The ideal material for nanophotonic applications will have a large refractive index at optical frequencies, respond to both the electric and magnetic fields of light, support large optical chirality and anisotropy, confine and guide light at the nanoscale, and be able to modify the phase and amplitude of incoming radiation in a fraction of a wavelength. Artificial electromagnetic media, or metamaterials, based on metallic or polar dielectric nanostructures can provide many of these properties by coupling light to free electrons (plasmons) or phonons (phonon polaritons), respectively, but at the inevitable cost of significant energy dissipation and reduced device efficiency. Recently, however, there has been a shift in the approach to nanophotonics. Low-loss electromagnetic responses covering all four quadrants of possible permittivities and permeabilities have been achieved using completely transparent and high-refractive-index dielectric building blocks. Moreover, an emerging class of all-dielectric metamaterials consisting of anisotropic crystals has been shown to support large refractive index contrast between orthogonal polarizations of light. These advances have revived the exciting prospect of integrating exotic electromagnetic effects in practical photonic devices, to achieve, for example, ultrathin and efficient optical elements, and realize the long-standing goal of subdiffraction confinement and guiding of light without metals. In this Review, we present a broad outline of the whole range of electromagnetic effects observed using all-dielectric metamaterials: high-refractive-index nanoresonators, metasurfaces, zero-index metamaterials and anisotropic metamaterials. Finally, we discuss current challenges and future goals for the field at the intersection with quantum, thermal and silicon photonics, as well as biomimetic metasurfaces.

All-dielectric metamaterials

Resonant plasmonic and metamaterial structures allow for control of fundamental optical processes such as absorption, emission and refraction at the nanoscale. Considerable recent research has focused on energy absorption processes, and plasmonic nanostructures have been shown to enhance the performance of photovoltaic and thermophotovoltaic cells. Although reducing metallic losses is a widely sought goal in nanophotonics, the design of nanostructured 'black' super absorbers from materials comprising only lossless dielectric materials and highly reflective noble metals represents a new research direction. Here we demonstrate an ultrathin (260 nm) plasmonic super absorber consisting of a metal–insulator–metal stack with a nanostructured top silver film composed of crossed trapezoidal arrays. Our super absorber yields broadband and polarization-independent resonant light absorption over the entire visible spectrum (400–700 nm) with an average measured absorption of 0.71 and simulated absorption of 0.85. Proposed nanostructured absorbers open a path to realize ultrathin black metamaterials based on resonant absorption.

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Broadband polarization-independent resonant light absorption using ultrathin plasmonic super absorbers

Anti-reflective coatings (ARCs) have evolved into highly effective reflectance and glare reducing components for various optical and opto-electrical equipments. Extensive research in optical and biological reflectance minimization as well as the emergence of nanotechnology over the years has contributed to the enhancement of ARCs in a major way. In this study the prime objective is to give a comprehensive idea of the ARCs right from their inception, as they were originally conceptualized by the pioneers and lay down the basic concepts and strategies adopted to minimize reflectance. The different types of ARCs are also described in greater detail and the state-of-the-art fabrication techniques have been fully illustrated. The inspiration that ARCs derive from nature (‘biomimetics’) has been an area of major research and is discussed at length. The various materials that have been reportedly used in fabricating the ARCs have also been brought into sharp focus. An account of application of ARCs on solar cells and modules, contemporary research and associated challenges are presented in the end to facilitate a universal understanding of the ARCs and encourage future research.

Anti-reflective coatings: A critical, in-depth review

A single-junction polymer solar cell with an efficiency of 10.1% is demonstrated by using deterministic aperiodic nanostructures for broadband light harvesting with optimum charge extraction. The performance enhancement is ascribed to the self-enhanced absorption due to collective effects, including pattern-induced anti-reflection and light scattering, as well as surface plasmonic resonance, together with a minimized recombination probability.

Single-Junction Polymer Solar Cells Exceeding 10% Power Conversion Efficiency

The study of ideal absorbers, which can efficiently absorb light over a broad range of wavelengths, is of fundamental importance, as well as critical for many applications from solar steam generation and thermophotovoltaics to light/thermal detectors. As a result of recent advances in plasmonics, plasmonic absorbers have attracted a lot of attention. However, the performance and scalability of these absorbers, predominantly fabricated by the top-down approach, need to be further improved to enable widespread applications. We report a plasmonic absorber which can enable an average measured absorbance of ~99% across the wavelengths from 400 nm to 10 μm, the most efficient and broadband plasmonic absorber reported to date. The absorber is fabricated through self-assembly of metallic nanoparticles onto a nanoporous template by a one-step deposition process. Because of its efficient light absorption, strong field enhancement, and porous structures, which together enable not only efficient solar absorption but also significant local heating and continuous stream flow, plasmonic absorber–based solar steam generation has over 90% efficiency under solar irradiation of only 4-sun intensity (4 kW m−2). The pronounced light absorption effect coupled with the high-throughput self-assembly process could lead toward large-scale manufacturing of other nanophotonic structures and devices.

Self-assembly of highly efficient, broadband plasmonic absorbers for solar steam generation.

Nature routinely produces nanostructured surfaces with useful properties, such as the self-cleaning lotus leaf, the colour of the butterfly wing, the photoreceptor in brittlestar and the anti-reflection observed in the moth eye. Scientists and engineers have been able to mimic some of these natural structures in the laboratory and in real-world applications. Here, we report a simple aperiodic array of silicon nanotips on a 6-inch wafer with a sub-wavelength structure that can suppress the reflection of light at a range of wavelengths from the ultraviolet, through the visible part of the spectrum, to the terahertz region. Reflection is suppressed for a wide range of angles of incidence and for both s- and p-polarized light. The antireflection properties of the silicon result from changes in the refractive index caused by variations in the height of the silicon nanotips, and can be simulated with models that have been used to explain the low reflection from moth eyes. The improved anti-reflection properties of the surfaces could have applications in renewable energy and electro-optical devices for the military.

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Chih Shan Lee

Papers

Improved broadband and quasi-omnidirectional anti-reflection properties with biomimetic silicon nanostructures