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 this paper, two different plasmonic metasurfaces are proposed to manipulate the optical properties of the corresponding thermal radiation. First of all, a tunable metasurface is proposed as a narrowband thermal emitter with wavelength tunability. Second, we demonstrate that the thermal radiation from an extending heated source can be focused to be with a designable pattern by modifying the spatial phase distribution of metasurfaces.

Temporal and Spatial Modification of Thermal Radiation

Meta-lens is an emerging optical device which composed of artificial nanostructures can freely manipulate the phase and amplitude of light. Meta-lenses show excellent performance and novel applications to meet the optical demands. The fascinating advantages of meta-lenses are their new properties, lighter weight, small size, high efficiency, better performance, broadband operation, lower energy consumption, data volume reduction, and CMOS compatibility for mass production. We demonstrate an intelligent depth-sensing system prototype applicable for diverse scenes, a switchable stereo vision system that adopts a 60 × 60 achromatic meta-lens array to measure depth over a 30 cm range with the support of deep learning. This system combines a light field camera and a structured light system to adapt to all light levels. The design, application, and experimental verification of the intelligent depth-sensing meta-device are reported in this talk. 

Meta-lens array for smart sensing (Conference Presentation)

Plasmonic structures offer a unique capability to generate electromagnetic oscillation in a tightly confined space at the metal-dielectric interface. This localized intense field can be used to significantly enhance light-matter interaction in an active material. Here, we use a hyperbolic metamaterial resonator on a UV AlGaN multiple quantum well to demonstrate a UV plasmonic nanolaser. The hyperbolic metamaterial consists of metal/dielectric multiple layer structure, which has dielectric permittivity tensor of opposite signs in two orthogonal directions. By proper design, it has an indefinite hyperbolic dispersion. We will discuss the resonator dimension design using this unique hyperbolic dispersion to enhance quantum well radiation for laser operation.

Hyperbolic metamaterial resonator for enhancing UV laser emission

Nano recording and readout on optical disk are investigated by a novel characterization method with ultrahigh resolution. Optical and thermal mechanisms of nano recording and readout on phase-change disk are discussed and proposed

Nano Recording and Readout on Optical Disk

In recent years, plasmonic nanostructured materials have been used to enhance light emission by creating localized electric fields that confine light fields to regions below the diffraction limit of the material, resulting in efficient lightmatter interactions.1 Plasmonic nanolasers based on these materials have been developed by using, for example, a dielectric nanowire or nanorod gain material—the laser amplification medium—placed on a metal film or silica/metal structure to form a Fabry-Pérot cavity resonator (an arrangement of mirrors for multiple light reflection).2, 3 However, the nanowire or nanorod length in these plasmonic nanolasers is often fairly long (several micrometers) and it is not easy to control the nanowire/nanorod orientation, which limits the potential applications of these devices. Here, we discuss our recent work using a metal-dielectric hyperbolic metamaterial (HMM)—a material engineered to exhibit extreme anisotropy upon interaction with light—as a plasmonic cavity to demonstrate a 289nm UV plasmonic nanolaser. Although the quantum well heterostructures used in these nanolasers, which increase the strength of electro-optical interactions, have a low internal quantum efficiency of 30%, the strong light-matter coupling introduced by the HMM plasmonic cavity can still bring the devices above the lasing threshold. The dispersion relation (the effect of a dispersive medium on the properties of a light wave) of the stacked metal-dielectric HMM is given by:

Din Ping Tsai

Papers

Temporal and Spatial Modification of Thermal Radiation

Meta-lens array for smart sensing (Conference Presentation)

Hyperbolic metamaterial resonator for enhancing UV laser emission

Nano Recording and Readout on Optical Disk

A deep-UV plasmonic nanolaser with hyperbolic metamaterials