Metamaterials, artificial composites with properties not found in nature, have revolutionised the way to control electromagnetic radiation. When implemented at infrared and visible frequencies, the properties of metamaterials ultimately rely on the plasmonic resonances of their constituent meta-atoms, typically consisting of nanostructured metals with subwavelength size. Such plasmonic nanostructures also exhibit intriguing features when isolated, such as the possibility to control the colour of light at the nanoscale. In this work, Alejandro Martínez will discuss the main features of plasmonic metamaterials at optical frequencies and will show how such properties can be employed to build novel optical security features with unprecedented performance.

Metamaterials
Metamaterials are artificial structures formed by periodic arrays of subwavelength meta-atoms that are tailored to display electromagnetic properties not reached by conventional materials in nature.[1] The response of the metamaterial to an incoming electro-magnetic wave is given by the interaction of its constituent meta-atoms, instead of the materials (typically, metals) composing them. This way, both the electric permittivity εr and the magnetic permeability µr, which account respectively for the material response to incoming electric and magnetic fields, can be tailored independently at will. Remarkably, this exciting possibility opens the door to get values of the refractive index (n = εrµr) and wave impedance (Z = µrεr Zvacuum) – the main parameters used by engineers to build optical devices – not attainable via natural materials.
Achievable values of εr and µr are quite constrained at optical frequencies (infrared and visible regimes), which limits the possibilities for designing optical devices. The available values of εr depend on the material. In metals, we have εr < 0 below the so called plasma frequency, which means that light cannot propagate through them (they act as mirrors and are used to build gratings and holograms). In transparent materials such as dielectrics, we have εr > 1, which allows lossless light propagation. However, even for materials with a high refractive index, such as some semiconductors, maximum values of εr are about 20. Magnetic activity is negligible in all known materials, leading to µr = 1 and n = εr.

 

figure 1.

Metamaterials enable any possible value of εr and µr, including negative, very high and even zero. Interestingly, by doing εr = µr impedance matching with free-space is achieved, leading to devices with any refractive index but showing no reflection. As in natural materials, a key feature in metamaterials is that the meta-atoms’ size as well as the spacing between them need to be much smaller than the operational wavelength, so that the metamaterial behaves as an effective medium (εr, e f f, µr, e f f). By downscaling the meta-atoms’ size down to the micro- and nanoscale, the pursued unusual responses – not achievable by conventional materials – can be attained at optical frequencies.

Negative refraction and optical magnetism with metamaterials
The refractive index in transparent natural materials is positive because at optical frequencies they show εr > 1 and µr = 1. However, n > 0 is not a fundamental requirement in electromagnetism. Indeed, a hypothetical material having both εr and µr simultaneously negative exhibits a negative refractive index. In such a material, the phase form evolves backwards, which produces a ‘negative’ bending of a light beam when passing from free-space to such a medium and vice versa. This unexpected property, known as negative refraction, is not attainable with conventional materials, but can be achieved via properly designed metamaterials. To do so, the key idea is to build a periodic lattice of subwave length size meta-atoms consisting of two metallic nanoparticles: one exhibiting an effective electrical response leading to εr, e f f < 0 and the other one exhibiting an effective magnetic response leading to µr, e f f < 0. 2D and 3D lattices of thin metal wires behave as metamaterials with εr, e f f < 0 below a certain frequency that can be tailored. Achieving optical magnetism is more challenging, since natural materials do not respond to the optical magnetic field. However, if we create metallic meta-atoms supporting virtual current loops (therefore imitating the current flow producing magnetism at low frequencies) we can get effective magnetism even at visible wavelengths. This can be achieved via the so called split-ring resonators or by coupled nanoring. Finally, by interlacing the lattices of electric and magnetic meta-atoms, a negative-index metamaterial becomes feasible. 

Invisibility cloaks with metamaterials
The possibility to build metamaterials in which both εr, e f f and µr, e f f can be tailored locally at will leads to unconventional ways of manipulating light beams.
For instance, an object can be surrounded by a meta-material anisotropic cloak in which εr, e f f and µr, e f f change with coordinates so that light beams impinging on it go around the object without touching it and finally recover their original trajectory. If the cloak is designed to work for all visible wavelengths, the object would be completely invisible to the human eye. Noticeably, since the light cannot penetrate the cloak, it is not possible to see anything from inside the cloak. This exciting possibility, still far from experimental demonstrations in the visible regime, has fuelled the research field known as transformation optics, which enables the design of arbitrary optical devices for controlling light propagation using metamaterials.

Metasurfaces
Building true metamaterials requires complex 3D fabrication processes. However, scientists have also proposed that metamaterials created on a surface – metasurfaces – can provide an unprecedented control on the propagation of light, whilst alleviating challenging 3D na  cturing. Metasurfaces are 2D arrays of meta-atoms that are independently designed to modify the amplitude and phase of the light transmitted, reflected or scattered from them. This way, striking phenomena such as negative refraction or reflection for normally incident beams become attainable using a properly designed metasurface with subwavelength thickness.

Plasmonic resonances and subwavelength colour control
The meta-atoms forming a metamaterial also display intriguing features when isolated. Indeed, since they are metallic, they support resonances of a kind of wave known as surface plasmon, which propagates at a metal-dielectric interface. This is why metamaterials working in the optical regime are also usually called plasmonic metamaterials. Such resonances, typically referred to as localised surface plasmon resonances (LSPRs), have frequencies in the infrared and visible regimes, depending on the size, shape and composition of the metallic nanostructures. At resonance, such nanostructures exhibit enhanced/suppressed light reflection, scattering and/or absorption, which in the visible regime can be used for controlling the colour at a local level. Moreover, the size of the nanostructure is much smaller than the LSPR wavelength, so the colour is locally manipulated in a subwavelength size, since surface plasmons go beyond the Abbe’s diffraction limit. 

Using metamaterials and plasmonics in optical security
It is clear that some of the metamaterials’ properties presented above may be used as optical security features: their intriguing properties cannot be mimicked by using natural materials, which should hinder counterfeiting. For instance, a metamaterial providing negative refraction could certainly be used as an optical security feature since it refracts light in a direction opposite to conventional materials. Indeed, there is no need to go so far: a metamaterial exhibiting optical magnetism could be used for optical security purposes. This is the idea we used in a security label formed by a metamaterial consisting of coupled silver nanorings shows negative values for
µr, e f f in the visible domain. Since all known natural media show µr = 1, the magnetic response could not be produced by any other means, profoundly restricting the counterfeiting possibilities. By changing the inner radius of the rings, the µr, e f f value at a certain wavelength is modified, so by locally controlling the permeability in certain areas it would be feasible to write a ‘magnetic light’ code on the label, which indeed would increase the protection.

In general, the feasibility to modify εr, e f f and µr, e f f at our will using metamaterials could be used to create optical structures with unprecedented levels of security against counterfeiting. Notice that a security element based on optical magnetism would require an external sensor for reading out the value of µr, e f f. But noticeably, this metamaterial would consist of a periodic array of scatterers, so it would also display iridescence effects behaving in parallel as an optical variable device, which would be visible to the human eye. Therefore, we would have two different security features on a single device. 
The ability for manipulating the phase but also the amplitude of the scattered light provided by meta-surfaces has been used to develop ultra‑thin holograms with high resolution and low noise. Even though the thickness of the metasurface hologram can be as small as 100 nm, efficiencies beyond 80% are achievable,which proves the potential of metamaterials for advanced holographic designs. Noticeably, meta-surfaces also enable 3D holography.

Subwavelength metallic nanostructures supporting LSPRs provide a way to manipulate the colour of an image in a size smaller than the wavelength, opening the way towards imaging with ultra-high resolution beyond the Abbe’s diffraction limit. Recently, metallic nanostructures supporting LSPRs have been used for printing with pixel sizes as small as 250 x 250 nm2 (100,000 dpi resolution). The key idea is illustrated below: metallic nanodisks (diameter ~ 100 nm) supporting LSPRs in the visible regime are placed on top of a metal back reflector. The LSPR wavelength is characterised by a minimum in the reflectance, since the energy is scattered and absorbed by the nanodisk. By changing the diameter, the reflectance minimum is shifted, so we can properly select the colour in a region smaller than the light wavelength.
Extended (instead of localised) surface plasmon resonances in metallic films drilled by an array of holes are responsible for the phenomenon known as extraordinary optical transmission. Such structures can also be used to implement colour filters, although in this case the minimum pixel size would be of the order of tens of microns. The use of plasmon-based metallic hole arrays as optical security features has been recently proposed.

Nanofabrication of metamaterials and plasmonics: state of the art and future developments
Fabrication of metamaterials, metasurfaces and plasmonic nanostructures working at the infrared and visible domain require sophisticated processes with resolutions down to the 50‑nm level because of the subwavelength nature of the employed metallic nano-particles. For research, high‑resolution tools such as focused ion beam or, more frequently, electron beam lithography (EBL) have been typically used. For instance, we have used EBL to create metamaterials working at infrared wavelengths and exhibiting LSPRs, negative index or optical magnetism.. Unfortunately, EBL is a low-throughput technique not suitable for the large-volume and low-cost production required for most optical security features. However, it is feasible to create a master by means of EBL and then using techniques such as nanoimprint lithography to replicate this master in any kind of substrate, including flexible platforms. A metamaterial built on a flexible substrate with negative index at a wavelength of 2 µm has been demonstrated using this technique. As mentioned above, this metamaterial – which, using this fabrication technique, is built on areas larger than 75 cm – also displays iridescent features because of its periodicity, so a dual optical security feature would be present. This technology could open the door towards the deployment of metamaterials and plasmonics in the field of optical security. 

Conclusion
In summary, metamaterials based on metallic nano-structures with subwavelength dimensions provide unnatural features which could be used for the implementation of advanced optical security devices. Such nanostructures could also be used on surfaces (metasurfaces) or even isolated to manipulate the amplitude, phase and wavelength of light beams. Fabrication techniques such as nanoimprint lithography could enable these nanotechnology advances to be deployed in the field of optical security. 

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