Propagation loss in a two-dimensional triangular photonic crystal waveguide

By means of a plane-wave-based transfer-matrix method, we present an extensive study of propagation loss in two-dimensional (2D) photonic crystal waveguides with limited cladding wall thickness. We examine the dependence of propagation loss on both the propagation direction and the waveguide wall thickness. It is shown that the propagation loss is a function of excitation frequency, wall thickness and waveguide outermost surface morphology. The conclusion that the propagation loss essentially decays exponentially with respect to the cladding wall thickness of a waveguide is valid only to a certain degree. In addition, we find that the propagation loss exhibit very complex behavior with respect to the excitation frequency.     

Lamellar model of the left-handed metamaterials composed of metallic split-ring resonators and wires

Left-handed materials composed of split ring resonators (SRRs) and wires are investigated in the viewpoint of lamellar composite with epsilon-negative (ENG) and mu-negative (MNG) materials staked alternatively. Several configurations of the SRR-wire metamaterial are numerically simulated to confirm its left-handed response as an analogy to the ENG-MNG lamellar model.     

Relativistic energy loss and induced photon emission in the interaction of a left-handed sphere with an external electron beam

Relativistic energy loss and photon emission in the interaction of a coated sphere containing a left-handed material with an external electron beam are studied based on the classical electrodynamics. Both of electric modes and magnetic modes are not only found in the spectra of energy loss and photon emission for different combinations of electron velocity and sphere radius, the new excitation modes can be also observed. Our results show that different excitation modes for the left-hand materials and the structure information of coated sphere can be explored by scanning transmission electron microscopy (STEM).     

Fabrication of two-dimensional infrared photonic crystals by deep reactive ion etching on Si wafers and their optical properties

We report the fabrication and characterization of two-dimensional silicon-based photonic crystal structures realized by deep reactive ion etching. Photonic crystals with square and triangular lattices with very high aspect ratios up to 33 have been achieved. Photonic bandgap behaviors are identified by the transmission and reflection spectra measured by using a Fourier transform infrared spectrometer. The experimental results are in good agreement with the calculated band structures.     

Linewidth Narrowing in Microstrip Resonator Using Medium

An effective highly dispersive medium is proposed by utilizing strong dispersion of localized defect mode in a microstrip photonic crystal. Linewidth narrowing in a composite microstrip resonator using this photonic- crystal-based effective medium is investigated and its dependence on the structure parameters is presented. Both numerical simulations and experimental results demonstrate that compared to the traditional resonator, the Qfactor increases much more quickly and the peak transmission decreases more slowly at the same time for the composite resonator.     

Photonic bandgap properties of 8-fold symmetric photonic quasicrystals

The bandgap properties of 2D 8-fold symmetric photonic quasicrystals (PQCs) composed of a set of rods are numerically investigated for various fill factors, dielectric constants and propagation angles using the finite difference time domain (FDTD) method. Some interesting properties are found besides the known results: (i) the central frequency of the bandgap shifts to low frequency as r/a increases; (ii) an optimum fill factor to obtain the largest gap width exists, which decreases with increasing dielectric constant; (iii) compared with the behavior for a given fill factor, the variation of bandgap width for the optimum fill factor increases much more significantly with the dielectric constant. Selection guidelines for 8-fold symmetric PQC designs are provided for optimum fill factors and variation of the relative bandgap width at different dielectric constants.     

Negative refractive index in the near‐UV from Au‐coated CuCl nanoparticle superlattices

Nanoparticle superlattices consisting of CuCl spheres coated with a gold (Au) nanoshell are shown to exhibit negative index of refraction in the near‐UV regime. These predictions are based on calculations of the effective refractive index by use of the extended Maxwell–Garnett theory. The results for the refractive index are supplemented by multiple‐scattering calculations of light‐transmittance curves from finite slabs of nanoparticle superlattices. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ultra‐subwavelength focusing of light by a monolayer of metallic nanoshells with an adsorbed defect

We study theoretically the focusing properties of a two‐dimensional periodic array of silver‐coated silica nanospheres. The array contains an adsorbed nanosphere of the same type which serves as point source when the array is illuminated by a plane wave. By using a rigorous multiple‐scattering theory we show that under plane wave illumination, the array focuses the incident light in a tight area with size of about 1/70 of the wavelength. At the same time, the near‐field is amplified by three orders of magnitude. The frequency corresponding to the focusing and amplification effects depends on the thickness of the silver nanoshell offering a tunable response of the structure. The latter can find application in optical trapping, lithography and imaging below the diffraction limit. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Control of reflectivity and stop bands in a Bragg stack with a four-layer period

We provide a general expression for the band dispersion in a Bragg stack with a four-layer period, and show how the expression is considerably simplified in the case of a perturbed two-layer Bragg stack, where every second layer has refractive index n₁ and n₂, respectively. We present a range of analytical expressions to describe key features of the band dispersion, including the dependence of the band gap and of the evanescent decay length on variations in layer thickness for TE and TM polarized light incident perpendicular and at an angle to the Bragg stack.     

Characteristics of guided waves in indefinite-medium waveguides

We have presented a discussion on the guided waves in the indefinite-medium waveguides. The characteristic equations describing TE and TM guided waves have been derived. Aiming mainly at the TE guided waves, we have analyzed the existence conditions for the guided and surface modes for four distinct cases, respectively. A number of exotic properties have been revealed in such waveguides, such as the coexistence of forward and backward modes, the absence of fundamental modes, the existence of surface modes and the relaxed requirements for waveguide core constitutive parameters. Numerical results have confirmed our analyses.     

Focusing Properties in Photonic Crystal Structure Formed by Air Holes in Dielectric Background with Concave Interface

We investigate the focusing properties of photonic crystal structures with a concavo-concave as the photonic crystal boundary. The photonic crystal is constituted by air holes parallelly distributed in a uniform dielectric. A good-quality focus of a plane wave can appear out of the photonic crystal structure, and a strong far field focus is also formed from the photonic crystal interface. A negative-refractive beam at the frequency 0.195 （2πc/α） for the excited Bloch wave mode near the Brillouin zone edges under all incident angles are obtained by simulation.     

Optical Tamm States in Dielectric Photonic Crystal Heterostructure

We investigate one-dimensional dielectric photonic crystal and optical Tamm modes formed by superposition of two band gaps and find that this kind of mode can be explained by the single negative materials tunnelling effect. A finite-size dielectric photonic band gap can mimic one kind of effective single negative material and this property sensitively depends on the frequency location in stop-band regions and surface termination and so on. The effective impedance match and effective phase match give the precise position of the optical Tamm mode. Complete transparency via tunnelling is achieved by two opaque media and demonstrates the validity of our approach.     

Effective dielectric constant of two-dimensional photonic crystals in optical bands

By using the Fourier expansion method, we have developed an approach to calculate the effective dielectric index of a two-dimensional photonic crystal. The approach is very general: it can take into account various Bravais lattice structure as well as arbitrary spatial variation of the dielectric index. It has been found that near a nondegenerate frequency ω_n(Γ) at Γ point, the transverse magnetic (TM) mode is ordinary, as it is independent of the propagation direction, whereas in general the transverse electric (TE) mode depends on the propagation direction, it is extraordinary. Therefore, a two-dimensional photonic crystal can always be described by an effective dielectric index for TM mode near the nondegenerate frequency ω_n(Γ). However, the TE mode is much more complicated unless the lattice structure is highly symmetric. Moreover, a two-dimensional square photonic crystal has been identified as an effective birefringent crystal having two negative refractive indexes from the perspective of Snell's law.     

Complete evanescent tunneling gaps in one-dimensional photonic crystals

Complete band gaps are found in one-dimensional photonic crystals composed of negative-permittivity and negative-permeability materials. The mechanism of these complete band gaps, unlike the Bragg complete gaps formed by interferences of forward/backward propagating waves, originate from the evanescent wave tunneling in the single-negative materials is reported for the first time. Moreover, it is also reported for the first time that both Bragg complete gaps and evanescent wave tunneling complete gaps exist in a three-constituent one-dimensional photonic structure simultaneously.     

Study on maximum band gap of two-dimensional photonic crystal with elliptical holes

In this paper, the maximum photonic band gap (PBG) of two-dimensional (2D) photonic crystal (PC) with elliptical air holes was studied by the finite-difference time-domain (FDTD) method based on changing the ratio (semi-major axis length of elliptical hole to the filling ratio) and azimuth angle of elliptical holes, respectively. It is shown that the PBG exhibits a peak value when the ratio of semi-major axis length to the filling ratio is equal to 0.86 approximately by increasing the filling ratio, and central frequency and the low boundary frequency of PBG decrease linearly with the increasing of semi-major axis length. In the aspect of the influence of azimuth angle from 0 to 90°, the PBG presents a minimum value, and central frequency and the low boundary frequency of PBG become high non-linearly by the increasing of azimuth angle to any filling ratio.     

Disorders influences on the focusing effect of all-dielectric photonic crystal slab superlens

The influences of structure disorders on the subwavelength focusing properties of an all-dielectric photonic crystal slab superlens are theoretically studied. The structure disorders are considered as randomly perturbing the position or diameter of air holes of the photonic crystal slab. The results show that the photonic crystal slab superlens can tolerate within 10% degree of positional disorder or 15% degree of diameter disorder without destroying the focusing function.     

Tunable multichannel filter in photonic crystal heterostructure containing permeability-negative materials

A tunable multichannel filter is demonstrated theoretically based on a one-dimensional photonic crystal heterostructure containing permeability-negative material. The filtering properties of the photonic crystal filter, including the channel number and frequency, can be tuned by adjusting the structure parameters or by a pump laser. The angular response of the photonic crystal filter and the influences of the losses on the filtering properties are also analyzed.     

Investigation of one-dimensional photonic crystals composed of dispersive materials

One-dimensional photonic crystals (1DPCs) composed of dispersive materials (including negative refractive index materials, negative μ materials, and negative ? materials) are proposed. The dependence of the band gaps on the angle of incidence and thickness scale are investigated by using the transfer matrix method. Simulation results show that the band gaps of these dispersive material 1DPC are insensitive to the thickness scale. The defect modes of these doped 1DPCs behave specially when the thickness of the defect layer, the angle of incidence and the thickness scale of PC change.     

Energy compensated mode in a waveguide composed of lossy left-handed metamaterial

We reveal that stimulated Raman process can be one of the effective ways to compensate the energy loss intrinsic to the optical modes in waveguides composed of left-handed metamaterial. We evaluate the effect of the energy compensating process by using the guided mode’s wave functions multiplied by the envelope functions, which describe the stimulated Raman energy compensating process. Such an evaluation shows that the energy of the optical signal mode in the waveguide with lossy left-handed media can successfully be compensated.     

Multichannel filtering properties of photonic crystals consisting of single-negative materials

The multichannel filtering properties have been investigated by means of the transfer-matrix method in the one-dimensional periodic structures containing two kinds of single-negative materials. Two sets of comb-like filtering channels appear simultaneously on the two sides of the bandgap with zero-effective phase. One is omnidirectional in the low frequency, and the other is angle-sensitive in the high frequency. The multichannel filtering properties of the structures, such as the channel number, the central frequencies and quality factors, may be modified by adjusting the period number and the layer thicknesses. It thus provides a simple way to design multichannel filter with specific channels.