Omnidirectional Reflector Research Articles

Enhancement of GaN-Based Light-Emitting Diodes Transferred From Si (111) Substrate Onto Electroplating Cu Submount With TiO$_{2}$/SiO$_{2}$–Al Omnidirectional Reflector

In this paper, crack-free InGaN/GaN multiple quantum wells (MQWs) light-emitting diodes (LEDs) were transferred from Si (111) substrate onto the electroplating Cu submount, with inserting a 5-pair TiO $_{2}$ /SiO $_{2}$ omnidirectional reflector (ODR) and an Al mirror, defined as TiO $_{2}$ /SiO $_{2}$ –Al ODR. The reflectivity in the blue spectral range of the TiO $_{2}$ /SiO $_{2}$ –Al ODR was as high as 97%. The current–voltage (I–V) characteristics showed no obvious degradation of electrical properties for the ODR-based LED on Cu. In comparison to conventional LEDs on Si without ODRs, the light output of ODR-based LEDs on Cu was enhanced by 59.3% at a driving current of 350 mA. The improvement is mainly attributed to the following factors: the removal of the opaque Si substrate, the inserting of the high-reflectivity TiO $_{2}$ /SiO $_{2}$ –Al ODR, and the transfer onto the Cu submount with better thermal conductivity.

Graded Index Transparent Conducting Materials with Enhanced Optical Functionality

Transparent conducting oxides (TCO) have been extensively used in various technologically important applications including solar cells, flat panel displays, antireflective coatings, (organic)light emitting diodes and many other uses as advanced optical materials. Nanoporous and nanostructured films, assemblies and arrangements are important from an applied point of view in microelectronics, photonics and optical materials.The ability to minimize reflection, control light output and use contrast and variation of the refractive index to modify photonic characteristics can provide routes to enhanced photonic crystal devices, omnidirectional reflectors, antireflection coatings and broadband absorbing materials.This work demonstrates very high quality epitaxially grown (MBE) indium oxide, and indium tin oxide nanostructures and nanowires that are defect free and grown using homogeneous ITO seeds from glass or semiconducting surfaces. These branched and graded porosity layers grown directly on LEDs, glass coatings and other devices are shown to significantly enhance transparency or absorbance, depending on the visible wavelength range of interest and suppress Fresnel reflections for antireflection coatings. The benefit of controlling optical properties of nanoporous transparency conducting materials are shown for SiGe MQW LEDs, transparent Li-ion battery electrodes, antireflection coatings, and selective frequency absorbers across the visible range, and also demonstrates control over IR transmission without adversely affecting electronic conductivity.ReferencesC. M. Eliason and M. D. Shawkey, Optics Express, 22 A642 (2014).C. O'Dwyer, M. Szachowicz, G. Visimberga, V. Lavayen, S. B. Newcomb and C. M. S. Torres, Nature Nanotech., 4, 239 (2009).C. O'Dwyer and C. M. S. Torres, Front. Physics, 1, 18 (2013).M. Osiak, W. Khunsin, E. Armstrong, T. Kennedy, C. M. S. Torres, K. M. Ryan and C. O'Dwyer, Nanotechnology, 24, 065401 (2013).M. Osiak, E. Armstrong, T. Kennedy, C. M. S. Torres, K. M. Ryan and C. O'Dwyer, ACS Appl. Mater. Interfaces, 5, 8195 (2013).P. D. C. King, T. D. Veal, J. Phys. 23, 334214 (2011).J. Q. Xi, M. F. Schubert, J. K. Kim, E. F. Schubert, M. Chen, S. Y. Lin, W. Liu and J. A. Smart, Nature Photonics, 1, 176 (2007).

Omnidirectional reflector using one-dimensional dispersive photonic heterostructure

We present theoretical design for omnidirectional reflector (ODR) using one-dimensional photonic heterostructure. With the help of transfer matrix approach, we present a simple design to achieve the ODR at a desired wavelength range of interest using a high refractive index material which is dispersive at that wavelength range. Taking material combinations of SiO2 and GaSb as low and high index medium a perfect ODR is designed in the 1480–1680nm wavelength region.

Analysis of Omnidirectional Dual-Reflector Antenna and Feeding Horn Using Method of Moments

A hybrid numerical technique is applied for the analysis of circularly symmetric dual reflector antenna fed by TEM coaxial horn to generate an omnidirectional radiation pattern. By following the steps of a technique developed for the analysis of horn antennas, it employs a stepped-waveguide method to analyze the interior surfaces of the TEM coaxial horn transition and the electric field integral equation (EFIE) is applied on the outer surfaces of the TEM coaxial horn including the radiating aperture and the reflectors. Simultaneously, the magnetic field integral equation (MFIE) is employed on the aperture to relate the aperture fields and those in the horn transition. Method of moments (MoM) is employed to numerically solve the resultant hybrid field integral equation. The technique is applied for electromagnetic analysis of a coaxial TEM horn antenna and an omnidirectional dual reflector antenna. Results for the antenna return loss at the input port and for the vertical radiation pattern are given and compared with measurements and with commercial software results.

“Seeing” the Resonant SPP Modes Confined in Metal Nanocavity via Cathodoluminescne Spectroscopy

ABSTRACT:Surface plasmon polaritons (SPPs), which are coupled excitations of electrons bound to a metal-dielectric interface, show great potential for application in future nanoscale photonic systems due to the strong field confinement at the nanoscale, intensive local field enhancement, and interplay between strongly localized and propagating SPPs. The fabrication of sufficiently smooth metal surface with nanoscale feature size is crucial for SPPs to have practical applications. A template stripping (ST) method combined with PMMA as a template was successfully developed to create extraordinarily smooth metal nanostructures with a desirable feature size and morphology for plasmonics and metamaterials. The advantages of this method, including the high resolution, precipitous top-to bottom profile with a high aspect ratio, and three-dimensional characteristics, make it very suitable for the fabrication of plasmonic structures. By using this ST method, boxing ring-shaped nanocavities have been fabricated and the confined modes of surface plasmon polaritons in these nanocavities have been investigated and imaged by using cathodoluminescence (CL) spectroscopy, which has been turned out to be a powerful means to characterize the resonant SPPs modes confined in metal nanocavities [1∼5] . The mode of the out-of-plane field components of surface plasmon polaritons dominates the experimental mode patterns, indicating that the electron beam locally excites the out-of-plane field component of surface plasmon polaritons. Quality factors can be directly acquired from the spectra induced by the ultrasmooth surface of the cavity and the high reflectivity of the silver (Ag) reflectors. Because of its three-dimensional confined characteristics and the omnidirectional reflectors, the nanocavity exhibits a small modal volume, small total volume, rich resonant modes, and flexibility in mode control. Numerous applications, such as plasmonic filter, nanolaser, and efficient light-emitting devices, can be expected to arise from these developments.

级联一维光子晶体全方位反射器的带宽最大化

级联一维光子晶体全方位反射器的带宽最大化

Light propagation in anisotropic metamaterials. II. Reflection from the half-space

We consider reflection of light from half-space of anisotropic metamaterial at arbitrary direction of optical axis in the plane of light incidence. We obtain conditions at which total reflection and refraction do not depend on the polarization of incident light and the reflection does not depend on also the angle of incidence. We also study the possibilities and conditions for using the considered system as beam splitter, omnidirectional reflector, phase retarder, and so on.

Design of omnidirectional reflectors based on a cascaded one-dimensional photonic crystal structure

The omnidirectional transmission properties of photonic heterostructures composed of cascaded one-dimensional photonic crystals (1DPCs) with the same materials and different thickness ratios of the alternate high- and low-refractive index layers were studied theoretically. A criterion for designing omnidirectional reflectors with a maximum bandgap width is presented for heterostructures with an arbitrary number of cascaded 1DPCs. Omnidirectional reflectors based on two and three cascaded 1DPCs are designed according to the criterion.

Silicon Based One-Dimensional Photonic Crystal as a TM-Mode Filter

The effect of temperature and angle of incidence on the transmission spectra of a one dimensional photonic crystal for TE and TM polarizations has been studied. For a range of angle of incidence from 0° to 60°, the proposed structure works as an omni-directional reflector. For other angles of incidence more than 60°, the structure works as a selective TM-polarization filter i.e. a filter which completely blocks TE-polarized waves but allows certain frequencies of TM-polarized waves. At an angle of incidence of 89°, the structure works as a multi channel optical filter without introducing any defect in the geometry. Also, this multi-channel filter can work as a tunable filter by varying the temperature of the structure and the number of layers in the geometry. Only for two bi-layers the proposed structure works as a single channel filter. Therefore, such types of structure are easier to fabricate and may be cost effective as compared to defect one-dimensional photonic crystals.

Enhancement in light output power of LEDs with reflective current blocking layer and backside hybrid reflector

A GaN-based light-emitting diode (LED) with reflective current blocking layer (CBL) underneath p-electrode pad and backside hybrid reflector was fabricated and investigated. With a 20 mA injection current, the LED with SiO2/ITO/Al reflective CBL deposited on naturally textured p-InGaN/p-GaN surface exhibited a light output power that was 7.6% and 18.5% higher than those of the textured LEDs with SiO2 CBL and without SiO2 CBL, respectively. The LED with backside hybrid reflector exhibited a light output power that was 30% higher than that of LED without the hybrid reflector. The enhancement in light output power is attributed to the improved current spreading performance via the SiO2 CBL, the Al omnidirectional metal reflector to prevent the light absorption by the opaque p-electrode pad, and the backside hybrid reflector to extract bottom-emitting light.

Omni-directional reflectors for deep blue LED using symmetric autocloning method

Omni-directional reflectors (ODRs) for deep blue LED were designed and fabricated using symmetric autocloning method. The symmetric stack multi-layers for the reflectors were designed by finite-difference time-domain simulation. The fabricating process of ODR is combined with the techniques of anodic aluminum oxide (AAO) process and autocloning method. The two-dimensional structure template of nano-channel array was grown using AAO with the period of 150 nm. Then the shaping layer was deposited on the AAO template by evaporation deposition. Besides, the ion etching was applied to modify the apex angle to the triangle shape at 100°. Finally, the sub/(0.5TiO2 SiO2 0.5TiO2)8 multi-layer stack was deposited on the shaping layer using autocloning method to achieve the ODR. The results show the reflective spectra of ODR at the incident angles of 0, 30, 45, and 60° had high values within the range 400–450 nm. Besides, the central wavelength shifting is not obvious which is very good for keeping the color of LED stable.

Omni-directional reflection bands in one-dimensional plasma dielectric photonic crystals

In this paper the omni-directional reflection bands in one-dimensional plasma photonic crystal (PPC) have been studied theoretically. We present the study of plasma photonic crystal, having alternate regions of plasma⿿dielectric (Al2O3 or ZnS). Reflectances from this periodic multilayered structure in TE- and TM-modes are calculated for different angles of incidence in microwave region for omni-directional reflection bands. The reflectance is obtained by solving a Maxwell's equation using a translational matrix method. In addition to this, we have also studied the effect of variation of plasma width as well as plasma density on the reflection properties of plasma dielectric photonic crystal in TE- and TM-modes. The study of reflectance bands of such plasma photonic crystals shows that it can be used as omni-directional reflector.

Epitaxial growth of an antireflective, conductive, graded index ITO nanowire layer

Nanoporous and nanostructured films, assemblies and arrangements are important from an applied point of view in microelectronics, photonics and optical materials. The ability to minimize reflection, control light output and use contrast and variation of the refractive index to modify photonic characteristics can provide routes to enhanced photonic crystal devices, omnidirectional reflectors, antireflection coatings and broadband absorbing materials. This work shows how multiscale branching of defect-free ITO NWs grown as a layer with a graded refractive index improves antireflection properties and shifts the transparency window into the near-infrared (NIR). The measurements confirm the structural quality and growth mechanism of the NW layer without any heterogeneous seeding for NW growth. Optical reflectance measurements confirm broadband antireflection down to <5% between 1.3-1.6 um which is tunable with the NW density. The work also outlines how the suppression of the Burstein-Moss shifts using refractive index variation allows transparency in a conductive NW layer into NIR range.

Nanostructured Transparent Conductive Oxides for Photovoltaic Applications

ABSTRACTOblique-angle deposition is used to fabricate indium tin oxide (ITO) optical coatings with a porous, columnar nanostructure. Nanostructured ITO layers with a reduced refractive index are then incorporated into antireflection coating (ARC) structures with a step-graded refractive index design, enabling increased transmittance into an underlying semiconductor over a wide range of wavelengths of interest for photovoltaic applications. Low-refractive index nanostructured ITO coatings can also be combined with metal films to form an omnidirectional reflector (ODR) structure capable of achieving high internal reflectivity over a broad spectrum of wavelengths and a wide range of angles. Such conductive high-performance ODR structures on the back surface of a thin-film solar cell can potentially increase both the current and voltage output by scattering unabsorbed and emitted photons back into the active region of the device.

One dimensional photonic crystals from As2S3 and PMMA films for photonic and sensor applications

One dimensional photonic crystal (1D-PhC) in form of 19 layered stack was prepared through layer-by-layer deposition of spin coated Poly (methyl methacrylate) (n = 1.49 at 1.55 μm) and vacuum deposited As2S3 (n = 2.27 at 1.55 μm) as low and high refractive index materials, respectively. The optical properties of the single layers and the multilayered stack are studied at normal and oblique incidence of linearly polarized light. The possibility of using the 1D-PhC as an omnidirectional reflector was exploited theoretically and confirmed experimentally. Besides, the prospect of using the studied structure for chemical sensing with optical read-out is demonstrated.

Dispersion surfaces and light propagation in homogeneous dielectric-magnetic uniaxial medium

We investigated light propagation in a homogeneous medium having double anisotropy (i.e. with anisotropy of both dielectric and magnetic permittivities) and an arbitrary orientation of its optical axis in the plane of incidence. We investigated wave surfaces for the considered medium. We showed that only some groups of surfaces can arise. We investigated the conditions for total independent of polarization reflection and the conditions for total transmission, as well as possibilities of the subject system of serving as: an omnidirectional reflector, a beam splitter and a phase retarder.

Wave Transmission in Dispersive Si-Based One Dimensional Photonic Crystal

Transmission of electromagnetic waves through a Si-based one dimensional photonic crystal has been investigated. The proposed structure works as an omni-directional reflector for a certain range of wavelength for an angle of incidence up to 55?. The structure works as a narrow band TM-polarization filter for an angle of incidence more than 55?, i.e. a filter which completely blocks TE-polarized waves but allows certain wavelengths of TM-polarized waves. But at an angle of incidence of 89?, the structure works as a multiple narrow band TM-polarization filter even though no defect layer is introduced inside the structure. It is also found that this multiple narrow pass-bands of TM-polarized waves can be tuned to a desired range of wavelength by changing the temperature of the structure.

Tunable Reflection Bands and Defect Modes in One-Dimensional Tilted Photonic Crystal Structure

We show theoretically that range of reflection bands and defect modes inside the band gap can be tuned by using a one-dimensional tilted photonic crystal (TPC) structure. A TPC structure is similar to the conventional PC structure with the only difference that in this case alternate layers are inclined at certain angle in the direction of periodicity of the structure. In order to obtain the reflectance spectra of the proposed structure transfer matrix method (TMM) has been employed. From the analysis of the reflectance curve, it is found that 100% reflectance range can be varied and enhanced by using TPC structure for both (TE- and TM-) polarizations. The enhancement in reflection bands increases as the tilt angle increases for both the polarizations and hence the enlarged omni-reflectance bands are obtained. Further, we study the properties of the defect modes in TPC structure by introducing the tilted defect at the different tilt angle. The results show that defect modes (tunneling modes) can be tuned at different wavelengths by changing the tilt angle of the structure without changing other parameters. Finally, the effect of variation thickness of defect layers on the tunneling mode has been studied for both TPC and conventional PC structure. The proposed model might be used as a tunable broadband omnidirectional reflector as well as tunable tunneling or transmission mode, which has potential applications in the field of photonics and optoelectronics.

One dimensional photonic band gap material as an omnidirectional reflector

In this article, the reflection properties in one-dimensional dielectric-dielectric photonic band gap (PBG) structure have been studied. We have used SiO2 as material of low refractive index and Te as a high refractive index material. Reflectivity of proposed PBG structure is plotted as a function of wavelength and angle of incidence and omni-directional PBGs are computed theoretically. To obtain reflectance, we used transfer matrix method for solving Maxwell’s equations for electromagnetic wave in PBG structures. For a large range of frequency, the PBG structure is found to exhibit omni-directional reflection which can be exploited in devices such as optical resonators, mirrors, etc.

Experimental confirmation of strong fluorescence enhancement using one-dimensional GaP/SiO_2 photonic band gap structure

In this paper we report the experimental confirmation of the fluorescence enhancement effect using a one-dimensional photonic band gap (1D PBG) structure. This 1D PBG structure consists of periodic multilayer thin films with gallium phosphide (GaP) and silicon dioxide (SiO2) as the alternating high and low index materials. Strong evanescent field enhancement can be generated at the last interface due to the combination of total internal reflection and photonic crystal resonance for the excitation wavelength. In addition, the 1D PBG structure is designed as an omnidirectional reflector for the red-shifted fluorescent signal emitted from the surface bounded molecules. This omnidirectional reflection function helps to improve the collection efficiency of the objective lens and further increase the detected fluorescent signal. Compared with the commonly used bare glass substrate, an average enhancement factor of 69 times has been experimentally verified with quantum dots as the fluorescent markers. This fluorescence enhancer may find broad applications in single molecular optical sensing and imaging.