Kuang-Yu Yang

High-Efficiency Broadband Anomalous Reflection by Gradient Meta-Surfaces

We combine theory and experiment to demonstrate that a carefully designed gradient meta-surface supports high-efficiency anomalous reflections for near-infrared light following the generalized Snells law, and the reflected wave becomes a bounded surface wave as the incident angle exceeds a critical value. Compared to previously fabricated gradient meta-surfaces in infrared regime, our samples work in a shorter wavelength regime with a broad bandwidth (750-900 nm), exhibit a much higher conversion efficiency (∼80%) to the anomalous reflection mode at normal incidence, and keep light polarization unchanged after the anomalous reflection. Finite-difference-time-domain (FDTD) simulations are in excellent agreement with experiments. Our findings may lead to many interesting applications, such as antireflection coating, polarization and spectral beam splitters, high-efficiency light absorbers, and surface plasmon couplers.

High-Efficiency Broadband Meta-Hologram with Polarization- Controlled Dual Images

Holograms, the optical devices to reconstruct predesigned images, show many applications in our daily life. However, applications of hologram are still limited by the constituent materials and therefore their working range is trapped at a particular electromagnetic region. In recent years, the metasurfaces, an array of subwavelength antenna with varying sizes, show the abilities to manipulate the phase of incident electromagnetic wave from visible to microwave frequencies. Here, we present a reflective-type and high-efficiency meta-hologram fabricated by metasurface for visible wavelength. Using gold cross nanoantennas as building blocks to construct our meta-hologram devices with thickness ∼ λ/4, the reconstructed images of meta-hologram show polarization-controlled dual images with high contrast, functioning for both coherent and incoherent light sources within a broad spectral range and under a wide range of incidence angles. The flexibility demonstrated here for our meta-hologram paves the road to a wide range of applications related to holographic images at arbitrary electromagnetic wave region.

Seedless, silver-induced synthesis of star-shaped gold/silver bimetallic nanoparticles as high efficiency photothermal therapy reagent

This work demonstrates a simple method for synthesizing a shape-controllable bimetallic gold/silver nanostructured material. Spiky star-shaped gold/silver nanoparticles are obtained by mixing HAuCl4, AgNO3 and ascorbic acid with shaking for 20 s. The wide range of star shapes and irregular quasi-spherical nanoparticles is tailored by tuning the ratio of metal precursors. The wavelengths absorbed by the nanoparticles can be tuned from visible light to near infrared by controlling their shape. To maintain the morphology of the nanoparticles, enhance their thermal stability and support their application in biological systems, modified chitosan was utilized for the properties and to keep the material well dispersed in solution in deionized water. The moderate concentration of modified chitosan capped bimetallic star-shaped nanoparticles not only ensured non-toxicity to normal cells and cancer cells, but also promoted high efficiency photothermal ablation of cancer cells. Ultimately, this nanotechnology-driven assay has huge potential for application in rapid synthesis, tunable absorption and non-cytotoxic photothermal therapy for the effective treatment of cancer.

Optical magnetic response in three-dimensional metamaterial of upright plasmonic meta-molecules

We report the first three-dimensional photonic metamaterial, an array of erected U-shape plasmonic gold meta-molecules, that exhibits a profound response to the magnetic field of light incident normal to the array. The metamaterial was fabricated using a double exposure e-beam lithographic process. It was investigated by optical measurements and finite-element simulations, and showed that the magnetic field solely depends on the plasmonic resonance mode showing either enhanced in the centre of the erected U-shape meta-molecule (16 times enhancement) or enhanced around two prongs of erected U-shape meta-molecule (4 times enhancement).

Magnetic plasmon induced transparency in three-dimensional metamolecules

Abstract In a laser-driven atomic quantum system, a continuous state couples to a discrete state resulting in quantum interference that provides a transmission peak within a broad absorption profile the so-called electromagnetically induced transparency (EIT). In the field of plasmonic metamaterials, the sub-wavelength metallic structures play a role similar to atoms in nature. The interference of their near-field coupling at plasmonic resonance leads to a plasmon induced transparency (PIT) that is analogous to the EIT of atomic systems. A sensitive control of the PIT is crucial to a range of potential applications such as slowing light and biosensor. So far, the PIT phenomena often arise from the electric resonance, such as an electric dipole state coupled to an electric quadrupole state. Here we report the first three-dimensional photonic metamaterial consisting of an array of erected U-shape plasmonic gold nanostructures that exhibits PIT phenomenon with magnetic dipolar interaction between magnetic metamolecules. We further demonstrate using a numerical simulation that the coupling between the different excited pathways at an intermediate resonant wavelength allows for a π phase shift resulting in a destructive interference. A classical RLC circuit was also proposed to explain the coupling effects between the bright and dark modes of EIT-like electromagnetic spectra. This work paves a promising approach to achieve magnetic plasmon devices.

Vertical split-ring resonator based nanoplasmonic sensor

Split-ring resonators (SRRs) have been the subject of investigation as plasmonic sensors that operate by sensing plasmon resonance shift δλ when exposed to a medium with a refractive index change δn. However, conventional planar SRRs have their plasmon fields spread into the substrates, reducing accessible sensing volume and its sensing performance. Such a limitation can be eradicated with vertical SRRs in which the plasmon fields localized in SRR gaps are lifted off from the substrate, allowing for greatly enhanced sensitivity. Here, we demonstrate the highest sensitivity among reported SRR-based sensors in optical frequencies.

Design of high birefringence and low confinement loss photonic crystal fibers with five rings hexagonal and octagonal symmetry air-holes in fiber cladding

We present a new cladding design for high birefringence and low confinement loss photonic crystal fibers (PCFs) using a full-vector finite element method with anisotropic perfectly matched boundary layer. Six cases of PCFs are proposed for comparison. The proposed cladding in PCFs is composed of five rings of air-holes. Air-holes on the inner two rings are arranged in a hexagonal symmetry whereas, air-holes on the outer three rings are arranged in an octagonal symmetry in fused silica. Results show that suitable design air-holes on the inner two rings will significantly increase the birefringence, whereas, elliptical holes with major axis along x-axis on the outer three rings will provide strong confinement ability. The highest modal birefringence and lowest confinement loss of our proposed case five structure at the excitation wavelength of λ = 1550 nm can be achieved at a magnitude of 0.87 × 10−2 and less than 0.01 dB/km with only five rings of air-holes in fiber cladding.

Optical magnetic response of upright plasmonic molecules in 3D metamaterial

An array of fabricated U-shaped split-ring resonators shows a profound response to the magnetic field of light incident normal to the sample.

Highly efficient anomalous reflection by an optical metasurface

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 urchin-like bimetallic nanoparticles for photothermal cancer therapy

Many metal nanoparticles (NPs) are being studied for use in photothermal cancer therapy, a non-invasive technique where specific wavelengths of light can be used to excite the NPs, causing local heating that selectively kills cancer cells. These NPs can convert light to heat because of surface plasmon resonance, where electrons oscillate at the surface of the NPs resulting in specific absorbance or scattering. Plasmonic metal NPs have attracted enormous interest because their physical and chemical properties can be tuned by changing the size, shape, and composition of the nanostructures. Experimental and theoretical results have shown that nonspherical NPs, such as branched or urchin-like ones, exhibit stronger electromagnetic behavior than spherical NPs, making them more effective for photothermal therapy.1 Moreover, anisotropic gold NPs exhibit numerous plasmon resonances in the visible and near-IR range, which depend strongly on polarization. However, anisotropic NPs require long synthesis times, and they are unstable at room temperature. These issues limit their use. Branched gold NPs can be obtained by several methods, the most common is the seeded growth approach. While the morphology of the resulting NPs can be tuned by several factors in the seeded growth approach,2 the process is lengthy and complicated. Seedless synthesis is another route to save time, but the NP morphology cannot be well-tuned, which limits its efficiency in bio-applications.3 We created a new technique to grow tunable, non-spherical gold/silver NPs using a simple, rapid Figure 1. Transmission-electron-microscopy bright-field images of gold/silver nanocrystals synthesized with various gold/silver ratios: (a) 3, (b) 10, (c) 15, (d) 30, (e) 50 and (f) 100.5