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Heteroepitaxial crystalline films underlie many electronic and optical technologies but are prone to forming defects at their heterointerfaces. Atomic‐scale defects such as threading dislocations that propagate into a film impede the flow of charge carriers and light degrading electrical/optical performance of devices. Diagnosis of subsurface defects traditionally requires time‐consuming invasive techniques such as cross‐sectional transmission electron microscopy. Using III–V films grown on Si, noninvasive, bench‐top diagnosis of subsurface defects have been demonstrated by optical second‐harmonic scanning probe microscope. A high‐contrast pattern is observed of subwavelength "hot spots" caused by scattering and localization of fundamental light by defect scattering sites. Size of these observed hotspots are strongly correlated to the density of dislocation defects. The results not only demonstrate a global and versatile method for diagnosing subsurface scattering sites but uniquely elucidate optical properties of disordered media. An extension to third harmonics would enable irregularities detection in non‐χ(2) materials making the technique universally applicable. [ABSTRACT FROM AUTHOR]

Heteroepitaxial crystalline films underlie many electronic and optical technologies but are prone to forming defects at their heterointerfaces. Atomic‐scale defects such as threading dislocations that propagate into a film impede the flow of charge carriers and light degrading electrical/optical performance of devices. Diagnosis of subsurface defects traditionally requires time‐consuming invasive techniques such as cross‐sectional transmission electron microscopy. Using III–V films grown on Si, noninvasive, bench‐top diagnosis of subsurface defects have been demonstrated by optical second‐harmonic scanning probe microscope. A high‐contrast pattern is observed of subwavelength "hot spots" caused by scattering and localization of fundamental light by defect scattering sites. Size of these observed hotspots are strongly correlated to the density of dislocation defects. The results not only demonstrate a global and versatile method for diagnosing subsurface scattering sites but uniquely elucidate optical properties of disordered media. An extension to third harmonics would enable irregularities detection in non‐χ(2) materials making the technique universally applicable. [ABSTRACT FROM AUTHOR]

In this work, acoustic scattering cancellation is expanded to cases of acoustic waves interacting with non-spherical elastic objects and collections of closely packed objects. The proposed method for achieving this is by using spherical fluid layers to represent the objects as effective spherical targets, with multiple fluid layers and an elastic core, for which scattering cancellation is used to determine the necessary layer properties. Three representative objects are considered here to illustrate this approach: a sphere with dimples, a sphere with bumps, and a bicone. Theoretical results are presented for each of the non-spherical objects in water, which indicate a reduction of 30-40 dB in scattering strength when coated with a two-layer fluid cancellation shell. This cancellation is achieved for objects in the resonant scattering regime, which is characterized by constructive and destructive interference patterns that arise from higher scattering modes. The analytical results are verified using three-dimensional finite element simulations. [ABSTRACT FROM AUTHOR]

Metasurfaces consisting of active, non-liner and time-varying elements [[5], [7]] offer a plethora of new opportunities for wireless communication systems, inspiring novel communication models and wireless architectures (Fig. The pace of development in the world of 5G communication systems has proven to be much more demanding than previous generations, with 5G-Advanced seemingly around the corner [[1]]. Despite these ever-growing developments and demands for wireless communications, the wireless propagation environment has so far been largely determined by nature, making it the most challenging portion of the communication system to control, and thereby requiring the end devices to adapt their communication strategies to uncontrollable and fast-changing environments. [Extracted from the article]

Nonreciprocal optical devices based on magneto-optical ferrites in their low-loss regimes have been widely investigated as a promising platform for integrated photonics. Nonreciprocity in such devices originates from circular birefringence, leading to frequency splitting of forward and backward modes and, as a result, nonreciprocal transmission. In this paper, we propose an alternative approach to realize nonreciprocal devices based on magneto-optical circular dichroism and relying on the very presence of optical absorption. Our approach relies on the phenomenon of spin-Hall effect of light, which gives rise to chiral near field interactions of light carrying transverse angular momentum with matter, which, in lossy regimes, yields a disparate absorption for forward and backward optical modes. As an example of practical application, we design an optical isolator based on ring resonator integrating Ce:YIG ferrite, and we demonstrate isolation near 880 nm absorption line due to the ionic electric dipole transition. A Ce:YIG film asymmetrically placed on the inner side of the ring yields different critical coupling conditions due to the chiral nature of evanescent light for forward and backward waves, leading to nonreciprocal absorption and transmission. The proposed approach to nonreciprocity may significantly broaden the possible choice of magneto-optical materials for nonreciprocal devices, enabling operation even in lossy regimes. [ABSTRACT FROM AUTHOR]

In analogy to Lifshitz transitions in electronic systems, topological transitions have recently attracted widespread attention in photonic metamaterials, metasurfaces, and two-dimensional materials, enabling exotic regimes for light-matter interactions. Here, we discuss and study enhanced photonic local density of states in twisted hyperbolic bilayers, enabled by topological transitions emerging at specific twist angles. Our results enhance the understanding of nanoscale light-matter interactions in stacked optical materials as they are rotated with respect to each other in the context of twistronics and suggest emerging applications of these concepts for photonics, including for the manipulation of radiative heat transfer and the control and harvesting of light at the nanoscale. [ABSTRACT FROM AUTHOR]

Quasi-bound states in the continuum (QBICs) are Fano resonant states with long optical lifetimes controlled by symmetry-breaking perturbations. While conventional Fano responses are limited to linear polarizations and do not support tailored phase control, here we introduce QBICs born of chiral perturbations that encode arbitrary elliptical polarization states and enable geometric phase engineering. We thereby design metasurfaces with ultrasharp spectral features that shape the impinging wave front with near-unity efficiency. Our findings extend Fano resonances beyond their conventional limits, opening opportunities for nanophotonics, classical and quantum optics, and acoustics. [ABSTRACT FROM AUTHOR]

Organic light emitting diodes can achieve close to unitary internal quantum efficiency; however, their external quantum efficiency is much lower due to losses within the device. Gradient metasurfaces and metagratings can be utilized to achieve substantial electromagnetic field manipulation and enhancement of the local density of photonic states, thereby improving the external quantum efficiency of organic light emitting devices. In this work, we show how suitably designed reflecting plasmonic metagratings can be potentially incorporated into a top-emitting organic light emitting device, resulting in large enhancement of light emissivity by effectively coupling out the light trapped in the waveguide modes. We describe how both periodic and quasiperiodic metagratings can be used to improve device performance. Electromagnetic simulations and measurements show that our metagratings greatly enhance the electromagnetic field intensity in the light emission layer, leading to increased emission normal to the plane of the layers (by up to a factor of 4.8) and suppression of light trapping typically lost into waveguide modes. [ABSTRACT FROM AUTHOR]

We show that tri-layer structures combining epsilon-negative and magneto-optical material layers can exhibit unidirectional resonant photon tunneling phenomena that can discriminate between circularly polarized (CP) waves of given handedness impinging from opposite directions, or between CP waves with different handedness impinging from the same direction. This physical principle, which can also be interpreted in terms of a Fabry-Perot-type resonance, may be utilized to design compact optical isolators for CP waves. Within this framework, we derive simple analytical conditions and design formulae, and quantitatively assess the isolation performance, also taking into account the unavoidable imperfections and nonidealities. [ABSTRACT FROM AUTHOR]

We have modeled, fabricated, and characterized a plasmonic metasurface with subwavelength features, whose dominant resonance is the independent of incident angle and polarization, and sensitive only to the material composition and geometry of a single element. Higher-order resonances, associated with surface plasmon polariton (SPP) coupling and higher diffraction orders, are sensitive to the incident angle and the array periodicity and less pronounced compared with the metasurface resonance. Numerical simulations and theoretical analysis highlight a clear physical difference between the SPP resonances and the dominant metasurface collective resonance, whose properties may be of great interest for plasmonic solar cells and subwavelength color filters. [ABSTRACT FROM AUTHOR]

Chu's limit determines the minimum radiation quality factor Q of an electrically small resonator, and hence its maximum operational bandwidth, which is inversely proportional to its volume. This bound imposes severe restrictions in several areas of technology, from wireless communications to nanophotonics and metamaterials. We show that a suitably tailored temporal modulation of the matching network, combined with proper detuning of the feeding impedance, can overcome this limit and enable radiation over broader bandwidths, which scale as 1/√Q, ensuring at the same time stability. Our findings open opportunities for communication systems, nanophotonics, and sensor technology. [ABSTRACT FROM AUTHOR]

Mechanical motion can break the symmetry in which sound travels in a medium, but significant nonreciprocity is typically achieved only for large motion speeds. We combine moving media with zero-index acoustic propagation, yielding extreme nonreciprocity and induced bianisotropy for modest applied speeds. The metamaterial is formed by an array of waveguides loaded by Helmholtz resonators, and it exhibits opposite signs of the refractive index sustained by asymmetric Willis coupling for propagation in opposite directions. We use this response to induce nonreciprocal positive-to-negative sound refraction, and we propose a nonreciprocal metamaterial lens focusing only with excitation from one side based on asymmetric Willis coupling. [ABSTRACT FROM AUTHOR]

Following our recently introduced analytical tools to model and design conformal mantle cloaks based on metasurfaces [Padooru et al., J. Appl. Phys. 112, 034907 (2012)], we investigate their performance and physical properties when excited by an electric line source placed in their close proximity. We consider metasurfaces formed by 2-D arrays of slotted (meshes and Jerusalem cross slots) and printed (patches and Jerusalem crosses) sub-wavelength elements. The electromagnetic scattering analysis is carried out using a rigorous analytical model, which utilizes the two-sided impedance boundary conditions at the interface of the sub-wavelength elements. It is shown that the homogenized grid-impedance expressions, originally derived for planar arrays of sub-wavelength elements and plane-wave excitation, may be successfully used to model and tailor the surface reactance of cylindrical conformal mantle cloaks illuminated by near-field sources. Our closed-form analytical results are in good agreement with full-wave numerical simulations, up to sub-wavelength distances from the metasurface, confirming that mantle cloaks may be very effective to suppress the scattering of moderately sized objects, independent of the type of excitation and point of observation. We also discuss the dual functionality of these metasurfaces to boost radiation efficiency and directivity from confined near-field sources. [ABSTRACT FROM AUTHOR]

The article presents physics research on reconfigurable Floquet elastodynamic topological insulator based on synthetic angular momentum bias. Topics discussed include originating with the discovery of the quantum Hall effect (QHE) in condensed matter physics, topological order has been receiving increased attention also for classical wave phenomena; and topological protection enables efficient and robust signal transport; mechanical topological insulators (TIs).

In this paper, the analysis of patch antennas with U-shaped metalization loaded by bianisotropic media is developed. A theoretical study is performed to obtain transmission-line equations for bianisotropic materials, and then a method-of-lines (MoL) procedure is applied to obtain the radiative characteristics. Presented results assure, for particular patch dimensions and feed application geometry, a multifrequency operation in the microwave range, even for isotropic substrates. The application of bianisotropic materials gives better performances for bandwidth and input impedance at the central frequency, but degrades other frequency operation performances. © 2002 John Wiley & Sons, Inc. Microwave Opt Technol Lett 32: 3–5, 2002. [ABSTRACT FROM AUTHOR]

Phase, polarization, amplitude, and frequency represent the basic dimensions of light, playing crucial roles for both fundamental light–material interactions and all major optical applications. Metasurfaces have emerged as a compact platform to manipulate these knobs, but previous metasurfaces have limited flexibility to simultaneous control them. A multi‐freedom metasurface that can simultaneously and independently modulate phase, polarization, and amplitude in an analytical form is introduced, and frequency multiplexing is further realized by a k‐space engineering technique. The multi‐freedom metasurface seamlessly combines geometric Pancharatnam–Berry phase and detour phase, both of which are frequency independent. As a result, it allows complex‐amplitude vectorial hologram at various frequencies based on the same design strategy, without sophisticated nanostructure searching of massive geometric parameters. Based on this principle, full‐color complex‐amplitude vectorial meta‐holograms in the visible are experimentally demonstrated with a metal–insulator–metal architecture, unlocking the long‐sought full potential of advanced light field manipulation through ultrathin metasurfaces. [ABSTRACT FROM AUTHOR]

Electromagnetic fields interacting with microscopic structural features in a composite material provide emerging optical properties that surpass those offered by the individual components. However, composite materials can be generally lossy due to the scattering effects induced by inhomogeneities at the interfaces between different compounds. To overcome such problems, complicated and costly manufacturing procedures, such as top‐down approaches, are generally required. In contrast, here ZnO–ZnWO4 eutectic self‐organized composites grown by the micropulling method are considered, displaying sharp and strongly polarized transmission at 397 nm. Such an optical response is notable because it is not observed in either ZnO or ZnWO4 single crystals. The optical response is due to the refractive index matching of the two constituents, which self‐organize into ordered structures via a micropulling down method. The optical behavior reported here can directly lead to applications, such as tunable narrowband filters with bandpass of 3 nm and polarizers, paving the way to a new self‐organization route for manufacturing optical components. [ABSTRACT FROM AUTHOR]

Phonon polaritons provide useful opportunities, complementary to those provided by plasmon polaritons, in the study of the interaction of light with matter at small scales. The focus of this review is on phonon polaritons in low‐dimensional van der Waals (vdW) materials and heterostructures. Phonon polaritons confined in vdW materials exhibit large electromagnetic localization and are easy to hybridize with other collective modes. The extreme optical anisotropy in vdW systems produces the natural hyperbolic dispersion, enabling the access to deep subdiffractional optics and often yielding improved figures of merit over hyperbolic metamaterials. These virtues hold promises for practical nanophotonic applications, including optical sensing, super‐resolution imaging, energy and emission engineering, quantum optics, and a next generation of optical circuit elements. [ABSTRACT FROM AUTHOR]

Lasers and light emitters do not typically radiate fields with orbital angular momentum (OAM). Here we show that a suitable scheme of spatiotemporal modulation of a microring cavity laser can impart a synthetic angular momentum, resulting in beams with well-defined OAM. The phenomenon relies on a traveling wave modulation of the refractive index of the microring, which breaks the degeneracy of oppositely oriented whispering gallery modes. In parallel, a static structural grating on the periphery of the microring enables efficient vertical radiation. The proposed structure is inherently tunable and can also emit fields with zero net OAM while retaining toroidal energy distributions similar to the effect of an axicon lens. [ABSTRACT FROM AUTHOR]

Commercially available nonreciprocal devices, such as isolators and circulators, play a fundamental role in communication systems. Since they commonly rely on magnetic materials, they tend to become bulky, expensive, and difficult to be integrated in conventional microelectronic circuits. Here, we explore the functionality of a magnetic-free circulator where reciprocity is broken by suitable geometric asymmetries combined with tailored nonlinearities. We show that it is possible to operate a fully passive coupled resonator system without external bias like a circulator for pulsed signals impinging at its ports within a desired range of intensities. The functionality can be applied to a variety of physical systems, ranging from electronics to photonics and acoustics. [ABSTRACT FROM AUTHOR]

Chirality is a ubiquitous phenomenon in the natural world. Many biomolecules without inversion symmetry, such as amino acids and sugars, are chiral molecules. Measuring and controlling molecular chirality at a high precision down to the atomic scale that are highly desired in physics, chemistry, biology, and medicine, however, have remained challenging. Herein, we achieve all-optical reconfigurable chiral meta-molecules experimentally using metallic and dielectric colloidal particles as artificial atoms or building blocks to serve at least two purposes. One is that the on-demand meta-molecules with strongly enhanced optical chirality are well-suited as substrates for surface-enhanced chiroptical spectroscopy of chiral molecules and as active components in optofluidic and nanophotonic devices. The other is that the bottom-up-assembled colloidal meta-molecules provide microscopic models to better understand the origin of chirality in the actual atomic and molecular systems. [ABSTRACT FROM AUTHOR]

Various concepts related to parity-time symmetry, including anti–parity-time symmetry, have found broad applications in wave physics. Wave systems are fundamentally described by Hermitian operators, whereas their unusual properties are introduced by incorporation of gain and loss. We propose that the related physics need not be restricted to wave dynamics, and we consider systems described by diffusive dynamics. We study the heat transfer in two countermoving media and show that this system exhibits anti–parity-time symmetry. The spontaneous symmetry breaking results in a phase transition from motionless temperature profiles, despite the mechanical motion of the background, to moving temperature profiles. Our results extend the concepts of parity-time symmetry beyond wave physics and may offer opportunities to manipulate heat and mass transport. [ABSTRACT FROM AUTHOR]

Optical non-linear phenomena are typically observed in natural materials interacting with light at high intensities, and they benefit a diverse range of applications from communication to sensing. However, controlling harmonic conversion with high efficiency and flexibility remains a major issue in modern optical and radio-frequency systems. Here, we introduce a dynamic time-domain digital-coding metasurface that enables efficient manipulation of spectral harmonic distribution. By dynamically modulating the local phase of the surface reflectivity, we achieve accurate control of different harmonics in a highly programmable and dynamic fashion, enabling unusual responses, such as velocity illusion. As a relevant application, we propose and realize a novel architecture for wireless communication systems based on the time-domain digital-coding metasurface, which largely simplifies the architecture of modern communication systems, at the same time yielding excellent performance for real-time signal transmission. The presented work, from new concept to new system, opens new pathways in the application of metamaterials to practical technology. [ABSTRACT FROM AUTHOR]

A new class of phenomena stemming from topological states of quantum matter has recently found a variety of analogies in classical systems. Spin-locking and one-way propagation have been shown to drastically alter scattering of electromagnetic waves, thus offering an unprecedented robustness to defects and disorder. Despite these successes, bringing these new ideas to practical grounds meets a number of serious limitations. In photonics, when it is crucial to implement topological photonic devices on a chip, two major challenges are associated with electromagnetic dissipation into heat and out-of-plane radiation into free space. Both these mechanisms may destroy the topological state and seriously affect the device performance. Here, we demonstrate experimentally that the topological order for light can be implemented in all-dielectric on-chip prototype metasurfaces, which mitigate the effect of Ohmic losses by using exclusively structured dielectric materials, and we reveal that coupling of the system to the radiative continuum does not affect topological properties. We demonstrate the spin-Hall effect of light for spin-polarized topological edge states through near-field spectroscopy measurements. [ABSTRACT FROM AUTHOR]

The Aharonov-Bohm effect is a well-established quantum phenomenon that relates the behaviour of an electron not only to the local electromagnetic field but also to the associated potentials. An important consequence is that electron beams travelling through an arbitrary medium can Fcarry information not only about the properties of the materials along with their trajectories but also about those of the entire background around them. Based on this established principle, the authors propose an inverse formulation for the estimation of a sample's magnetic permeability based on measurements of the wave function of a split electron beam. For one-dimensional distributions, the analysed concept is shown to lead to the ideal retrieval of the sample's texture for a sufficiently large number of measurements. Two- and three-dimensional permeability distributions can be successfully estimated by employing sparsification transformations associated with compressive imaging approaches. The proposed configuration and formulation may be useful in non-destructive detection and sensing of biological, chemical or ferrimagnetic samples. [ABSTRACT FROM AUTHOR]

Abstract: Due to their ability to strongly modify the local optical field through the excitation of surface plasmon polaritons (SPPs), plasmonic nanostructures are often used to reshape the emission direction and enhance the radiative decay rate of quantum emitters, such as semiconductor quantum dots (QDs). These features are essential for quantum information processing, nanoscale photonic circuitry, and optoelectronics. However, the modification and enhancement demonstrated thus far have typically led to drastic alterations of the local energy density of the emitters, and hence their intrinsic optical properties, leaving little room for active control. Here, dynamic tuning of the energy states of a single semiconductor QD is demonstrated by optically modifying its local dielectric environment with a nearby plasmonic structure, instead of directly coupling it to the QD. This technique leaves intact the intrinsic optical properties of the QD, while enabling a reversible all‐optical control mechanism that operates below the diffraction limit at low power levels. [ABSTRACT FROM AUTHOR]

Graded metasurfaces exploit the local momentum imparted by an impedance gradient to mold the impinging wave front. This approach suffers from fundamental limits on the overall conversion efficiency, and it is challenged by fabrication limitations on the spatial resolution. Here, we introduce the concept of metagratings, formed by periodic arrays of carefully tailored bianisotropic inclusions and show that they enable wave front engineering with unitary efficiency and significantly lower fabrication demands. We employ this concept to design reflective metasurfaces for wave front steering without limitations on efficiency. A similar approach can be extended to transmitted beams and arbitrary wave front transformation, opening opportunities for highly efficient metasurfaces for extreme wave manipulation. [ABSTRACT FROM AUTHOR]

In this work, we experimentally demonstrate metasurface-enhanced photoresponse in organic photodetectors. We have designed and integrated a metasurface with broadband functionality into an organic photodetector, with the goal of significantly increasing the absorption of light and generated photocurrent from 560 up to 690 nm. We discuss how the metasurface can be integrated with the fabrication of an organic photodiode. Our results show large gains in responsivity from 1.5× to 2× between 560 and 690 nm. [ABSTRACT FROM AUTHOR]

The article cites a research study that describes a new class of photonic systems result in bandgap opening at one of the spin-valley photonics, whereas the other valley exhibits Dirac-like conical dispersion. It shows a robust pseudospin- and valley-polarized one-way Klein tunneling, exhibiting marked implications on photon transport.

Confining electromagnetic energy is crucial to enhance light-matter interactions, with important implications for science and technology. Here, the opportunities offered by trapping and confining light in open structures, based on the concept of embedded eigenstates within the radiation continuum enabled by zero-index metamaterials, are discussed. Building upon the physical insights offered by the analysis, a general platform is put forward that allows the realization of extremely high field enhancements in open structures under external illumination. Structures supporting embedded eigenstates represent a rare example of physical systems in which extreme-in principle unbounded-responses can be tamed. The proposed design recipe to realize bound states in the continuum also offers a simple model that allows testing of important questions that surround the concept of embedded eigenstates, such as their effect on the local density of photonic states. The findings help clarify which nano-optical and radio-wave applications may benefit from this unusual and singular response. [ABSTRACT FROM AUTHOR]

Although cloaks are effective at suppressing the observability of static objects, they can be defeated when in motion. Here we discuss a general technique to cloak the motion of objects from static observers, based on compensating the Doppler shift associated with their motion with frequency conversion sustained by a spatiotemporally modulated cover. The concept is theoretically and numerically demonstrated in a system composed of a planar reflector covered by a spatiotemporally modulated slab. It is shown that, for properly selected modulation frequency, the composite system can appear to an external observer as stationary, even though it is actually moving. This concept may pave the way to the minimization of clutter produced by moving objects as well as to new directions in the science of cloaking. [ABSTRACT FROM AUTHOR]

Tailoring of electromagnetic spontaneous emission predicted by E. M. Purcell more than 50 years ago has undoubtedly proven to be one of the most important effects in the rich areas of quantum optics and nanophotonics. Although during the past decades the research in this field has been focused on electric dipole emission, the recent progress in nanofabrication and study of magnetic quantum emitters, such as rare-earth ions, has stimulated the investigation of the magnetic side of spontaneous emission. Here, we review the state-of-the-art advances in the field of spontaneous emission enhancement of magnetic dipole quantum emitters with the use of various nanophotonics systems. We provide the general theory describing the Purcell effect of magnetic emitters, overview realizations of specific nanophotonics structures allowing for the enhanced magnetic dipole spontaneous emission, and give an outlook on the challenges in this field, which remain open to future research. [ABSTRACT FROM AUTHOR]

All-optical ultrafast signal modulation and routing by low-loss nanodevices is a crucial step towards an ultracompact optical chip with high performance. Here, we propose a specifically designed silicon dimer nanoantenna, which is tunable via photoexcitation of dense electron-hole plasma with ultrafast relaxation rate. On the basis of this concept, we demonstrate the effect of beam steering by up to 20 degrees through simple variation of the intensity of incident light. The effect, which is suitable for ultrafast light routing in an optical chip, is demonstrated both in the visible and near-IR spectral regions for silicon- and germanium-based nanoantennas. We also reveal the effect of electron-hole plasma photoexcitation on the local density of states (LDOS) in the dimer gap and find that the orientation averaged LDOS can be altered by 50%, whereas modification of the projected LDOS can be even more dramatic, almost five-fold for transverse dipole orientation. Moreover, our analytical model sheds light on the transient dynamics of the studied nonlinear nanoantennas, yielding all temporal characteristics of the suggested ultrafast nanodevice. The proposed concept paves the way to the creation of low-loss, ultrafast, and compact devices for optical signal modulation and routing. [ABSTRACT FROM AUTHOR]

On-Chip Plasmonic Vortex Interferometers (Laser Photonics Rev. 16(10)/2022) B Optical Interferometers b In article number 2200242, Yuan-Hao Lang, Quan Xu, and colleagues have developed a new form of optical interferometers by customizing the plasmonic vortex generation from two spin channels. In several square wavelengths, plasmonic vortex interferometer constructs interference field with subwavelength fine structure, demonstrating the measurement of polarization state, spin, and orbital angular momentum of the incident light beam. [Extracted from the article]

Drexhage's seminal observation that spontaneous emission rates of fluorophores vary with distance from a mirror uncovered the fundamental notion that a source's environment determines radiative linewidths and shifts. Further, this observation established a powerful tool to determine fluorescence quantum yields. We present the direct analogue for sound. We demonstrate that a Chinese gong at a hard wall experiences radiative corrections to linewidth and line shift, and extract its intrinsic radiation efficiency. Beyond acoustics, our experiment opens new ideas to extend the Drexhage experiment to metamaterials, nanoantennas, and multipolar transitions. [ABSTRACT FROM AUTHOR]

The canonical Su-Schrieffer-Heeger (SSH) array is one of the basic geometries that have spurred significant interest in topological band-gap modes. Here, we show that the judicious inclusion of third-order Kerr nonlinearities in SSH arrays opens rich physics in topological insulators, including the possibility of supporting self-induced topological transitions, as a function of the applied intensity. We highlight the emergence of a class of topological solutions in nonlinear SSH arrays localized at the array edges and with unusual properties. As opposed to their linear counterparts, these nonlinear states decay to a plateau of nonzero amplitude inside the array, highlighting the local nature of topologically nontrivial band gaps in nonlinear systems. We study the conditions under which these states can be excited and their temporal dynamics as a function of the applied excitation, paving the way to interesting directions in the physics of topological edge states with robust propagation properties based on nonlinear interactions in suitably designed periodic arrays. [ABSTRACT FROM AUTHOR]

Optical metasurfaces, typically referred to as two-dimensional metamaterials, are arrays of engineered subwavelength inclusions suitably designed to tailor the light properties, including amplitude, phase and polarization state, over deeply subwavelength scales. By exploiting anomalous localized interactions of surface elements with optical waves, metasurfaces can go beyond the functionalities offered by conventional diffractive optical gratings. The innate simplicity of implementation and the distinct underlying physics of their wave-matter interaction distinguish metasurfaces from three-dimensional metamaterials and provide a valuable means of moulding optical waves in the desired manner. Here, we introduce a general approach based on the electromagnetic equivalence principle to develop and synthesize graded, non-periodic metasurfaces to generate arbitrarily prescribed distributions of electromagnetic waves. Graded metasurfaces are realized with a single layer of spatially modulated, electrically polarizable nanoparticles, tailoring the scattering response of the surface with nanoscale resolutions. We discuss promising applications based on the proposed local wave management technique, including the design of ultrathin optical carpet cloaks, alignment-free polarization beam splitters and a novel approach to enable broadband light absorption enhancement in thin-film solar cells. This concept opens up a practical route towards efficient planarized optical structures with potential impact on the integrated nanophotonic technology. [ABSTRACT FROM AUTHOR]

Metamaterials made of nanoscale inclusions or artificial unit cells exhibit exotic optical properties that do not exist in natural materials. Promising applications, such as super-resolution imaging, cloaking, hyperbolic propagation, and ultrafast phase velocities have been demonstrated based on mostly micrometer-scale metamaterials and few nanoscale metamaterials. To date, most metamaterials are created using costly and tedious fabrication techniques with limited paths toward reliable large-scale fabrication. In this work, we demonstrate the one-step direct growth of self-assembled epitaxial metal-oxide nanocomposites as a drastically different approach to fabricating large-area nanostructured metamaterials. Using pulsed laser deposition, we fabricated nanocomposite films with vertically aligned gold (Au) nanopillars (~20 nm in diameter) embedded in various oxide matrices with high epitaxial quality. Strong, broad absorption features in the measured absorbance spectrum are clear signatures of plasmon resonances of Au nanopillars. By tuning their densities on selected substrates, anisotropic optical properties are demonstrated via angular dependent and polarization resolved reflectivity measurements and reproduced by full-wave simulations and effective medium theory. Our model predicts exotic properties, such as zero permittivity responses and topological transitions. Our studies suggest that these self-assembled metal-oxide nanostructures provide an exciting new material platform to control and enhance optical response at nanometer scales. [ABSTRACT FROM AUTHOR]

We show that properly designed mantle cloaks, consisting of patterned metallic sheets placed around cylindrical monopoles, allow tightly packing the same antennas together in a highly dense telecommunication platform. Our experimental demonstration is applied to the relevant example of two cylindrical monopole radiators operating for 3G and 4G mobile communications. The two antennas are placed in close proximity, separated by 1/10 of the shorter operational wavelength, and, after cloaking, are shown to remarkably operate as if isolated in free-space. This result paves the way to unprecedented co-siting strategies for multiple antennas handling different services and installed in overcrowded platforms, such as communication towers, satellite payloads, aircrafts, or ship trees. More broadly, this work presents a significant application of cloaking technology to improve the efficiency of modern communication systems. [ABSTRACT FROM AUTHOR]

We apply the Pancharatnam-Berry phase approach to plasmonic metasurfaces loaded by highly nonlinear multiquantum-well substrates, establishing a platform to control the nonlinear wave front at will based on giant localized nonlinear effects. We apply this approach to design flat nonlinear metasurfaces for efficient second-harmonic radiation, including beam steering, focusing, and polarization manipulation. Our findings open a new direction for nonlinear optics, in which phase matching issues are relaxed, and an unprecedented level of local wave front control is achieved over thin devices with giant nonlinear responses. [ABSTRACT FROM AUTHOR]

Nonlinear metasurfaces based on coupling a locally enhanced plasmonic response to intersubband transitions of n-doped multi-quantum-wells (MQWs) can provide second-order susceptibilities orders of magnitude larger than any other nonlinear flat structure measured so far. Here we present a comprehensive theory to characterize the electromagnetic response of nonlinear processes occurring in ultrathin MQW-based plasmonic metasurfaces, providing a homogeneous model that takes phase matching at the unit-cell level and the influence of saturation and losses into account. In addition, the limits imposed by saturation of the MQW transitions on the nonlinear response of these metasurfaces are analytically derived, revealing useful guidelines to design devices with enhanced performance. Our approach is first validated using experimental data and then applied to theoretically investigate novel designs able to achieve significant second-harmonic generation efficiency in the infrared frequency band. [ABSTRACT FROM AUTHOR]

Inspired by the parity-time symmetry concept, we show that a judicious spatial modulation of gain and loss in ε-near-zero metamaterials can induce the propagation of exponentially bound interface modes characterized by zero attenuation. With specific reference to a bilayer configuration, via analytical studies and parametrization of the dispersion equation, we show that this waveguiding mechanism can be sustained in the presence of moderate gain/loss levels, and it becomes leaky (i.e., radiative) below a gain/loss threshold. Moreover, we explore a possible rod-based metamaterial implementation, based on realistic material constituents, which captures the essential features of the waveguiding mechanism, in good agreement with our theoretical predictions. Our results may open up possibilities for the design of optical devices and reconfigurable nanophotonics platforms. [ABSTRACT FROM AUTHOR]

Non-reciprocal components, which are essential to many modern communication systems, are almost exclusively based on magneto-optical materials, severely limiting their applicability. A practical and inexpensive route to magnetic-free non-reciprocity could revolutionize radio-frequency and nanophotonic communication networks. Angular-momentum biasing was recently proposed as a means of realizing isolation for sound waves travelling in a rotating medium, and envisaged as a path towards compact, linear integrated non-reciprocal electromagnetic components. Inspired by this concept, here we demonstrate a subwavelength, linear radio-frequency non-reciprocal circulator free from magnetic materials and bias. The scheme is based on the parametric modulation of three identical, strongly and symmetrically coupled resonators. Their resonant frequencies are modulated by external signals with the same amplitude and a relative phase difference of 120°, imparting an effective electronic angular momentum to the system. We observe giant non-reciprocity, with up to six orders of magnitude difference in transmission for opposite directions. Furthermore, the device topology is tunable in real time, and can be directly embedded in a conventional integrated circuit. [ABSTRACT FROM AUTHOR]

Electrically tunable mid‐infrared metasurfaces with nanosecond response time and broad tuning range are reported. Electrical tuning is achieved by employing strong polaritonic coupling of electromagnetic modes in metallic nanoresonators with voltage‐tunable inter‐subband transitions in semiconductor heterostructures, tailored for a giant quantum‐confined Stark effect. Experimentally, a 220‐nm‐thick multi‐quantum‐well semiconductor layer is sandwiched between a ground plane and a metal layer patterned with plasmonic nanoresonators. Approximately 300 nm absorption peak tuning and over 30% absorption change are demonstrated at around 7 μm wavelength at normal incidence by changing the DC bias voltage from 0 to +5 V. Fast reflectivity modulation of the metasurface is shown with a response time less than ∼10 ns using bias voltage modulation between +2 V and +4 V. Since the bias affects the optical response at the individual nanoresonator level, this approach may be used to create metasurfaces for ultrafast electrical wavefront tuning and beam steering. [ABSTRACT FROM AUTHOR]

Intersubband transitions in n-doped multi-quantum-well semiconductor heterostructures make it possible to engineer one of the largest known nonlinear optical responses in condensed matter systems-but this nonlinear response is limited to light with electric field polarized normal to the semiconductor layers. In a different context, plasmonic metasurfaces (thin conductor-dielectric composite materials) have been proposed as a way of strongly enhancing light-matter interaction and realizing ultrathin planarized devices with exotic wave properties. Here we propose and experimentally realize metasurfaces with a record-high nonlinear response based on the coupling of electromagnetic modes in plasmonic metasurfaces with quantum-engineered electronic intersubband transitions in semiconductor heterostructures. We show that it is possible to engineer almost any element of the nonlinear susceptibility tensor of these structures, and we experimentally verify this concept by realizing a 400-nm-thick metasurface with nonlinear susceptibility of greater than 5 × 104 picometres per volt for second harmonic generation at a wavelength of about 8 micrometres under normal incidence. This susceptibility is many orders of magnitude larger than any second-order nonlinear response in optical metasurfaces measured so far. The proposed structures can act as ultrathin highly nonlinear optical elements that enable efficient frequency mixing with relaxed phase-matching conditions, ideal for realizing broadband frequency up- and down-conversions, phase conjugation and all-optical control and tunability over a surface. [ABSTRACT FROM AUTHOR]

We show that obliquely incident, transversely magnetic-polarized plane waves can be totally transmitted (with zero reflection) through epsilon-near-zero (ENZ) bilayers characterized by balanced loss and gain with parity-time (PT) symmetry. This tunneling phenomenon is mediated by the excitation of a surface wave localized at the interface separating the loss and gain regions. We determine the parameter configurations for which the phenomenon may occur and, in particular, the relationship between the incidence direction and the electrical thickness. We show that, below a critical threshold of gain and loss, there always exists a tunneling angle which, for moderately thick (wavelength-sized) structures, approaches a critical value dictated by the surface-wave phase-matching condition. We also investigate the unidirectional character of the tunneling phenomenon, as well as the possible onset of spontaneous symmetry breaking, typical of PT-symmetric systems. Our results constitute an interesting example of a PT-symmetry-induced tunneling phenomenon, and may open up intriguing venues in the applications of ENZ materials featuring loss and gain. [ABSTRACT FROM AUTHOR]

Acoustic isolation and nonreciprocal sound transmission are highly desirable in many practical scenarios. They may be realized with nonlinear or magneto-acoustic effects, but only at the price of high power levels and impractically large volumes. In contrast, nonreciprocal electromagnetic propagation is commonly achieved based on the Zeeman effect, or modal splitting in ferromagnetic atoms induced by a magnetic bias. Here, we introduce the acoustic analog of this phenomenon in a subwavelength meta-atom consisting of a resonant ring cavity biased by a circulating fluid. The resulting angular momentum bias splits the ring's azimuthal resonant modes, producing giant acoustic nonreciprocity in a compact device. We applied this concept to build a linear, magnetic-free circulator for airborne sound waves, observing up to 40-decibel nonreciprocal isolation at audible frequencies. [ABSTRACT FROM AUTHOR]

We study resonant tunnelling effects that can occur in trilayer structures featuring a dielectric layer sandwiched between two magneto-optical(MO)-metal layers. We show that the resonance splitting associated with these phenomena can be exploited to enhance Faraday rotation at optical frequencies. Our results indicate that, in the presence of realistic loss levels, a tri-layer structure of subwavelength thickness is capable of yielding sensible (∼10°) Faraday rotation with transmittance levels that are an order of magnitude larger than those attainable with a standalone slab of MO metal of the same thickness. [ABSTRACT FROM AUTHOR]