Omnidirectional Reflector Research Articles

Design and fabrication of white light emitting diodes with an omnidirectional reflector

This study investigates white light emitting diodes (LEDs), packaged with an omnidirectional reflector (ODR) and UV LED of 378 nm. The current work designs an ODR to solve the problems of UV leak and low phosphors conversion efficiency with a thin film structure of S/(HLH)n/air modified from a quarter-wave stack. Since an ODR is a perfect dielectric mirror for UV light, the UV leak can be eliminated and recycled to enhance phosphors conversion efficiency. By measuring the emission spectrum of a white LED with an ODR, the orientation distribution of the color properties, and the radiation power, the current study did not detect any UV leak. The average luminous intensity enhancement is 15%.

Chip-scale spectrometry based on tapered hollow Bragg waveguides

We describe a micro-spectrometer that exploits out-of-plane radiation at mode cutoff in a tapered leaky waveguide clad by omnidirectional Bragg reflectors. The device can be viewed as a side-coupled, tapered Fabry-Perot cavity. An effective-index transfer-matrix model reveals that optimal resolution is dependent on the reduction or mitigation of back-reflection and standing waves leading up to the cutoff point. We address this by insertion of low numerical aperture optics between the taper and the detector, and demonstrate an experimental resolution as small as approximately 1 nm and operating bandwidth >100 nm in the 1550 nm range, from a tapered waveguide with footprint approximately 50 microm x 500 microm. The device combines the small size of a Fabry-Perot instrument with the detector array compatibility and fixed optics of a grating-based instrument.

Omnidirectional Reflector Based on Dual Effect of Leaky Mode Resonance and Surface Plasmon Resonance

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Design of broad omnidirectional total reflectors based on one-dimensional dielectric and magnetic photonic crystals

Omnidirectional total reflectors based on one-dimensional dielectric and magnetic photonic crystals are investigated in this paper. We adopt the transmission matrix method to analyze the transmission properties of such reflectors and use the decimal genetic algorithm to obtain broad omnidirectional total reflection (ODTR) bandwidth. Finally, an omnidirectional total reflector with very broad ODTR bandwidth of 1.34 ω 0 is achieved.

Light extraction enhancement of sapphire‐free InGaN LEDs using single‐ and double‐side surface roughening techniques

AbstractWe compare the performance of three types of GaN‐based light‐emitting diodes (LEDs) epitaxially grown on patterned sapphire substrates (PSSs). For the devices with a single roughened p‐GaN surface, the light output power of the original LEDs with the PSS and Ag reflector (Sample A) is lower than that of thin‐film LEDs transferred onto Si carriers via glue bonding (Sample B). While the thin GaN LEDs with double‐side roughening (both p‐GaN and micropillar undoped‐GaN surfaces) and glue/Ag omnidirectional reflector (Sample C) shows much higher light output power than that of Sample B. As compared with the Sample A, the Sample C shows a performance enhancement of 50.3% in light output power and 46.3% in power efficiency at 350 mA. These results reflect that the contribution of the micropillar undoped‐GaN texturing for the light extraction is dominant. The employment of Si carrier decreases the junction temperature of the Sample C, a 46.6 °C lower than that of the conventional LEDs on PSS. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Low refractive index contrast porous silicon omnidirectional reflectors

We report on the fabrication and characterization of a porous silicon omnidirectional reflector formed by periodic substructures stacked together. For these substructures, a low refractive index contrast has been used, resulting in substructures without omnidirectional reflectivity band. The use of a low refractive index contrast involves the reduction of the requirements to obtain omnidirectional reflectors. We demonstrate the existence of an omnidirectional reflectivity range with a high gap-to-midgap ratio by means of reflectivity spectra measurements for a range of incidence angles. The results are in good agreement with a theoretical model of the reflector. The fabricated structure is the first reported porous silicon reflector suitable for 1.55 μm applications.

Electron-Beam and Sputter-Deposited Indium–Tin Oxide Omnidirectional Reflectors for High-Power Wafer-Bonded AlGaInP Light-Emitting Diodes

Performance variation of high-power wafer-bonded AlGaInP light-emitting diode (LED) with an emission wavelength of induced by E-beam and sputter-deposited indium–tin oxide (ITO) films, one component of triple-layer omnidirectional reflectors (ODRs), has been investigated in detail. The entire ODRs consist of p-GaP, dispersive dot contacts of acting as ohmic contacts, an intermediate low-refractive index ITO, and a silver (Ag) layer. The results show that annealing under nitrogen atmosphere yields a much rougher surface for the E-beam evaporated ITO than that for the magnetron sputtered one, which leads to a lower reflectivity of the ITO/Ag system. A similar resistivity of the two ITO films after annealing confirms no influence on the current-voltage characteristics of the corresponding devices. The smoother surface morphology of the sputter-deposited ITO after annealing enhances the light output power of 18% as compared to the one with the E-beam evaporated ITO at for the ITO ODR-based AlGaInP LEDs.

Modified L-shape Hollow Waveguides with Two Types of 458-chamfers

In this paper, two types of 45°-chamfers are embedded to an L-shape (90°-bent) hollow waveguide which is formed by omni-directional reflector to improve the transmission efficiency. Compared with that of an abrupt 90°-bent hollow waveguide, the proposed structures have enhanced the transmission efficiency by a factor of more than 4. The design rules of the 45°-chamfer and the fabrication considerations are discussed in detail.

Enhanced Luminance Efficiency of Wafer-Bonded InGaN–GaN LEDs With Double-Side Textured Surfaces and Omnidirectional Reflectors

A p-side-up GaN-based light-emitting diode (LED) on a silicon substrate was designed and fabricated using a combination of omnidirectional reflector (ODR) and double-side textured surface (both p-GaN and undoped-GaN) structures via surface-roughening, laser lift-off (LLO) and wafer-bonding technologies. The reflectivity of the designed ODR can reach 99.1% at a wavelength of 460 nm. The textured surface of top p-GaN was achieved under low temperature (LT) conditions using metalorganic chemical vapor deposition. It was found that the GaN LED with an extra 200-nm-thick LT p-GaN layer exhibits a 50% enhancement in luminance intensity. The luminance efficiency of double-side roughened silicon-ODR-GaN LED with a small chip size of 250 mum times 500 mum can be improved from 23.2% to 28.2% at an injection current of 20 mA. For the case of 1 mm times 1 mm in chip size, the saturation behavior of the light output power is not observed when an injection current increased from 20 to 350 mA, where the luminance efficiency at 20 mA can reach 28.9%, demonstrating an enhancement by 46%, as compared with that of the conventional GaN-sapphire LEDs. These enhanced results can be attributed to higher reflectivity from the ODR and multiple chances of light emitted from the active region to escape, as well as a centralizing effect of light along the vertical direction.

Investigation of InGaN/GaN power chip light emitting diodes with TiO2/SiO2 omnidirectional reflector

Enhancements of light extraction of GaN-based power chip (PC) LEDs with and without rough surface on p-GaN and TiO2/SiO2 omnidirectional reflector (ODR) on the bottom are presented. Motivated by phosphor-conversion white light applications, the peak-emitting wavelength of our studied PC LEDs is chosen to be 455 nm and the fabricated ODR is designed for the same wavelength regime. At a driving current of 350 mA and a chip size of 1 mm × 1 mm on a TO-can package, the light output power of the PC LED with ODR on the bottom and pit type of rough surface on p-GaN is enhanced by 67% when compared with the same device without ODR and rough surface. Furthermore, by examining the radiation patterns, the PC LED with the ODR and rough surface shows stronger enhancement around the vertical direction. Our results provide promising potential to increase output powers of commercial light emitting devices, especially for white light applications.

Improved Light Extraction Efficiency in AlGaInP Light-Emitting Diodes by Applying a Periodic Texture on the Surface

In this letter, AlGaInP-GaP-based light-emitting diodes (LEDs) were fabricated with an Si substrate and an SiO2-ITO-Ag omni-directional reflector using a metal-to-metal bonding technique. To enhance light extraction efficiency, a periodic texture was applied to the (Al0.5Ga0.5)0.5In0.5P surface layer of AlGaInP-Si LEDs by photolithography and a wet etching process. The exterior of the etched texture consists of a series of bowl-shaped recesses. With a 350-mA current injection, the typical output power of the AlGaInP-Si LEDs with and without the textured surface (LED-I and LED-II, respectively) were measured at approximately 118 and 81 mW, respectively, when the LED chips were bonded on the TO 46 without resin encapsulation. The enhancement of output power in LED-I can be attributed to a multitude of bowl-shaped notches on the surface, which resulted in a reduction of the reabsorption probability of the photons due to the fact that the photon path length in LED-I is shorter than in LED-II before the photons escape into the free space.

Enhancement in output power of blue gallium nitride-based light-emitting diodes with omnidirectional metal reflector under electrode pads

In this study, we demonstrate a GaN-based light-emitting diode (LED) with nonalloyed metal contacts onto the n+-GaN surface and transparent contact layer (indium tin oxide) to serve as the n-type electrode (cathode) and the p-type electrode pad (anode), respectively. Comparing with the conventional LEDs, which the electrode pads and/or Ohmic contacts form through conventional Cr∕Au metal contacts, the nonalloyed metal contacts (Ag∕Cr∕Au or Al∕Cr∕Au) used in the present experimental blue LEDs also play the role of reflector to prevent the emitted light from absorption by the opaque electrode pads with low reflectivity (Cr∕Au). With an injection current of 20mA, the enhancement in the light output power has approximately a 14% magnitude compared to the GaN-based LEDs without Ag or Al reflectors under the Cr∕Au electrode pads.

Light extraction enhancement of GaN-based light emitting diodes using MgF2/Al omnidirectional reflectors

We report on the enhancement of the light extraction of GaN-based light emitting diodes (LEDs) by using MgF2/Al omnidirectional reflectors (ODRs). The ODRs consisting of a quarter-wavelength-thick MgF2 having a refractive index of 1.39 and Al metal produce a high-angle-integrated reflectivity of 96.6%. To optimize the electrical injection and light reflection, the MgF2/Al ODRs are combined with Pd/Ag metallic reflectors using mesh configuration. Compared to reference LEDs, LEDs fabricated with the MgF2/Al ODRs show an enhanced output power by 23% and a slight increase in the forward voltage by 0.18 V, leading to the improvement in power efficiency by 17%.

High-Frequency Bistatic Scattering by Depolarizing, Nearly Omnidirectional Reflectors: Higher Order Polyhedral Reflectors

The bistatic scattering by perfectly conducting higher order polyhedral reflectors of large dimensions with respect to the wavelength, is studied. These particular faceted polyhedra, composed of several identical trihedral corner reflectors (TCR), are expected to exhibit a large RCS over a wide angular range with specific properties in terms of directivity and depolarization. Since numerical methods can not be applied to these complex structures (owing to the number of unknowns involved), an asymptotic method based on the physical optics combined with the geometrical optics approximations and the method of equivalent currents, recently developed for a single TCR, is used. The validity of the extended method is discussed and results for higher order (first-, second- and fourth-order) octahedral reflectors and icosahedral reflectors are also studied, in particular in terms of depolarization.

Broad Omnidirectional Reflector Design for Optical Telecommunications Band Using Genetic Algorithm

A novel application of genetic algorithm (GA) in designing omnidirectional reflectors with broad reflection bandwidth is reported in this paper. The key is to search the best combination style for different one-dimensional (1D) quasi-periodic PCs to form a heterostructure. Optimization based on Thus–Morse (TM) sequence has proved the effectiveness of GA. Finally, a high reflector described as [air∣(HLLH)5(HLLHLHHL)3(HL)7∣air, where nL = 1.46, nH = 4.6, dL = 0.220λ0/nL, and dH = 0.158λ0/nL] is achieved to provide a perfect omnidirectional reflection (ODR) bandwidth of 100% in optical telecommunications band (800–2000 nm).

Experimental investigation of a metamaterial omnidirectional reflector

We demonstrate experimentally the existence of an omnidirectional reflection band of a two-dimensional metamaterial structure consisting of split ring resonators and thin conducting wires. Experimental angular reflectance data agree reasonably well with simulations using model dispersion relations for the effective dielectric permittivity and magnetic permeability that were fitted to the experimentally retrieved material parameters. The tunability of the reflection bandwidth is demonstrated by changing the effective electric plasma frequency. These findings are also relevant to optical frequencies where similar metamaterial composites have been fabricated.

Light extraction enhancement of InGaN light-emitting diode by roughening both undoped micropillar-structure GaN and p-GaN as well as employing an omnidirectional reflector

Light extraction enhancement of InGaN–GaN light-emitting diode (LED) is demonstrated with double-side roughening both on the p-GaN surface and the micropillar undoped GaN as well as an omnidirectional reflector via patterning sapphire substrate, wafer-bonding, laser lift-off, and chemical wet etching technologies. This device design enhances the light output power up to 77% compared to the conventional LED with a single roughened p-GaN on patterned sapphire substrate at an injection current of 350mA. Due to the employment of Si carrier, the junction temperature measurement at 350mA yields a 46.6°C lower than that of the conventional LEDs.

Broad omnidirectional reflectors design using genetic algorithm

A application of genetic algorithm in designing omnidirectional reflectors is reported in this paper. Based on different quasi-periodic dielectric multilayer stacks, we give some optimization examples concerning both quart-wave and non quart-wave systems, and finally we achieve a photonic heterostructure with very broad omnidirectional reflection bandwidth.

Enhanced Vertical Extraction Efficiency From a Thin-Film InGaN–GaN Light-Emitting Diode Using a 2-D Photonic Crystal and an Omnidirectional Reflector

An InGaN-GaN thin-film vertical-type light-emitting diode with a two-dimensional photonic crystal (PC) on the emitting surface and a TiO-SiO omnidirectional reflector on the bottom was fabricated. The device was investigated by performing a series of experiments and numerical computations. Electroluminescence measurement revealed a strong extraction enhancement in the vertical direction at 433-nm wavelength. The emission spectrum of the light was found to be strongly modified by the PC to have a significantly narrow linewidth of 5 nm. Our experimental results were in accord with those obtained from our numerical findings.

Enhanced light extraction of InGaN-based green LEDs by nano-imprinted 2D photonic crystal pattern

In this paper, we propose a simple, low cost and mass producible nanoimprint lithography (NIL) method to texture the surface of GaN-based light emitting diodes (LEDs) with a two-dimensional photonic crystal (2DPC). Such a 2DPC structure not only enhanced the light output power but also changed the far-field pattern simultaneously. Also, a TiO2/SiO2 omnidirectional reflector (ODR) was deposited on the backside of the LEDs to further increase the light output power. Under 350 mA current injection, it was found that forward voltages were 3.35, 3.34 and 3.75 V while the light output powers of the LEDs were 59.5, 92.5 and 112.1 mW for the conventional LED, the PCLED with 20 nm depth, and the PCLED with 120 nm depth all with chip size of 1 mm × 1 mm, respectively. A 88.4% enhancement in light output power of PCLED with a 120 nm depth and ODR on the backside could be achieved when compared to the conventional LED under the driving current of 350 mA. From the measurement results, it was also found that the NIL process does not degrade the electrical properties of the fabricated LEDs.