The inevitable feedback between the environmental and energy crisis within the next decades can probably trigger and/or promote a global imbalance in both financial and public health terms. To handle this difficult situation, in the last decades, many different classes of materials have been recruited to assist in the management, production, and storage of so-called clean energy. Probably, ferromagnets, superconductors and ferroelectric/piezoelectric materials stand at the frontline of applications that relate to clean energy. For instance, ferromagnets are usually employed in wind turbines, superconductors are commonly used in storage facilities and ferroelectric/piezoelectric materials are employed for the harvesting of stray energy from the ambient environment. In this work, we focus on the wide family of ferroelectric/piezoelectric materials, reviewing their physical properties in close connection to their application in the field of clean energy. Among other compounds, we focus on the archetypal compound Pb(Zr,Ti)O3 (or PZT), which is well studied and thus preferred for its reliable performance in applications. Also, we pay special attention to the advanced ferroelectric relaxor compound (1−x)Pb(Mg1/3Nb2/3)O3−xPbTiO3 (or PMN-xPT) due to its superior performance. The inhomogeneous composition that many kinds of such materials exhibit at the so-called morphotropic phase boundary is reviewed in connection to possible advantages that it may bring when applications are considered.

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 is based on analytical techniques that allow multipolar decomposition of the scattered field in agreement with full-wave simulations, and lays the foundations for a light management method.

Metasurfaces are capable of fully reshaping the wavefronts of incident beams in desiredmanners.However,therequirementforexternallightexcitationand the resonant nature of their meta-atoms, make challenging their on-chip integration. Here, we introduce the conceptanddesignofafreshclassof metasurfaces, driven by unidirectional guided waves, capable of arbitrary wavefront control based on the uniquedispersion properties of unidirectional guided waves rather than resonant meta-atoms. Upon experimentally demonstrating the feasibility of our designs in the microwave regime, we numerically validate the introduced principle through the design of several microwave meta-devices using metal-air-gyromagnetic unidirectional surface magneto-plasmons, agilely converting unidirectional guided modes into the wavefrontsof3DBesselbeams,focusedwaves,andcontrollablevortexbeams. We, further, numerically demonstrate sub-diffraction focusing, which is beyond the capability of conventional metasurfaces. Our unfamiliar yet prac tical designs may enable full, broadband manipulation of electromagnetic waves on deep subwavelength scales.

In this letter, we demonstrate that a set of absorbing boundary conditions (ABCs) for numerical simulations of waves, proposed originally by Engquist and Majda and later generalized by Trefethen and Halpern, can alternatively be derived with the use of Pauli matrices algebra. Hence a novel approach to the derivation of one-way wave equations in electromagnetics is proposed. That is, the classical wave equation can be factorized into two two-dimensional wave equations with first-order time derivatives. Then, using suitable approximations, not only Engquist and Majda ABCs can be obtained, but also generalized ABCs proposed by Trefethen and Halpern, which are applicable to simulations of radiation problems.

In the past two decades, index-near-zero (INZ) modes and materials, with their spatial phase invariance and super coupling, gained increasing attention for applications in all-optical/quantum computing and communication. However, the modulation of INZ modes is typically complex and discontinuous, often achieved through intricate experimental methods, thereby hindering their widespread application. Here, we propose two deep-subwavelength magneto-optical one-way waveguides and discover three broadband tunable INZ modes, exhibiting predict able behavior dependent on the external magnetic field (EMF). By utilizing these INZ modes, we design broadband tunable all-optical phase modulators through straightforward EMF control. The tunable and predictable nature of INZ modes, combined with deep-subwavelength phase modulators, may advance miniaturized all-optical communication and computation.

All-optical logic gates have been studied intensively owing to their potential to enable broadband, low-loss and high-speed communications. However, poor tunability has remained a key challenge in this field. In this work, we propose a Y-shaped structure composed of Yttrium Iron Garnet (YIG) layers that can serve as tunable all-optical logic gates, including, but not limited to, OR, AND and NOT gates, by applying external magnetic fields to magnetize the YIG layers. Our findings reveal that these logic gates are founded on protected one-way edge modes, where by tuning the wavenumber k of the operating mode to a sufficiently small (or even zero) value, the gates can become nearly immune to nonlocal effects. This not only enhances their reliability but also allows for maintaining extremely high precision in their operations. Furthermore, the operating band itself of the logic gates is also shown to be tunable. We introduce a straightforward and practical method for controlling and switching these gates between "work", "skip", and "stop" modes. These findings have potentially significant implications for the design of high-performance and robust all-optical microwave communication systems.

Extraordinary optical transmission (EOT) is a hallmark of surface plasmons and a precursor to nanoplasmonics and metamaterials. However, to the best of our knowledge, this effect has never been topologically protected in three dimensions, leaving it vulnerable to structural imperfections, nonlocal effects, and backreflections. We report broadband, three-dimensional unidirectional structures that allow for EOT (normalized transmission > 1) through deep-subdiffractional single holes, immune to these deleterious effects. These structures avoid unnecessary propagation losses and achieve maximum transmission through a single hole, limited only by unavoidable dissipative losses. In the limit of vanishing losses, the transmission through a deep-subdiffractional hole can approach unity, significantly surpassing existing devices, and rivaling the performance of negative-index ‘perfect’ lenses. The topological stability of these structures renders them robust against surface roughness, defects, and nonlocality, without the need for elaborate meta-structures or tapering.

Standard electronic computing based on nanoelectronics and logic gates has upended our lives in a profound way. However, suffering from, both, Moore’s law and Joule’s law, further development of logic devices based solely on elec-tricity has gradually stuck in the mire. All-optical logic devices are believed to be a potential solution for such a problem. This work proposes an all-optical digital logical system (AODLS) based on unidirectional (one-way propagation) modes in the microwave regime. In a Y-shaped module of the AODLS, the basic seven logic gates, including OR, AND, NOT, NOR, NAND, XOR, and XNOR gates, are achieved for continuous broadband operation relying on the existence of unidi-rectional electromagnetic signals. Extremely large extinction and contrast ratios are found in these logic gates. The idea of “negative logic” is used in designing the AODLS. Moreover, the authors further demonstrate that the AODLS can be assembled to multi-input and/or multi-output logical functionalities, which is promising for parallel computation. Besides, numerical simulations perfectly fit with and corroborate the theoretical analyses presented here. The low-loss, broadband, and robust characteristics of this system are outlined and studied in some detail. The AODLS consisting of unidirectional structures may open a new route for all-optical calculation and integrated optical circuits.

This monograph is a detailed introduction to the nascent and ever-evolving fields of metamaterials and nanophotonics, with key techniques and applications needed for a comprehensive understanding of these fields all detailed. These include the 'standard' and high-accuracy 'nonstandard' FDTD techniques, finite-difference frequency-domain mode solvers, the transfer matrix method, analytic calculations for dielectric and plasmonic waveguides, dispersion, Maxwell-Bloch and density functional theory, as well as design methods for constructing metamaterials and nanolasers, and quantum plasmonics. The book is intended for final-year undergraduates, as well as postgraduates or active researchers who wish to understand and enter these fields in a 'user-friendly' manner, and who have a basic understanding of and familiarity with electromagnetic theory.

Τhe incumbent technology for bringing light to the nanoscale, the near-field scanning optical microscope, has notoriously small throughput efficiencies of the order of 104 −105, or less. We report on a broadband, topological, unidirectionally guiding structure, not requiring adiabatic tapering and, in principle, enabling near-perfect (∼100%) optical transmission through an unstructured single arbitrarily subdiffraction slit at its end. Specifically, for a slit width of just λeff/72 (λ0/138) the attained normalized transmission coefficient reaches a value of 1.52, while for a unidirectional-only (nontopological) device the normalized transmission through a λeff/21 (∼λ0/107) slit reaches 1.14; both limited only by inherent material losses, and with zero reflection from the slit. The associated, under ideal (ultralow-loss) conditions, near-perfect optical extraordinary transmission has implications, among diverse areas in wave physics and engineering, for high-efficiency, maximum-throughput nanoscopes and heat-assisted magnetic recording devices.

Topological features, in particular distinct band intersections known as nodal rings, usually requiring three-dimensional structures, have now been demonstrated experimentally in an elegantly simple one-dimensional photonic crystal.

The classical “time-bandwidth” limit for linear time-invariant (LTI) devices in physics and engineering asserts that it is impossible to store broadband propagating waves (large Δω">Δω’s) for long times (large Δt’s). For standing (non-propagating) waves, i.e., vibrations, in particular, this limit takes on a simple form, ΔtΔω=1">ΔtΔω=1, where Δω">Δω is the bandwidth over which localization (energy storage) occurs, and Δt">Δt is the storage time. This is related to a well-known result in dynamics, namely that one can achieve a high Q-factor (narrowband resonance) for low damping, or small Q-factor (broadband resonance) for high damping, but not simultaneously both. It thus remains a fundamental challenge in classical wave physics and vibration engineering to try to find ways to overcome this limit, not least because that would allow for storing broadband waves for long times, or achieving broadband resonance for low damping. Recent theoretical studies have suggested that such a feat might be possible in LTI terminated unidirectional waveguides or LTI topological “rainbow trapping” devices, although an experimental confirmation of either concept is still lacking. In this work, we consider a nonlinear but time-invariant mechanical system and demonstrate experimentally that its time-bandwidth product can exceed the classical time-bandwidth limit, thus achieving values both above and below unity, in an energy-tunable way. Our proposed structure consists of a single-degree-of-freedom nonlinear oscillator, rigidly coupled to a nondispersive waveguide. Upon developing the full theoretical framework for this class of nonlinear systems, we show how one may control the nonlinear flow of energy in the frequency domain, thereby managing to disproportionately decrease (increase) Δt">Δt, the storage time in the resonator, as compared with an increase (decrease) of the system’s bandwidth Δω">Δω. Our results pave the way toward conceiving and harnessing hitherto unattainable broadband and simultaneously low-loss wave-storage devices, both linear and nonlinear, for a host of key applications in wave physics and engineering.

We comprehensively review several general methods and analytical tools used for causality evaluation of photonic materials. Our objective is to call to mind and then formulate, on a mathematically rigorous basis, a set of theorems which can answer the question whether a considered material model is causal or not. For this purpose, a set of various distributional theorems presented in literature is collected as the distributional version of the Titchmarsh theorem, allowing for evaluation of causality in complicated electromagnetic systems. Furthermore, we correct the existing material models with the use of distribution theory in order to obtain their causal formulations. In addition to the well-known Kramers–Krönig (K–K) relations, we overview four further methods which can be used to assess causality of given dispersion relations, when calculations of integrals involved in the K–K relations are challenging or even impossible. Depending on the given problem, optimal approaches allowing us to prove either the causality or lack thereof are pointed out. These methodologies should be useful for scientists and engineers analyzing causality problems in electrodynamics and optics, particularly with regard to photonic materials, when the involved mathematical distributions have to be invoked.

Topologically protected transport has recently emerged as an effective means to address a recurring problem hampering the field of slow light for the past two decades: its keen sensitivity to disorders and structural imperfections. With it, there has been renewed interest in efforts to overcome the delay-time-bandwidth limitation usually characterizing slow-light devices, on occasion thought to be a fundamental limit. What exactly is this limit, and what does it imply? Can it be overcome? If yes, how could topological slow light help, and in what systems? What applications might be expected by overcoming the limit? Our Perspective here attempts addressing these and other related questions while pointing to important new functionalities both for classical and quantum devices that overcoming the limit can enable.

Metasurfaces constitute an emerging technology, allowing for compact manipulation of all degrees of freedom of an incident lightwave. A key ongoing challenge in the design of these structures is how to allow for energy-efficient dynamic (active) operation, particularly for the polarization of incident light, which other standard devices typically cannot efficiently act upon. Here, we present a quasi-two-dimensional magneto-optic metasurface capable of simultaneously high-contrast on/off operation, as well as rotation of the polarization angle of a linearly polarized wave—that is, without converting the incident linear polarization to elliptical, which is normally particularly challenging. Furthermore, the device’s operation is broadband, with a bandwidth of around 5 µm, and can be conveniently manipulated using an external magnetic bias. Our findings, corroborated using two different full-wave simulation approaches, may allow for functional metasurfaces operating in the terahertz (THz) regime, giving rise to robust, energy-efficient, and high-dynamic-range broadband isolation, to be used for a wealth of optoelectronic and communication applications.

An optical analogue of magic-angle twisted graphene bilayer gives rise to rigorously stopped light, which coupled with gain allows for a new type of a nanolaser with remarkable figures of merit.

We report the experimental realization of a multifunctional microscale plasmonic metasurface capable of sampling a light beam and performing five functionalities, while allowing high direct transmission and maintenance of the properties of the input light beam. The plasmonic metasurface integrates light-to-surface-plasmon coupling, two-channel wavelength demultiplexing with a channel spacing smaller than 44 nm, wavelength and polarization controllable beam splitting of a monochromatic, single polarization signal, and four-level polarization and wavelength-polarization demultiplexing in an all-in-one structure. Such a device can play a key role for on-chip adaptable integrated circuits for parallel signal processing, communications, and nondestructive sensing.

The discrimination of enantiomers is crucial in biochemistry. However, chiral sensing faces significant limitations due to inherently weak chiroptical signals. Nanophotonics is a promising solution to enhance sensitivity thanks to increased optical chirality maximized by strong electric and magnetic fields. Metallic and dielectric nanoparticles can separately provide electric and magnetic resonances. Here we propose their synergistic combination in hybrid metal–dielectric nanostructures to exploit their dual character for superchiral fields beyond the limits of single particles. For optimal optical chirality, in addition to maximization of the resonance strength, the resonances must spectrally coincide. Simultaneously, their electric and magnetic fields must be parallel and π/2 out of phase and spatially overlap. We demonstrate that the interplay between the strength of the resonances and these optimal conditions constrains the attainable optical chirality in resonant systems. Starting from a simple symmetric nanodimer, we derive closed-form expressions elucidating its fundamental limits of optical chirality. Building on the trade-offs of different classes of dimers, we then suggest an asymmetric dual dimer based on realistic materials. These dual nanoresonators provide strong and decoupled electric and magnetic resonances together with optimal conditions for chiral fields. Finally, we introduce more complex dual building blocks for a metasurface with a record 300-fold enhancement of local optical chirality in nanoscale gaps, enabling circular dichroism enhancement by a factor of 20. By combining analytical insight and practical designs, our results put forward hybrid resonators to increase chiral sensitivity, particularly for small molecular quantities.

We report that the fundamental three-dimensional (3-D) scattering single-channel limit can be exceeded in magneto-optical assisted systems by inducing non-degenerate magnetoplasmonic modes. In addition, we propose a 3-D active (magnetically assisted) forward-superscattering to invisibility switch, functioning at the same operational wavelength. Our structure is composed of a high-index dielectric core coated by indium antimonide (InSb), a semiconductor whose permittivity tensorial elements may be actively manipulated by an external magnetic bias B0. In the absence of B0, InSb exhibits isotropic epsilon-near-zero (ENZ) and plasmonic behavior above and below its plasma frequency, respectively, a frequency band which can be utilized for attaining invisibility using cloaks with permittivity less than that of free space. With realistic B0 magnitudes as high as 0.17 T, the gyroelectric properties of InSb enable the lift of mode degeneracy, and the induction of Zeeman-split type magnetoplasmonic modes that beat the fundamental single-channel limit. Moreover, we show that chains of such particles, where each one operates in its superscattering regime, enable giant off-to-on enhancement in scattering efficiency, as well as unprecedentedly high forward scattering. These all-in-one designs allow for the implementation of functional and readily tunable optical devices.

Plasmonic metasurfaces are promising as enablers of nanoscale nonlinear optics and flat nonlinear optical components. Nonlinear optical responses of such metasurfaces are determined by the nonlinear optical properties of individual plasmonic meta-atoms. Unfortunately, no simple methods exist to determine the nonlinear optical properties (hyperpolarizabilities) of the meta-atoms hindering the design of nonlinear metasurfaces. Here, we develop the equivalent RLC circuit (resistor, inductor, capacitor) model of such meta-atoms to estimate their second-order nonlinear optical properties, that is, the first-order hyperpolarizability in the optical spectral range. In parallel, we extract from second-harmonic generation experiments the first-order hyperpolarizabilities of individual meta-atoms consisting of asymmetrically shaped (elongated) plasmonic nanoprisms, verified with detailed calculations using both nonlinear hydrodynamic-FDTD and nonlinear scattering theory. All three approaches, analytical, experimental, and computational, yield results that agree very well. Our empirical RLC model can thus be used as a simple tool to enable an efficient design of nonlinear plasmonic metasurfaces. © 2020 American Chemical Society.

We report the experimental realization of periodically perforated plasmonic metasurfaces capable of integrating several key functionalities, such as light-to-surface plasmon coupling, controllable beam-splitting, wavelength filtering and routing, high resolution differential wavelength measurement, and vectorial displacement sensing. The plasmonic metasurfaces operate at telecom wavelengths, at the vicinity of the eigenmode crossing points where zero group velocity is experienced, and their functionality parameters, such as sensitivity to misalignment, prong angular separation, power ratio, polarization, and bandwidth, can be adjusted by designing the boundary shape and by conveniently manipulating their alignment with the illuminating light beam. In the same context, a circular plasmonic metasurface could also serve as a vectorial displacement sensor capable of monitoring simultaneously the magnitude and direction of the displacement between its center and that of the illuminating beam. The compact, easily controllable, and all-in-one nature of our devices can enable on-chip integrated circuits with adaptable functionality for applications in sensing and optical signal processing. © 2020 American Chemical Society.

The time-bandwidth limit is a mathematical tenet that affects all reciprocal resonators, stating that the product of the spectral bandwidth that can couple into a resonant system and its characteristic energy decay time is always equal to 1. Here, we develop an analytical and numerical model to show that introducing nonreciprocal coupling to a generalized resonator changes the power balance between the reflected and intracavity fields, which consequently overcomes the time-bandwidth limit of the resonant system. By performing a full evaluation of the time-bandwidth product (TBP) of the modeled resonator, we show that it represents a measure of the increased delay imparted to a light wave, with respect to what the bandwidth of the reciprocal resonant structure would allow to the same amount of in-coupled power. No longer restricted to the value 1, we show that the TBP can instead be used as a figure of merit of the improvement in intracavity power enhancement due to the nonreciprocal coupling. © 2021 American Physical Society.

Invisibility cloaking devices constitute a unique and potentially disruptive technology, but only if they can work over broad bandwidths for electrically-large objects. So far, the only known scheme that allows for broadband scattering cancellation from an electrically-large object is based on an active implementation where electric and magnetic sources are deployed over a surface surrounding the object, but whose 'switching on' and other characteristics need to be known (determined) a priori, before the incident wave hits the surface. However, until now, the performance (and potentially surprising) characteristics of these devices have not been thoroughly analysed computationally, ideally directly in the time domain, owing mainly to numerical accuracy issues and the computational overhead associated with simulations of electrically-large objects. Here, on the basis of a finite-difference time-domain (FDTD) method that is combined with a perfect (for FDTD's discretized space) implementation of the total-field/scattered-field (TFSF) interface, we present detailed, time- and frequency-domain analyses of the performance and characteristics of active cloaking devices. The proposed technique guarantees the isolation between scattered- and total-field regions at the numerical noise level (around -300 dB), thereby also allowing for accurate evaluations of the scattering levels from imperfect (non-ideal) active cloaks. Our results reveal several key features, not pointed out previously, such as the suppression of scattering at certain frequencies even for imperfect (time-delayed) sources on the surface of the active cloak, the broadband suppression of back-scattering even for imperfect sources and insufficiently long predetermination times, but also the sensitivity of the scheme on the accurate switching on of the active sources and on the predetermination times if broadband scattering suppression from all angles is required for the electrically-large object. © 2021 Optical Society of America.

Most present-day resonant systems, throughout physics and engineering, are characterized by a strict time-reversal symmetry between the rates of energy coupled in and out of the system, which leads to a trade-off between how long a wave can be stored in the system and the system’s bandwidth. Any attempt to reduce the losses of the resonant system, and hence store a (mechanical, acoustic, electronic, optical, or of any other nature) wave for more time, will inevitably also reduce the bandwidth of the system. Until recently, this time-bandwidth limit has been considered fundamental, arising from basic Fourier reciprocity. In this work, using a simple macroscopic, fiber-optic resonator where the nonreciprocity is induced by breaking its time-invariance, we report, in full agreement with accompanying numerical simulations, a time-bandwidth product (TBP) exceeding the ‘fundamental’ limit of ordinary resonant systems by a factor of 30. We show that, although in practice experimental constraints limit our scheme, the TBP can be arbitrarily large, simply dictated by the finesse of the cavity. Our results open the path for designing resonant systems, ubiquitous in physics and engineering, that can simultaneously be broadband and possessing long storage times, thereby offering a potential for new functionalities in wave-matter interactions. © 2020, The Author(s).

Magneto-optical materials have become a key tool in functional nanophotonics, mainly due to their ability to offer active tuning between two different operational states in subwavelength structures. In the long-wavelength limit, such states may be considered as the directional forward- and back-scattering operations, due to the interplay between magnetic and electric dipolar modes, which act as equivalent Huygens sources. In this work, on the basis of full-wave electrodynamic calculations based on a rigorous volume integral equation (VIE) method, we demonstrate the feasibility of obtaining magnetically-tunable directionality inversion in spherical microresonators (THz antennas) coated by magnetooptical materials. In particular, our analysis reveals that when a high-index dielectric is coated with a magnetooptical material, we can switch the back-scattering of the whole particle to forward-scattering simply by turning off/ on an external magnetic field bias. The validity of our calculations is confirmed by reproducing the above two-state operation, predicted by the VIE, with full-wave finite-element commercial software. Our results are of interest for the design of state-of-the-art active metasurfaces and metalenses, as well as for functional nanophotonic structures, and scattering and nanoantennas engineering. © 2020 Grigorios P. Zouros et al., published by De Gruyter. This work is licensed under the Creative Commons Attribution 4.0 International License.

In their paper in Optica 6, 104 (2019), Mann et al. claim that linear, time-invariant nonreciprocal structures cannot overcome the time-bandwidth limit and do not exhibit an advantage over their reciprocal counterparts, specifically with regard to their time-bandwidth performance. In this Comment, we argue that these conclusions are unfounded. On the basis of both rigorous full-wave simulations and insightful physical justifications, we explain that the temporal coupled-mode theory, on which Mann et al. base their main conclusions, is not suited for the study of nonreciprocal trapped states, and instead direct numerical solutions of Maxwell’s equations are required. Based on such an analysis, we show that a nonreciprocal terminated waveguide, resulting in a trapped state, clearly outperforms its reciprocal counterpart; i.e., both the extraordinary time-bandwidth performance and the large field enhancements observed in such modes are a direct consequence of nonreciprocity. © 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

We report a thorough theoretical investigation of magnon-assisted photon transitions in magnetic garnet micron-sized spheres, which operate as optomagnonic resonators. In this case, matching the intraband splitting of optical Mie modes, induced by particle magnetization, to the eigenfrequency of the uniform-precession spin wave, high-efficiency triply resonant optical transitions between these modes, through respective emission or absorption of a cavity magnon, are enabled. By means of rigorous full electrodynamic computations, supported by corresponding approximate analytical calculations, we provide compelling evidence of greatly increased optomagnonic interaction, compared to that in similar processes between whispering gallery modes of corresponding submillimeter spheres, due to the reduced magnon mode volume. We explain the underlying mechanisms to a degree that goes beyond existing interpretation, invoking group theory to derive general selection rules and highlighting the role of the photon spin as the key property for maximizing the respective coupling strength. © 2020 American Physical Society.

We present a theoretical and numerical model to show how the nonreciprocal coupling in a generalized resonator affects the balance between the reflected and intracavity power as well as the time-bandwidth product of the system. © OSA 2020 © 2020 The Author(s)

We present a theoretical and numerical model to show how the nonreciprocal coupling in a generalized resonator affects the balance between the reflected and intracavity power as well as the time-bandwidth product of the system. © 2020 OSA.

Detection and differentiation of enantiomers in small quantities are crucially important in many scientific fields, including biology, chemistry, and pharmacy. Chiral molecules manifest their handedness in their interaction with the chiral state of light (e.g., circularly polarized light), which is commonly leveraged in circular dichroism (CD) spectroscopy. However, compared to the linear refractive index molecular chirality is extremely weak, resulting in low detection efficiencies. Recently, it has been shown that these weak chiroptical signals can be enhanced by increasing the optical chirality of the electromagnetic fields interacting with chiral samples. Here, we show numerically and analytically that dielectric structures can provide an optimum chiral sensing platform by offering uniform superchiral near-fields. To illustrate this, we first study a simple dielectric dimer and show that circularly polarized light can induce parallel and out of phase electric and magnetic fields, which are spectrally and spatially overlapped, and therefore produce superchiral fields at the midpoint of the dimer. This behavior is in contrast to, for example, a plasmonic dimer, where the optical chirality is limited by the electric dipolar field, which is not completely out of phase with the incident magnetic field. With the insights gained from this analysis, next we develop an approach for overlapping electric and magnetic fields in a single particle, based on Kerker effect. In particular, we introduce a Kerker-inspired metasurface consisting of holey dielectric disks, which offers uniform and accessible superchiral near-fields with CD signal enhancements of nearly 24 times. © 2019 American Chemical Society.

We analyze the engineering of subwavelength nanoantennas composed of anisotropic nanospheroids, for the development of photonic devices. Instead of using conventional isotropic dielectrics, we introduce resonant anisotropic nanoparticles, allowing for shifting Kerker condition points further inside the visible. To address this study, we construct a perturbation-based discrete eigenfunction method, for the electromagnetic scattering of a plane wave by a prolate or oblate uniaxial anisotropic spheroid. The method is fast and yields the solution for the bistatic radar and total scattering cross sections, which is valid for small eccentricities of the spheroid. The validity of this technique is verified by the alternative general purpose discrete dipole approximation method. We investigate the engineering of subwavelength nanoantennas due to material and geometry shaping, like the change of anisotropy type, anisotropy ratio, and deviation of the nanoantenna from sphericity. © 1995-2012 IEEE.

In this article, a rigorous analytical methodology is introduced for designing near-zero refractive index metamaterials (NZIMs). Our proposed NZIM media is realized by three stacked layers of perforated metallic surfaces, each layer composed of a fishnet-like periodic array of square holes. By a proper design of such structures, a low refractive index medium is achieved at their corresponding plasma frequency. The low refractive index property is studied by retrieving the effective parameters of NZIM via inversion techniques, which gives an effective near-zero refractive index, at an operating frequency of 1.5 GHz. Then, the designed NZIM is used for gain enhancement of a circular waveguide antenna. The analysis shows that the proposed platform can enhance the directivity of our antenna by 3 dB while maintaining the return loss <−20 dB. © 2019 Wiley Periodicals, Inc.

All resonant systems throughout science and engineering, independent of their physical implementation have a bandwidth that is inversely related to the decay time. A similar relation exists in all slow-light systems, where the group index (and therefore the delay for a given footprint) is inversely related to the bandwidth. Therefore, resonant or slow-light systems can either store a broad signal for a short time, or a narrow signal for a long time, but cannot achieve large delay for broad bandwidth signals.Here we discuss our recent work on non-reciprocal optical systems that are not constrained by the delay-bandwidth limit. We show that large, broadband optical delay is not a pipe dream and is achievable with current optical technology. We discuss the underlying physics of delay and bandwidth in non-reciprocal optical systems and present an experimental implementation, based on a figure-9 cavity. We demonstrate a delay-bandwidth product 30 times above the seemingly fundamental time-bandwidth limit of traditional systems. Furthermore, we show that the optical pulse can be released after an arbitrary number of round trips, providing the control and tunability lacking from conventional spiral waveguide or fibre loop delay lines. © 2019 IEEE.

We are so accustomed to the time-bandwidth limit that we often take it to be a fundamental relation that cannot be overcome. Contrary to this belief we here propose that the storage/delay time of a system can in fact be decoupled from the operating bandwidth, by breaking Lorentz reciprocity. We discuss two different mechanisms for breaking Lorentz reciprocity and show that - in the correct system layout - both can result in systems that exceed the conventional time bandwidth limit by orders of magnitude. © 2018 IEEE.

An invisibility cloak should completely hide an object from an observer, ideally across the visible spectrum and for all angles of incidence and polarizations of light, in three dimensions. However, until now, all such devices have been limited to either small bandwidths or have disregarded the phase of the impinging wave or worked only along specific directions. Here, we show that these seemingly fundamental restrictions can be lifted by using cloaks made of fast-light media, termed tachyonic cloaks, where the wave group velocity is larger than the speed of light in vacuum. On the basis of exact analytic calculations and full-wave causal simulations, we demonstrate three-dimensional cloaking that cannot be detected even interferometrically across the entire visible regime. Our results open the road for ultrabroadband invisibility of large objects, with direct implications for stealth and information technology, non-disturbing sensors, near-field scanning optical microscopy imaging, and superluminal propagation. © 2019, The Author(s).

We demonstrate that a non-reciprocal, time-variant fiber cavity can operate above the "fundamental" time-bandwidth limit (TBL) of reciprocal structures by more than two orders of magnitude. © 2018 The Author(s).

Chirality plays an essential role in life, providing unique functionalities to a wide range of biomolecules, chemicals, and drugs, which makes chiral sensing and analysis critically important. The wider application of chiral sensing continues to be constrained by the involved chiral signals being inherently weak. To remedy this, plasmonic and dielectric nanostructures have recently been shown to offer a viable route for enhancing weak circular dichroism (CD) effects, but most relevant studies have thus far been ad hoc, not guided by a rigorous analytical methodology. Here, we report the first analytical treatment of CD enhancement and extraction from a chiral biolayer placed on top of a nanostructured substrate. We derive closed-form expressions of the CD and its functional dependence on the background-chiroptical response, substrate thickness and chirality, as well as on the optical chirality and intensity enhancement provided by the structure. Our results provide key insights into the trade-offs that are to be accommodated in the design and conception of optimal nanophotonic structures for enhancing CD effects for chiral molecule detection. Based on our analysis, we also introduce a practical, dielectric platform for chiral sensing featuring large CD enhancements while exhibiting vanishing chiroptical background noise. © 2018 American Chemical Society.

Self-organized criticality emerges in dynamical complex systems driven out of equilibrium and characterizes a wide range of classical phenomena in physics, geology, and biology. We report on a quantum coherence-controlled selforganized critical transition observed in the light localization behavior of a coherence-driven nanophotonic configuration. Our system is composed of a gain-enhanced plasmonic heterostructure controlled by a coherent drive, in which photons close to the stopped-light regime interact in the presence of the active nonlinearities, eventually synchronizing their dynamics. In this system, on the basis of analytical and corroborating full-wave Maxwell-Bloch computations, we observe quantum coherence-controlled self-organized criticality in the emergence of light localization arising from the synchronization of the photons. It is associated with two first-order phase transitions: one pertaining to the synchronization of the dynamics of the photons and the second pertaining to an inversionless lasing transition by the coherent drive. The so-Attained light localization, which is robust to dissipation, fluctuations, and many-body interactions, exhibits scale-invariant power laws and absence of finely tuned control parameters. We also found that, in this nonequilibrium dynamical system, the effective critical "temperature" of the system drops to zero, whereupon one enters the quantum self-organized critical regime. Copyright © 2018 The Authors, some rights reserved.

In this paper, we suggest the design of wavelength tunable, polarization-sensitive optical nanoswitches based on plasmonic spheroidal core-shell nanoparticles. Using a quasi-static approximation, we derive closed-form expressions for short and open circuit conditions, which respectively provide extremely high and low permittivity values at optical frequencies. Owing to the anisotropic nature of spheroidal particles, the analysis are performed in longitudinal and transversal polarizations, where the electric field of the impinging wave is along the major and minor axes of the spheroid, respectively. The derived formulas analytically elucidate this anisotropy, which has been implied by different short and open circuit conditions for two states of polarization. Our results show that by exploiting eccentricity in the spheroidal core-shells (i.e. compared to the spherical core-shells), the switching conditions can be transferred to infrared (IR) and ultraviolet (UV) frequencies, in longitudinal and transversal polarizations, respectively. Finally, the effective permittivity of the core-shell is extracted analytically using the concept of internal homogenization, which gains insight into the optical response of particle. Our analyses pave the way towards realizing tunable and polarization dependent components in UV, optical and IR frequencies for sensing and nanocircuitry applications. © 2018 IOP Publishing Ltd.

A century-old tenet in physics and engineering asserts that any type of system, having bandwidth Dw, can interact with a wave over only a constrained time period Dt inversely proportional to the bandwidth (Dt·Dw ∼ 2p). This law severely limits the generic capabilities of all types of resonant and wave-guiding systems in photonics, cavity quantum electrodynamics and optomechanics, acoustics, continuum mechanics, and atomic and optical physics but is thought to be completely fundamental, arising from basic Fourier reciprocity.We propose that this "fundamental" limit can be overcome in systems where Lorentz reciprocity is broken. As a system becomes more asymmetric in its transport properties, the degree to which the limit can be surpassed becomes greater. By way of example, we theoretically demonstrate how, in an astutely designed magnetized semiconductor heterostructure, the above limit can be exceeded by orders of magnitude by using realistic material parameters. Our findings revise prevailing paradigms for linear, time-invariant resonant systems, challenging the doctrine that high-quality resonances must invariably be narrowband and providing the possibility of developing devices with unprecedentedly high time-bandwidth performance.

Metamaterials provide functionalities that are not present in naturally occurring materials, including negative refraction, super-lenses or hyper-lens, cloaking, and ultraslow and stopped light [1-5]. Usually, both the electric permittivity and the magnetic permeability need to be controlled to realize metamaterials. © 2017 IEEE.

There has recently been a surge of interest in the physics and applications of broadband ultraslow waves in nanoscale structures operating below the diffraction limit. They range from light waves or surface plasmons in nanoplasmonic devices to sound waves in acoustic-metamaterial waveguides, as well as fermions and phonon polaritons in graphene and van der Waals crystals and heterostructures.We review the underlying physics of these structures, which upend traditional wave-slowing approaches based on resonances or on periodic configurations above the diffraction limit. Light can now be tightly focused on the nanoscale at intensities up to  1000 times larger than the output of incumbent near-field scanning optical microscopes, while exhibiting greatly boosted density of states and strong wave-matter interactions. We elucidate the general methodology by which broadband and, simultaneously, large wave decelerations, well below the diffraction limit, can be obtained in the above interdisciplinary fields.We also highlight a range of applications for renewable energy, biosensing, quantum optics, high-density magnetic data storage, and nanoscale chemical mapping.

Recent progress in the design and realization of optical antennas enclosing fluorescent materials has demonstrated large spontaneous-emission enhancements and, simultaneously, high radiation efficiencies. We discuss here that an important objective of such work is to increase spontaneous-emission rates to such a degree that light-emitting diodes (LEDs) can possess modulation speeds exceeding those of typical semiconductor lasers, which are usually in the range ∼20-50 GHz. We outline the underlying physics that enable large spontaneous-emission enhancements in metallic nanostructures, and we then discuss recent theoretical and experimentally promising results, where enhancements larger than a factor of ∼300 have been reported, with radiation efficiencies exceeding 50%. We provide key comparative advantages of these structures in comparison to conventional dielectric microcavity designs, namely the fact that the enhancement of spontaneous emission can be relatively nonresonant (i.e., broadband) and that the antenna nanostructures can be spectrally and structurally compatible for integration with a wide class of emitters, including organic dyes, diamond nanocrystals and colloidal quantum dots. Finally, we point out that physical insight into the underlying effects can be gained by analyzing these metallic nanostructures in their equivalent-circuit (or nano-antenna) model, showing that all main effects (including the Purcell factor) can adequately be described in that approach. ©2016 Optical Society of America.

We introduce a new method to localize lightwaves. We show how the interaction of a gain medium with a planar deep-UV plasmonic heterostructure, at its zero-vg point, strongly localizes light. A quantum-coherent drive provides a means to control the localization and dramatically improve the dynamics. © OSA 2015.

We introduce a new method to localize lightwaves. We show how the interaction of a gain medium with a planar deep-UV plasmonic heterostructure, at its zero-vg point, strongly localizes light. A quantum-coherent drive provides a means to control the localization and dramatically improve the dynamics. © 2015 OSA.

We introduce a scheme where a time-dependent source excites "complex-frequency" modes in uniform plasmonic heterostructures, enabling complete and dispersionless stopping of light pulses, resilient to realistic levels of dissipative, radiative, and surface-roughness losses. Using transparent conducting oxides at telecommunication wavelengths we show how, without increasing optical losses, multiple light pulses can decay with time precisely at their injection points, unable to propagate despite the complete absence of barriers in front or behind them. Our results theoretically demonstrate extraordinary large light-deceleration factors (of the order of 1.5×107) in integrated nanophotonic media, comparable only to those attainable with ultracold atomic vapors or with quantum coherence effects, such as coherent population oscillations, in ruby crystals. © 2014 American Physical Society.

We introduce a quantum-coherence driven many-body photonic nanostructure, in which we observe self-organized phase-transitions to a new type of non-potential light localization, resilient to dissipation, fluctuations, and nonlinear interactions. © OSA 2016.

The extremely large speed of light is a tremendous asset but also makes it challenging to control, store or shrink beyond its wavelength. Particularly, reducing the speed of light down to zero is of fundamental scientific interest that could usher in a host of important photonic applications, some of which are hitherto fundamentally inaccessible. These include cavity-free, low-threshold nanolasers, novel solar-cell designs for efficient harvesting of light, nanoscale quantum information processing (owing to the enhanced density of states), as well as enhanced biomolecular sensing. We shall here present nanoplasmonic-based schemes where timedependent sources excite “complex-frequency†modes in uniform (plasmonic) heterostructures, enabling complete and dispersion-free stopping of light pulses, resilient to realistic levels of dissipative, radiative and surface-roughness losses. Our theoretical and computational results demonstrate extraordinary large lightdeceleration factors (of the order of 15,000,000) in integrated nanophotonic media, comparable only to those attainable with ultracold atomic vapours or with quantum coherence effects, such as coherent population oscillations, in ruby crystals. © 2014 SPIE.

Integrating amplifying media with metamaterials allows loss-free plasmonic operation and opens a route for controlling nanoscale quantum emitters.

Optical metamaterials and nanoplasmonics bridge the gap between conventional optics and the nanoworld. Exciting and technologically important capabilities range from subwavelength focusing and stopped light to invisibility cloaking, with applications across science and engineering from biophotonics to nanocircuitry. A problem that has hampered practical implementations have been dissipative metal losses, but the efficient use of optical gain has been shown to compensate these and to allow for loss-free operation, amplification and nanoscopic lasing. Here, we review recent and ongoing progress in the realm of active, gain-enhanced nanoplasmonic metamaterials. On introducing and expounding the underlying theoretical concepts of the complex interaction between plasmons and gain media, we examine the experimental efforts in areas such as nanoplasmonic and metamaterial lasers. We underscore important current trends that may lead to improved active imaging, ultrafast nonlinearities on the nanoscale or cavity-free lasing in the stopped-light regime. © 2012 Macmillan Publishers Limited. All rights reserved.

Nanoplasmonic metamaterials are an exciting new class of engineered media that promise a range of important applications, such as subwavelength focusing, cloaking, and slowing/stopping of light. At optical frequencies, using gain to overcome potentially not insignificant losses has recently emerged as a viable solution to ultra-low-loss operation that may lead to next-generation active metamaterials. Maxwell-Bloch models for active nanoplasmonic metamaterials are able to describe the coherent spatiotemporal and nonlinear gain-plasmon dynamics. Here, we extend the Maxwell-Bloch theory to a Maxwell-Bloch Langevin approach-a spatially resolved model that describes the light field and noise dynamics in gain-enhanced nanoplasmonic structures. Using the example of an optically pumped nanofishnet metamaterial with an embedded laser dye (four-level) medium exhibiting a negative refractive index, we demonstrate the transition from loss-compensation to amplification and to nanolasing. We observe ultrafast relaxation oscillations of the bright negative-index mode with frequencies just below the THz regime. The influence of noise on mode competition and the onset and magnitude of the relaxation oscillations is elucidated, and the dynamics and spectra of the emitted light indicate that coherent amplification and lasing are maintained even in the presence of noise and amplified spontaneous emission. © 2012 American Chemical Society.

Active nanoplasmonic metamaterials support bright and dark modes that compete for gain. Using a Maxwell-Bloch approach incorporating Langevin noise we study the lasing dynamics in an active nanofishnet structure. We report that lasing of the bright negative-index mode is possible if the higher-Q dark mode is discriminated by gain, spatially or spectrally. The nonlinear competition during the transient phase is followed by steady-state emission where bright and dark modes can coexist. We analyze the influence of pump intensity and polarization and explore methods for mode control. © 2012 American Physical Society.

Plasmonic metamaterials form an exciting new class of engineered media that promise a range of important applications, such as subwavelength focusing, cloaking and slowing/stopping of light. Recently it has been shown that the internal losses due to the natural absorption of metals at optical frequencies can be compensated by gain. Here, we employ a Maxwell-Bloch methodology which allows us to study the dynamics of the coherent plasmon-gain interaction, nonlinear saturation, field enhancement and radiative damping. Using numerical pump-probe experiments on a double-fishnet metamaterial with dye-molecule inclusions we investigate the buildup of the inversion and the formation of the plasmonic modes in the low-Q fishnet cavity. We find that loss compensation occurs in the negative-refractive-index regime and that, due to the loss compensation and the associated sharpening of the resonance, the real part of the refractive index of the metamaterial becomes more negative compared to the passive case. Furthermore, we investigate the behavior of the metamaterial above the lasing threshold, and we identify the occurrence of a far-field lasing burst and gain depletion. Our results provide deep insight into the internal processes that affect the macroscopic properties of active metamaterials. This could guide the development of amplifying and lasing plasmonic nanostructures. © 2012 Springer-Verlag.

We present an overview of recent advances within the field of slow- and stopped-light in metamaterial and plasmonic waveguides. We start by elucidating the mechanisms by which these configurations can enable complete stopping of light. Decoherence mechanisms may destroy the zero-group-velocity condition for real-frequency/complex-wavevector modes, but we show that metamaterial and nanoplasmonic waveguides also support complex-frequency/real-wavevector modes that uphold the light-stopping condition. A further point of focus is how, by using gain, dissipative losses can be overcome in the slow- and stopped-light regimes. To this end, on the basis of full-wave finite-difference time-domain (FDTD) simulations and analytic transfer-matrix calculations, we show that the incorporation of thin layers made of an active medium, placed adjacently to the core layer of a negative-refractive-index waveguide, can fully remove dissipative losses in a slow- or stopped-light regime where the effective index of the guided lightwave remains negative. © 2012 Elsevier B.V. All rights reserved.

Using a full-wave Maxwell-Bloch Langevin approach we show how, by using gain, we may overcome losses in double-fishnet negative-index metamaterials, as well as achieve net steady-state amplification and nanoscopic lasing over a broad ultrathin area. © OSA 2012.

Active nanoplasmonic metamaterials, pumped above lasing threshold, can exhibit dynamic competition between bright, radiative and dark, trapped modes of the structure. We study the spatio-temporal mode competition and explore methods of mode control. © 2011 Optical Society of America.

Active nanoplasmonic metamaterials, pumped above lasing threshold, can exhibit dynamic competition between bright, radiative and dark, trapped modes of the structure. We study the spatio-temporal mode competition and explore methods of mode control. © 2012 OSA.

We outline recent advances in active gain-enhanced plasmonic metamaterials revealing and elucidating the inherent complex interplay of light, surface plasmon polaritons and gain materials to allow a compensation of dissipative losses in negative-refractive-index optical metamaterials and to achieve net steady-state amplification and nanoscopic lasing over a broad but ultrathin area. On the basis of a fully 3-dimensional Maxwell-Bloch Langevin approach we then demonstrate that in a suitably designed gain-enhanced plasmonic/ metamaterial heterostructure light pulses can be completely stopped at well-accessed complex-frequency zero-group-velocity points leading to thresholdless nanolasers that beat the diffraction limit via a novel, stopped-light mode-locking mechanism. © 2012 IEEE.

We present a plasmonic waveguide where light pulses are stopped at well-accessed complex-frequency zero-group-velocity points. Introducing gain at such points results in cavity-free, "thresholdless" nanolasers beating the diffraction limit via a novel, stopped-light mode-locking mechanism. © OSA 2012.

A Comment on the Letter by Mark I. Stockman, Phys. Rev. Lett.PRLTAO0031-9007 106, 156802 (2011)10.1103/PhysRevLett.106.156802. The author of the Letter offers a Reply. © 2011 American Physical Society.

Plasmonic metamaterials form an exciting new class of engineered media that promise a range of important applications, such as subwavelength focusing, cloaking and slowing/stopping of light. At optical frequencies, using gain to overcome potentially not insignificant losses has recently emerged as a viable solution to ultralow-loss operation that may lead to next-generation active metamaterials. Here, we employ a Maxwell-Bloch methodology for the analysis of these gain-enhanced optical nanomaterials. The method allows us to study the dynamics of the coherent plasmon-gain interaction, nonlinear saturation, field enhancement as well as radiative and non-radiative damping such as tunnelling and Förster coupling. Using numerical pump-probe experiments on a double-fishnet metamaterial with dye-molecule inclusions we investigate the build-up of the inversion and the formation of the plasmonic modes in the low-Q fishnet cavity. We find that loss compensation occurs in the negative-refractiveindex regime and that, due to the loss compensation and the associated sharpening of the resonance, the real part of the refractive index of the metamaterial becomes more negative compared to the passive case. Furthermore, we investigate the behaviour of the metamaterial above the lasing threshold, and we identify the occurrence of a far-field lasing burst and gain depletion when higher dye densities are used. Our results provide deep insight into the internal processes that affect the macroscopic properties of active metamaterials. This could guide the development of amplifying and lasing plasmonic nanostructures. © 2011 Copyright Society of Photo-Optical Instrumentation Engineers (SPIE).

We theoretically and numerically analyze a five-layer "trapped rainbow" waveguide made of a passive negative refractive index (NRI) core layer and gain strips in the cladding. Analytic transfer-matrix calculations and full-wave time-domain simulations are deployed to calculate, both in the frequency and in the time domain, the losses or gain experienced by complex-wave-vector and complex-frequency modes. We find excellent agreement between five distinct sets of results, showing that the use of evanescent pumping (gain) can compensate the losses in the NRI slow- and stopped-light regimes. © 2011 American Physical Society.

Photonic metamaterials allow for a range of exciting applications unattainable with ordinary dielectrics. However, the metallic nature of their meta-atoms may result in increased optical losses. Gain-enhanced metamaterials are a potential solution to this problem, but the conception of realistic, three-dimensional designs is a challenging task. Starting from fundamental electrodynamic and quantum mechanical equations, we establish and deploy a rigorous theoretical model for the spatial and temporal interaction of lightwaves with free and bound electrons inside and around metallic (nano-) structures and gain media. The derived numerical framework allows us to self-consistently study the dynamics and impact of the coherent plasmon-gain interaction, nonlinear saturation, field enhancement, radiative damping and spatial dispersion. Using numerical pump-probe experiments on a double-fishnet metamaterial structure with dye molecule inclusions, we investigate the build-up of the inversion profile and the formation of the plasmonic modes in a low-Q cavity. We find that full loss compensation occurs in a regime where the real part of the effective refractive index of the metamaterial becomes more negative compared to the passive case. Our results provide a deep insight into how internal processes affect the overall optical properties of active photonic metamaterials fostering new approaches to the design of practical, loss-compensated plasmonic nanostructures. © 2011 The Royal Society.

We outline the theory of slow-light propagation in waveguides featuring negative electromagnetic parameters (permittivity, permeability and/or refractive index). We explain the mechanism by which these heterostructures can enable stopping of light even in the presence of disorder and, simultaneously, dissipative losses. Using full-wave numerical simulations and analytical transfer-matrix calculations we show that the incorporation of thin layers made of an active medium adjacently to the core layer of a negative-refractive-index waveguide can completely remove dissipative losses - in a slow- or stopped-light regime where the effective index of the guided lightwave remains negative. We also review and compare several 'trapped rainbow' schemes that have recently been proposed for slowing and stopping waves. © 2011 SPIE.

We establish the theory of light amplification and lasing in optical metamaterials. We show how loss compensation in slow-light metamaterial heterostructures becomes possible, and elucidate light amplification and lasing in active nanoplasmonic double-fishnet metamaterials. © 2011 Optical Society of America.

We establish a theory that traces light amplification in an active double-fishnet metamaterial back to its microscopic origins. Based on ab initio calculations of the light and plasmon fields we extract energy rates and conversion efficiencies associated with gain and loss channels directly from Poynting's theorem. We find that for the negative refractive index mode both radiative loss and gain outweigh resistive loss by more than a factor of 2, opening a broad window of steady-state amplification (free of instabilities) accessible even when a gain reduction close to the metal is taken into account. © 2011 American Physical Society.

Using full-wave simulations we show how the incorporation of thin layers made of an active medium adjacently to the core layer of a negative-index slow-light waveguide can completely remove dissipative optical losses. © 2010 Optical Society of America.

Metamaterials exhibiting negative electromagnetic parameters can enable a multitude of exciting applications, but currently their performance is limited by the occurrence of losses-particularly radiation losses, which dominate over their dissipative counterparts even in the optical regime. Here, a metamaterial configuration is conceived that judiciously generalizes the traditional electromagnetically induced transparency (EIT) scheme-by which radiation losses can be restrained-in such a way that EIT can be observed and exploited in negative-magnetic metamaterials. Analytic theory and three-dimensional simulations unveil the required route: introduction of poor-conductor meta-atoms next to the good-conductor meta-atoms of a magnetic metamaterial. This setup results in a frequency band where the metamaterial remains negative-magnetic, while its loss-performance dramatically improves owing to suppression of radiation damping. Furthermore, we show that placing the two meta-atoms on orthogonal planes gives rise to a passive anisotropic metamaterial exhibiting permeabilities with negative real parts (Re {μ} <0) and active imaginary parts (Im {μ} >0 for an e+iωt time dependence) along its principal crystallographic axes. © 2010 The American Physical Society.

We review recent theoretical and experimental breakthroughs in the realm of slow and stopped light in structured photonic media featuring negative electromagnetic parameters (permittivity/permeability and/or refractive index). We explain how and why these structures can enable complete stopping of light even in the presence of disorder and, simultaneously, dissipative losses. Using full-wave numerical simulations we show that the incorporation of thin layers made of an active medium adjacently to the core layer of a negative-refractive- index waveguide can completely remove dissipative losses - in a slow-light regime where the effective index of the guided wave is negative. We, also, review and compare several 'trapped rainbow' schemes that have recently been proposed for slowing and stopping light. © 2010 Copyright SPIE - The International Society for Optical Engineering.

We review recent theoretical and experimental in progress in the realisation of slow and stopped light by the 'trapped rainbow' principle in optical metamaterials featuring negative electromagnetic parameters (permittivity/permeability and/or refractive index). We explain how and why these structures can enable complete stopping of light even in the presence of disorder and, simultaneously, dissipative losses. Using full-wave numerical simulations we show that the incorporation of thin layers made of an active medium adjacently to the core layer of a negative-refractive-index waveguide can completely remove dissipative losses - in a slow-light regime where the effective index of the guided wave is negative. © 2010 SPIE.

We investigate on the basis of a full three-dimensional spatio-temporal Maxwell-Bloch approach the possibility of complete loss compensation in non-bianisotropic negative refractive index (NRI) metamaterials. We show that a judicious incorporation of optically pumped gain materials, such as laser dyes, into a double-fishnet metamaterial can enable gain in the regime where the real part n′ of the resulting effective refractive index (n = n′ + in″) is negative. It is demonstrated that a frequency band exists for realistic opto-geometric and material (gain/loss) parameters where n′ < 0 and simultaneously n″ < 0 hold, resulting in a figure-of-merit that diverges at two distinct frequency points. Having ensured on the microscopic, meta-molecular level that realistic levels of losses and even gain are accessible in the considered optical frequency regime we explore the possibility of compensating propagation losses in a negative refractive index slow-light metamaterial heterostructure. The heterostructure is composed of a negative refractive index core-layer bounded symmetrically by two thin active cladding layers providing evanescent gain to the propagating slow light pulses. It is shown that backward-propagating light - having anti-parallel phase and group velocities and experiencing a negative effective refractive index - can be amplified inside this slow-light waveguide structure. Our results provide a direct and unambiguous proof that full compensation of losses and attainment of gain are possible on the microscopic as well as the macroscopic level in the regime where the non-bianisotropic refractive index is negative - including, in particular, the regime where the guided light propagates slowly. © 2010 SPIE.

On the basis of a full-vectorial three-dimensional Maxwell-Bloch approach we investigate the possibility of using gain to overcome losses in a negative refractive index fishnet metamaterial. We show that appropriate placing of optically pumped laser dyes (gain) into the metamaterial structure results in a frequency band where the nonbianisotropic metamaterial becomes amplifying. In that region both the real and the imaginary part of the effective refractive index become simultaneously negative and the figure of merit diverges at two distinct frequency points. © 2010 The American Physical Society.

A comprehensive theoretical study of the optical properties and switching competence of double-shell photonic crystals (DSPC) and doubleinverse-opal photonic crystals (DIOPC) is presented. Our analysis reveals that a DIOPC structure with a silicon (Si) background exhibits a complete photonic bandgap (PBG), which can be completely switched on and off by moving the core spheres inside the air pores of the inverse opal. We show that the size of this switchable PBG assumes a value of 3.78% upon judicious structural optimization, while its existence is almost independent of the radii of the interconnecting cylinders, whose sizes are difficult to control during the fabrication process. The Si-based DIOPC may thus offer a novel and practical route to complete PBG switching and optical functionality.

We discuss the optical properties metal nanoforests - a composite metamaterial in which silver nanowires are aligned inside a finite-thickness dielectric host medium. Using finite-element modelling and a self-consistent extraction of effective-medium parameters, we find that this structure can enable an effective optical diamagnetic response that is orders of magnitude stronger compared to that of naturally occurring diamagnetic materials. Our analysis reveals that there is a frequency region where the nanoforest exhibits strong diamagnetic response while simultaneously allowing for high transmission of incident electromagnetic waves. Our analysis shows that the phenomena are robust to the presence of disorder, in the occurrence of which it can still facilitate high figure-of-merit diamagnetic responses. © 2009 SPIE.

We provide an overview of 'trapped rainbow' techniques for stopping light. We show that guided modes with real propagation consrtant and complex frequency can be 'trapped rainbow'-stopped even in the presence of waveguide losses. © 2009 Optical Society of America.

We review recent progress in the realm of ultra-slow and stored light inside metamaterial waveguides. We elucidate a number of critical issues pertaining to the study of light propagation in various slowlight metamaterial structures. © 2008 Optical Society of America.

Recently there has been a considerable interest in metamaterial waveguide structures capable of dramatically slowing down or, even, completely stopping light. Here, we shall explain in some detail the working principle behind the deceleration and/or stopping of light in metamaterial structures, and review the various, metamaterial-enabled, methods that have been proposed thus far towards achieving such a goal. Further, we will concisely describe how one can construct zero-loss metamaterials over a continuous and broad (but not infinite) range of frequencies, which is an essential prerequisite for any slow-light system. Moreover, it will be explained that inside such waveguide structures light can in principle be stopped (zero group velocity, νg = 0) even in the presence of losses. By nature, metamaterial-enabled schemes for stopping/storing light invoke solid-state materials and, as such, are not subject to low-temperature or atomic coherence limitations. Furthermore, these methods simultaneously allow for broad bandwidth operation, since they do not rely on group index resonances; large delay-bandwidth products, since a wave packet can, in principle, be completely stopped and buffered indefinitely; and (for the case, in particular, where a negative-index metamaterial is used) high, almost 100%, in/out-coupling efficiencies. Thus, we conclude that these methods for trapping photons, which can be realised using existing technology, could open the way to a multitude of hybrid optoelectronic devices to be used in 'quantum information' processing, communication networks and signal processors and may conceivably herald a new realm of combined metamaterials and slow light research. © 2009 SPIE.

Using finite-difference time-domain (FDTD) simulations we investigate the propagation of light pulses in waveguides having a core made of a negative-refractive-index metamaterial. In order to validate our model we carry out separate simulations for a variety of waveguide core thickness. The numerical results not only qualitatively confirm that light pulses travel slower in waveguides with thinner cores, but further they reveal that the effective refractive indices experienced by the propagating pulses compare favourably with exact theoretical predictions. We also examine the propagation of light pulses in waveguides with adiabatically, longitudinally varying core refractive index. The effective refractive indices extracted from these simulations confirm previous theoretical predictions while, both a slowing and an increase in the amplitude of the pulses are observed. © 2009 IOP Publishing Ltd.

We review recent progress in the realm of ultra-slow and stored light inside metamaterial waveguides. We elucidate a number of critical issues pertaining to the study of light propagation in various slowlight metamaterial structures. © 2008 Optical Society of America.

We review recent progress in the realm of ultra-slow and stored light inside metamaterial waveguides. We elucidate a number of critical issues pertaining to the study of light propagation in various slow-light metamaterial structures. ©2008 Optical Society of America.

A comprehensive investigation of the optical properties of a composite metamaterial in which silver nanowires are aligned inside a finite-thickness dielectric host medium is presented. Using a rigorous finite-element based modelling approach, together with a self-consistent process for the extraction of effective-medium parameters, we find that this structure can enable an effective optical diamagnetic response that is orders of magnitude stronger compared to that of naturally occurring diamagnetic materials. Interestingly, our analysis reveals that there is a frequency region where the nanoforest exhibits a strong diamagnetic response while simultaneously allowing for high transmission of incident electromagnetic waves. We examine the physical origin behind the magnetic properties of this structure, as well as its resilience to fabrication imperfections. Our analysis shows that the structure is robust to the presence of disorder, in the occurrence of which it can still facilitate high figure-of-merit diamagnetic responses. © 2009 IOP Publishing Ltd.

The authors theoretically demonstrate a practical scheme for robust and complete photonic bandgap switching using a three-dimensional double-inverse-opal photonic crystal. The investigated structure consists of a close-packed face-centered-cubic arrangement of spherical air pores, interconnected via air channels and embedded in a high-index (tin disulfide) backbone. We show that by placing lower-index movable dielectric scatterers (titania) inside the air pores, a complete photonic bandgap opens for certain positions of the scatterers, which altogether closes for other positions. Our analysis reveals that this switching scheme is robust to geometric imperfections and allows for sizeable bandgap switching. © 2008 American Institute of Physics.

A proposal for transporting photons invisibly between two unconnected points in space seems worthy of a Star Trek plot. But it is in principle wholly realizable, and could open up new vistas - literally. ©2008 Nature Publishing Group.

Lightwaves guided along an adiabatically tapered negative-index heterostructure can efficiently be brought to a complete halt. We prove this conclusion by means of, both, full-wave and pertinent ray-tracing analyses. © 2007 Optical Society of America.

We demonstrate the deceleration of guided electromagnetic waves propagating along an adiabatically tapered negative-refractive-index metamaterial heterostructure and show that light can ideally be brought to a complete halt. It is analytically shown that, in principle, this method simultaneously allows for broad bandwidth operation (since it does not rely on group index resonances), large delay-bandwidth products (since a wave packet can be completely stopped and buffered indefinitely) and high, almost 100 % in/out-coupling efficiencies. The halting of a monochromatic field component travelling along the heterostructure is demonstrated on the basis of a wave analysis and confirmed in a pertinent ray analysis, which unmistakably illustrates the trapping of the associated light-ray and the formation of a double light-ray cone (optical clepsydra) at the point where the ray is trapped. This method for trapping photons conceivably opens the way to a multitude of hybrid optoelectronic devices to be used in quantum information processing, communication networks and signal processors and may herald a new realm of combined metamaterials and slow light research © 2008 IEEE.

We show how guided electromagnetic waves propagating along an adiabatically tapered negative-refractive-index metamaterial heterostructure can be brought to a complete halt. It is analytically shown that, in principle, this method simultaneously allows for broad bandwidth operation (since it does not rely on group index resonances), large delaybandwidth products (since a wave packet can be completely stopped and buffered indefinitely) and high, almost 100%, in/out-coupling efficiencies. By nature, the presented scheme invokes solid-state materials and, as such, is not subject to low-temperature or atomic coherence limitations. A wave analysis, which demonstrates the halting of a monochromatic field component travelling along the heterostructure, is followed by a pertinent ray analysis, which unmistakably illustrates the trapping of the associated light-ray and the formation of a double light-ray cone ('optical clepsydra') at the point where the ray is trapped. This method for trapping photons conceivably opens the way to a multitude of hybrid optoelectronic devices to be used in 'quantum information' processing, communication networks and signal processors and may herald a new realm of combined metamaterials and slow light research.

A competent method for slowing and completely stopping light, based on wave propagation along an adiabatically tapered negative-refractive-index metamaterial heterostructure, is presented. It is analytically shown that, in principle, this method simultaneously allows for broad bandwidth operation (since it does not rely on group index resonances), large delay-bandwidth products (since a wave packet can be completely stopped and buffered indefinitely) and high, almost 100%, in/out-coupling efficiencies. Moreover, by nature, the presented scheme invokes solid-state materials and, as such, is not subject to low-temperature or atomic coherence limitations. A wave analysis, which demonstrates the halting of a monochromatic field component travelling along the heterostructure, is followed by a corresponding ray analysis that illustrates the trapping of the associated light-ray and the formation of a double light-ray cone ('optical clepsydra'). This method for trapping photons conceivably opens the way to a multitude of hybrid, optoelectronic devices to be used in 'quantum information' processing, communication networks and signal processors, and may herald a new realm of combined metamaterials and slow light research.

The trend in silicon photonics, in the last few years has been to reduce waveguide size to obtain maximum gain in the real estate of devices as well as to increase the performance of active devices. Using different methods for the modulation, optical modulators in silicon have seen their bandwidth increased to reach multi GHz frequencies. In order to simplify fabrication, one requirement for a waveguide, as well as for a modulator, is to retain polarisation independence in any state of operation and to be as small as possible. In this paper we provide a way to obtain polarisation independence and improve the efficiency of an optical modulator using a V-shaped pn junction base on the natural etch angle of silicon, 54.7 deg. This modulator is compared to a flat junction depletion type modulator of the same size and doping concentration. © 2007 Optical Society of America.

We introduce an efficient method for slowing and stopping/storing light, which is based on wave propagation along a slowly axially varying, adiabatically tapered, negative refractive index metamaterial heterostructure. We analytically show that the present method can, in principle, simultaneously allow for broad bandwidth operation (since it does not rely on group index resonances), large delay-bandwidth products (since a wave packet can be completely stopped and buffered indefinitely) and high, almost 100%, in/out-coupling efficiencies. Moreover, by nature, the presented scheme invokes solid-state materials and, as such, is not subject to low-temperature or atomic coherence limitations. This method for trapping photons conceivably opens the way to a multitude of hybrid, optoelectronic devices to be used in 'quantum information' processing, communication networks and signal processors, and may herald a new realm of combined metamaterials and slow light research.

We analytically demonstrate that a lightwave propagating along an adiabatically tapered waveguide with a core of negative refractive index (NRI) material can efficiently be brought to a complete standstill, while allowing for more than 90% in-coupling from an ordinary dielectric waveguide. © 2007 Optical Society of America.

Light usually propagates inside transparent materials in well known ways. However, recent research has examined the possibility of modifying the way the light travels by taking a normal transparent dielectric and inserting tiny metallic inclusions of various shapes and arrangements. As light passes through these structures, oscillating electric currents are set up that generate electromagnetic field moments; these can lead to dramatic effects on the light propagation, such as negative refraction. Possible applications include lenses that break traditional diffraction limits and 'invisibility cloaks' (refs 5, 6). Significantly less research has focused on the potential of such structures for slowing, trapping and releasing light signals. Here we demonstrate theoretically that an axially varying heterostructure with a metamaterial core of negative refractive index can be used to efficiently and coherently bring light to a complete standstill. In contrast to previous approaches for decelerating and storing light, the present scheme simultaneously allows for high in-coupling efficiencies and broadband, room-temperature operation. Surprisingly, our analysis reveals a critical point at which the effective thickness of the waveguide is reduced to zero, preventing the light wave from propagating further. At this point, the light ray is permanently trapped, its trajectory forming a double light-cone that we call an 'optical clepsydra'. Each frequency component of the wave packet is stopped at a different guide thickness, leading to the spatial separation of its spectrum and the formation of a 'trapped rainbow'. Our results bridge the gap between two important contemporary realms of science - metamaterials and slow light. Combined investigations may lead to applications in optical data processing and storage or the realization of quantum optical memories. ©2007 Nature Publishing Group.

We show that, with judicious choice of opto-geometrical parameters, oscillatory waves guided by generalized left-handed slab waveguides can attain zero group velocity. Advantages compared to previous methods of slowing or stopping light are concisely discussed. © 2006 Optical Society of America.

Three-dimensional full-wave electromagnetic analysis of a travelling-wave heterojunction phototransistor (HPT) is presented. Employing the finite-difference time-domain method and run on a fast, parallel processing machine the simulation herein allowed, for the first time to our knowledge, the simultaneous investigation of the optical and electrical characteristics of the travelling-wave structure. Snapshots of the field propagation inside the device provide valuable insight into its passive behaviour and conclusively demonstrate the velocity mismatch between the optical wave and the photogenerated electrical signal. Numerical results are presented for the device's output characteristic impedance, photocurrent and effective refractive indices of the optical and electrical signal that quantify the difference in the velocities of the two waves. Moreover, results obtained from the method's initial test in the simulation of an asymmetric planar optical waveguide, similar to the one integrated within the HPT's structure, compare very favourably with the theory. © 2006 IOP Publishing Ltd.

The authors present an exact, analytic study of oscillatory modes guided by generalized asymmetric two-dimensional planar heterostructures with negative refractive index in either the core or the cladding. It is shown that, in sharp contrast to normal dielectric configurations, these waveguides always possess a frequency region where the second-order oscillatory mode may exist alone and allow for attaining zero group velocity under weak guidance conditions. In addition the mode has a field distribution that renders it excitable with an end-fire approach, making such structures attractive for applications requiring slow light. Advantages compared to previous methods of slowing or stopping light are discussed. © 2006 American Institute of Physics.

We present a detailed analytical study of surface plasmon polaritons (SPPs) in generalized asymmetric slab waveguides with a core of negative permittivity and permeability. Profiting from the duality principle, we confine ourselves to the analysis of p -polarized (TM) SPP eigenmodes, which also occur in thin metallic films. It is shown that the left-handed (LH) structures considered here support a richer variety of SPPs when compared to their metallic counterparts. Depending on the refractive index distribution, the permittivity of each medium and the thickness of the core, a total of 30 solutions to the involved characteristic equation are identified in a unified manner and classified systematically. In order to identify conclusively all SPPs, we follow an analytical methodology based directly on the solution constraints inherent in the associated transcendental equation. This treatment reveals striking features of the formed SPP eigenmodes, such as the existence of "supermodes" when no SPP is supported at one of the slab interfaces. Moreover, our study reveals the opening of gaps in the SPP dispersion diagrams, occurrence of monomodal propagation for specific choices of the material parameters, presence of SPPs with no cutoff thickness and coexistence of three eigenmodes, with double mode-degeneracy points occurring twice. The eigenmodes with negative energy flux that give rise to negative group velocity are identified via a closed-form expression for the time-averaged power flow P in the guide. For each eigenmode, we examine the variation of P with the reduced slab thickness and discuss key features of the effective index geometric dispersion diagram, most of which are unique to the generalized structures studied herein. © 2006 The American Physical Society.

A robust methodology is presented for the accurate modal analysis of three-dimensional dielectric waveguides with the finite-difference time-domain (FDTD) method. We investigate the propagation of well-defined vector modes along strongly guiding rectangular waveguides. Eigensolutions of the vectorial wave equation are utilized in the simulations to accurately launch the fundamental and higher order eigenmodes. Results for their FDTD-computed propagation constant are found to be in excellent agreement with existing mode-solving techniques. Improved accuracy or significant computational savings are achieved when the nonstandard FDTD concepts are incorporated in the context of the present analysis. © 2005 IEEE.

We investigate generalized asymmetric left-handed (LH) slab waveguides that constitute an extension to the symmetric structure initially proposed by Pendry. By utilizing the asymmetry it has been shown that they have the potential for increased image resolution. This is due to the amplification of the formed surface waves (SWs) that is able to compensate material losses. Some preliminary studies in this direction have been reported in the literature but, to the best of our knowledge, there is no complete study of all possible S W eigenmodes supported by such LH waveguides. A rigorous theoretical investigation is presented herein that offers clear mathematical explanation and detailed physical insight into the formation of surface polaritons (SPs) in these heterostructures and provides the conditions for their existence. It is found that a rich variety of SP modes can exist (30 in total) that depend critically on the combination of the different refractive indices, constitutive parameters ε(ω) and μ(ω) the sample geometry. For each case we provide the geometric dispersion diagram and the profile of the corresponding stable field configuration from which the various characteristics of the mode (enhancement, phase reversal) are apparent, An interesting result of the above analysis is that, for certain choices of the material parameters, the coupling between the interfaces allows the existence of new 'supermodes' when no stable solutions exist at the isolated interfaces of the slab alone. Finally, the modes that give rise to negative group velocity are identified and key features of the dispersion diagrams are discussed, most of which are unique to the structures studied herein and have not been previously recovered with symmetrical studies.

The development and application of a three-dimensional finite-difference time-domain (FDTD) model for a travelingwave heterojunction phototransistor (TW-HPT) are presented. The model is enhanced using effective permittivity schemes at the dielectric interfaces and special techniques for the treatment of very thin material sheets. The 3D fullwave electgromagnetic model allows the numerical calculation of the output photocurrent, electrical characteristic impedance, light absorption, microwave losses as well as microwave and optical dispersion. Run in a fast, parallel processing, machine the simulation herein allowed (for the first time, to the best of the authors' knowledge) the simultaneous investigation of the optical and microwave characteristics of the traveling-wave structure. This is in contrast to the approach followed by other researchers in the past, as well as by popular simulation packages, based on which results only for the microwave property can be obtained. Snapshots of the field propagation inside the device provide valuable insight into its passive behavior and clearly demonstrate the device's velocity mismatch between the optical signal and the photogenerated electrical pulse. Numerical results for the effective refractive indices of the optical and electrical wave quantify the difference in the velocities of the two waves.

Three-dimensional finite-difference time-domain modelling of a travelling-wave heterojunction phototransistor is presented. The electromagnetic model allows the simultaneous simulation of the optical and microwave properties of the travelling-wave structure. The results clearly demonstrate the effect of velocity mismatch between the optical wave and the photogenerated electrical wave.