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.

By learning from nature, it may become possible to realize efficient, stable energy conversion by developing photosynthesis technologies that use only earth-abundant materials and operate under mild conditions. In past decades, the photoelectrolysis of water has attracted continual interest as a potential route to sustainable hydrogen production for low-cost green energy.1–4 Water splitting is regarded as artificial photosynthesis in which materials convert the energy of sunlight into chemical energy to produce chemical fuels (hydrogen and oxygen). Our goal is to improve the efficiency of water splitting by photoelectrolysis using plasmonic resonance, that is, the coherent oscillation of free electrons in a noble metal driven by incident electromagnetic waves. In a recent publication,5 we demonstrated the use of direct writing by a femtosecond (fs) laser to fabricate patternable plasmonic silver (Ag) particles from a silver(I,III) oxide (AgO) film. Figure 1(a) shows an artist’s impression of a sample and its operating principle. The embedded Ag nanostructures serve as plasmonic couplers to enhance the photoactivity of a photoelectrode consisting of a zinc oxide (ZnO) nanorod array. In this configuration, plasmon damping can originate in the Ag nanostructures, producing both scattering and absorption behavior. The embedded plasmonics act as subwavelength scattering elements to trap randomly propagating plane waves into an absorbing semiconductor material by multiple high-angle scattering into the ZnO nanorods. This increases the effective optical path length in the photoelectrode. In addition, scattering from the plasmonics on Figure 1. (a) Artist’s impression of zinc oxide/silver (ZnO/Ag) plasmonic photoelectrode. CB: Conduction band. VB: Valence band. e: Electron. h: Hole. H2: Molecular hydrogen. H+: Hydrogen ions. (b) Scanning electron microscopy (SEM) image of sample.

Plasmonic zinc oxide/silver photoelectrode for green hydrogen production

We demonstrate strong optical activity (and circular dichroism) for both microwave and photonic achiral planar metamaterials. The effect arises from extrinsic chirality resulting from oblique incidence of light onto the metamaterial structures.

Optical activity in achiral metamaterials

Abstract. Integrated-resonant units (IRUs), associating various meta-atoms, resonant modes, and functionalities into one supercell, have been promising candidates for tailoring composite and multifunctional electromagnetic responses with additional degrees of freedom. Integrated-resonant metadevices can overcome many bottlenecks in conventional optical devices, such as broadband achromatism, efficiency enhancement, response selectivity, and continuous tunability, offering great potential for performant and versatile application scenarios. We focus on the recent progress of integrated-resonant metadevices. Starting from the design principle of IRUs, a variety of IRU-based characteristics and subsequent practical applications, including achromatic imaging, light-field sensing, polarization detection, orbital angular momentum generation, metaholography, nanoprinting, color routing, and nonlinear generation, are introduced. Existing challenges in this field and opinions on future research directions are also provided.

Integrated-resonant metadevices: a review

A new framework of light coherence optimization is proposed to design non-ideal broadband achromatic lenses, enabling large-scale flat lenses' implementation and high performance. The strategy paves the way for practical planar optical devices and full-color imaging systems. 

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Advance of large-area achromatic flat lenses

A new type of laser diodes with good beam quality is introduced. The far-field divergence angle can be close to diffraction-limited value. In the new design, the direction of the waveguide on a broad area Fabry-Perot laser diode is tilted at an angle from the facet normal. This design is called “angled broad area laser diode”. In this tilted waveguide device, filamentation is not observed. The far-field divergence angle is generally within 5 times the diffraction-limited
value. This tilted broad area laser is advantageous over the angled grating DFB laser because the difficulty of matching the grating period with peak gain wavelength is avoided.

Din Ping Tsai

Papers

Plasmonic zinc oxide/silver photoelectrode for green hydrogen production

Optical activity in achiral metamaterials

Integrated-resonant metadevices: a review

Advance of large-area achromatic flat lenses

Angled broad-area semiconductor lasers to emit high output power with good beam quality