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.

The super-resolution near-field structure (super-RENS) is an ultrahigh-density near-field optical disk data storage medium which can achieve superior spatial resolution. Our previous studies found that enhanced local optical intensity occurred at the near field of the super-RENS disk, and the nonlinear near-field optical enhancement is related to the localized surface plasmons of silver clusters dissociated from the AgO x layer of the super-RENS disk. In this paper, we report the near-field and far-field properties of AgO x -type super-RENS with different embedded silver nanoparticles using two-dimensional finite-difference time-domain (FDTD) calculations. Highly localized enhancements are found between adjacent silver nanoparticles in the near fields. The far-field signals of different types silver nano scatters confirm the super-resolution capability of AgO x -type Super-RENS disks. The behaviors of far-field signals indicate the correlation between the enhanced localized surface plasmons and the super-resolution capabilities of AgO x -type super-RENS.

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Far-field optical response of localized near-field enhancements induced by nano scatters in AgOx super-resolution near-field structure

The super-resolution near-field structure (super-RENS) of the AgOx-type can perform the task of the high-density near-field optical recording in a more feasible way and has demonstrated better signal-to-noise ratio. For further exploration of the optical resolution and controllability of the super-RENS disks, we studied the near-field and far-field optical properties of the AgOx-type super-RENS embedded with periodic silver nanoparticles or nanoclusters with finite-difference time-domain (FDTD) simulations. It was found that the period of nanoclusters could selectively manipulate the resolution of the super-RENS disk.

Enhanced resolution of AgOx-type super-RENS disks with periodic silver nanoclusters

Conductive-tip atomic force microscopy (C-AFM) is a powerful tool for investigating the electrical characteristics of phase-change materials commonly used for electronic and optical data storage. We demonstrate the usefulness of this technique by examining the electrical conductivity of crystalline marks recorded with a focused laser pulse on a thin Ge2Sb2Te5 film.

Characterization of Ge2Sb2Te5 thin film alloys using conductive‐tip atomic force microscopy

We use the three-dimensional finite-difference time-domain method to investigate surface plasmon polariton coupling between two nano-recording marks which are of different shapes. The different coupling characteristics and the influence of these coupling effects on the read-out reflection signal will be discussed.

Surface plasmon polariton coupling between nano recording marks and their effect on optical read-out signal

A photocatalytic reactor is disclosed herein, which includes a rotatable light guide plate and a light source. The light guide plate is coated with a photocatalyst film on a surface thereof. A light beam emitted from the light source can be coupled into and propagate inside the light guide plate to activate the photocatalysis of the photocatalyst film. Additionally, the rotation of the light guide plate further expedites the photocatalysis so as to enhance the reaction efficiency of the photocatalytic reactor.

Din Ping Tsai

Papers

Far-field optical response of localized near-field enhancements induced by nano scatters in AgOx super-resolution near-field structure

Enhanced resolution of AgOx-type super-RENS disks with periodic silver nanoclusters

Characterization of Ge2Sb2Te5 thin film alloys using conductive‐tip atomic force microscopy

Surface plasmon polariton coupling between nano recording marks and their effect on optical read-out signal

Photocatalytic reactor with movable conformal light guiding plate