Yuan-Fong Chau

Design of plasmonic toroidal metamaterials at optical frequencies

Toroidal multipoles are the subject of growing interest because of their unusual electromagnetic properties different from the electric and magnetic multipoles. In this paper, we present two new related classes of plasmonic metamaterial composed of purposely arranged of four U-shaped split ring resonators (SRRs) that show profound resonant toroidal responses at optical frequencies. The toroidal and magnetic responses were investigated by the finite-element simulations. A phenomenon of reversed toroidal responses at higher and lower resonant frequencies has also been reported between this two related metamaterials which results from the electric and magnetic dipoles interaction. Finally, we propose a physical model based on coupled LC circuits to quantitatively analyze the coupled system of the plasmonic toroidal metamaterials.

Near-field optical properties and surface plasmon effects generated by a dielectric hole in a silver-shell nanocylinder pair

Near-field optical properties and surface plasmon effects in a silver-shell nanocylinder pair with five different dielectric holes (DHs) that interact with a transverse magnetic mode incident plane wave are simulated by use of the finite-element method, which includes the investigation of particle-particle interaction. The proposed structure exhibits a redshifted localized surface plasmon that can be tuned over an extended wavelength range by varying the dielectric constant in DHs and the thickness of the nanocylinder silver shell. The increase in the near-field intensity is attributed to a larger effective size of DH that is filled with a higher refractive medium.

High birefringence photonic crystal fiber with a complex unit cell of asymmetric elliptical air hole cladding

High birefringence induced by elliptical air hole photonic crystal fibers (EHPCFs) is analyzed numerically using the finite-element method. Statistical correlations between the birefringence and the various parameters are obtained. We found that the complex elliptical air hole is better than that of a circular one to obtain high birefringence in photonic crystal fibers. Our suggested structures can considerably enhance the birefringence in EHPCFs and show that the birefringence can be as high as 1.1294 x 10(-2), which is higher than the birefringence obtained from conventional step-index fiber (5 x 10(-4)), circular air holes PCF (3.7 x 10(-3)), and elliptical hollow PCF (2.35 x 10(-3)).

Fabrication of three dimensional split ring resonators by stress-driven assembly method

We demonstrate a self-assembly strategy for fabricating three dimensional (3D) metamaterials. This strategy represents the desired 3D curving prongs of the split ring resonators (SRRs) erected by metal stress force with appropriate thin film parameters. Transmittance spectra and field patterns corresponding to each resonance modes are calculated by finite element method (FEM). The eigen-modes of the SRRs can be excited by normal illumination with polarization state parallel to the erected SRRs, which are unlike for the cases of planar SRRs. This method opens a promising fabrication process for the application of tailored 3D SRRs.

Three-dimensional analysis of surface plasmon resonance modes on a gold nanorod

We investigate the surface optical properties of a gold (Au) nanorod by using the finite-element method in a three-dimensional model. Results from the near-field optical images show spatially oscillatory patterns (nodal fields) on the surface, and these are attributable to plasmon-mode wave functions. We interpret these phenomena in terms of the electric field nodes on the surface of the rod, and the results show good agreement with our experimental observation in the optical images.

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

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.

Highly Birefringent Index-Guiding Photonic Crystal Fiber with Squeezed Differently Sized Air-Holes in Cladding

We propose a novel high-birefringence index-guiding photonic crystal fiber (PCF). This PCF is composed of a solid silica core and a cladding with two differently sized squeezed elliptical air-holes. The mode birefringence of a fundamental mode in such PCFs is analyzed numerically by the finite-element method. Numerical results reveal that an extraordinarily high modal birefringence at the excitation wavelength of λ=1550 nm, 2.6×10-2, is acquired. The contributions of the cladding with two different sizes of air-hole ellipticity, the center-to-center distance between the air-holes, and the the number of cladding rings as well as the confinement loss to the birefringence are systematically evaluated. The evolution of birefringence with the structural variations shows that our highly birefringent fiber can be designed in a controlled manner.

Significantly Enhanced Birefringence of Photonic Crystal Fiber Using Rotational Binary Unit Cell in Fiber Cladding

We propose a novel high-birefringence index-guiding photonic crystal fiber (PCF). This PCF is composed of a solid silica core and a cladding with rotational squeezed-triangular-lattice elliptical air holes, which consist of binary unit cells. The birefringence of a fundamental mode in such a PCF is analyzed numerically using the finite element method. A binary unit cell in a PCF cladding that combines its rotational effects can enhance the birefringence as high as a magnitude of the order of 10-2. This study provides a new viewpoint for the characterization and design of a high-birefringence PCF.

50/50 beam splitter using a one-dimensional metal photonic crystal with parabolalike dispersion

The spatial dispersion properties of a one-dimensional metal photonic crystal have been analyzed and five types of dispersion curves have been shown at a normalized frequency less than 1. It is demonstrated that by exploiting a parabolalike dispersion behavior, a metal photonic crystal slab can be used to realize an exactly 50∕50 beam splitter.

Electromagnetic energy vortex associated with sub-wavelength plasmonic Taiji marks

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.