Omnidirectional Reflector Research Articles

Designing of Broadband Solar Omnidirectional Reflector with a Chirped Multilayer Using an Optimization Procedure Based on a Genetic Algorithm

Designing of Broadband Solar Omnidirectional Reflector with a Chirped Multilayer Using an Optimization Procedure Based on a Genetic Algorithm

Enhanced light extraction efficiency for the inclined-sidewall-shaped AlGaN-based DUV LED by using an omni-directional reflector with a thin hybrid dielectric layer.

In this Letter, an omni-directional reflector (ODR) with a thin hybrid dielectric layer (hybrid-ODR) is proposed to enhance the light extraction efficiency (LEE) for inclined-sidewall-shaped AlGaN-based deep ultraviolet light-emitting diode (DUV LED) by inserting a thin diamond with high refraction index into a conventional Al/Al2O3-based ODR. The three-dimensional finite-difference time-domain (3D FDTD) simulation results show that the LEE of TM-polarized light for the DUV LED with hybrid-ODR is enhanced by 18.5% compared with Al/Al2O3-based ODR. It is because the diamond can transform the evanescent wave in Al2O3 into the propagating light wave in diamond, thereby preventing effective excitation of the surface plasmon polariton (SPP) on the surface of the metal Al. Moreover, the Brewster's angle effect causes the TM-polarized light in diamond to propagate effectively into AlGaN. Furthermore, decreasing the total thickness of the dielectric layer also improves the scattering effect of the inclined sidewall. However, the utilization of hybrid-ODR results in a slight reduction in the LEE for transverse electric (TE) polarized light because the light is confined to the diamond layer and eventually absorbed by the metal Al.

Enhancing the quantum efficiency of a silicon solar cell using one dimensional thin film interferometry

Crystalline silicon solar cells are prominent renewable energy sources that convert solar energy into electrical energy without emitting greenhouse gases. They are used to energize satellites as well as provide electricity for buildings, local areas/settlements, and remote locations. However, the efficiency of these solar cells is limited by two factors: light reflection at the Si-Air interface, which reduces energy absorption, and the recombination rate of electron-hole pairs that reduces the mobile charge carriers available for electricity conduction. Solar cell expenses are comparable to conventional energy resources, motivating research to increase solar cell efficiency for commercial viability. The goal of our study was to increase crystalline solar cell efficiency theoretically by simulation. We mathematically simulated an omni-directional reflector with alternate Si/SiO2 layers using multi-layered thin film interferometry in MATLAB, which was designed to reflect wavelengths in the range of 628 nm to 808 nm. We found that the ODR’s energy reflected more energy at a central wavelength of 800 nm and 8 total layers. We also simulated an anti-reflection coating layer for the top of the solar cell using alternate ZnO-SiO2 multilayers for enhanced transmission in the range of 481 nm to 2000 nm to increase solar cell efficiency for various optoelectronic devices and thermo-photovoltaic applications. We found that this reduced the reflectivity of the Si-Air interface by 49%.

Transformation one-dimensional photonic crystal omnidirectional reflector

In this paper, a concept of transformation one-dimensional photonic crystal is proposed to design omnidirectional reflector, which has a reduced thickness compared with that of the omnidirectional reflector using conventional one-dimensional photonic crystal structure. At first, the omnidirectional reflection band of an original one-dimensional photonic crystal is simulated using the transfer matrix method for comparison. Next, the transformation one-dimensional photonic crystal is designed based on transformation optics. Simulation results indicate that the transformation one-dimensional photonic crystal structures with arbitrary shrunken thickness can omnidirectionally reflect the incident TM waves that operates in the same omnidirectional reflection band of the original one-dimensional photonic crystal structure. Such a concept of transformation one-dimensional photonic crystal also can be expanded to any omnidirectional reflector designed by using other multilayer dielectric systems.

Improved Optical Efficiency of 850-nm Infrared Light-Emitting Diode with Reflective Transparent Structure.

This study investigated a reflective transparent structure to improve the optical efficiency of 850 nm infrared light-emitting diodes (IR-LEDs), by effectively enhancing the number of extracted photons emitted from the active region. The reflective transparent structure was fabricated by combining transparent epitaxial and reflective bonding structures. The transparent epitaxial structure was grown by the liquid-phase epitaxy method, which efficiently extracted photons emitted from the active area in IR-LEDs, both in the vertical and horizontal directions. Furthermore, a reflective bonding structure was fabricated using an omnidirectional reflector and a eutectic metal, which efficiently reflected the photons emitted downwards from the active area in an upward direction. To evaluate reflective transparent IR-LED efficiency, a conventional absorbing substrate infrared light-emitting diode (AS IR-LED) and a transparent substrate infrared light-emitting diode (TS IR-LED) were fabricated, and their characteristics were analyzed. Based on the power-current (L-I) evaluation results, the output power (212 mW) of the 850 nm IR-LED with the reflective transparent structure increased by 76% and 26%, relative to those of the AS IR-LED (121 mW) and TS IR-LED (169 mW), respectively. Furthermore, the reflective transparent structure possesses both transparent and reflective properties, as confirmed by photometric and radial theta measurements. Therefore, light photons emitted from the active area of the 850 nm IR-LED were efficiently extracted upward and sideways, because of the reflective transparent structure.

Large omnidirectional mid-infrared photonic bandgap in a one-dimensional ternary photonic crystal consisting of isotropic dielectric, elliptical metamaterial and plasma

Photonic bandgaps (PBGs) in traditional one-dimensional (1-D) binary photonic crystals (PhCs) consisting of two kinds of isotropic dielectrics strongly shift towards shorter wavelengths as incident angle increases. Such blueshift property of PBGs intensively limits the widths of omnidirectional photonic bandgaps (OPBGs). Very recently, researchers achieved a special kind of PBGs called angle-insensitive PBGs in novel 1-D binary PhCs consisting of isotropic dielectric and elliptical metamaterial (EMM). The emergence of such angle-insensitive PBGs provides us an opportunity to achieve large OPBGs. Herein, we periodically introduce plasma layers into a 1-D binary PhC consisting of isotropic dielectric and EMM with an angle-insensitive PBG to achieve a large OPBG at mid-infrared wavelengths. The EMM is mimicked by an all-dielectric subwavelength multilayer. The broaden effect of the OPBG originates from the plasmonic property of plasma and the angle-insensitive property of the PBG. The width of the OPBG reaches 4.19 μm. Our work provides a feasible route to achieving large OPBGs in 1-D PhCs and would promote the development of OPBG-based devices, such as omnidirectional broadband reflectors and omnidirectional filters.

Shaped Omnidirectional Reflector Fed by a Dielectric Lens Associated with a Coaxial Feed Horn

The paper explores an omnidirectional antenna configuration composed of a reflector fed by a shaped dielectric lens associated with a coaxial TEM horn. A simple formula describes the lens shape obtained by applying Fermat’s principle to control the rays’ caustic emerging from the dielectric interface. Based on geometrical optics (GO) principles, a synthesis technique defines the reflector shape to control the antenna radiation pattern in the vertical plane. Concatenated conic describes the reflector generatrix. The study employs a full-wave analysis to validate the designs and explores the proposed configuration to attend two distinct far-field specifications.

Ultra-broadband polarization-independent omnidirectional reflectors via dielectric-reversal quasicrystal heterostructure multilayer films

Since quasicrystals provide additional freedom to expand photonic bandgap, dielectric quasiperiodic sequences of broadbands become crucial for implementing low-loss omnidirectional reflectors or filters. The current major challenge is to find an elegant scheme to construct angle-insensitive multilayer structures with fewer layer numbers. In this work, we create an appealing quasiperiodic ordering of the dielectric-reversal Fibonacci sequence to optimize the properties of omnidirectional reflectors. We have concatenated it to a periodic sequence to construct the omnidirectional reflector of hybrid multilayers with thin thickness and reflectivity above 99% over a visible scope from 436 to 559 nm. An ultrabroad polarization-independent omnidirectional bandgap of the relative bandwidth of 39.8% has been achieved by cascading two dielectric-reversal ordering sequences. The self-similarity of Fibonacci multilayer structures along with this characteristic quasiperiodic order guarantees the existence of perfect omnidirectional reflections at any scale. This ordering strategy of dielectric-reverse essentially differs from previous conjugate and recursion methods, which will significantly enrich the variety of quasiperiodic orders in theory and applied science domains.

Performance improvement of AlGaInP based red LEDs by combination of omnidirectional mirror and AuZn micro-contacts

An omnidirectional reflector (ODR) consisted of (MgF2)/Ag was employed in AlGaInP-based vertical red light-emitting diodes (LEDs) in order to increase the light extraction hence the wall-plug efficiency. In this structure, the dielectric MgF2 layer with low refractive index acted as current blocking and reflection enhancing layer. The cylindrical AuZn alloy micro-contacts perforated through the MgF2 layer and played roles in current spreading and injection. The area ratio of AuZn micro-contacts was optimized to improve the performance of the LED. The results indicated that increasing the area ratio of micro-contacts led to the improvement of current injection uniformity and reduction of forward voltage especially at higher injection current, but at the expense of light extraction by reducing reflection. After optimized, the best performance of red-light LEDs can be obtained with 3% area ratio of micro-contacts.

Study on combined reflectors for improving efficiency of high power near-infrared emitters

In this study, a combined reflector was suggested to improve the efficiency of high-power near-infrared (NIR) emitters. The combined reflector was fabricated by combining a distributed Bragg reflector (DBR) and an omnidirectional reflector (ODR). In the wafer bond process required for fabricating a high-power NIR emitter, the combined reflector was fabricated such that the developed DBR, which grew as the lowest layer of the epitaxial structure, bonded directly to the ODR deposited on the silicon wafer. The DBR in the combined reflector efficiently reflected photons emitted downward at a certain angle of 20° from the active region. A large number of photons, except at a certain angle by the DBR, are reflected by the other ODR. Therefore, the combined reflector might have favorable reflectivity properties produced by either DBRs or ODRs. As a result, photons emitted downward from the active region of the near-infrared emitter could be efficiently reflected upward and sideways by using a combined reflector of DBRs and ODR. From the results of the characteristics, the NIR emitter with the efficient combined reflector displays a 166% and 118% higher output power than that with either DBRs or ODR. Further, it was re-proved that there exist paramount characteristics of DBRs and ODR in a combined reflector through measurement of photometric and radial theta.

Design of inclined omni-directional reflector for sidewall-emission-free micro-scale light-emitting diodes

Micro-scale light emitting diode (µLED) reduces the front self-luminous display area by orders of magnitude, which brings the increase of sidewall emission and the resulting crosstalk between adjacent sub-pixels of displays. Reflector coating on the µLED’s sidewall is effective for the collection and utilization of side emission light. However, the existing multilayer periodic dielectric mirrors are undesirable for all angular reflection. In this paper, an omni-directional reflector (ODR) is designed for the µLED display pixel to effectively utilize light leakage from the sidewall emission and improve the total light extraction efficiency (LEE). The designed ODR is insensitive to the angular distribution of the incident light, and has the capability to reflect the light reaching the µLED’s sidewall into the escape cone for front emission. By introducing the two-dimensional finite-difference time-domain method, the µLED laminated structure is established to analyze the sidewall emission with an optimal inclination angle. By introducing the ODR for the inclined µLED pixel, the overall LEE can be increased by 2.24 times and the sidewall emission is reduced by 99.6% compared with conventional vertical µLEDs. Simulation and experimental results are in good agreement. The proposed ODR is expected to achieve the increase of LEE and sidewall-emission-free µLED displays.

Enhanced Light Output Power on Near-Infrared Light-Emitting Diodes with TITO/Ag Multilayer Reflector.

A titanium–indium tin oxide (TITO) multilayer reflector was investigated to improve the light efficiency of high-power, near-infrared, light-emitting diodes (NIR-LEDs). The TITO/Ag was fabricated by combining a patterned TITO and an omnidirectional reflector (ODR). For fabricating a high-power NIR-LED, the wafer bond process required the TITO reflective structure, which has patterns filled by AlAu contact metal, bonded directly to the Ag reflector deposited on the silicon wafer. Among Ag-based single- and multilayer reflectors, the TITO/Ag showed the highest reflectance (R = 96%), which was favorable for wafer-bonded high-power NIR-LEDs. Therefore, the TITO/Ag reflector enabled the production of wafer-bonded NIR-LED chips that exhibit superior output performance (190 mW) compared with conventional cases using a single Ag reflector.

Analysis of transmission spectra in one-dimensional ternary photonic crystals with complex unit cell

We investigate the light transmission spectra of a one-dimensional semiconductor ternary photonic crystal with a complex unit cell composed of different combinations of two three-layers (symmetric and asymmetric) using the transfer matrix method. The transmittance properties of such photonic crystals are analyzed numerically for the normal and oblique incidence angles of electromagnetic radiation. The complete transmission spectra for the complex ternary photonic structures have been verified from the Brewster angle. Special attention is paid to the study of light transmission spectra dependencies as a function of the thicknesses of layers in the complex unit cell. The proposed structure can be used to design the optical wide-band filters, omnidirectional reflectors at the infrared and terahertz regimes.

Omnidirectional High Reflectors Using Silica/Superconductor Fibonacci Photonic Crystal for Optical Communication Applications

Using the transfer matrix method and Fibonacci structure, an omnidirectional high reflector that covers the fiber optics wavelengths is searched. We aim to make the reflector covers the operating wavelengths of Plastic and Glass fiber optics (650, 850, 1300 and 1550nm). Firstly, an ordinary Fibonacci structure composed of TiO2 and SiO2 slabs materials was studied and for TE and TM polarization modes the reflectance spectra cover only the 850, 1300 and 1550 nm wavelengths and for a limited range of wave-angle of incidence. Secondly, the SiO2 slabs were replaced with the superconductor YBCO slabs which allows the extension of the reflectance bands to cover all fiber optics wavelengths for the entire angle of incidence interval [0-1.5 radian] and for both polarization modes. During that, the structure thickness was optimized by varying the iteration number and the optical thickness of Fibonacci sequence. Finally, the optical behavior of the reflector was tested when changing the ambient temperature.

The properties of mid infrared surface modes at the interface of air and one dimensional ternary photonic crystal

In the present manuscript, theoretical design of truncated one dimensional ternary photonic crystal is proposed for analyzing the properties of transverse electric (TE) surface modes viz. band structures, its localization in photonic band gaps (PBGs), field profiles and energy carried by these modes. The periodicity of the structure (comparable to the wavelength of incident electromagnetic waves) is chosen as such the modes are localized in mid-infrared frequency domain (600cm−1 to 1800cm−1 or 5μm to 16μm). The dispersion relation of surface modes is derived by using continuity condition of tangential components of electric and magnetic fields and transfer matrix method. It is found that by varying the cap layer thicknessdc and thickness of first layerdt of each unit cell, surface modes are either strongly (near center of PBG with decay length ~8.8μm) or weakly (near edge of PBG with decay length ~42.5μm) bounded to the interface. The energy carried by the surface modes along the interface plane is around 3.3mJ while across the interface plane it is of the order of10−14. It has been proved that the material rich modes (~2V/m) have high field enhancement compared to material deficient modes (~1V/m). The study stresses on different aspects of the proposed model that can be sought for suitable practicable applications in mid-infrared photonics, especially in design of mid-infrared bio-chemical sensors, mid-infrared temperature sensors, waveguides, omnidirectional reflectors, optical filters, band edge lasers, multiplexers, mitigators, compensators, etc.

Omnidirectional photonic bandgap in one-dimensional photonic crystals containing hyperbolic metamaterials.

We theoretically and experimentally investigate the angle-dependent omnidirectional photonic bandgap (PBG) in one-dimensional photonic crystals (PCs) comprising hyperbolic metamaterials (HMMs) for TM polarization, which is different from blue-shifted PBG in conventional all-dielectric photonic crystals. The frequency range of PBG increases when the incident angles increase, owing to the red-shift and blue-shift of the long-wavelength and short-wavelength band edges, respectively. The red-shifted band edge originates from the phase-variation compensation mechanism between the HMMs and dielectric material. The experimental values are in good agreement with the simulation results. These nanostructures are ideal for fabricating photonic devices such as omnidirectional reflectors.

Anomalous Reflection with Omnidirectional Active Metasurfaces Operating in Free Space

Metasurfaces that manipulate the reflection of impinging sound have recently received significant attention, but virtually all past designs are passive structures with high temporal and spatial dispersion and thus have narrow bandwidths and are unidirectional. In this work, we introduce a method to design omnidirectional and broadband anomalous acoustic reflectors composed of nonresonant active scatterers arranged in a periodic lattice of subwavelength periodicity. The method relies on the manipulation of the wave-vector component parallel to the reflector's surface (i.e., transverse component) in a broadband manner and for arbitrary impinging sound directions. To illustrate our method experimentally, we design and measure an anomalous reflecting metasurface operating in free space for which the transverse wave-vector components of the incident and reflected waves differ by a prescribed constant. We show that the effect does not depend on the direction of incidence and has a bandwidth an order of magnitude larger than passive anomalous reflectors. This work shows that the active scatterers behave similarly to the atoms of a natural material rather than the unit cells of gratings or phononic crystals in that rearranging the active scatterers changes the metasurface geometry while preserving the device's desired functionality.

Vector coherence imaging for enhancement of small omni-directional scatterers and suppression of geometric reflections

Ultrasonic imaging with full-matrix capture (FMC) datasets using the total focusing method (TFM) has been demonstrated to improve the detection, sizing and characterization of small defects in the area of Nondestructive Evaluation (NDE). Despite the general improvements in resolution and signal to noise ratio (SNR) offered by FMC/TFM, detection of small flaws remains challenging - particularly when flaws are in immediate proximity to the bounding surfaces of a test piece. In this paper, it is shown that by retaining only phase information during the application of the TFM algorithm, it is possible increase the image intensity of small omni-directional reflectors with respect to large specular reflectors (i.e. backwall/frontwall). The proposed algorithm, called Vector Coherence Imaging, is derived from the Vector Coherence Factor phase coherence metric and represents the amount of statistical (Shannon) information in favour of the presence of a coherent scatterer everywhere in the imaging domain. The VCI algorithm is shown to aid in detection of small defects located close to the boundaries of the specimen. Notably, VCI is seen to facilitate detection tip diffraction/scattering originating from surface connected fatigue cracks (less than 2 ultrasonic wavelengths in height) and early stage high temperature hydrogen attack (HTHA) damage (void nucleation and micro-fissuring).

Nonreciprocal absorption and omnidirectional band gap in the biaxial hyperbolic metamaterials with black phosphorus

Hyperbolic metamaterials (HMs) stacked by black phosphorus (BP) and silver are proposed, whose permittivities of three orthogonal directions are theoretically investigated. Four structures are designed by the HMs, of which the quasi-periodic structure 4 with gradient thickness and the filter element is adopted. The simulated results show that the hyperbolic properties can be found in several distinct regions for the dielectric constant of BP. In the proposed structure 4, the absorption peaks of transverse electric (TE) wave and transverse magnetic (TM) wave can be observed at different incident angles and frequency positions during the forward transmission. The peak value of TM wave is larger than that of TE wave at a sharp angle, which is more suitable for realizing the sensors and its sensitivity is 1.49 THz·degree−1 in the linear range. For the backward transmission, the entire structure is presented with the photonic band gap in the incident angle range of 0°–70° and in the frequency range of 630–740 THz, which can be called the omnidirectional band gap. Thus, the phenomenon of the nonreciprocal transmission of electromagnetic waves along two directions can be obtained. The cross stacking of the x- and y-directions of the dielectric constant of BP and the x- and y-axes of the coordinate axis is also considered. There is little difference with the above results, only the sensitivity of the sensor is enhanced slightly. The calculated results can apply for realizing the nonreciprocal absorbers, sensor, and an omnidirectional reflector.

Optimized Omnidirectional High-Reflectance Using Octonacci Photonic Crystal for Thermographic Sensing Applications

The transmittance of waves through one-dimensional periodic and Octonacci photonic structures was studied using the theoretical transfer matrix method for both wave-polarization-modes. The first structures were made up of the SiO2 and TiO2 materials. The objective here was to obtain a broad omnidirectional high reflector covering the infrared spectrum of a thermographic camera [1–14µm] and, especially, to prevent the transmission of emitted human body peak radiation λmax = 9.341 µm. By comparing the periodic and Octonacci structures, we found that the last structure presented a main and wide photonic band gap near this human radiation. For that, we kept only the Octonacci structure for the rest of the study. The first structure did not give the aspired objective; thus, we replaced the TiO2 layers with yttrium barium copper oxide material, and a significant enhancement of the omnidirectional photonic band gap was found for both TE and TM polarization modes. It was shown that the width of this band was sensitive to the Octonacci iteration number and the optical thickness (by changing the reference wavelength), but it was not affected by the ambient temperature. The number of layers and the thickness of the structure was optimized while improving the omnidirectional high reflector properties.