Francesco Monticone

Performing Mathematical Operations with Metamaterials

Computational Metamaterials Optical signal processing of light waves can represent certain mathematical functions and perform computational tasks on signals or images in an analog fashion. However, the complex systems of lenses and filters required are bulky. Metamaterials can perform similar optical processing operations but with materials that need only be a wavelength thick. Silva et al. (p. 160; see the Perspective by Sihvola) present a simulation study that shows how an architecture based on such metamaterials can be designed to perform a suite of mathematical functions to create ultrathin optical signal and data processors. An approach is described whereby metamaterials can be designed to perform a suite of mathematical functions. [Also see Perspective by Sihvola] We introduce the concept of metamaterial analog computing, based on suitably designed metamaterial blocks that can perform mathematical operations (such as spatial differentiation, integration, or convolution) on the profile of an impinging wave as it propagates through these blocks. Two approaches are presented to achieve such functionality: (i) subwavelength structured metascreens combined with graded-index waveguides and (ii) multilayered slabs designed to achieve a desired spatial Green’s function. Both techniques offer the possibility of miniaturized, potentially integrable, wave-based computing systems that are thinner than conventional lens-based optical signal and data processors by several orders of magnitude.

A subwavelength plasmonic metamolecule exhibiting magnetic-based optical Fano resonance

We demonstrate that a subwavelength plasmonic metamolecule consisting of four nanoparticles supports a magnetic response spectrally overlapped with the electric dipole resonance. Small structural asymmetries lead to interference and thus a Fano resonance in scattering.

Ultrathin pancharatnam-berry metasurface with maximal cross-polarization efficiency.

Novel ultrathin dual-functional metalenses are proposed, fabricated, tested, and verified in the microwave regime for the first time. The significance is that their anomalous transmission efficiency almost reaches the theoretical limit of 25%, showing a remarkable improvement compared with earlier ultrathin metasurface designs with less than 5% coupling efficiency. The planar metalens proposed empowers significant reduction in thickness, versatile focusing behavior, and high transmission efficiency simultaneously.

Hybrid bilayer plasmonic metasurface efficiently manipulates visible light

Two highly coupled plasmonic metasurfaces exhibit much higher conversion efficiency and extinction ratio than individual ones. Metasurfaces operating in the cross-polarization scheme have shown an interesting degree of control over the wavefront of transmitted light. Nevertheless, their inherently low efficiency in visible light raises certain concerns for practical applications. Without sacrificing the ultrathin flat design, we propose a bilayer plasmonic metasurface operating at visible frequencies, obtained by coupling a nanoantenna-based metasurface with its complementary Babinet-inverted copy. By breaking the radiation symmetry because of the finite, yet small, thickness of the proposed structure and benefitting from properly tailored intra- and interlayer couplings, such coupled bilayer metasurface experimentally yields a conversion efficiency of 17%, significantly larger than that of earlier single-layer designs, as well as an extinction ratio larger than 0 dB, meaning that anomalous refraction dominates the transmission response. Our finding shows that metallic metasurface can counterintuitively manipulate the visible light as efficiently as dielectric metasurface (~20% in conversion efficiency in Lin et al.’s study), although the metal’s ohmic loss is much higher than dielectrics. Our hybrid bilayer design, still being ultrathin (~λ/6), is found to obey generalized Snell’s law even in the presence of strong couplings. It is capable of efficiently manipulating visible light over a broad bandwidth and can be realized with a facile one-step nanofabrication process.

Negative refraction, gain and nonlinear effects in hyperbolic metamaterials

The negative refraction and evanescent-wave canalization effects supported by a layered metamaterial structure obtained by alternating dielectric and plasmonic layers is theoretically analyzed. By using a transmission-line analysis, we formulate a way to rapidly analyze the negative refraction operation for given available materials over a broad range of frequencies and design parameters, and we apply it to broaden the bandwidth of negative refraction. Our analytical model is also applied to explore the possibility of employing active layers for loss compensation. Nonlinear dielectrics can also be considered within this approach, and they are explored in order to add tunability to the optical response, realizing positive-to-zero-to-negative refraction at the same frequency, as a function of the input intensity. Our findings may lead to a better physical understanding and improvement of the performance of negative refraction and subwavelength imaging in layered metamaterials, paving the way towards the design of gain-assisted hyperlenses and tunable nonlinear imaging devices.

Leaky-Wave Theory, Techniques, and Applications: From Microwaves to Visible Frequencies

Leaky waves have been among the most active areas of research in microwave engineering over the second half of the 20th century. They have been shown to dominate the near-field of several open wave-guiding structures, of great interest to tailor their radiation, guidance and filtering properties. The elegant theoretical analyses and deep physical insights in this area, developed in an era in which computational resources were limited, represent a fundamental scientific legacy that is still extremely relevant in todays engineering society and beyond. In this regard, the relevance of leaky-wave concepts has been increasingly recognized in recent times over a broader scientific community, including optics and physics societies. In this paper, after revisiting the fundamental concepts of leaky-wave theory, we discuss and connect different relevant research activities in which leaky-wave concepts have been applied, with the goal of facilitating multidisciplinary interactions on these topics. In addition to the canonical microwave applications of leaky waves, particular attention is devoted to a few areas of interest in modern optics, such as directive optical antennas, extraordinary optical transmission, and embedded scattering eigenvalues, in which leaky waves play a fundamental role.

Interplay of Magnetic Responses in All-Dielectric Oligomers To Realize Magnetic Fano Resonances

We study the interplay between collective and individual optically induced magnetic responses in quadrumers made of identical dielectric nanoparticles. Unlike their plasmonic counterparts, all-dielectric nanoparticle clusters are shown to exhibit multiple dimensions of resonant magnetic responses that can be employed for the realization of anomalous scattering signatures. We focus our analysis on symmetric quadrumers made from silicon nanoparticles and verify our theoretical results in proof-of-concept radio frequency experiments demonstrating the existence of a novel type of magnetic Fano resonance in nanophotonics.

Metamaterials and plasmonics: From nanoparticles to nanoantenna arrays, metasurfaces, and metamaterials

The rise of plasmonic metamaterials in recent years has unveiled the possibility of revolutionizing the entire field of optics and photonics, challenging well-established technological limitations and paving the way to innovations at an unprecedented level. To capitalize the disruptive potential of this rising field of science and technology, it is important to be able to combine the richness of optical phenomena enabled by nanoplasmonics in order to realize metamaterial components, devices, and systems of increasing complexity. Here, we review a few recent research directions in the field of plasmonic metamaterials, which may foster further advancements in this research area. We will discuss the anomalous scattering features enabled by plasmonic nanoparticles and nanoclusters, and show how they may represent the fundamental building blocks of complex nanophotonic architectures. Building on these concepts, advanced components can be designed and operated, such as optical nanoantennas and nanoantenna arrays, which, in turn, may be at the basis of metasurface devices and complex systems. Following this path, from basic phenomena to advanced functionalities, the field of plasmonic metamaterials offers the promise of an important scientific and technological impact, with applications spanning from medical diagnostics to clean energy and information processing.

The quest for optical magnetism: from split-ring resonators to plasmonic nanoparticles and nanoclusters

Natural materials exhibit negligible magnetism at optical frequencies, since the direct effects of the optical magnetic field on matter are much weaker than electric ones. In the last decades, however, scientists and engineers have been trying to tackle and overcome these limitations by designing artificial subwavelength meta-molecules that support a strong magnetic response, even if made of non-magnetic materials. These issues have become popular because of the excitement around magnetic metamaterials and their exotic wave interaction. Here, we review recent efforts on this topic, with particular focus on magnetic metamaterials operating at optical frequencies. We discuss how the concept of split-ring resonators, introduced at microwaves to realize artificial magnetic inclusions with subwavelength footprint, can be translated to the optical region exploiting the plasmonic features of metallic nanoparticles suitably arranged in nanoclusters and nanorings. We also show that Fano interference effects, triggered by small symmetry-breaking in nanoclusters, may be the key to largely boost the magnetic response in optical meta-molecules. All these findings show that the alliance of optical metamaterials and advanced nanotechnology holds the promise to provide significant advances in the quest towards low-loss optical magnetic materials.

Multilayered Plasmonic Covers for Comblike Scattering Response and Optical Tagging

We discuss the potential of multilayered plasmonic particles to tailor the optical scattering response. The interplay of plasmons localized in thin stacked shells realizes peculiar degenerate cloaking and resonant states occurring at arbitrarily close frequencies. These concepts are applied to realize ultrasharp comblike scattering responses and synthesize staggered, ideally strong superscattering states closely coupled to invisible states. We demonstrate robustness to material losses and to variations in the background medium, properties that make these structures ideal for optical tagging.