Lower Band Edge Research Articles

One-dimensional photonic crystals bound by light

Through rigorous simulations, the light scattering induced optical binding of one-dimensional (1D) dielectric photonic crystals is studied. The optical forces corresponding to the pass band, band gap, and band edge are qualitatively different. It is shown that light can induce self-organization of dielectric slabs into stable photonic crystals, with its lower band edge coinciding with the incident light frequency. Incident light at normal and oblique incidence and photonic crystals with parity-time symmetry are also considered.

Design of a Low Profile and Compact Omnidirectional Filtering Patch Antenna

A low profile omnidirectional patch antenna with filtering response is investigated in this paper. The triangular patch antenna is axially fed by a probe at its center, exciting both its TM10 and TM11 modes. Comparing with the traditional circular patch, the triangular patch can not only minimize the patch size but also can generate a radiation null at the upper band edge. A ring slot and a series of shorting vias are introduced into the patch to merge the two modes, enhancing the bandwidth of the passband. The combination of the two elements simultaneously generates a radiation null at the lower band edge. Consequently, a compact filtering patch antenna with quasi-elliptic bandpass response is obtained without involving specific filtering circuits. The prototype with profile of $0.03~\lambda _{0}$ has a 10-dB impedance bandwidth of 8.9% (4.3–4.7 GHz), an average gain of 6.0 dBi within passband, an out-of-band suppression level of more than 30 dB within lower stopband (0–3.6 GHz), and more than 20 dB within upper stopband (5.1–6.3 GHz).

Dual-Polarized Filtering Antenna With High Selectivity and Low Cross Polarization

This paper presents a dual-polarized patch antenna with quasi-elliptic bandpass responses. The proposed antenna is mainly composed of a feeding network, a driven patch, and a stacked patch, with its entire height being $0.09\lambda $ . The feeding network consists of two orthogonal H-shaped lines that coupled to the driven patch, each for one polarization. The elaborately-designed feeding lines not only ensure a sharp roll-off rate at the lower band edge, but also help to achieve low cross polarization and high isolation between two feeding ports. On the other hand, the upper stacked patch provides improved suppression levels at the upper stopband and also an enhanced gain within passband. Consequently, a compact dual-polarized antenna with satisfying filtering performance is obtained, without using extra filtering circuits. For demonstration, an antenna is designed to fit the specification of LTE band (2.49–2.69 GHz). The implemented antenna achieves an average a gain of 9 dBi, a cross-polarization ratio of 29 dB, an isolation of 35 dB within LTE band. The out-of-band suppression level is more than 40 dB within the 2G and 3G frequency bands from 1.71–2.17 GHz. It can be used as the antenna elements in multiband base station antenna arrays to reduce the mutual coupling.

Electronic properties of highly-active Ag3AsO4 photocatalyst and its band gap modulation: an insight from hybrid-density functional calculations.

The electronic structures of highly active Ag-based oxide photocatalysts Ag3AsO4 and Ag3PO4 are studied by hybrid-density functional calculations. It is revealed that Ag3AsO4 and Ag3PO4 are indirect band gap semiconductors. The Hartree-Fock mixing parameters are fitted for experimental band gaps of Ag3AsO4 (1.88 eV) and Ag3PO4 (2.43 eV). The smaller electron effective mass and the lower valence band edge of Ag3AsO4 are likely to be responsible for the superior photocatalytic oxidation reaction to Ag3PO4. The comparable lattice constant and analogous crystal structure between the two materials allow the opportunities of fine-tuning the band gap of Ag3AsxP1-xO4 using a solid-solution approach. The development of Ag3AsxP1-xO4 should be promising for the discovery of novel visible-light sensitized photocatalysts.

High-Gain Filtering Patch Antenna Without Extra Circuit

This paper presents a novel kind of patch antenna with high-selectivity filtering responses and high-gain radiation performance. The proposed antenna is mainly composed of a driven patch and a stacked patch, with its entire height being ${0.09\lambda }$ . Three shorting pins and a U-slot are embedded in the driven patch to enhance out-of-band suppression levels and skirt selectivity near the lower band-edge, whereas the stacked patch provides a sharp roll-off rate at the upper band-edge and also an enhanced gain. Without using extra filtering circuits, the proposed antenna exhibits a quasi-elliptic boresight gain response with three radiation nulls. For demonstration, an antenna is implemented covering the LTE band (2.3–2.7 GHz). The antenna achieves an average gain of 9.7 dBi within passband, and out-of-band suppression levels of more than 21 dB.

Absolute Instability near the Band Edge of Traveling-Wave Amplifiers.

Applying the Briggs-Bers "pole-pinch" criterion to the exact transcendental dispersion relation of a dielectric traveling wave tube (TWT), we find that there is no absolute instability regardless of the beam current. We extend this analysis to the circuit band edges of a linear beam TWT by approximating the circuit mode as a hyperbola in the frequency-wave-number (ω-k) plane and consider the weak coupling limit. For an operating mode whose group velocity is in the same direction as the beam mode, we find that the lower band edge is not subjected to absolute instability. At the upper band edge, we find a threshold beam current beyond which absolute instability is excited. The nonexistence of absolute instability in a linear beam TWT and the existence in a gyrotron TWT, both at the lower band edge, is contrasted. The general study given here is applicable to some contemporary TWTs such as metamaterial-based and advanced Smith-Purcell TWTs.

Dispersion, decaying length and localization of transverse magnetic surface modes in one dimensional plasma photonic crystals

The dispersion behavior, localization and total energy flow of transverse magnetic surface modes supported at the interface of air and a semi-infinite one dimensional plasma photonic crystals truncated with varying cap layer is analyzed by transfer matrix method. It is observed that properties of surface modes highly depend upon parallel wave vector, thickness of cap layer and plasma frequency. By altering thickness of cap layer from lower to higher values, all of the surface modes shift towards lower band edge, and vice versa. Also, it is found that localization of transverse magnetic surface modes are controlled by plasma frequency, parallel wave vector and thickness of cap layer.

Ultra wide band vertex truncated printed pentagon monopole antenna

ABSTRACTPrinted vertex truncated pentagon monopole antenna has been proposed, which yields ultra wide bandwidth. It gives bandwidth (BW) for VSWR ≤ 2 of 1.5–12 GHz. This new configuration has approximate omnidirectional radiation patterns in the azimuth plane throughout the BW. Simulated as well as measured results are presented, which are in agreement. The design equation for calculating the lower band edge frequency has also been derived for the printed pentagon monopole antenna. Systematic explanation has been given using impedance and surface current variations for ultra wide response. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:2228–2234, 2014

Substrate integrated waveguide loaded by 3-dimensional embedded split ring resonators

A substrate integrated waveguide (SIW) loaded by three-dimensional embedded split ring resonators (3-DESRRs) is discussed theoretically and experimentally. The 3-DESRR structure which has proved to be an alternative magnetic metamaterial resonator for SIW structures is able to provide negative transversal permeability if excited properly. The 3-DESRR loaded SIW shows a bandpass-like response as an effect of loading the 3-DESRRs in the SIW. The lower and upper band edges of the resulting transmission line (TL) are determined by the SIW cutoff (fc) and the resonance frequency of the 3-DESRRs, respectively. Finally, a prototype of the proposed 3-DESRR loaded SIW TL is fabricated and tested. The measured results are in good agreement with those obtained by theory and simulation.

The Anderson model with missing sites

In the present note we show dynamical localization for an Anderson model with missing sites in a discrete setting at the bottom of the spectrum in arbitrary dimension d . In this model, the random potential is defined on a relatively dense subset of Zd , not necessarily periodic, i.e., a Delone set in Zd . To work in the lower band edge we need no further assumption on the geometric complexity of the Delone set. We use a spatial averaging argument by BourgainKenig to obtain a uniform Wegner estimate and an initial length scale estimate, which yields localization through the Multiscale Analysis for non ergodic models. This argument gives an explicit dependence on the maximal distance parameter of the Delone set for the Wegner estimate. We discuss the case of the upper spectral band edge and the arising need of imposing the (complexity) condition of strict uniform pattern frequency on the Delone set. Mathematics subject classification (2010): 82B44 (Primary), 47B80, 47H40 (Secondary).

Cu(i)-based delafossite compounds as photocathodes in p-type dye-sensitized solar cells

The research of p-type dye-sensitized solar cells (p-DSSCs) has attracted growing attention because of the potential for integration with conventional n-type DSSCs (n-DSSCs) into the more efficient tandem-DSSCs. However, to date the performance of p-DSSCs is lagging behind that of n-DSSCs. One main reason is the lack of optimal photocathode materials. This article reviews the most recent progress in utilizing Cu(I)-based delafossite compounds, CuMO2 (M = Al, Ga or Cr), as photocathodes in p-DSSCs. As alternative materials to the commonly used NiO, the CuMO2 compounds have their intrinsic advantages such as lower valence band edge, larger optical bandgap and higher conductivity. By providing an insight into these materials and their applications in p-DSSCs, this perspective aims to stimulate more exciting research in the development of p-DSSCs as well as of tandem-DSSCs.

Fabrication of CPW-Fed Fractal Antenna for UWB Applications with Omni-Directional Patterns

Novel and compact CPW-fed antennas are proposed comprised of a fractal patch and modified ground-plane. The ground-plane is truncated at the center and includes dielectric notches at its side to enhance the antenna's impedance bandwidth. The dimensions of the notches effectively control the upper and lower band edges of the antenna. The optimized antenna operates across 2.95–12.81 GHz for S 11 ≤ −10 dB. Omnidirectional radiation pattern is achieved over the full UWB frequency range. The miniaturized antenna has a total size of 14 × 18 × 1 mm3. The characteristics of the proposed antenna are suitable for UWB wireless communication requiring low profile antennas.

Design of Photonic Crystals Waveguide Using Microfluidic Infiltration

The use of photonic crystals (PCS) in biosensor applications has lead to the development of highly sensitive and selective microfluidic sensor elements. Two main advantages of these devices for sensing applications are their high sensitivity and their reduced size, which makes it possible, in one hand, to detect very small analytes without the need of markers (label-free detection), and to integrate many of these devices on a single chip to perform a multi-parameter detection on the other hand. In the present paper, we analyze the design of a highly sensitive microfluidic sensors based on 2D photonic crystal slab waveguide formed by increasing the radii of air holes localized at each side of the line defect and filling with homogenous de-ionized water (nc =1.33). The transmission spectrum of the sensor has been obtained with the use of Finite Difference Time Domain (FDTD) method and it has been observed that a 306 nm wavelength position of the lower band edge shift was observed corresponding to a sensitivity of more than 927 nm per refractive index unit (RIU). Development of microfluidic sensor designs that enhance sensitivity is especially important because it allows detection of lower concentrations of analytes.

Experimental verification of directional liquid surface wave emission at band edge frequencies

Directional liquid surface wave emission at band edge frequencies is an interesting physical phenomenon and has already been studied in theoretical research. There has been no experimental validation of it to date, however. This paper has as its subject the experimental investigation of the emission effect when a point source is placed inside a finite square array of rigid cylinders standing vertically in liquid. Both the wave patterns and spatial intensities are obtained by experiment and compared with simulated results calculated by using the finite element method. We can see from this comparison that the two results correspond closely both at lower and upper band edge frequency. Obvious directional wave emission along a desired direction is observed in the source structures, confirming previous theoretical predictions. In the future, this method could serve as a directional liquid wave source in applications used in hydraulic and ocean engineering for the concentration of wave energy.

Flexible Transparent Metal/Polymer Composite Materials Based on Optical Resonant Laminate Structures

Suitable design of periodic metal/polymer composite materials is shown to facilitate resonant tunneling of light at absorbing wavelengths and to provide a means to significantly reduce optical absorption losses in polymer-based metallodielectric composite structures. The conditions for resonant tunneling are established based on the concept of "photonic band edge alignment" in 1D-periodic systems. For the particular case of a four-layer gold/polystyrene laminate structure, it is shown that the matching of the lower band edge of the 1D-periodic structure with the plasma frequency of the metal component facilitates the increase of optical transmission by about 500% as compared to monolithic film structures of equal total thickness. The effect of sheet thickness on the optical properties of thin metal films is determined and shown to be an important prerequisite for the reliable prediction of resonant metallodielectric structures. The resonant 1D-periodic metal/polymer heterostructures are shown to retain the flexural stability of the polymer matrix and thus could find application as flexible transparent conductors in areas such as "plastic electronics".

Effect Of Thickness On Structural And Optical Properties Of ZnxCd1-xS Thin Films Prepared By Chemical Spray Pyrolysis

Zn0.5Cd0.5S polycrystalline thin films of different thickness were deposited on cleaned glass substrates by spray pyrolysis technique. X-ray diffraction was used to characterize the thin films. X-ray diffraction study showed that, all the films have the hexagonal, wurtzite structure. Grain sizes calculated from Scherer relation are in the range of 47.02-48.21 nm and the grain size of the thin films are observed to increase with the increase in the thickness of the sample. The optical properties of the polycrystalline thin films were investigated by the UV- spectroscopy. The band gap of the thin films is found to be direct allowed transition and decreases with the increase of thickness of films in the range of 2.85- 2.7 eV. Extinction coefficient k showed oscillatory behavior in lower band edge region. The Zn0.5Cd0.5S polycrystalline thin films are suitable for solar cell application.

Impurity modes in Frenkel exciton systems with dipolar interactions and cubic symmetry

We introduce a continuum model for impurity modes of Frenkel excitons in fully occupied face-centered and body-centered cubic lattices with dipole-dipole interactions and parallel moments. In the absence of impurities, the model reproduces the small-k behavior found in numerical calculations of dipolar lattice sums. The exciton densities of states near the upper and lower band edges are calculated and compared with the corresponding results for a random array of dipoles. The Green function obtained with the continuum model, together with a spherical approximation to the Brillouin zone, is used to determine the conditions for the formation of a localized exciton mode associated with a shift in the transition energy of a single chromophore. The dependence of the local mode energy on the magnitude of the shift is ascertained. The formation of impurity bands at high concentrations of perturbed sites is investigated using the coherent potential approximation. The contribution of the impurity bands to the optical absorption is calculated in the coherent potential approximation. The locations of the optical absorption peaks of the dipolar system are shown to depend on the direction of propagation of the light relative to the dipolar axis, a property that is maintained in the presence of short-range interactions.

Nonlinear Schrödinger solitons oscillate under a constant external force

odinger equation with an external timeindependent force of the form f (x) = r exp(−iKx). Here the solitons travel with an oscillating velocity and all other characteristics of the solitons (amplitude, width, momentum, and phase) also oscillate. This behavior was predicted by a collective variable theory and confirmed by simulations. However, the reason for these oscillations remains unclear. Moreover, the spectrum of the oscillations exhibits a second strong peak, in addition to the intrinsic soliton peak. We show that the additional frequency belongs to a certain extended linear mode (which we refer to as a phonon for short) close to the lower band edge of the phonon continuum. Initially the soliton is at rest. When it starts to move it is deformed, begins to oscillate, and excites the above phonon mode such that the total momentum in a certain moving frame is conserved. In this frame the phonon does not move. However, not only does the soliton move in the homogeneous, time-periodic field of the phonon, but it also oscillates.

Ultra-Wideband Bandpass Filter Based on Composite Right/Left Handed Transmission-Line Unit-Cell

This paper presents a novel high performance ultra-wideband (UWB) bandpass filter that satisfies FCC UWB specifications. The filter is based on a composite right/left handed (CRLH) structure consisting of an asymmetric unit-cell with a short circuited inductive stub that exhibits stopband rejection level greater than 25 dB at both the lower and upper band edges while maintaining an insertion-loss of less than 0.5 dB across its passband. In addition, a simple and effective technique is proposed to create and control sharp rejection notch-bands within the filter's passband in order to provide interference immunity from undesired radio signals, such as wireless local area networks (WLAN) that co-exist within the UWB spectrum. The proposed technique was verified practically to validate the design methodology. The experimental results of the prototype circuit are presented and compared with the simulation results. The dimensions of the proposed filter are 0.37λ × 0.10λ (i.e., 16.4 × 4.8 mm2). The filter's characteristics and compact size make it suitable for UWB systems.

Joint Optimization of Microstrip Patch Antennas Using Particle Swarm Optimization for UWB Systems

Ultra wideband (UWB) systems are the most appropriate for high data rate wireless transmission with low power consumption. However, the antenna design for UWB has been a challenging task. Moreover, it is always desirable to have more freedom by designing different shape antennas with identical characteristics so that they can be used in either transmitter or receiver depending on other physical constraints such as area. To tackle these issues, in this paper, we have investigated a joint optimization of three different shape-printed monopole antennas, namely, printed square monopole antenna, printed circular monopole antenna and printed hexagonal monopole antenna, for UWB applications. More specifically, we have obtained the optimized geometrical parameters of these antennas by minimizing the mean-square-error for desired lower band edge frequency, quality factor, and bandwidth. The objective of joint optimization is to have identical frequency characteristics for the aforementioned three types of PMA which will give a freedom to interchangeably use them at either side, transmitting or receiving. Moreover, we employ particle swarm optimization (PSO) algorithm for our problem as it is well known in the literature that PSO performs well in electromagnetic and antenna applications. Simulation results are presented to show the performance of the proposed design.