Reflection Bandwidth Research Articles

Diffusive–Reflective Metasurface With Dual Independent Reflection Bands for RCS Reduction

A diffusive–reflective metasurface with dual independent bands is designed in this letter, which can be utilized to achieve specular reflection in the middle band and radar cross-section (RCS) reduction in the sidebands. The metasurface is made up of two elements, one of which consists of two diagonally arranged arrows with two tails of different lengths, and another is its mirror structure. Two independent bands are realized based on the resonances of two different-length tails, and their current distributions and parameter analysis are given for the comprehensible theory. Then, a prototype of the designed metasurface with one wide specular reflection band is fabricated and measured, and the 10.3% middle reflection band as well as 29.5% and 35.8% side RCS reduction bands are achieved.

Influence of the Output Window Reflection on the Performance of W-Band Gyrotron Traveling Wave Tubes

Effects of the output window reflection instability on the performance of our designed <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${W}$ </tex-math></inline-formula> -band broadband gyrotron traveling wave tube (gyro-TWT) were studied by the simulation and hot test experiments. Strong oscillations and frequency self-modulation (FSM) in the after-cavity interaction (ACI) region were observed in the zero-driven state with a two-layer window and are verified with the beam-wave particle-in-cell simulation. Oscillations and FSM of 80–81.8 GHz and 91.03–94.68 GHz are, respectively, generated from the TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">22</sub> and TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">02</sub> modes in the ACI region due to the high window reflection level. By using the “sandwich” and meta-surface windows with low reflection and broad bandwidth, the parasitic ACI oscillations can be effectively suppressed. The maximum saturated output power is improved to 156.8 kW at 94 GHz and is zero-driven stable in our recent hot test experiments.

A Broadband Antenna Based on a Single CRLH TL Unit Cell and a Parasitic Strip for Sub-1GHz DVB and LTE Applications

A Broadband and slim profile antenna based-on a single composite right/left-handed transmission line unit cell and parasitic shorted strip is presented for sub-1 GHz digital video broadcasting and long-term evolution applications (470-970 MHz). It is comprised of an ungrounded coplanar waveguide that feeds a meandered strip (distributed series inductors), which is connected through capacitive coupling (series capacitance) with a virtual small ground, while an upper shorted meandered strip represents a parallel inductor. A shorted meandered strip (parasitic inductor) is added to create a third resonant frequency. A broadband operation is achieved by merging three different resonant modes frequencies (zeroth-order mode and two quarter wavelength monopole modes) to satisfy 6 dB bandwidth criterion. A prototype is constructed and measured in order to verify the achieved results. The results of reflection parameter, gain, efficiency, patterns and bandwidth efficiency product are presented, discussed, and evaluated with respect to literature. The results show that the proposed antenna is a potential candidate for recent broadband digital video broadcasting applications.

Tunable Circularly Polarized Luminescence with a High Dissymmetry Factor Emitted from Luminogen-Bonded and Electrically Controlled Polymer-Stabilized Cholesteric Liquid Crystals

Circularly polarized luminescence (CPL)-active materials with high dissymmetry factor (glum) values show great potential in photonic devices. In this study, electric-field-driven systems with tunable CPL signals are successfully fabricated based on polymer-stabilized cholesteric liquid crystal (PSChLC) doping by fluorescent molecules. By constructing a gradient helical superstructure of PSChLCs, distinctive CPL emission from two sides of a single sample is realized, in which the |glum| values were measured to be 0.6 and 1.5, respectively. Herein, we discussed the possible mechanism of this phenomenon. In addition, an applied electric field could broaden the reflection bandwidth of PSChLCs from 150 to 500 nm, covering the whole visible light region. Furthermore, this electric field-induced behavior leads to the variation of CPL signals and corresponding glum values, indicating the potential of the novel materials in the design and preparation of CPL-emitting devices with electrically tunable CPL intensity and glum.

Toward an optimum design of fractal sausage Minkowski antenna for GPS applications

Fractal is one of the important tools to produce efficient and well-suited antennas for navigation systems. A novel design of broadband fractal sausage Minkowski antenna is described in this research article. Sausage Minkowski is designed and simulated for global positioning system (GPS) application that relies on fractal geometry using computer simulation studio microwave (CST MW) software. The substrate sheet is Roger TMM4 has of 4.5 dielectric constant and 1.6 mm altitude. A small metallic patch over a large metallic ground layer is made of perfect electric conductor (PEC) material. The suggested fractal antenna can be used efficiently in the military applications which demand a narrow spectrum and a tiny antenna profile. By introducing four various levels of fractal iteration models and compared each one to the other. The main aim of this work is to increase antenna gain and reduce the antenna size, in addition to improving the semi-flat voltage standing wave ratio (VSWR) with respect to decreasing the resonance frequency is. For 10% decrease in the frequency for the 3rd iteration value led to a clear improvement in the antenna characteristics such as directivity, gain, and VSWR. The reflection coefficient and bandwidth remain in range.

Electrically controllable reflection bandwidth polymer-stabilized cholesteric liquid crystals with low operating voltage

ABSTRACT An electrically controllable reflection bandwidth polymer-stabilised cholesteric liquid crystals (PSCLCs) with low operating voltage is reported. This work focuses on the contribution of the crosslink density of the polymer network to broaden the bandwidth and the ability of the polymer network to trap ionic impurities under direct-current (DC) electric fields. Furthermore, it is shown that the appearance of nonideal optical properties in some samples, such as scatter and haze, is correlated to the polymer/LC compatibility and the soft segments motion under DC electric field. Based on these results, a model relating the motion of the polymer network with the broadening of the reflection bandwidth and the baseline optical properties is developed. This study provides useful guidelines for future PSCLCs production and performance optimisation.

High‐Efficiency Vivid Color CIGS Solar Cell Employing Nondestructive Structural Coloration

Structural coloration is an effective method for coloring a solar cell with minimum optical loss. For the application of building integrated photovoltaics, it is desirable to integrate structural colors directly onto the solar cell without damaging it with an economical process. Herein, a solution‐processed distributed Bragg reflector (sDBR) is introduced to realize efficient and vividly colored CuInxGa(1−x)Se2 (CIGS) solar cells. The sDBR is directly combined with the CIGS solar cell at room temperature without damaging the photoactive layer. The sDBR is fabricated on CIGS solar cells by spin coating alternate layers of solution‐processable TiO2 nanoparticles bound with acetylacetone and a polymethyl methacrylate solution. The vividly colored CIGS solar cells exhibit high reflectivity and narrow bandwidth at the designated wavelength, resulting in pure red, green, and blue. The reduction in the photocurrent by the coloring is minimized due to the narrow bandwidth characteristics of the sDBR, while a gain in the photocurrent is obtained at wavelengths outside of the range used for coloring due to the antireflection effect of the sDBR. As a result, the power conversion efficiency of R, G, and B sDBR/CIGS solar cells is 97.2%, 99.4% and 102%, respectively, compared to a conventional CIGS solar cell.

Flexible and broadband absorbing woven fabric using carbon-based resistive frequency selective surface

In this paper, a flexible and broadband absorbing woven fabric with good mechanical property is proposed. It is composed of two-layer carbon-based resistive patterns printed on plain weave quartz fiber cloth, sandwiched by 3D woven fiber fabric through stitching process. An equivalent circuit model is introduced to guide the broadband absorption design. Both simulation and measured results have verified that such absorber can achieve −10 dB reflection bandwidth ranging from 7.1 to 35.4 GHz with the total thickness of 5.5 mm. The measured tensile strength reaches as high as 112 MPa, exhibiting the excellent mechanical performance compared with the traditional rubber absorbers. In addition, this absorber is also demonstrated to perform well under circumstances of oblique incidence and different polarizations. The above results suggest that our design is promising for the further development of novel flexible absorbers in wearable electronics and communication fields.

Modified theory of coupled waves and a numerical method for the analysis of Bragg mirrors with an arbitrary refractive index profil

An analytical and effective numerical approach to the analysis of the optical characteristics of Bragg mirrors with an arbitrary refractive index profile is developed. Analytical expressions obtained using the modified theory of coupled waves for the coefficients of reflection / transmission of light from through the BM with high accuracy predict its main characteristics at both low and high contrast of the refractive index. The hybrid numerical method includes the numerical calculation of the transfer matrix for one period of the knowledge base and its analytical multiplication for an arbitrary number of periods. The developed methods are applied to the analysis of the properties of practically important BMs made on the basis of sprayed pairs of Ta2O5/SiO2 dielectrics, Group III nitrides and arsenides. The characteristics of the knowledge base with smooth (gradient) interfaces are considered in detail. Keywords: vertically emitting lasers, reflection coefficient, reflection bandwidth.

A compact design of a modified Bragg grating filter based on a metal-insulator-metal waveguide for filtering and temperature sensing applications

In this work, we presented a numerical study on the modified Bragg grating structure based on a metal-insulator-metal (MIM) waveguide (WG) that can be simultaneously employed as an optical filter and temperature sensor. The device optimization is performed by utilizing the 2D-finite element method. The standard Bragg grating structure is composed of a periodic arrangement of nanoblocks (NBs) decorated in the center of the MIM WG resulting in the formation of a broadband Bragg reflection bandwidth. However, in the proposed study, the NBs are displaced to one side of the MIM WG creating a different optical path with unequal indices on both sides of the grating, as a result, a narrow MZI dip is obtained along with the Bragg reflection spectrum. To functionalize the MZI dip for temperature sensing application, the MIM WG slot is filled with a temperature-sensitive polydimethylsiloxane (PDMS) polymer. Consequently, the modified Bragg grating structure can be simultaneously employed as a bandstop filter (bandwidth~200 nm) and temperature sensor with an attractive sensitivity of 0.47 nm/°C.

Development of colored perovskite solar cells using cholesteric helicoidal superstructures

Perovskite solar cells (PeSCs) are promising candidates for building-integrated photovoltaics (BIPV) because of their excellent performance and processability. In this study, the first colored PeSCs that use liquid crystalline cholesteric phase-based reflective filters (ChRFs), which were inspired by the cuticle of a beetle, are developed. The reflective wavelengths, bandwidth, and reflectance of ChRFs are easier to modulate than those of reflective filters based on metals or oxides. Reflective wavelengths affect the hue, and they can be modulated by tuning the contents of a chiral dopant. The reflective bandwidth affects the power conversion efficiency, and it can be modulated by tuning the birefringence of the host liquid crystalline molecules. The reflectance also affects the chromaticity, and it can be controlled by modifying the simple optical design of the ChRF structure. ChRFs are suitable for large and flexible PeSCs because they are constructed using only organic components and wet processes.

Enhanced performance of GaN-based visible flip-chip mini-LEDs with highly reflective full-angle distributed Bragg reflectors

High-efficiency GaN-based visible flip-chip miniaturized-light emitting diodes (FC mini-LEDs) are desirable for developing white LED-backlit liquid crystal displays. Here, we propose a full-angle Ti3O5/SiO2 distributed Bragg reflector (DBR) for blue and green FC mini-LEDs to enhance the device performance. The proposed full-angle Ti3O5/SiO2 DBR is composed of different single-DBR stacks optimized for central wavelength in blue, green, and red light wavelength regions, resulting in wider reflective bandwidth and less angular dependence. Furthermore, we demonstrate two types of GaN-based FC mini-LEDs with indium-tin oxide (ITO)/DBR and Ag/TiW p-type ohmic contacts. Experimental results exhibit that the reflectivity of full-angle DBR is higher than that of Ag/TiW in the light wavelength range of 420 to 580 nm as the incident angle of light increases from 0° to 60°. As a result, the light output powers (LOPs) of blue and green FC mini-LEDs with ITO/DBR are enhanced by 7.7% and 7.3% in comparison to blue and green FC mini-LEDs with Ag/TiW under an injection current of 10 mA. In addition, compared with FC mini-LED with Ag/TiW, light intensity of FC mini-LED with ITO/DBR is improved in side direction, which is beneficial to mix light in backlight system of liquid crystal displays (LCDs).

Prediction of material composition for microwave absorption through mathematical modelling

In this paper, we analyse the suitability of various elements and their composition for microwave absorption. This is done by developing a mathematical model based on transmission line theory, polarizability and dielectric and magnetic response of the material. The inputs to this model are the desired specifications of the absorber like reflection loss, resonance frequency and bandwidth. The corresponding output parameters are the range of the atomic radii and magnetic dipole moment of suitable elements and their effective energy parameters. This mathematical model has been verified for compounds like oxides and ferrites that have been proven posses microwave absorption properties through previous experimental research. This mathematical model has been verified for compounds like oxides and ferrites that have been proven to possess microwave absorption properties through previous experimental research. Through numerical computations, we propose new material compositions with various elements that can be used as effective microwave absorbers.

2.58 kW Narrow Linewidth Fiber Laser Based on a Compact Structure with a Chirped and Tilted Fiber Bragg Grating for Raman Suppression

We report a high power, narrow linewidth fiber laser based on oscillator one-stage power amplification configuration. A fiber oscillator with a center wavelength of 1080 nm is used as the seed, which is based on a high reflection fiber Bragg grating (FBG) and an output coupling FBG of narrow reflection bandwidth. The amplifier stage adopted counter pumping. By optimizing the seed and amplifier properties, an output laser power of 2276 W was obtained with a slope efficiency of 80.3%, a 3 dB linewidth of 0.54 nm and a signal to Raman ratio of 32 dB, however, the transverse mode instability (TMI) began to occur. For further increasing the laser power, a high-power chirped and tilted FBG (CTFBG) was inserted between the backward combiner and the output passive fiber, experimental results showed that both the threshold of Stimulated Raman scattering (SRS) and TMI increased. The maximum laser power was improved to 2576 W with a signal to Raman ratio of 42 dB, a slope efficiency of 77.1%, and a 3 dB linewidth of 0.87 nm. No TMI was observed and the beam quality factor M2 maintained about 1.6. This work could provide a useful reference for obtaining narrow-linewidth high-power fiber lasers with high signal to Raman ratio.

A temperature-insensitive FBG acceleration sensor with sinusoid-shaped curved beams

PurposeThis study aims to solve the problem of temperature cross sensitivity of fiber Bragg grating in structural health monitoring, proposing a novel acceleration sensor based on strain chirp effect which is insensitive to temperature.Design/methodology/approachA kind of M-shaped double cantilever beam structure is developed. The fiber grating is pasted in the gradient strain region of the beam, and the chirp effect is produced under the action of non-uniform stress, and then the change of acceleration is converted into the change of reflection bandwidth to demodulate and eliminate the temperature interference. Through theoretical analysis, simulation and experimental verification with rectangular beam sensor.FindingsThe results show that the sinusoidal curvature beam sensor is insensitive to the change of temperature and is more likely to produce chirp effect. The sensitivity is about 317 pm/g, and the natural frequency is 56 Hz.Originality/valueThis paper fulfils an insensitive to temperature changes sensor which has effectively solved the temperature cross-sensitivity problem in building structure health monitoring.

Bandwidth control of loop type frequency selective surfaces using dual elements in various arrangements

Frequency selective surfaces (FSSs) have applications across multiple disciplines due to their unique electromagnetic properties. This paper investigates the use of both rounded square loops, and simple loop type dual elements arranged in unique patterns, to control the transmission and reflection bandwidth and resonant frequencies over KU and K frequency bands supported by equivalent circuit models. The FSSs were fabricated using laser engraving to create conductive loop type elements on a thin, flexible and optically transparent Mylar substrate (relative permittivity of 2.7 and thickness of 65 μm). The frequency response of the surfaces are controlled through the element self-inductance and capacitive coupling with neighbouring elements. This work shows that different arrangements result in the formation of multiple distinct resonances. The theoretical and experimental results were in good agreement where rounded squares and dual element arrays were employed to create broadband and multiband band-stop FSSs. A polarization sensitive surface exhibited stop-bands at 12 and 16 GHz in transverse electric polarization and a stop-band at 14.4 GHz in transverse magnetic polarization. This technique can be applied to any periodic array through careful selection of the individual elements in the array, as well as their arrangement.

A Comparison of Top Distributed Bragg Reflector for 1300 nm Vertical Cavity Semiconductor Optical Amplifiers Based on III–V Compound

In this work, the design of GaAs/AlGaAs distributed Bragg reflector (DBR) has been implemented for 1300 nm vertical cavity semiconductor optical amplifiers (VCSOAs) for optical fiber communication applications. The top DBR period and Al concentration are varied, the peak reflectivity of the DBR is increasing from 50% to 97.5% for 13 periods with increasing Al concentration, whereas the reflectivity bandwidth is increased to almost 190 nm. The relation between wavelength and incidence angle variation on DBR reflectivity is increasing with the incident angle (0°, 20°, 30°, and 50°), the resonant wavelength and bandwidth of the measured reflectance spectra shifts to shorter wavelength and wider bandwidth, respectively. In addition, a comparison between the linear, the graded, and the parabolic DBRs has been achieved with transfer matrix method using MATLAB software to show the influence of layer in DBRs and its effect on lasing wavelength. It is shown that using grading DBR mirror is much more beneficial compared to abrupt DBR, whereas it has lower reflectivity of almost 10% due to VCSOAs device which needs less number of top layers until prevent reaching lasing threshold.

Polariton condensation in an organic microcavity utilising a hybrid metal-DBR mirror

We have developed a simplified approach to fabricate high-reflectivity mirrors suitable for applications in a strongly-coupled organic-semiconductor microcavity. Such mirrors are based on a small number of quarter-wave dielectric pairs deposited on top of a thick silver film that combine high reflectivity and broad reflectivity bandwidth. Using this approach, we construct a microcavity containing the molecular dye BODIPY-Br in which the bottom cavity mirror is composed of a silver layer coated by a SiO2 and a Nb2O5 film, and show that this cavity undergoes polariton condensation at a similar threshold to that of a control cavity whose bottom mirror consists of ten quarter-wave dielectric pairs. We observe, however, that the roughness of the hybrid mirror—caused by limited adhesion between the silver and the dielectric pair—apparently prevents complete collapse of the population to the ground polariton state above the condensation threshold.

Sensing characteristics of grating assisted counterpropagating cladding modes in optical fibers.

In this paper, using full vector modal field analysis, we present physical parameter sensing, e.g., temperature, axial strain, bulk refractive index, and affinity sensing characteristics of counterpropagating cladding modes in optical fibers. Counterpropagating cladding modes (HEmn) are considered to be excited by resonant power coupling from the fundamental core mode using a suitably designed fiber Bragg grating (FBG). We show that for such couplings, the bandwidths of reflection spectra are much smaller than those obtained for conventional core mode reflecting FBGs. Next, we also show that the reflection bandwidth increases linearly with increasing grating strength, whereas it decreases exponentially with increasing grating length. The reflection spectra of such FBGs, as a function of different ambient physical perturbation parameters, are obtained, and then the grating sensitivity corresponding to those parameters is established. Interestingly, we have noticed that despite having the same evanescent field in the analyte region, unlike the long-period grating assisted co-propagating cladding modes (HEmn), the same cladding modes while propagating counter to the core mode are weakly sensitive to changes in the ambient refractive index. The reason behind this observation is discussed.

Higher–Order Bragg Reflection Colors in Polymer–Stabilized Cholesteric Liquid Crystals

The dynamic electro‐optic (EO) response of polymer‐stabilized cholesteric liquid crystals prepared using unpolarized UV light (U–PSCLC), such as reflection bandwidth broadening and either red or blue tuning of the reflection peak, has been previously reported. Herein, recent efforts to use a polarized single argon‐ion laser beam to create PSCLCs (L–PSCLCs) with higher‐order reflections are described. The L–PSCLCs exhibit a primary reflection peak in the near‐infrared (NIR) regime and a second‐order reflection band with a narrow bandwidth in the visible regime that results from a deformed in‐plane CLC helical structure. The initial positions of the reflection bands are adjusted by the chiral dopant concentrations of the CLC mixture, and red, green, and blue reflection colors from the second‐order Bragg reflection are demonstrated. The primary and the second‐order reflection bands can be shifted to longer wavelengths by application of a DC electric field. The reflection efficiency of the higher‐order reflection notch increases with polymer concentration, which affects the degree of in‐plane deformation and fixation of the CLC helix. Modeling is used to further explain the formation of the higher‐order reflection bands of PSCLCs observed experimentally.