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.

In the past decade or two, the field of nanophotonics has seen rapid development, empowered by introducing the concepts of plasmonics and metamaterials. The enabling feature behind this progress has been the use of noble metals that exhibit negative dielectric permittivities over a wide spectral range of visible and infrared wavelengths that allowed for manipulating the light on the subwavelength scale. Consequently, numerous interesting phenomena that otherwise do not exist in nature have been demonstrated in laboratories, but their transitions to practical applications have been painfully slow due to the large ohmic losses that are inherent in all metals. Doped semiconductors with lower losses have been proposed as new plasmonic materials to replace noble metals. Because the electron densities that are introduced with the artificial doping are always a few order of magnitude lower than what naturally available in metals, their plasma frequencies are shifted considerably toward longer wavelengths to the ...

Comparative Analysis of Metals and Alternative Infrared Plasmonic Materials

We numerically compare the mode birefringence and conflnement loss with four patterns (case A{D) of index-guiding photonic crystal flbers (PCF) using the flnite element method. These PCFs are composed of a solid silica core surrounded by difierent sizes of elliptical air holes and a cladding which consist of the same elliptical air holes in flber cladding with tetragonal lattice. The maximal modal birefringence and lowest conflnement loss of our proposed case A structure at the excitation wavelength of ‚ = 1550nm can be achieved at a magnitude of 5:3 £ 10 i2 (which is the highest value to our knowledge) and less than 0.051dB/km (an acceptable value less than 0.1dB/km) with only four rings of air holes in flber cladding, respectively. The merit of our designed PCFs is that the birefringence and conflnement loss can be easily controlled by turning the pitch (hole to hole spacing) of elliptical air holes in PCF cladding.

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A Comparative Study of High Birefringence and Low Confinement Loss Photonic Crystal Fiber Employing Elliptical Air Holes in Fiber Cladding with Tetragonal Lattice

Enhanced surface plasmon resonances in a silvershell nanocylindrical pair connected by a different type of nanobar that interacts with incident plane wave of transverse magnetic polarization are simulated by use of the finite element method. Arrays of silver nanoshells connected by silver nanobars are also investigated. The proposed structure exhibits a red-shifted localized surface plasmon that can be tuned over an extended wavelength range by varying the width of the nanobar and the dielectric constant in dielectric holes (DHs). The increase in the scattering cross sections is attributed to the effects of surface plasmon on the nanobar surface and a larger effective size of DH that is filled with a higher refractive medium. The predictive character of these calculations allows one to tailor the shape of the nanoparticle to achieve excitation spectra on demand with a controlled field enhancement.

Enhanced surface plasmon resonance based on the silver nanoshells connected by the nanobars

Surface plasmon effects excitation from the three-pair arrays of silver-shell nanocylinders are investigated numerically by using the finite-element method. Effects from different illumination wavelengths, interparticle distance, interpair distance, and the radii of air hole in nanocylinders are studied. Compared to the three-pair arrays of solid silver cylinder, the near-field optical response of the shell exhibits electric field enhancements and redshift which are found to be strongly influenced by tuning the radius of the air-hole in the nanocylinders.

Surface plasmon effects excitation from three-pair arrays of silver-shell nanocylinders

http://ntur.lib.ntu.edu.tw/bitstream/246246/164277/1/39.pdf

Din Ping Tsai

Papers

Comparative Analysis of Metals and Alternative Infrared Plasmonic Materials

A Comparative Study of High Birefringence and Low Confinement Loss Photonic Crystal Fiber Employing Elliptical Air Holes in Fiber Cladding with Tetragonal Lattice

Enhanced surface plasmon resonance based on the silver nanoshells connected by the nanobars

Surface plasmon effects excitation from three-pair arrays of silver-shell nanocylinders

Three-dimensional analysis of silver nano-particles doping effects on super resolution near-field structure