suitable for a wide range of applications, including tunable full-color printing and displays, information encryption, and anti-counterfeiting.

Despite recent advances in active metaoptics, wide dynamic range combined with high-speed reconfigurable solutions is still elusive. Phase-change materials (PCMs) offer a compelling platform for metasurface optical elements, owing to the large index contrast and fast yet stable phase transition properties. Here, we experimentally demonstrate an in situ electrically-driven reprogrammable metasurface by harnessing the unique properties of a phase-change chalcogenide alloy, Ge$_{2}$Sb$_{2}$Te$_{5}$ (GST), in order to realize fast, non-volatile, reversible, multilevel, and pronounced optical modulation in the near-infrared spectral range. Co-optimized through a multiphysics analysis, we integrate an efficient heterostructure resistive microheater that indirectly heats and transforms the embedded GST film without compromising the optical performance of the metasurface even after several reversible phase transitions. A hybrid plasmonic-PCM meta-switch with a record electrical modulation of the reflectance over eleven-fold (an absolute reflectance contrast reaching 80%), unprecedented quasi-continuous spectral tuning over 250 nm, and switching speed that can potentially reach a few kHz is presented. Our work represents a significant step towards the development of fully integrable dynamic metasurfaces and their potential for beamforming applications.

Optical skyrmions have recently been constructed by tailoring electric or spin field distributions through the interference of multiple surface plasmon polaritons, offering promising features for advanced information processing, transport and storage. Here, we construct topologically robust plasmonic skyrmions in a wisely tailored space-coiling meta-structure supporting magnetic localized spoof plasmons (LSPs), which are strongly squeezed down to {\lambda}3/106 and do not require stringent external interference conditions. By directly measuring the spatial profile of all three vectorial magnetic fields, we reveal multiple {\pi}-twist target skyrmion configurations mapped to multi-resonant near-equidistant LSP eigen-modes. The real-space topological robustness of these skyrmion configurations is confirmed by arbitrary deformations of the meta-structure, demonstrating flexible skyrmionic textures with arbitrary shapes. The observed magnetic LSP skyrmions pave the way to ultra-compact and topologically robust plasmonic devices, such as flexible sensors, wearable electronics and ultra-compact antennas.

Two dimensional (2D) transition metal dichalcogenide (TMDC) materials, such as MoS2, WS2, MoSe2, and WSe2, have received extensive attention in the past decade due to their extraordinary physical properties. The unique properties make them become ideal materials for various electronic, photonic and optoelectronic devices. However, their performance is limited by the relatively weak light-matter interactions due to their atomically thin form factor. Resonant nanophotonic structures provide a viable way to address this issue and enhance light-matter interactions in 2D TMDCs. Here, we provide an overview of this research area, showcasing relevant applications, including exotic light emission, absorption and scattering features. We start by overviewing the concept of excitons in 1L-TMDC and the fundamental theory of cavity-enhanced emission, followed by a discussion on the recent progress of enhanced light emission, strong coupling and valleytronics. The atomically thin nature of 1L-TMDC enables a broad range of ways to tune its electric and optical properties. Thus, we continue by reviewing advances in TMDC-based tunable photonic devices. Next, we survey the recent progress in enhanced light absorption over narrow and broad bandwidths using 1L or few-layer TMDCs, and their applications for photovoltaics and photodetectors. We also review recent efforts of engineering light scattering, e.g., inducing Fano resonances, wavefront engineering in 1L or few-layer TMDCs by either integrating resonant structures, such as plasmonic/Mie resonant metasurfaces, or directly patterning monolayer/few layers TMDCs. We then overview the intriguing physical properties of different types of van der Waals heterostructures, and their applications in optoelectronic and photonic devices. Finally, we draw our opinion on potential opportunities and challenges in this rapidly developing field of research., Comment: 101 pages, 25 figures

The physics of systems that cannot be described by a Hermitian Hamiltonian, has been attracting a great deal of attention in recent years, motivated by their nontrivial responses and by a plethora of applications for sensing, lasing, energy transfer/harvesting, topology and quantum networks. Electromagnetics is an inherently non-Hermitian research area because all materials are lossy, loss and gain distributions can be controlled with various mechanisms, and the underlying systems are open to radiation. Therefore, the recent developments in non-Hermitian physics offer exciting opportunities for a broad range of basic research and engineering applications relevant to the antennas and propagation community. In this work, we offer a tutorial geared at introducing the unusual electromagnetic phenomena emerging in non-Hermitian systems, with particular emphasis on a sub-class of these systems that obey parity-time (PT) symmetry. We discuss the basic concepts behind this topic and explore their implications for various phenomena. We first discuss the basic features of P, T and PT operators applied to electromagnetic and quantum mechanical phenomena. We then discuss the exotic response of PT-symmetric electromagnetic structures and their opportunities, with particular attention to singularities, known as exceptional points, emerging in these systems, and their unusual scattering response., Comment: To be submitted to IEEE journal

Epsilon-near-zero and epsilon near-pole materials enable reflective systems supporting a class of symmetry-protected and accidental embedded eigenstates (EE) characterized by a diverging phase-resonance. Here we show that pairs of topologically protected scattering singularities necessarily emerge from EEs when a non-Hermitian parameter is introduced, lifting the degeneracy between oppositely charged singularities. The underlying topological charges are characterized by an integer winding number and appear as phase vortices of the complex reflection coefficient. By creating and annihilating them, we show that these singularities obey charge conservation, and provide versatile control of amplitude, phase and polarization in reflection, with potential applications for polarization control and sensing.

Electromagnetic embedded eigenstates, also known as bound states in the continuum (BICs), hold a great potential for applications in sensing, lasing, enhanced nonlinearities and energy harvesting. However, their demonstrations so far have been limited to large-area periodic arrays of suitably tailored elements, with fundamental restrictions on the overall footprint and performance in presence of inevitable disorder. In this work, we demonstrate a BIC localized in a single subwavelength resonator obtained by suitably tailoring the boundaries around it, enabling a new degree of control for on-demand symmetry breaking. We experimentally demonstrate how boundary-induced BICs open exciting opportunities for sensing by tracing the dissolution of NaCl in water, determining evaporation rates of distilled and saltwater with a resolution of less than 1e-6 L using a tabletop experimental setup.

Modern photonic and quantum technologies demand reciprocity breaking, e.g., isolators, full-duplex systems, noise isolation in quantum computers, motivating searching for practical approaches beyond magnet-based devices. This work overviews the up-and-coming advances in optical nonreciprocity, including new materials (Weyl semimetals, topological insulators, metasurfaces), active structures, time-modulation, PT-symmetry breaking, nonlinearity, quantum nonlinearity, unidirectional gain and loss, chiral quantum states, and valley polarization.

Coupling emitters with nanoresonators is an effective strategy to control light emission at the subwavelength scale with high efficiency. Low-loss dielectric nanoantennas hold particular promise for this purpose, owing to their strong Mie resonances. Herein, we explore a highly miniaturized platform for the control of emission based on individual subwavelength Si nanospheres (SiNSs) to modulate the directional excitation and exciton emission of two-dimensional transition metal dichalcogenides (2D TMDs). A modified Mie theory for dipole-sphere hybrid systems is derived to instruct the optimal design for desirable modulation performance. Controllable forward-to-backward intensity ratios are experimentally validated in 532 nm laser excitation and 635 nm exciton emission from a monolayer WS2. Versatile light emission control along all device orientations is achieved for different emitters and excitation wavelengths, benefiting from the facile size control and isotropic shape of SiNSs. Simultaneous modulation of excitation and emission via a single SiNS at visible wavelengths significantly improves the efficiency and directivity of TMD exciton emission and leads to the potential of multifunctional integrated photonics. Overall, our work opens promising opportunities for nanophotonics and polaritonic systems, enabling efficient manipulation, enhancement and reconfigurability of light-matter interactions.

Optical supercavity modes (superstates), i.e., hybrid modes emerging from the strong coupling of two nonorthogonal modes of an open cavity, can support ultranarrow lines in scattering spectra associated with quasi bound states in the continuum (quasi-BIC). These modes are of great interest for sensing applications as they enable compact systems with unprecedented sensitivity. However, these quasi-BIC sensors obey the shot-noise limit, which may be overcome only in quantum sensors. Here, we unveil that a three-level quantum system (e.g., atom, quantum dot, superconducting qubit) can be tailored to support the quantum analog of an embedded superstate with an unboundedly narrow emission line in the strong coupling regime. Remarkably, we demonstrate that the coupling of such a system with a cavity (e.g., plasmonic or dielectric nanoparticle, microcavity, microwave resonator) enables sensing properties with significantly reduced noise. Our results can be applied to a plethora of quantum platforms from microwave superconductors to cold atoms and quantum dots, opening interesting opportunities for quantum sensing and computing.

Optical nanoantennas, i.e., elements transforming localized light or waveguide modes into freely propagating fields and vice versa, are vital components for modern nanophotonics. Optical antennas have been demonstrated to cause the Dicke superradiance effect, i.e., collective spontaneous emission of quantum sources. However, the impact of coherent excitation on the antenna performance, such as directivity, efficiency, and Purcell effect, remains mostly unexplored. Herein, using full-wave numerical simulations backed by a quantum model, we unveil that coherent excitation allows controlling antenna multipoles, on-demand excitation of nonradiative states, enhanced directivity and improves antenna radiation efficiency. This collective excitation corresponds to the states with nonzero dipole moment in the quantum picture, where the quantum phase is well defined. The results of this work bring another degree of freedom - the collective phase of an ensemble of quantum emitters - to control optical nanoantennas and, as such, pave the way to the use of collective excitations for nanophotonic devices with superb performance. To make the discussion independent of the frequency range, we consider the all-dielectric design and use dimensionless units., Comment: arXiv admin note: substantial text overlap with arXiv:1909.06010

Twisted two-dimensional bi-layers offer exquisite control on the electronic bandstructure through the interlayer rotation and coupling, enabling magic-angle flat-band superconductivity and moir\'e excitons. Here, we demonstrate how analogous principles, combined with large anisotropy, enable extreme control and manipulation of the photonic dispersion of phonon polaritons (PhPs) in van der Waals (vdW) bi-layers. We experimentally observe tunable topological transitions from open (hyperbolic) to closed (elliptic) dispersion contours in twisted bi-layered {\alpha}-MoO3 at photonic magic angles, induced by polariton hybridization and robustly controlled by a topological quantity. At these transitions the bilayer dispersion flattens, exhibiting low-loss tunable polariton canalization and diffractionless propagation with resolution below {\lambda}0/40. Our findings extend twistronics and moir\'e physics to nanophotonics and polaritonics, with great potential for nano-imaging, nanoscale light propagation, energy transfer and quantum applications., Comment: Nature, in press

Embedded eigenstates are nonradiative modes of an open structure with momentum compatible with radiation, yet characterized by unboundedly large Q-factors. Traditionally, these states originate from total destructive interference of radiation from two or more non-orthogonal modes in periodic structures. In this work, we demonstrate a novel class of embedded eigenstates based on Berreman modes in epsilon-near-zero (ENZ) layered materials and propose realistic silicon carbide (SiC) structures supporting high-Q (~10^3) resonances based on these principles. The proposed structures demonstrate strong absorption in a narrow spectral and angular range, giving rise to quasi-coherent and highly directive thermal emission.

Electromagnetic resonators are a versatile platform to harvest, filter and trap electromagnetic energy, at the basis of many applications from microwaves to optics. Resonators with a large intrinsic quality factor (Q) are highly desirable since they can store a large amount of energy, leading to sharp filtering and low loss. Exciting high-Q cavities with monochromatic signals, however, suffers from poor excitation efficiency, i.e., most of the impinging energy is lost in the form of reflection, since high-Q resonators are weakly coupled to external radiation. Although critical coupling eliminates reflections in steady-state by matching the intrinsic and coupling decay rates, this approach requires the introduction of loss in the resonator, causing dissipation and lowering the overall Q-factor. Here, we extend the notion of critical coupling to high-Q lossless resonators based on tailoring the temporal profile of the excitation wave. Utilizing coupled-mode theory, we demonstrate an effect analogous to critical coupling by mimicking loss with non-monochromatic excitations at complex frequencies. Remarkably, we show that this approach enables unitary excitation efficiency in open systems, even in the limit of extreme quality factors in the regime of quasi-bound states in the continuum.

Recent advances in twistronics of low-dimensional materials, such as bilayer graphene and transition-metal dichalcogenides, have enabled a plethora of unusual phenomena associated with moir\'e physics. However, several of these effects require demanding manipulation of superlattices at the atomic scale, such as the careful control of rotation angle between two closely spaced atomic lattices. Here, we study moir\'e hyperbolic plasmons in pairs of hyperbolic metasurfaces (HMTSs), unveiling analogous phenomena at the mesoscopic scale. HMTSs are known to support confined surface waves collimated towards specific directions determined by the metasurface dispersion. By rotating two evanescently coupled HMTSs with respect to one another, we unveil rich dispersion engineering, topological transitions at magic angles, broadband field canalization, and plasmon spin-Hall phenomena. These findings open remarkable opportunities to advance metasurface optics, enriching it with moir\'e physics and twistronic concepts., Comment: 17 pages, 4 figures

All-dielectric nanoantennas have recently opened exciting opportunities for functional nanophotonics, owing to their strong optical resonances along with low material loss in the near-infrared range. Pushing these concepts to the visible range is hindered by a larger absorption coefficient of Si and other high-index semiconductors, thus encouraging the search for alternative dielectrics for nanophotonics. In this paper, we employ bandgap engineering to synthesize hydrogenated amorphous Si nanoparticles (a-Si:H NPs) offering ideal features for functional nanophotonics. We observe significant material loss suppression in a-Si:H NPs in the visible range caused by hydrogenation-induced bandgap renormalization, producing resonant modes in single a-Si:H NPs with Q factors up to ~100, in the visible and near-IR range for the first time. In order to demonstrate light-matter interaction enhancement, we realize highly tunable all-dielectric nanoantennas coupling them to photochromic spiropyran (SP) molecules. We show ~70% reversible all-optical tuning of light scattering at the high-Q resonant mode, along with minor tunability when out of resonance. This remarkable all-optical tuning effect is achieved under a low incident light intensity ~3.8 W/cm2 for UV light and ~1.1*10^2 W/cm2 for green light.

Recent progress in nanofabrication has led to tremendous technological developments in nanophotonics, which rely on the interaction of light with nanostructured matter. Nanophotonics has experienced a large surge of interest in recent years, from basic research to applied technology. The increased importance of extremely low-energy data processing at ultra-fast speeds has been encouraging the use of light for signal transport and processing. Energy demands and interaction time scales become smaller with the physical size of the nanostructures, hence nanophotonics opens the opportunity of integrating a large number of devices that can generate, control, modulate, sense and process light signals at ultrafast speeds and below femtojoule/bit energy levels. However, losses and diffraction pose fundamental challenges to the fundamental ability of nanophotonic structures to confine light efficiently in smaller and smaller volumes. In this framework, active nanophotonics, which combines the latest advances in nanotechnology with gain materials has become in recent years a vital area of optics research, both from the physics, material science, and engineering standpoint. In this paper, we review recent efforts in enabling active nanodevices for lasing and optical sources, loss compensation, and to realize new optical functionalities, like PT-symmetry, exceptional points and nontrivial lasing based on suitably engineered distributions of gain and loss in nanostructures., Comment: IEEE will be the copyright holder

Parity-time (PT) symmetry has been opening exciting opportunities in optics, yet the required careful balance of loss and gain has been hindering its practical implementations. Here, we propose a gain-free route to PT-symmetry based on non-monochromatic excitations that mimic loss and gain in passive systems. Based on the concept of virtual absorption, extended here to implement also virtual gain, we induce PT-symmetry and its landmark effects, such as broken phase transitions, anisotropic transmission resonances and laser-absorber pairs, in a fully passive, hence inherently stable, system. These results open a path to establish PT-symmetric phenomena in purely passive platforms., Comment: 18 pages, 4 figures, and the supplement is not included

The tremendous progress in light scattering engineering made it feasible to develop optical tweezers allowing capture, hold, and controllable displacement of submicronsize particles and biological structures. However, the momentum conservation law imposes a fundamental restriction on the optical pressure to be repulsive in paraxial fields. Although different approaches to get around this restriction have been proposed, they are rather sophisticated and rely on either wavefront engineering or utilize active media. Herein, we revisit the issue of optical forces by their analytic continuation to the complex frequency plane and considering their behavior in transient. We show that the exponential excitation at the complex frequency offers an intriguing ability to achieve a pulling force for a passive resonator of any shape and composition even in the paraxial approximation, the remarkable effect which is not reduced to the Fourier transform. The approach is linked to the virtual gain effect when an appropriate transient decay of the excitation signal makes it weaker than the outgoing signal that carries away greater energy and momentum flux density. The approach is implemented for the Fabry-Perot cavity and a high refractive index dielectric nanoparticle, a fruitful platform for intracellular spectroscopy and lab-on-a-chip technologies where the proposed technique may found unprecedented capabilities.

Antennas, i.e., elements transforming localized or waveguide modes into freely propagating fields and vice versa, are vital components for wireless technologies across the entire spectrum of electromagnetic waves, including microwaves and optics. Although optical antennas are usually fed by either a single source or many sources incoherently, recent studies demonstrate an ability of resonant nanostructures to cause the synchronization of quantum source spontaneous emission, i.e., Dicke superradiance effect. What remains poorly explored is how the coherent excitation can affect antenna performance: its multipole composition, directivity, efficiency, and Purcell effect. In this paper, we investigate an antenna excited by two coherent sources and demonstrate that this approach can boost antenna performance. To make the discussion independent of the frequency range, we restrict the consideration by all-dielectric antennas, which attract significant interest in both microwaves and optics. We explore that coherent excitation of a dielectric antenna by two dipole sources makes it feasible to control the multipoles resulting in boosted superradiance and enhanced directivity. Interestingly, it makes possible excitation of nonradiative field configuration, anapole state, and turning it on and off on demand. Remarkably, this approach also allows reducing the quenching effect and enhancing the radiation efficiency without changing the antenna geometry. We design a dielectric subwavelength coherently driven antenna operating in both superdirective and superradiative regimes simultaneously with the total enhancement factor over 2000.

Wireless power transfer (WPT) technologies are becoming ubiquitous, but unlike wire technologies, they do not allow efficient energy transfer through barely transparent barriers. The proposed solutions to this issue rely on resonantly enhanced transparency of specially designed obstacles and hence are narrowband and require using artificially tailored materials. In this work, we suggest a fundamentally different approach to wireless power transfer through such barriers based on tailoring of a coherent auxiliary wave with a certain amplitude and phase from the side where the energy should be delivered to. This wave facilitates signal transmission and reflects back to the receiver allowing archiving of 100% energy transfer efficiency even for barely transparent barriers. In contrast to the traditional solution, this approach is general, applicable to lossless and lossy barriers and does not require cost-ineffective and complicated artificial structures.

In this Letter, we discuss the general problem of exciting radiationless field distributions in open cavities, with the goal of clarifying recent findings on this topic. We point out that the radiationless scattering states, like anapoles, considered in several recent studies, are not eigenmodes of an open cavity; therefore, their external excitation is neither surprising nor challenging (similar to the excitation of nonzero internal fields in a transparent, or cloaked, object). Even more, the radiationless anapole field distribution cannot be sustained without the actual presence of external incident fields. Conversely, we prove that the Lorentz reciprocity theorem prevents the external excitation of radiationless optical eigenmodes, as in the case of embedded eigenstates and bound states in the continuum in open cavities. Our discussion clarifies the analogies and differences between invisible bodies, nonradiating sources, anapole scatterers and emitters, and embedded eigenstates, especially in relation to their external excitation.

Scattering of electromagnetic waves lies at the heart of most experimental techniques over nearly the entire electromagnetic spectrum, ranging from radio waves to optics and X-rays. Hence, deep insight into the basics of scattering theory and understanding the peculiar features of electromagnetic scattering is necessary for the correct interpretation of experimental data and an understanding of the underlying physics. Recently, a broad spectrum of exceptional scattering phenomena attainable in suitably engineered structures has been predicted and demonstrated. Examples include bound states in the continuum, exceptional points in PT-symmetrical non-Hermitian systems, coherent perfect absorption, virtual perfect absorption, nontrivial lasing, non-radiating sources, and others. In this paper, we establish a unified description of such exotic scattering phenomena and show that the origin of all these effects can be traced back to the properties of poles and zeros of the underlying scattering matrix. We provide insights on how managing these special points in the complex frequency plane provides a powerful approach to tailor unusual scattering regimes.