Modern nanophotonic and meta-optical devices utilize a tremendous number of structural degrees of freedom to enhance light--matter interactions. A fundamental question is how large such enhancements can be. We develop an analytical framework to derive upper bounds to single-frequency electromagnetic response, across near- and far-field regimes, for any materials, naturally incorporating the tandem effects of material- and radiation-induced losses. Our framework relies on a power-conservation law for the polarization fields induced in any scatterer. It unifies previous theories on optical scattering bounds and reveals new insight for optimal nanophotonic design, with applications including far-field scattering, near-field local-density-of-states engineering, optimal wavefront shaping, and the design of perfect absorbers. Our bounds predict strikingly large minimal thicknesses for arbitrarily patterned perfect absorbers, ranging from 50--100 nm for typical materials at visible wavelengths to micrometer-scale thicknesses for polar dielectrics at infrared wavelengths. We use inverse design to discover metasurface structures approaching the minimum-thickness perfect-absorber bounds.
We derive upper bounds to free-space concentration of electromagnetic waves, mapping out the limits to maximum intensity for any spot size and optical beam-shaping device. For sub-diffraction-limited optical beams, our bounds suggest the possibility for orders-of-magnitude intensity enhancements compared to existing demonstrations, and we use inverse design to discover metasurfaces operating near these new limits. We also demonstrate that our bounds may surprisingly describe maximum concentration defined by a wide variety of metrics. Our bounds require no assumptions about symmetry, scalar wav…
We investigate a dynamically controllable electromagnetically induced transparency-like metamaterial. The unit structure of the metamaterial consists of a low-quality-factor resonator and a high-quality-factor resonator. The field gradient of the incident electromagnetic wave in the transverse direction induces coupling between these two types of resonators and causes a transparency phenomenon. We present the simulation and experimental results for the dynamic control of the electromagnetic response.
Designing objects with predefined optical properties is a task of fundamental importance for nanophotonics, and chirality is a prototypical example of such a property, with applications ranging from photochemistry to nonlinear photonics. A measure of electromagnetic chirality with a well-defined upper bound has recently been proposed. Here, we optimize the shape of silver helices at discrete frequencies ranging from the far infrared to the optical band. Gaussian process optimization, taking into account also shape derivative information of the helices scattering response, is used to maximize t…
Toroidal dipole, first described by Ia. B. Zeldovich [Sov. Phys. JETP 33, 1184 (1957)], is a distinct electromagnetic excitation that differs both from the electric and the magnetic dipoles. It has a number of intriguing properties: static toroidal nuclear dipole is responsible for parity violation in atomic spectra; interactions between static toroidal dipole and oscillating magnetic dipole are claimed to violate Newton's Third Law while non-stationary charge-current configurations involving toroidal multipoles have been predicted to produce vector potential in the absence of electromagnetic …
We propose a scheme for realizing subwavelength electromagnetic diode by employing cascading nonlinear meta-atoms. One-way response is demonstrated on a microwave transmission line comprising of three metallic ring resonators acting as meta-atoms and a varactor as the nonlinear medium inclusion. Experiments show that our implementation can operate simultaneously as forward diode and backward diode at different frequencies. A transmission contrast of up to 14.7dB was achieved between forward and backward transmission. Subwavelength size of our diode should be useful for miniaturization of integ…
In this Comment I argue that the method of images used by Huang, Yu, Gu and co-authors [Phys Rev. E, 65, 21401 (2002); Phys Rev. E., 69, 51402 (2004)] to calculate electromagnetic properties of interacting spheres at finite frequencies is inapplicable and does not provide a physically meaningful approximation.
We theoretically investigate the electromagnetic response of a novel class of multi-layered metamaterials obtained by alternating graphene sheets and dielectric layers, the whole structure not exhibiting a plane of reflection symmetry along the stacking direction. We show that the electromagnetic response of the structure is characterized by a magneto-electric coupling described by an effective chiral parameter. Exploiting the intrinsic tunability of the graphene-light coupling, we prove that one can tune both the dielectric and the chiral electromagnetic response by varying the graphene chemi…
The coherence length of the thermal electromagnetic field near a planar surface has a minimum value related to the nonlocal dielectric response of the material. We perform two model calculations of the electric energy density and the field's degree of spatial coherence. Above a polar crystal, the lattice constant gives the minimum coherence length. It also gives the upper limit to the near field energy density, cutting off its $1/z^3$ divergence. Near an electron plasma described by the semiclassical Lindhard dielectric function, the corresponding length scale is fixed by plasma screening to t…
This article reviews the material properties that enable maximum optical response. We highlight theoretical results that enable shape-independent quantification of material "figures of merit," ranging from classical sum rules to more recent single-frequency scattering bounds. A key delineation at optical frequencies is between polaritonic materials that support highly subwavelength resonances and dielectric materials that can have vanishingly small loss rates. We discuss the key metrics that enable comparisons both within these material classes and between them. We discuss analogous metrics fo…
Optical metasurfaces with high-Q chiral resonances can boost light-matter interaction for various applications of chiral response for ultrathin, active, and nonlinear metadevices. Usually, such metasurfaces require sophisticated depth-resolved nanofabrication to realize subwavelength stereo-nanostructures, posing overwhelming challenges, especially in the short-wavelength range. Here, we suggest a novel planar design for chiral metasurfaces supporting bound states in the continuum (BICs) and demonstrate experimentally chiroptical responses with record-high Q-factors (Q=390) and near-perfect ci…
We develop a general formalism to describe the interactions between a discrete set of plasmonic resonators mediated by the electromagnetic field. The resulting system of equations for closely-spaced meta-atoms represents a cooperative metamaterial response and we find that collective interactions between asymmetric split-ring resonators arranged in a 2D lattice explains the recent experimental observations of system-size dependent narrowing of the transmission resonance linewidth. We further show that this cooperative narrowing depends sensitively on the lattice spacing.
Nonlocal electromagnetic effects of graphene arise from its naturally dispersive dielectric response. We present semi-analytical solutions of nonlocal Maxwell's equations for graphene nano-ribbons array with features around 100 nm, where we found prominent departures from its local response. Interestingly, the nonlocal corrections are stronger for light polarization parallel to the ribbons, which manifests as additional broadening of the Drude peak. For the perpendicular polarization case, nonlocal effects lead to blue-shifts of the plasmon peaks. These manifestations provide a physical measur…
Absorption and scattering of the impinging electromagnetic waves are the two fundamental operations describing the energy exchange of any, organic or inorganic, particle with its environment. In the case of virion cells, substantial extinction power, counting both absorbing and scattering effects, is a prerequisite for performing a variety of coupling actions against the viral particles and, thus, a highly sought-after feature. By considering realistic dispersion for the dielectric permittivity of proteins and a core-shell modeling allowing for rigorous formulation via Mie theory, we report op…
In media with strong spatial dispersion the electric displacement vector and the electric field are typically linked by a partial differential equation in the bulk region. The objective of this work is to highlight that in the vicinity of an interface the relation between the macroscopic fields cannot be univocally determined from the bulk response of the involved materials, but requires instead the knowledge of internal degrees of freedom of the materials. We derive such relation for the particular case of "wire media", and describe a numerical formalism that enables characterizing the electr…
By clarifying the approach of Luan (Phys. Rev. E, 2009), we can generalize the analysis of dispersive (meta)materials, and treat other material responses involving not only loss, but also gain and coherent response.
Nonlinear metasurfaces based on coupling a locally enhanced plasmonic response to intersubband transitions of n-doped multi-quantum-wells (MQWs) have recently provided second-order susceptibilities orders of magnitude larger than any other nonlinear flat structure measured so far. Here, we present a comprehensive theory to characterize the electromagnetic response of nonlinear processes occurring in ultrathin MQW-based plasmonic metasurfaces, providing a homogeneous model that takes phase-matching at the unit-cell level and the influence of saturation and losses into account. In addition, the …
Photonic devices play an increasingly important role in advancing physics and engineering, and while improvements in nanofabrication and computational methods have driven dramatic progress in expanding the range of achievable optical characteristics, they have also greatly increased design complexity. These developments have led to heightened relevance for the study of fundamental limits on optical response. Here, we review recent progress in our understanding of these limits with special focus on an emerging theoretical framework that combines computational optimization with conservation laws…
We present a thorough analysis of the electromagnetic response of strongly coupled neutral plasmas described by the gauge/gravity correspondence. The coupling of the external electromagnetic field with the tower of quasi-normal modes of the plasmas supports the presence of various electromagnetic modes with different properties. Among them we underline the existence of negative refraction with low dissipation for a transverse non-hydrodynamical mode. Previous hydrodynamical approaches have shown the ubiquitous character of negative refraction in charged plasmas and the absence thereof in neutr…
Applications of metallic metamaterials have generated significant interest in recent years. Electromagnetic behavior of metamaterials in the optical range is usually characterized by a local-linear response. In this article, we develop a finite-difference time-domain (FDTD) solution of the hydrodynamic model that describes a free electron gas in metals. Extending beyond the local-linear response, the hydrodynamic model enables numerical investigation of nonlocal and nonlinear interactions between electromagnetic waves and metallic metamaterials. By explicitly imposing the current continuity co…
Intense multi-THz pulses are used to study the coherent nonlinear response of bulk InSb by means of field-resolved four-wave mixing spectroscopy. At amplitudes above 5 MV/cm the signals show a clear temporal substructure which is unexpected in perturbative nonlinear optics. Simulations based on a two-level quantum system demonstrate that in spite of the strongly off-resonant character of the excitation the high-field pulses drive the interband resonances into a non-perturbative regime of Rabi flopping.
We develop a general formalism to describe the propagation of a near-resonant electromagnetic field in a medium composed of magnetodielectric resonators. As the size and the spatial separation of nanofabricated resonators in a metamaterial array is frequently less than the wavelength, we describe them as discrete scatterers, supporting a single mode of current oscillation represented by a single dynamic variable. We derive a Lagrangian and Hamiltonian formalism for the coupled electromagnetic fields and oscillating currents in the length gauge, obtained by the Power-Zienau-Woolley transformati…
The electromagnetic analog of an elastic spring-mass network is constructed. These electromagnetic circuits offer the promise of manipulating electromagnetic fields in new ways, and linear electrical circuits correspond to a subclass of them. They consist of thin triangular magnetic components joined at the edges by cylindrical dielectric components. (There are also dual electromagnetic circuits consisting of thin triangular dielectric components joined at the edges by cylindrical magnetic components.) Some of the edges can be terminal edges to which electric fields are applied. The response i…
We theoretically study the electromagnetic response of type-I and type-II centrosymmetric Weyl semimetals. We derive an anisotropic permittivity tensor with off-diagonal elements to describe such gyrotropic media. Our findings reveal that for appropriate Weyl cones tilts, the real part of the transverse component of the permittivity can exhibit an epsilon-near-zero response. The tilt parameter can also control the amount of loss in the medium, ranging from lossless to dissipative when transitioning from type-I to type-II. Similarly, by tuning either the frequency of the electromagnetic field o…
