Metasurfaces: From microwaves to visible

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

Thermal radiation is one of the most universal physical phenomena, and its study has played a key role in the history of modern physics. Our understanding of this subject has been traditionally bas...

Radiative Heat Transfer

In recent years we have learned to fabricate structures smaller than electromagnetic wavelengths, and to assemble them into metamaterials with exotic optical properties for previously unimaginable applications. One such property is perfect absorption of incident light, with no reflection or transmission, across many wavelengths. The authors review the physics, design principles, and classification of thin perfect absorbers, and outline avenues for progress.

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Thin perfect absorbers for electromagnetic waves: Theory, design, and realizations

In the wake of intense research on metamaterials the two-dimensional analogue, known as metasurfaces, has attracted progressively increasing attention in recent years due to the ease of fabrication and smaller insertion losses, while enabling an unprecedented control over spatial distributions of transmitted and reflected optical fields. Metasurfaces represent optically thin planar arrays of resonant subwavelength elements that can be arranged in a strictly or quasi periodic fashion, or even in an aperiodic manner, depending on targeted optical wavefronts to be molded with their help. This paper reviews a broad subclass of metasurfaces, viz. gradient metasurfaces, which are devised to exhibit spatially varying optical responses resulting in spatially varying amplitudes, phases and polarizations of scattered fields. Starting with introducing the concept of gradient metasurfaces, we present classification of different metasurfaces from the viewpoint of their responses, differentiating electrical-dipole, geometric, reflective and Huygens' metasurfaces. The fundamental building blocks essential for the realization of metasurfaces are then discussed in order to elucidate the underlying physics of various physical realizations of both plasmonic and purely dielectric metasurfaces. We then overview the main applications of gradient metasurfaces, including waveplates, flat lenses, spiral phase plates, broadband absorbers, color printing, holograms, polarimeters and surface wave couplers. The review is terminated with a short section on recently developed nonlinear metasurfaces, followed by the outlook presenting our view on possible future developments and perspectives for future applications.

Gradient metasurfaces: a review of fundamentals and applications.

Radiation absorbers have increasingly been attracting attention as crucial components for controllable thermal emission, energy harvesting, modulators, etc. However, it is still challenging to realize thin absorbers which can operate over a wide spectrum range. Here, we propose and experimentally demonstrate thin, broadband, polarization-insensitive and omnidirectional absorbers working in the near-infrared range. We choose titanium (Ti) instead of the commonly used gold (Au) to construct nano-disk arrays on the top of a silicon dioxide (SiO2) coated Au substrate, with the quality (Q) factor of the localized surface plasmon (LSP) resonance being decreased due to the intrinsic high loss of Ti. The combination of this low-Q LSP resonance and the propagating surface plasmon (PSP) excitation resonance, which occur at different wavelengths, is the fundamental origin of the broadband absorption. The measured (at normal light incidence) absorption is over 90% in the wavelength range from 900 nm to 1825 nm, with high absorption persisting up to the incident angle of ~40°. The demonstrated thin-film absorber configuration is relatively easy to fabricate and can be realized with other properly selected materials.

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Broadband near-infrared metamaterial absorbers utilizing highly lossy metals

We experimentally demonstrate a large area, optically opaque plasmonic absorber which can absorb 95% of visible light with an effective thickness of less than 150 nm. The absorber comprises, from top to bottom, a mono-layer of random gold nanoparticles, a dielectric spacer, and a bottom gold reflector. Reflectometry analyses show that its absorption is insensitive to the incidence's polarization or angle when the incident angle is less than 50°. At a larger incident angle, reflection increases and absorption spectra differ for two polarizations. Numerical simulations based on a 3D finite-element method suggest that the high absorbance is due to collective efforts of dipolar particle resonances, most often strongly coupled and forming chain resonances, as well as coupling of light to the surface plasmon polariton, irrespective of the incidence's polarization, through the top-layer particles. Similar high absorptivity is also demonstrated with silver or aluminum as the bottom reflector. These highly efficient visible light absorbers can be potential candidates for a range of passive and active photonic applications, including solar-energy harvesting as well as producing artificial colors on a large scale.

https://iopscience.iop.org/article/10.1088/2040-8978/16/2/025002/pdf

Lithography-free broadband visible light absorber based on a mono-layer of gold nanoparticles

We report the concept of a near-field memory device based on the radiative bistability effect in the system of two closely separated parallel plates of SiO2 and VO2 which exchange heat by thermal radiation in vacuum. We demonstrate that the VO2 plate, having metal-insulator transition at 340 K, has two thermodynamical steady-states. One can switch between the states using an external laser impulse. We show that due to near-field photon tunneling between the plates, the switching time is found to be only 5 ms which is several orders lower than in case of far field.

Near field thermal memory based on radiative phase bistability of VO2

We demonstrate with the finite-difference time-domain method that radiative heat transfer between two parallel gold plates can be significantly enhanced by engraving periodic grooves with a subwavelength width on the plate surfaces. The enhancement increases with a decrease in the separation distance at near-field regime and it can be further efficiently improved by having a supercell with multiple grooves with different depths. We attribute this near-field enhancement to coupling of thermally excited spoof surface plasmon polaritons, a type of artificial surface wave inherent to structured metal surfaces [J. B. Pendry, L. Martin-Moreno, and F. J. Garcia-Vidal, Science 305, 847 (2004)]. The frequency-dependent contribution to the heat transfer, or transmission-factor spectrum, is confirmed by calculating the dispersion relation of guided modes by the two parallel corrugated plates through a finite-element method. Especially, the photonic density of states derived from the dispersion relation is found to have excellent agreement to the transmission-factor spectrum.

Enhanced near-field radiative heat transfer between corrugated metal plates: Role of spoof surface plasmon polaritons

We systematically study a type of plasmonic light absorber based on a monolayer of gold nano-spheres with less than 30 nm in diameters deposited on top of a continuous gold substrate. The influences of particle size, inter-particle distance, particle-substrate spacer size etc on the resonance are studied thoroughly with a 3D finite-element method. We identified that the high-absorption resonance is mainly due to gap plasmon (coupled through particle bodies) when the separation between neighboring nano-spheres is small enough, such as close to 1 nm; at larger particle separations, the resonance is dominated by particle dipoles (coupled through the host dielectric). Experimentally, an absorber was fabricated based on chemically-synthesized gold nanoparticles coated with silica shell. The absorber shows a characteristic absorption band around 810 nm with a maximum absorbance of approximately 90%, which agrees reasonably well with our numerical calculation. The fabrication technique can be easily adapted for devising efficient light absorbers of large areas.

Jin Dai

Papers

Broadband near-infrared metamaterial absorbers utilizing highly lossy metals

Lithography-free broadband visible light absorber based on a mono-layer of gold nanoparticles

Near field thermal memory based on radiative phase bistability of VO2

Enhanced near-field radiative heat transfer between corrugated metal plates: Role of spoof surface plasmon polaritons

Light absorber based on nano-spheres on a substrate reflector