Showing papers by "Din Ping Tsai published in 2010"

Toroidal Dipolar Response in a Metamaterial

Abstract: Toroidal multipoles are fundamental electromagnetic excitations different from those associated with the familiar charge and magnetic multipoles. They have been held responsible for parity violation in nuclear and particle physics, but direct evidence of their existence in classical electrodynamics has remained elusive. We report on the observation of a resonant electromagnetic response in an artificially engineered medium, or metamaterial, that cannot be attributed to magnetic or charge multipoles and can only be explained by the existence of a toroidal dipole. Our direct experimental evidence of the toroidal response brings attention to the often ignored electromagnetic interactions involving toroidal multipoles, which could be present in naturally occurring systems, especially at the macromolecule level, where toroidal symmetry is ubiquitous.

Spectral Collapse in Ensembles of Metamolecules

Abstract: We report on the first direct experimental demonstration of a collective phenomenon in metamaterials: spectral line collapse with an increasing number of unit cell resonators (metamolecules). This effect, which is crucial for achieving a lasing spaser, a coherent source of optical radiation fuelled by coherent plasmonic oscillations in metamaterials, is linked to the suppression of radiation losses in periodic arrays. We experimentally demonstrate spectral line collapse at microwave, terahertz and optical frequencies.

CO2 photoreduction using NiO/InTaO4 in optical-fiber reactor for renewable energy

Abstract: The photocatalytic reduction of CO2 into fuels provides a direct route to produce renewable energy from sunlight. NiO loaded InTaO4 photocatalyst was prepared by a sol–gel method. Aqueous-phase CO2 photoreduction was performed in a quartz reactor to search for the highest photoactivity in a series of NiO/InTaO4 photocatalysts. Thereafter, the best NiO/InTaO4 was dip coated on optical fibers and calcined at 1100 °C. A uniform NiO/InTaO4 layer of 0.14 μm in thickness was observed on the optical fiber. An optical-fiber photoreactor, comprised of ∼216 NiO/InTaO4-coated fibers, was designed to transmit and spread light uniformly inside the reactor. The UV–vis spectra of powder InTaO4 as well as NiO loaded InTaO4 prepared via the same procedure indicated that both photocatalysts could absorb visible light. XRD confirmed that InTaO4 was in single phase. Vapor-phase CO2 was photocatalytically reduced to methanol using the optical-fiber reactor under visible light and real sunlight irradiation in a steady-state flow system. The rate of methanol production was 11.1 μmol/g h with light intensity of 327 mW/cm2 at 25 °C. Increasing the reaction temperature to 75 °C increased the production rate to 21.0 μmol/g h. Methanol production rate was 11.30 μmol/g h by utilizing concentrated sunlight which was comparable to the result of using artificial visible light. The quantum efficiencies were estimated to be 0.0045% and 0.063% in aqueous-phase and optical-fiber reactors, respectively, per gram NiO/InTaO4 photocatalyst. The quantum efficiency increased due to the superior light-energy utilization of NiO/InTaO4 thin film in the optical-fiber reactor

Optofluidic planar reactors for photocatalytic water treatment using solar energy.

Abstract: Optofluidics may hold the key to greater success of photocatalytic water treatment. This is evidenced by our findings in this paper that the planar microfluidic reactor can overcome the limitations of mass transfer and photon transfer in the previous photocatalytic reactors and improve the photoreaction efficiency by more than 100 times. The microreactor has a planar chamber (5 cm×1.8 cm×100 μm) enclosed by two TiO2-coated glass slides as the top cover and bottom substrate and a microstructured UV-cured NOA81 layer as the sealant and flow input/output. In experiment, the microreactor achieves 30% degradation of 3 ml 3×10−5M methylene blue within 5 min and shows a reaction rate constant two orders higher than the bulk reactor. Under optimized conditions, a reaction rate of 8% s−1 is achieved under solar irradiation. The average apparent quantum efficiency is found to be only 0.25%, but the effective apparent quantum efficiency reaches as high as 25%. Optofluidic reactors inherit the merits of microfluidics...

Laser-induced phase transitions of Ge_2Sb_2Te_5 thin films used in optical and electronic data storage and in thermal lithography

Abstract: Amorphous thin films of Ge2Sb2Te5, sputter-deposited on a ZnS-SiO2 dielectric layer, are investigated for the purpose of understanding the structural phase-transitions that occur under the influence of tightly-focused laser beams. Selective chemical etching of recorded marks in conjunction with optical, atomic force, and electron microscopy as well as local electron diffraction analysis are used to discern the complex structural features created under a broad range of laser powers and pulse durations. Clarifying the nature of phase transitions associated with laser-recorded marks in chalcogenide Ge2Sb2Te5 thin films provides useful information for reversible optical and electronic data storage, as well as for phase-change (thermal) lithography.

Chalcogenide glasses in active plasmonics

Abstract: The phase-change technology behind rewritable optical disks and the latest generation of electronic memories can also offer high contrast plasmonic switching functionality. Numerical simulations illustrate the extent of this potential while preliminary experiments show that a silver/gallium-lanthanum sulphide interface can support surface plasmon-polaritons and demonstrate the principle of plasmonic modulation through reversible photo-induced changes in the chalcogenide.

Controlling SERS intensity by tuning the size and height of a silver nanoparticle array

Abstract: It is demonstrated that the surface-enhanced Raman scattering (SERS) intensity of R6G molecules adsorbed on a Ag nanoparticle array can be controlled by tuning the size and height of the nanoparticles. A firm Ag nanoparticle array was fabricated on glass substrate by using nanosphere lithography (NSL) combined with reactive ion etching (RIE). Different sizes of Ag nanoparticles were fabricated with seed polystyrene nanospheres ranging from 430 nm to 820 nm in diameter. By depositing different thicknesses of Ag film and lifting off nanospheres from the surface of the substrate, the height of the Ag nanoparticles can be tuned. It is observed that the SERS enhancement factor will increase when the size of the Ag nanoparticles decreases and the deposition thickness of the Ag film increases. An enhancement factor as high as 2×106 can be achieved when the size of the polystyrene nanospheres is 430 nm in diameter and the height of the Ag nanoparticles is 96 nm. By using a confocal Raman mapping technique, we also demonstrate that the intensity of Raman scattering is enhanced due to the local surface plasmon resonance (LSPR) occurring in the Ag nanoparticle array.

Electromagnetic energy vortex associated with sub-wavelength plasmonic Taiji marks.

Abstract: The Taiji symbol is a very old schematic representation of two opposing but complementary patterns in oriental civilization. Using electron beam lithography, we fabricated an array of 70 × 70 gold Taiji marks with 30 nm thickness and a total area of 50 × 50 µm(2) on a fused silica substrate. The diameter of each Taiji mark is 500 nm, while the period of the array is 700 nm. Here we present experimental as well as numerical simulation results pertaining to plasmonic resonances of several Taiji nano-structures under normal illumination. We have identified a Taiji structure with a particularly interesting vortex-like Poynting vector profile, which could be attributed to the special shape and dimensions of the Taiji symbol.

Surface Plasmon Resonances Effects on Different Patterns of Solid-silver and Silver-shell Nanocylindrical Pairs

Abstract: Surface plasmon resonances effects on various geometries of solid-silver and silver-shell nanocylindrical pairs that interact with incident plane wave of transverse magnetic polarization are simulated by using the finite element method. Results show that the silver-shell nanocylindrical pair exhibits tunable plasmon resonances in the near-field zone that are not observed for the solid-silver case of similar size. It is also found that the facing corners of nanocylindrical pairs induce more surface plasmons in a wider range of wavelength.

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

Abstract: 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.

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

Abstract: 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.

A New Type of Optical Antenna: Plasmonics Nanoshell Bowtie Antenna with Dielectric Hole

Abstract: We first investigate plasmonic nanoshell bowtie nanoantenna with dielectric hole that interact with incident plane wave of transverse-magnetic polarization by use of the finite element method. The influence of the dielectric hole in nanoshell on the antenna field enhancement and spectral response is discussed. Results show that the proposed plasmonic nanoshell bowtie antenna can confine the field in a volume well below the diffraction limit and exhibit tunable surface plasmon resonances in a wider range of wavelength that are not observed for the conventional solid-gold case with the same volume. This offers a new possibility to further optimize the performance of optical nanoantenna.

Full Poincaré sphere coverage with plasmonic nanoslit metamaterials at Fano resonance

Abstract: A form-birefringent plasmonic metamaterial of the subwavelength thickness is used to convert the light's polarization state in a way to cover the whole Poincar\'e sphere's surface by adjusting the experimental configuration. This optical anisotropy is induced by grating surface plasmon polaritons of a nanoslit array made in a thin golden film with the narrow spectral Fano resonance. Phase delay between linearly polarized states introduced by the sample reaches the value of $0.85\ensuremath{\pi}$ in the visible corresponding to the effective ordinary-extraordinary refractive index difference of $\ensuremath{\Delta}n\ensuremath{\simeq}10.4$.

Controlling surface plasmon of several pair arrays of silver-shell nanocylinders.

Abstract: We studied, numerically, the characteristics of the surface plasmon of a system consisting of several pair arrays of silver-shell nanocylinders. Effects from different numbers of pair arrays, illumination wavelengths, and the core refractive index of silver-shell nanocylinders are studied by using the finite-element method. Results show that the peak wavelengths shift to shorter wavelengths (blueshifted) when the number of pair arrays increases from three to six. The near-field intensities in the gaps of the proposed type 1 structure can be tuned much stronger with a redshift by varying the wavelength of the incident light. The main features of surface plasmon effects can be qualitatively understood from some simple models of three, four, five, and six pairs of silver-shell nanocylinders.

Tuneable electron-beam-driven nanoscale light source

Abstract: Recently demonstrated ‘light-wells’—free-electron-driven tuneable nanoscale light sources—generate optical photons as electrons travel down a nano-hole through a metal‐dielectric multilayer structure. We report here on the application of boundary element modeling methods to the simulation of light-well output characteristics. The model is found to successfully reproduce the key features observed in experiment and as such will aid in the development and optimization of future device structures.

Pure angular momentum generator using a ring resonator.

Abstract: This paper reports a pure angular momentum generator using a ring resonator surrounded by a group of nano-rods. The evanescent waves of the circulating light in the ring are scattered by the nano-rods and generate a rotating electromagnetic field, which has only angular momentum but no linear momentum along the axis of rotation. The angular order is determined by the difference between the order of Whispering Gallery mode and the number of the rods, the rotating frequency is equal to the light frequency divided by the angular order. The maximum amplitude of the rotating electromagnetic fields can be 10 times higher than the amplitude of the input field when there are 36 rods (R(rod) = 120 nm, nr = 1.6). The pure angular momentum generator provides a new platform for trapping and rotation of small particles.

Enhanced Intermolecular Energy Transfer in the Vicinity of a Plasmonic Nanorice

Abstract: The problem of radiationless Forster energy transfer between a donor and an acceptor molecule is studied in the vicinity of a metallic nanorice. Using a recently formulated effective medium theory, the modified dipole–dipole interaction between the molecules in the vicinity of a spheroidal metallic nanoshell can be easily formulated, from which huge enhancement of the energy transfer rate is obtained due to the resonant excitation of the bonding and the antibonding plasmonic modes of the nanoshell. Effects due to the different locations and orientations of the molecules are also studied. The results show that the plasmonic resonances depend mainly on the nanorice geometry and much less on the configuration of the molecules, whereas the enhancement is more sensitive to the relative orientations and locations of the molecules.

Accurate description of the optical response of a multilayered spherical system in the long wavelength approximation

Abstract: The optical response of a multilayered spherical system of unlimited number of layers (a ``matryoshka'') in the long wavelength limit can be accounted for from the knowledge of the static multipole polarizability of the system to first-order accuracy. However, for systems of ultrasmall dimensions or systems with sizes not-too-small compared to the wavelength, this ordinary quasistatic long wavelength approximation (LWA) becomes inaccurate. Here we introduce two significant modifications of the LWA for such a nanomatryoshka in each of the two limits: the nonlocal optical response for ultrasmall systems $(l10\text{ }\text{nm})$, and the ``finite-wavelength corrections'' for systems $\ensuremath{\sim}100\text{ }\text{nm}$. This is accomplished by employing the previous work for a single-layer shell, in combination with a certain effective-medium approach formulated recently in the literature. Numerical calculations for the extinction cross sections for such a system of different dimensions are provided as illustrations for these effects. This formulation thus provides significant improvements on the ordinary LWA, yielding enough accuracy for the description of the optical response of these nanoshell systems over an appreciable range of sizes, without resorting to more involved quantum mechanical or fully electrodynamic calculations.

Near-field optical disk having surface plasmon amplification by stimulated emission of radiation

Abstract: A near-field optical disk having surface plasmon amplification by stimulated emission of radiation (SPASER) is provided. The near-field optical disk having SPASER includes a transparent substrate, a first transparent dielectric thin film layer formed on the transparent substrate, a SPASER thin film layer formed on the first transparent dielectric thin film layer, a second transparent dielectric thin film layer formed on the SPASER thin film layer, a recording thin film layer formed on the second transparent dielectric thin film layer, and a third transparent dielectric thin film layer formed on the recording thin film layer.

Design and fabrication of optical thin films for remote sensing instruments

Abstract: Optical thin films designed for multispectral band-pass filters and a primary Ag mirror were deposited on radiation-resistant glass by ion-beam-assisted deposition. The filters and mirror are for use in the optical payload of a remote sensing instrument. The optical parameters of the films and mirror were optimized by using admittance loci analysis. The simulation shows that the band-pass filters can achieve average transmittances of 95% in the blue (B1), green (B2), red (B3), near-infrared (B4), and panchromatic (400–900 nm) spectral ranges and an average reflectance of 99% for the primary Ag mirror in the visible spectrum. The corresponding properties of the films were investigated by in situ optical monitoring, spectrometry, and high-resolution transmission electron microscopy. It was found that the average transmittances are above 85% for all five band-pass filters, with a rejection transmittance below 1% in the spectral range of 350–1100 nm. The average reflectance of the primary Ag mirror (with a pr...

Approaches to the Synthesis and Characterization of Spherical and Anisotropic Noble Metal Nanomaterials

Abstract: The ability to tailor the shapes and sizes of inorganic nanostructures is critical for modern materials chemistry, because the intrinsic properties of most nanostructures are strongly dependent on their shapes and sizes, which can be further useful for various applications. This chapter provides an overview of the various synthesis methods of spherical and anisotropic noble metal nanomaterials. The shapes of resultant nanostructures can be controlled by manipulating different parameters, such as the crystallinity of seeds and the growth rates of different crystallographic facets. In this chapter, we discuss a number of useful parameters that can be tuned to control the formation of anisotropic silver and gold nanostructures with a specific shape in a solution-phase synthesis, using chemical and biological methods. A brief description of various synthesis methods and the characterization of the metal nanoparticles with a special emphasis on X-ray absorption measurements have been provided in order to understand the mechanistic aspects of crystal growth. Using the synthesis methods described herein, a variety of well-defined anisotropic shapes of silver and gold nanoparticles can be achieved and these methodologies can be directly extended to other metals. Keywords: nanostructures; spherical; anisotropic; gold; silver; chemical methods; biological methods; synthesis and characterization

Nonlocality and particle-clustering effects on the optical response of composite materials with metallic nanoparticles

Abstract: The optical properties of composites with metallic nanoparticles are studied, taking into account the effects due to the nonlocal dielectric response of the metal and the coalescing of the particles to form clusters. An approach based on various effective medium theories is followed, and the modeling results are compared with those from the cases with local response and particles randomly distributed through the host medium. Possible observations of our modeling results are illustrated via a calculation of the transmission of light through a thin film made of these materials. It is found that the nonlocal effects are particularly significant when the particles coalesce, leading to blue-shifted resonances and slightly lower values in the dielectric functions. The dependence of these effects on the volume fraction and fractal dimension of the metal clusters is studied in detail.

A combinatorial approach to metamaterials discovery

Abstract: We report a high through-put combinatorial approach to photonic metamaterial optimization. The new approach is based on parallel synthesis and consecutive optical characterization of large numbers of spatially addressable nano-fabricated metamaterial samples (libraries) with quasi-continuous variation of design parameters under real manufacturing conditions. We illustrate this method for Fano-resonance plasmonic nanostructures arriving at explicit recipes for high quality factors needed for switching and sensing applications.

Reciprocity theorem for nonlocal optics: completion of proof and application to spectroscopic analysis

Abstract: The problem of optical reciprocity in the long wavelength limit is considered in terms of the symmetry for the scalar Green's function under the Neumann boundary conditions, for materials with nonlocal and anisotropic dielectric responses. This extends the previous works in the literature which were limited either to the Dirichlet conditions or to cases of perfect conductor and local dielectric response. Application of this symmetry principle to the analysis of surface-enhanced spectroscopy for a molecule near a metallic nanoparticle is demonstrated, accounting for the nonlocal response of the particle.

Phase change gallium and germanium chalcogenides for optical electronic and plasmonic switching

Abstract: We show that the phase-change technology behind rewritable optical disks and the latest generation of electronic memories can also offer applications in active plasmonics and metamaterials. A range of chalcogenides have been fabricated and characterized for their optical, thermal and electrical properties. Experimental demonstrations show that excitation-induced refractive index changes in gallium lanthanum sulphide (Ga:La:S), a can provide high contrast switching functionality. We have fabricated a silver/GLS interface which can support surface plasmon-polaritons and also incorporated a Ga:La:S thin film with a metamaterials based on arrays of resonance cells. We demonstrate both plasmonic modulation and metamaterials switching through reversible refractive index changes in the chalcogenide.

Dynamic modifications of polarizability for large metallic spheroidal nanoshells

Abstract: We present an approach alternative to the hybridization model for the treatment of the coupled interfacial plasmon modes in spheroidal metallic nanoshells. Rather than formulating the problem from the Lagrangian dynamics of the free electronic fluid, we adopt an effective medium approach together with the uniqueness of the solutions to electromagnetic boundary value problem, from which the polarizability of the shells can then be systematically and efficiently derived; and the resonance frequencies for the coupled modes can be obtained from the poles in the polarizability. This approach can treat confocal nanoshells with different geometries for the spheroidal cavity and external surface and allow for a natural extension to incorporate corrections from the finiteness of the optical wavelength which are important for nanoparticles of larger sizes. This thus surpasses the hybridization model which is limited to incorporate only the electrostatic Coulomb interaction between the uncoupled plasmons. Numerical results will be provided for different nanoshell systems, and for the illustration of the various geometric and dynamic effects from our model.