The desire to use and control photons in a manner analogous to the control of electrons in solids has inspired great interest in such topics as the localization of light, microcavity quantum electrodynamics and near-field optics1,2,3,4,5,6. A fundamental constraint in manipulating light is the extremely low transmittivity of apertures smaller than the wavelength of the incident photon. While exploring the optical properties of submicrometre cylindrical cavities in metallic films, we have found that arrays of such holes display highly unusual zero-order transmission spectra (where the incident and detected light are collinear) at wavelengths larger than the array period, beyond which no diffraction occurs. In particular, sharp peaks in transmission are observed at wavelengths as large as ten times the diameter of the cylinders. At these maxima the transmission efficiency can exceed unity (when normalized to the area of the holes), which is orders of magnitude greater than predicted by standard aperture theory. Our experiments provide evidence that these unusual optical properties are due to the coupling of light with plasmons — electronic excitations — on the surface of the periodically patterned metal film. Measurements of transmission as a function of the incident light angle result in a photonic band diagram. These findings may find application in novel photonic devices.

Extraordinary optical transmission through sub-wavelength hole arrays

Nano-optics of surface plasmon polaritons

Plasmonic colours are structural colours that emerge from resonant interactions between light and metallic nanostructures. The engineering of plasmonic colours is a promising, rapidly emerging research field that could have a large technological impact. We highlight basic properties of plasmonic colours and recent nanofabrication developments, comparing technology-performance indicators for traditional and nanophotonic colour technologies. The structures of interest include diffraction gratings, nanoaperture arrays, thin films, and multilayers and structures that support Mie resonances and whispering-gallery modes. We discuss plasmonic colour nanotechnology based on localized surface plasmon resonances, such as gap plasmons and hybridized disk–hole plasmons, which allow for colour printing with sub-diffraction resolution. We also address a range of fabrication approaches that enable large-area printing and nanoscale lithography compatible with complementary metal-oxide semiconductor technologies, including nanoimprint lithography and self-assembly. Finally, we review recent developments in dynamically reconfigurable plasmonic colours and in the laser-induced post-processing of plasmonic colour surfaces. Plasmonic colours can be used to colour large surfaces, can be mass-produced and dynamically reconfigured, and can provide sub-diffraction resolution. In this Review, basic properties of plasmonic colours, different platforms supporting them and recent developments in the field are discussed.

Plasmonic colour generation

We introduce the first plasmonic palette utilizing color generation strategies for photorealistic printing with aluminum nanostructures. Our work expands the visible color space through spatially mixing and adjusting the nanoscale spacing of discrete nanostructures. With aluminum as the plasmonic material, we achieved enhanced durability and dramatically reduced materials costs with our nanostructures compared to commonly used plasmonic materials such as gold and silver, as well as size regimes scalable to higher-throughput approaches such as photolithography and nanoimprint lithography. These advances could pave the way toward a new generation of low-cost, high-resolution, plasmonic color printing with direct applications in security tagging, cryptography, and information storage.

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Plasmonic Color Palettes for Photorealistic Printing with Aluminum Nanostructures

Metallic nanostructures now play an important role in many applications. In particular, for the emerging fields of plasmonics and nanophotonics, the ability to engineer metals on nanometric scales allows the development of new devices and the study of exciting physics. This review focuses on top-down nanofabrication techniques for engineering metallic nanostructures, along with computational and experimental characterization techniques. A variety of current and emerging applications are also covered.

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Engineering metallic nanostructures for plasmonics and nanophotonics

We present a theoretical approach for calculating the fields diffracted by gratings made of highly conducting wires that have a rectangular shape. The fields between the wires are represented in terms of modal expansions that satisfy the approximated impedance boundary condition. Our results show that this procedure is particularly suited to dealing with gold gratings used in the infrared range, a spectral region where the assumption of a perfect conductor does not hold, and where the rigorous modal method assuming penetrable wires exhibits numerical instabilities linked with the high conductivity of gold. Numerical results are presented, and the theory is used to determine wire parameters by fitting theoretical and experimental data.

Highly conducting wire gratings in the resonance region.

A resonance phenomenon in the zeroth diffraction order of a gold-wire grating is explained by the excitation of surface polaritons. This effect is connected with a strong enhancement of the electromagnetic fields on the wire surface and consequently with a peak of power losses in the grating material. Measurements of the zeroth-order transmittance have been performed on gold gratings with periods of 1 and 2 micrometers in the near-infrared region which are in agreement with theoretical results. Furthermore, dispersion relations of the first-order coupling mode are presented having large energy gaps in the center of the Brillouin zone. It is shown that this energy gap strongly depends on the wire profile. In this coupling branch, however, practically no dispersion could be observed for optical wavelengths less than the grating period.

Surface polaritons on gold-wire gratings.

Gratings with periods smaller than visible wavelengths in ambient white light will exhibit enhanced colors if the profile is designed so that resonant light interaction occurs in the visible range. Resonances have a frequency-selective influence to the grating diffraction inducing colors in transmittance and reflectance, respectively. Apart from the well-known surface-plasmon polariton excitations and cavity resonances, newly discovered resonances in TE-polarization can be exploited for colorizing wire-gratings, when simply illuminated by unpolarized white light. Colors can be laterally tuned by varying the grating profile. The capability of generating images by sub-wavelength gratings is exemplified by a metallic wire grating embedded in a plastic foil with a lateral variable modulation depth. This method for producing colored images is predestined for industrial mass production and will have ever more practical applications such as for security features.

Colored images generated by metallic sub-wavelength gratings.

A theory describing the diffraction properties of highly conducting wire gratings with arbitrary cross-sections is presented. The method incorporates the surface impedance boundary condition on the field along metallic interfaces and relies on the solution of a coupled system of ordinary differential equations for finding the field between wires. The theory is exemplified numerically for different wire profiles. The numerical results for gold gratings in the resonance region are compared with those obtained from a theory valid for perfectly conducting wires. Comparisons to measurements validate the effectiveness of the developed algorithm and show that it is particularly suited for dealing with metallic gratings in the infrared region.

Diffraction from highly conducting wire gratings of arbitrary cross-section

The invention relates to a thin-layer element (30) having a multilayered structure for security papers, documents of value and the like, which appears to have a golden colour when viewed in incident light and which has at least two semitransparent mirror layers (34, 35) and at least one dielectric spacer layer (36) arranged between the at least two mirror layers, so that when measuring the transmission of un-polarised light in the blue wavelength range from 420 nm to 490 nm, a resonance having a half-value width of 70 to 150 nm is exhibited.

Hans Lochbihler

Papers

Highly conducting wire gratings in the resonance region.

Surface polaritons on gold-wire gratings.

Colored images generated by metallic sub-wavelength gratings.

Diffraction from highly conducting wire gratings of arbitrary cross-section

Gold-coloured thin-layer element having a multilayered structure