This article provides insufficient context for those unfamiliar with the subject.(May 2015) |
An electromagnetic metasurface refers to a kind of artificial sheet material with sub-wavelength features. Metasurfaces can be either structured or unstructured with subwavelength-scaled patterns. [1] [2] [3]
In electromagnetic theory, metasurfaces modulate the behaviors of electromagnetic waves through specific boundary conditions rather than constitutive parameters (such as refractive index) in three-dimensional (3D) space, which is commonly exploited in natural materials and metamaterials. Metasurfaces may also refer to the two-dimensional counterparts of metamaterials. [4] [5] There are also 2.5D metasurfaces that involve the third dimension as additional degree of freedom for tailoring their functionality. [6]
Metasurfaces have been defined in several ways by researchers.
1, “An alternative approach that has gained increasing attention in recent years deals with one- and two-dimensional (1D and 2D) plasmonic arrays with subwavelength periodicity, also known as metasurfaces. Due to their negligible thickness compared to the wavelength of operation, metasurfaces can (near resonances of unit cell constituents) be considered as an interface of discontinuity enforcing an abrupt change in both the amplitude and phase of the impinging light”. [7]
2, “Our results can be understood using the concept of a metasurface, a periodic array of scattering elements whose dimensions and periods are small compared with the operating wavelength”. [8]
3, “Metasurfaces based on thin films”. A highly absorbing ultrathin film on a substrate can also be considered as a metasurface, with properties not occurring in natural materials. [3] Following this definition, the thin metallic films such as that in superlens are also the early type of metasurfaces. [9]
The research of electromagnetic metasurfaces has a long history. Early in 1902, Robert W. Wood found that the reflection spectra of subwavelength metallic grating had dark areas. This unusual phenomenon was named Wood's anomaly and led to the discovery of the surface plasmon polariton (SPP), [10] a particular electromagnetic wave excited at metal surfaces. Subsequently, another important phenomenon, the Levi-Civita relation, [11] was introduced, which states that a subwavelength-thick film can result in a dramatic change in electromagnetic boundary conditions.
Generally speaking, metasurfaces could include some traditional concepts in the microwave spectrum, such as frequency selective surfaces (FSS), impedance sheets, and even Ohmic sheets. In the microwave regime, the thickness of these metasurfaces can be much smaller than the wavelength of operation (for example, 1/1000 of the wavelength) since the skin depth could be minimal for highly conductive metals. Recently, some novel phenomena were demonstrated, such as ultra-broadband coherent perfect absorption. The results showed that a 0.3 nm thick film could absorb all electromagnetic waves across the RF, microwave, and terahertz frequencies. [12] [13] [14]
In optical applications, an anti-reflective coating could also be regarded as a simple metasurface, as first observed by Lord Rayleigh.
In recent years, several new metasurfaces have been developed, including plasmonic metasurfaces, [15] [4] [7] [16] [17] metasurfaces based on geometric phases, [18] [19] metasurfaces based on impedance sheets, [20] [21] and glide-symmetric metasurfaces. [22]
One of the most important applications of metasurfaces is to control a wavefront of electromagnetic waves by imparting local, gradient phase shifts to the incoming waves, which leads to a generalization of the ancient laws of reflection and refraction. [18] In this way, a metasurface can be used as a planar lens, [23] [24] illumination lens, [25] planar hologram, [26] vortex generator, [27] beam deflector, axicon and so on. [19] [28]
Besides the gradient metasurface lenses, metasurface-based superlenses offer another degree of control of the wavefront by using evanescent waves. With surface plasmons in the ultrathin metallic layers, perfect imaging and super-resolution lithography could be possible, which breaks the common assumption that all optical lens systems are limited by diffraction, a phenomenon called the diffraction limit. [29] [30]
Another promising application is in the field of stealth technology. A target's radar cross-section (RCS) has conventionally been reduced by either radiation-absorbent material (RAM) or by purpose shaping of the targets such that the scattered energy can be redirected away from the source. Unfortunately, RAMs have narrow frequency-band functionality, and purpose shaping limits the aerodynamic performance of the target. Metasurfaces have been synthesized that redirect scattered energy away from the source using either array theory [31] [32] [33] or the generalized Snell's law. [34] [35] This has led to aerodynamically favorable shapes for the targets with reduced RCS.
Metasurface can also be integrated with optical waveguides for controlling guided electromagnetic waves. [36] [37] Applications for meta-waveguides such as integrated waveguide mode converters, [37] structured-light generations, [38] [39] versatile multiplexers, [40] [41] and photonic neural networks [42] can be enabled.
In addition, metasurfaces are also applied in electromagnetic absorbers, polarization converters, polarimeters, and spectrum filters. [43] Metasurface-empowered novel bioimaging and biosensing devices have also emerged and been reported recently. [44] [45] [46] [47] For many optically based bioimaging devices, their bulk footprint and heavy physical weight have limited their usage in clinical settings. [48] [49]
Various methods are available for simulating the interaction of electromagnetic waves on metasurfaces, and to enable their design, such as finite-difference time-domain (FDTD), finite-element methods (FEM) and rigorous coupled-wave analysis (RCWA).
For planar optical metasurfaces, prism-based algorithms allow for triangular prismatic space discretization, which is optimal for planar geometries. The prism-based algorithm has fewer elements than conventional tetrahedral methods, bringing higher computational efficiency. [50] A simulation toolkit has been released online, enabling users to efficiently analyze metasurfaces with customized pixel patterns. [51]
Characterizing metasurfaces in the optical domain requires advanced imaging methods since the involved optical properties often include both phase and polarization properties. Recent works suggest that vectorial ptychography, a recently developed computational imaging method, can be of relevance. It combines the Jones matrix mapping with a microscopic lateral resolution, even on large specimens. [52]
A metamaterial is a type of material engineered to have a property, typically rarely observed in naturally occurring materials, that is derived not from the properties of the base materials but from their newly designed structures. Metamaterials are usually fashioned from multiple materials, such as metals and plastics, and are usually arranged in repeating patterns, at scales that are smaller than the wavelengths of the phenomena they influence. Their precise shape, geometry, size, orientation, and arrangement give them their "smart" properties of manipulating electromagnetic, acoustic, or even seismic waves: by blocking, absorbing, enhancing, or bending waves, to achieve benefits that go beyond what is possible with conventional materials.
An optical vortex is a zero of an optical field; a point of zero intensity. The term is also used to describe a beam of light that has such a zero in it. The study of these phenomena is known as singular optics.
Federico Capasso is an Italian-American applied physicist and is one of the inventors of the quantum cascade laser during his work at Bell Laboratories. He is currently on the faculty of Harvard University.
Nanophotonics or nano-optics is the study of the behavior of light on the nanometer scale, and of the interaction of nanometer-scale objects with light. It is a branch of optics, optical engineering, electrical engineering, and nanotechnology. It often involves dielectric structures such as nanoantennas, or metallic components, which can transport and focus light via surface plasmon polaritons.
Extraordinary optical transmission (EOT) is the phenomenon of greatly enhanced transmission of light through a subwavelength aperture in an otherwise opaque metallic film which has been patterned with a regularly repeating periodic structure. Generally when light of a certain wavelength falls on a subwavelength aperture, it is diffracted isotropically in all directions evenly, with minimal far-field transmission. This is the understanding from classical aperture theory as described by Bethe. In EOT however, the regularly repeating structure enables much higher transmission efficiency to occur, up to several orders of magnitude greater than that predicted by classical aperture theory. It was first described in 1998.
A subwavelength-diameter optical fibre is an optical fibre whose diameter is less than the wavelength of the light being propagated through it. An SDF usually consists of long thick parts at both ends, transition regions (tapers) where the fibre diameter gradually decreases down to the subwavelength value, and a subwavelength-diameter waist, which is the main acting part. Due to such a strong geometrical confinement, the guided electromagnetic field in an SDF is restricted to a single mode called fundamental. In usual optical fibres, light both excites and feels shear and longitudinal bulk elastic waves, giving rise to forward-guided acoustic wave Brillouin scattering and backward-stimulated Brillouin scattering. In a subwavelength-diameter optical fibre, the situation changes dramatically.
A terahertz metamaterial is a class of composite metamaterials designed to interact at terahertz (THz) frequencies. The terahertz frequency range used in materials research is usually defined as 0.1 to 10 THz.
A photonic metamaterial (PM), also known as an optical metamaterial, is a type of electromagnetic metamaterial, that interacts with light, covering terahertz (THz), infrared (IR) or visible wavelengths. The materials employ a periodic, cellular structure.
The term chiral describes an object, especially a molecule, which has or produces a non-superposable mirror image of itself. In chemistry, such a molecule is called an enantiomer or is said to exhibit chirality or enantiomerism. The term "chiral" comes from the Greek word for the human hand, which itself exhibits such non-superimposeability of the left hand precisely over the right. Due to the opposition of the fingers and thumbs, no matter how the two hands are oriented, it is impossible for both hands to exactly coincide. Helices, chiral characteristics (properties), chiral media, order, and symmetry all relate to the concept of left- and right-handedness.
A metamaterial absorber is a type of metamaterial intended to efficiently absorb electromagnetic radiation such as light. Furthermore, metamaterials are an advance in materials science. Hence, those metamaterials that are designed to be absorbers offer benefits over conventional absorbers such as further miniaturization, wider adaptability, and increased effectiveness. Intended applications for the metamaterial absorber include emitters, photodetectors, sensors, spatial light modulators, infrared camouflage, wireless communication, and use in solar photovoltaics and thermophotovoltaics.
A coherent perfect absorber (CPA), or anti-laser, is a device which absorbs coherent waves, such as coherent light waves, and converts them into some form of internal energy, e.g. heat or electrical energy. It is the time-reversed counterpart of a laser. Coherent perfect absorption allows control of waves with waves without a nonlinear medium. The concept was first published in the July 26, 2010, issue of Physical Review Letters, by a team at Yale University led by theorist A. Douglas Stone and experimental physicist Hui W. Cao. In the September 9, 2010, issue of Physical Review A, Stefano Longhi of Polytechnic University of Milan showed how to combine a laser and an anti-laser in a single device. In February 2011 the team at Yale built the first working anti-laser. It is a two-channel CPA device which absorbs two beams from the same laser, but only when the beams have the correct phases and amplitudes. The initial device absorbed 99.4 percent of all incoming light, but the team behind the invention believe it will be possible to achieve 99.999 percent. Originally implemented as a Fabry-Pérot cavity that is many wavelengths thick, the optical CPA operates at specific optical frequencies. In January 2012, thin-film CPA has been proposed by utilizing the achromatic dispersion of metal-like materials, exhibiting the unparalleled bandwidth and thin profile advantages. Shortly after, CPA was observed in various thin film materials, including photonic metamaterial, multi-layer graphene, single and multiple layers of chromium, as well as microwave metamaterial.
A flat lens is a lens whose flat shape allows it to provide distortion-free imaging, potentially with arbitrarily-large apertures. The term is also used to refer to other lenses that provide a negative index of refraction. Flat lenses require a refractive index close to −1 over a broad angular range. In recent years, flat lenses based on metasurfaces were also demonstrated.
A plasmonic metamaterial is a metamaterial that uses surface plasmons to achieve optical properties not seen in nature. Plasmons are produced from the interaction of light with metal-dielectric materials. Under specific conditions, the incident light couples with the surface plasmons to create self-sustaining, propagating electromagnetic waves known as surface plasmon polaritons (SPPs). Once launched, the SPPs ripple along the metal-dielectric interface. Compared with the incident light, the SPPs can be much shorter in wavelength.
The orbital angular momentum of light (OAM) is the component of angular momentum of a light beam that is dependent on the field spatial distribution, and not on the polarization. OAM can be split into two types. The internal OAM is an origin-independent angular momentum of a light beam that can be associated with a helical or twisted wavefront. The external OAM is the origin-dependent angular momentum that can be obtained as cross product of the light beam position and its total linear momentum.
Photonic molecules are a form of matter in which photons bind together to form "molecules". They were first predicted in 2007. Photonic molecules are formed when individual (massless) photons "interact with each other so strongly that they act as though they have mass". In an alternative definition, photons confined to two or more coupled optical cavities also reproduce the physics of interacting atomic energy levels, and have been termed as photonic molecules.
A hybrid plasmonic waveguide is an optical waveguide that achieves strong light confinement by coupling the light guided by a dielectric waveguide and a plasmonic waveguide. It is formed by separating a medium of high refractive index from a metal surface by a small gap.
Plasmonics or nanoplasmonics refers to the generation, detection, and manipulation of signals at optical frequencies along metal-dielectric interfaces in the nanometer scale. Inspired by photonics, plasmonics follows the trend of miniaturizing optical devices, and finds applications in sensing, microscopy, optical communications, and bio-photonics.
Photonic topological insulators are artificial electromagnetic materials that support topologically non-trivial, unidirectional states of light. Photonic topological phases are classical electromagnetic wave analogues of electronic topological phases studied in condensed matter physics. Similar to their electronic counterparts, they, can provide robust unidirectional channels for light propagation. The field that studies these phases of light is referred to as topological photonics.
Spoof surface plasmons, also known as spoof surface plasmon polaritons and designer surface plasmons, are surface electromagnetic waves in microwave and terahertz regimes that propagate along planar interfaces with sign-changing permittivities. Spoof surface plasmons are a type of surface plasmon polariton, which ordinarily propagate along metal and dielectric interfaces in infrared and visible frequencies. Since surface plasmon polaritons cannot exist naturally in microwave and terahertz frequencies due to dispersion properties of metals, spoof surface plasmons necessitate the use of artificially-engineered metamaterials.
In photonics, a meta-waveguide is a physical structures that guides electromagnetic waves with engineered functional subwavelength structures. Meta-waveguides are the result of combining the fields of metamaterials and metasurfaces into integrated optics. The design of the subwavelength architecture allows exotic waveguiding phenomena to be explored.
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