Metamaterials are artificially fabricated materials that allow for the control of light and acoustic waves in a manner that is not possible in nature. This Review covers the recent developments in the study of so-called metasurfaces, which offer the possibility of controlling light with ultrathin, planar optical components. Conventional optical components such as lenses, waveplates and holograms rely on light propagation over distances much larger than the wavelength to shape wavefronts. In this way substantial changes of the amplitude, phase or polarization of light waves are gradually accumulated along the optical path. This Review focuses on recent developments on flat, ultrathin optical components dubbed 'metasurfaces' that produce abrupt changes over the scale of the free-space wavelength in the phase, amplitude and/or polarization of a light beam. Metasurfaces are generally created by assembling arrays of miniature, anisotropic light scatterers (that is, resonators such as optical antennas). The spacing between antennas and their dimensions are much smaller than the wavelength. As a result the metasurfaces, on account of Huygens principle, are able to mould optical wavefronts into arbitrary shapes with subwavelength resolution by introducing spatial variations in the optical response of the light scatterers. Such gradient metasurfaces go beyond the well-established technology of frequency selective surfaces made of periodic structures and are extending to new spectral regions the functionalities of conventional microwave and millimetre-wave transmit-arrays and reflect-arrays. Metasurfaces can also be created by using ultrathin films of materials with large optical losses. By using the controllable abrupt phase shifts associated with reflection or transmission of light waves at the interface between lossy materials, such metasurfaces operate like optically thin cavities that strongly modify the light spectrum. Technology opportunities in various spectral regions and their potential advantages in replacing existing optical components are discussed.

Flat Optics With Designer Metasurfaces

Recent years have seen a renewed interest in the harvesting and conversion of solar energy. Among various technologies, the direct conversion of solar to chemical energy using photocatalysts has received significant attention. Although heterogeneous photocatalysts are almost exclusively semiconductors, it has been demonstrated recently that plasmonic nanostructures of noble metals (mainly silver and gold) also show significant promise. Here we review recent progress in using plasmonic metallic nanostructures in the field of photocatalysis. We focus on plasmon-enhanced water splitting on composite photocatalysts containing semiconductor and plasmonic-metal building blocks, and recently reported plasmon-mediated photocatalytic reactions on plasmonic nanostructures of noble metals. We also discuss the areas where major advancements are needed to move the field of plasmon-mediated photocatalysis forward.

Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy

Since its discovery, the asymmetric Fano resonance has been a characteristic feature of interacting quantum systems. The shape of this resonance is distinctively different from that of conventional symmetric resonance curves. Recently, the Fano resonance has been found in plasmonic nanoparticles, photonic crystals, and electromagnetic metamaterials. The steep dispersion of the Fano resonance profile promises applications in sensors, lasing, switching, and nonlinear and slow-light devices.

The Fano resonance in plasmonic nanostructures and metamaterials

We present an introduction to surface-enhanced Raman scattering (SERS) which reviews the basic experimental facts and the essential features of the mechanisms which have been proposed to account for the observations. We then review very recent fundamental developments which include: SERS from single particles and single molecules; SERS from fractal clusters and surfaces; and new insights into the chemical enhancement mechanism of SERS.

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Surface-enhanced Raman scattering

Plasmons in strongly coupled metallic nanostructures.

We inspired and learned from nature to develop the meta-lens array for intelligent imaging and sensing. The design, fabrication, and applications of the intelligent meta-lens array are reported in this talk. We developed the meta-lens array based light field imaging system for digital focusing, full-color imaging, depth sensing for static and dynamic objects, and 1D to 3D edge detection. This research shows the importance of optical meta-devices for next-generation optical imaging and sensing. We believe this opens up an avenue for future applications of optical devices in micro-robotic vision, unmanned-vehicle sensing, virtual and augmented reality, drones, and miniature personal-security systems. 

Autonomous sensing by intelligent meta-lens array

We use 3-dimensional finite-difference time-domain method to investigate surface polariton coupling effects between two nano-recording marks which are of different shapes. The behavior of near-field optical interactions and their influence on reflected read-out signal will be discussed.

Near-field coupling between disk- or ring-shaped nano recording marks in phase-change material

We demonstrate experimentally and numerically the first ever observation of optical activity in a chiral metamaterial that is underpinned by the exotic resonant combination of an electric quadrupole and the elusive toroidal dipole.

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A new type of optical activity in a toroidal metamaterial

Nonlinear metasurface devices for generation and manipulation of coherent vacuum ultraviolet (VUV) light will be reported in this talk. The approaches for design, fabrication, and measurement of the metasurface devices with different working principles will be discussed. Our ultracompact metasurfaces are capable of efficiently generating VUV light via various nonlinear processes with effective nonlinear coefficients much better than general nonlinear medias in the targeted regime. They pave the way toward highly efficient and versatile miniaturized VUV nanodevices.

Design and Fabrication of the Vacuum Ultraviolet Nonlinear Metasurfaces

This feature issue is a partial collection of contributions from authors who presented their research at the 9th International Conference on Nanophotonics (ICNP 2016) held during March 21-25, 2016 at Academia Sinica, Taipei, Taiwan. ICNP is an independent conference series dedicated to nanophotonics research and applications. This feature issue collects 28 papers related to research presented at ICNP 2016.

Din Ping Tsai

Papers

Autonomous sensing by intelligent meta-lens array

Near-field coupling between disk- or ring-shaped nano recording marks in phase-change material

A new type of optical activity in a toroidal metamaterial

Design and Fabrication of the Vacuum Ultraviolet Nonlinear Metasurfaces

Introduction to the Feature Issue on nanophotonics.