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.

Photonics research and novel applications require an ability to manipulate light. Much can already be achieved with natural materials, but their permittivity and permeability are limited. However, artificial metamaterials (MTMs), which are made of electromagnetic (EM) microstructures in deep-subwavelength scales, operate as an effective medium with almost arbitrary permittivity and permeability and thus offer greater freedom to control light. Homogeneous MTMs have been shown to offer interesting effects such as negative refraction and a perfect lens.1, 2 Slowly varying inhomogeneous MTMs have separately realized invisibility cloaking for propagating waves (PWs)3–5 and surface waves (SWs).6, 7 Recently, the extraordinary light-manipulation abilities of ultrathin MTMs (i.e., metasurfaces) with abruptly varying material properties have attracted much attention. A V-shaped antenna array metastring supports anomalous reflection and refraction of incident light but has the drawback of low conversion efficiency.8, 9 In previous work, some of us introduced a new metasurface that functions as the perfect link between PWs and SWs.10 Unlike with slowly varying inhomogeneous MTMs,3–7 EM waves do not stay inside ultrathin gradient metasurfaces for a long time. As a result there is less scattering, phase distortion, and dissipation. The metasurfaces offer great versatility for manipulating light, for instance, for anomalous reflection,8, 9 an antireflection coating, and a highly efficient surface plasmon polariton (SPP) coupler (an SPP is a quasiparticle of light and electron waves).10 Figure 1. Schematics of the designed sample with a unit cell (inset) consisting of a gold (Au) nanorod (yellow) and a continuous Au film (yellow) separated by a magnesium fluoride (MgF2) spacer (blue). L, w, d1: Length, width, and depth of the nanorod, respectively. L1; 2: x, y dimensions of the unit cell, respectively. d2; 3: Depth of the MgF2 spacer and the Au film, respectively. Lx; y : x, y dimensions of the repeating unit, respectively.

Highly efficient anomalous reflection by an optical metasurface

Wurtzite InN films were prepared on a 6H-SiC substrate by a self-designed plasma-assisted metal-organic molecular-beam epitaxy system without a buffer layer. In this article, the authors investigate the structural and optical properties of InN films grown on a 6H-SiC substrate. The crystallinity and microstructure of the thin film were further characterized by x-ray diffraction (XRD), field-emission scanning-electron microscopy, and transmission-electron microscopy. Electrical and optical properties were evaluated by Hall and photoluminescence (PL) measurements. XRD results indicate that InN film grown at 500 °C is epitaxially grown along the c-axis orientation. The two-dimensional growth mode is clearly shown in scanning-electron microscope images. Room-temperature PL spectra show that the emission peak is located at ∼0.83 eV due to the Burstein–Moss effect. In addition, the crystalline InN samples crack and peel away from the substrate at elevated growth temperature. This phenomenon may be attributed to...

Direct growth of hexagonal InN films on 6H-SiC by radio-frequency metal-organic molecular-beam epitaxy

The appropriate technique for coupling light efficiently from the conventional silica waveguide (SWG) entrance and exit of a photonic crystal slab waveguide (PCSWG) in a square lattice of dielectric rods is determined by calculation using the two-dimensional (2D) and three-dimensional (3D) finite-difference time-domain method. Four types of taper structure as the interface between SWG and PCSWG have been investigated. Coupling efficiency is found to be highly sensitive to the taper structure, defect rod configuration, and slab thickness. We find that when the type D tapered structure with a pair of larger defect rods, which is more flexible during fabrication, is placed between input/output SWG and PCSWG, if alters the mode properties of the guide mode so that mode matching can be achieved by determining the optimum defect configurations. Also, detailed computations of the 2D device and 3D slab version are given.

Two-dimensional and three-dimensional analysis of taper structures for coupling into and out of photonic crystal slab waveguides

vii Conference Committee xi Introduction xiii Revivals of molecular nonlinear optics in physics, chemistry, and life sciences (Plenary Paper) [7040-100] J. Zyss, Lab. de Photonique Quantique et Moleculaire, CNRS, Institut dAlembert (France) NANOFABRICATION AND LITHOGRAPHY 7033 05 Hierarchical architectures based on optical near-field interactions (Invited Paper) [7033-04] N. Tate, W. Nomura, Univ. of Tokyo (Japan); T. Yatsui, Japan Science and Technology Agency (Japan); M. Naruse, Univ. of Tokyo (Japan) and National Institute of Information and Communications Technology (Japan); M. Ohtsu, Univ. of Tokyo (Japan) 7033 06 Photochemical nano-scale patterning with Rhodamine 6G dye from aqueous phase [7033-05] A. Kassu, F. A. Calzzani, Jr., J. M. Taguenang, R. K. Sileshi, A. Sharma, Alabama AM N. Hayazawa, RIKEN, The Institute of Physical and Chemical Research (Japan) and CREST, Japan Science and Technology Agency (Japan); M. Motohashi, Osaka Univ. (Japan); S. Kawata, RIKEN, The Institute of Physical and Chemical Research (Japan), Osaka Univ. (Japan), and CREST, Japan Science and Technology Agency (Japan) NANO-IMAGING I 7033 0E Ultra-high resolution Raman imaging by optically trapped dielectric microsphere (Invited Paper) [7033-13] Z. X. Shen, J. Kasim, Nanyang Technological Univ. (Singapore) NANO-IMAGING II 7033 0I Nanoscale coupling effects on single particle microscopy (Invited Paper) [7033-17] K. L. Shuford, K. A. Meyer, Oak Ridge National Lab. (United States); C. Li, S. O. Cho, Korea Advanced Institute of Science and Technology (Korea, Republic of); W. B. Whitten, R. W. Shaw, Oak Ridge National Lab. (United States)

Din Ping Tsai

Papers

Highly efficient anomalous reflection by an optical metasurface

Direct growth of hexagonal InN films on 6H-SiC by radio-frequency metal-organic molecular-beam epitaxy

Two-dimensional and three-dimensional analysis of taper structures for coupling into and out of photonic crystal slab waveguides

Plasmonics: Nanoimaging, Nanofabrication, and Their Applications IV

Near-field interactions between light and surface plasmons of a metal slab