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.

Optical meta-devices are composed of the collection of artificial subwavelength nanostructures. Phase, polarization, or amplitude of the incident electromagnetic waves can be manipulated by the specifically designed meta-devices. The demands of the new generation of photonics currently extend from classical to quantum optics. We report our progress in the design, fabrication, and application of the novel optical meta-devices from classical to quantum optics. We show a novel achromatic meta-lens array light field optical system for applications in imaging and sensing. We integrate a meta-lens array with a thin slice BBO nonlinear crystal to form a high-dimensional quantum entanglement optical chip. Results of the excellent mutual entanglement fidelity in 2-dimensional, 3-dimensional, and 4-dimensional experiments have successfully demonstrated the novel function of our high-dimensional optical quantum chip.

High dimensional optical meta-devices: classical to quantum

A process of growing large-area plasmonic-nano-particles-decorated ZnO nanorods on the polycarbonate optical disk substrate was developed, while a corresponding photocatalytic rotational reactor was fabricated. Hydrothermal process was adapted to grow ZnO nanorods perpendicular to the optical disk substrate at relatively lower temperature. The optical disk substrate has advantages of durable property in fast rotation and high impact-resistance. The plasmonic nano-particles, in this case, silver nano-particles, were deposed on the ZnO nanorods by direct sputtering. The morphology of ZnO nanorods and plasmonic nano-particles was investigated by Scanning Electron Microscope (SEM). The photocatalytic activity of the sample was evaluated by the degradation of methyl orange (MO) as a model compound in aqueous solution, and the decomposition rate of MO molecules is monitored by the optical spectroscopy measurements. In the optimized condition, less than 10% of the MO remained in the aqueous solution after a 20-minute treatment in the rotational reactor with our sample.

Plasmonic enhanced optical disk reactor for wastewater treatment

We describe the optimum design of the near-field scanning optical microscope (NSOM) based on a short probe tapping mode tuning-fork (TMTF) configuration and its applications in optoelectronic characterization and optical measurements. The short probe TMTF-NSOM is constructed to operate both in collection and excitation modes, in which a cleaved short fiber probe attached to one tine of the tuning fork is used as the light collector/emitter as well as the force sensing element. Interference fringes due to standing evanescent waves generated by total internal reflection are imaged by collection mode. On the other hand, excitation mode of short probe TMTF-NSOM is applied to perform near-field surface photovoltage measurements on AlGaInP light emitting diode structures.

Optimum instrumentation of a tapping mode, non-optically regulated near-field scanning optical microscope and its applications

An image-based refractive index measuring system comprises an optical device and an electronic device. The optical device is used to guiding an external light which is passed through an analyte. The electronic device comprises an image capture module, an image analyze module and a display module. The image capture module generates a first image by capturing the external light source. The image analyze module connects to the image capture module to receive the first image, and analyzes the first image in order to generate an analytical result comprising the refractive index of the analyte. The display module connects to the image analyze module to receive and display the analytical result.

Image-based refractive index measuring system

The toroidal spectral responses of toroidal metamolecule integrated by four gold U-shaped SRRs at optical frequencies are numerically studied. Downsizing the structure achieve observation of a plasmonic toroidal mode at optical frequencies.

Din Ping Tsai

Papers

High dimensional optical meta-devices: classical to quantum

Plasmonic enhanced optical disk reactor for wastewater treatment

Optimum instrumentation of a tapping mode, non-optically regulated near-field scanning optical microscope and its applications

Image-based refractive index measuring system

Plasmonic toroidal response of four U-shaped resonant rings at optical frequencies