Omnidirectional Reflector Research Articles

Omnidirectional Reflector with Total Internal Reflective Interface for Light Extraction Enhancement of Solid‐State Light Source

Light extraction efficiency is key to the performance of solid‐state light sources such as the light‐emitting diode. The reflectance of the bottom mirror has a significant impact on the light extraction efficiency because a large difference in the refractive index between the light source and air leads to multiple internal reflections and optical loss. Therefore, efficiency can be drastically improved with even a slight increase in the bottom mirror reflectance. This paper focuses on wide angular light incidences. An omnidirectional reflector (ODR) is then designed that included a low‐index thick layer (MgF2), a pair of quarter‐wavelength‐thick layers (Ta2O5/MgF2), and a metal layer (Ag) on a solid‐state light source with n = 2.4. The first thick layer works as a perfect reflector for wide angular and broadband incident light since total internal reflection (TIR) occurs at the interface. In the simulations, the angular integrated reflectance for our designed reflector (TI‐ODR) reaches 99.7%. The reflectance is higher than the typical ODR that is composed of a quarter‐wavelength‐thick layer and Ag (98.6%) or an Ag reflector alone (97.9%). The TI‐ODR is then fabricated onto a gallium nitride (GaN)‐based light‐emitting diode and obtained a light extraction efficiency enhancement factor of approximately 1.1.

One-dimensional dielectric bi-periodic photonic structures based on ternary photonic crystals

We investigate the transmittivity spectra, fields, and energy distribution of the electromagnetic eigenwaves propagating in a one-dimensional (1D) dielectric photonic crystal [(TiO2/SiO2)NAl2O3]M with two periods formed by unit cells TiO2/SiO2 and (TiO2/SiO2)NAl2O3. Spectra of TE- and TM-modes depend on the geometric parameters of the structure and undergo modifications with the change in the period numbers, layer thicknesses, and incidence angle. Special attention is paid to the applicability of the hybrid effective medium approximation comprising the long-wave approximation and two-dimensional (2 × 2) transfer matrix method. We demonstrate spectral peculiarities of the bi-periodic structure and also show the differences between the band gap spectra of the bi-periodic and ternary 1D dielectric photonic crystals. The presented photonic crystal structure can find its applications in optoelectronics and nanophotonics areas as omnidirectional reflectors, optical ultra-narrow bandpass filters, and antireflection coatings.

Experimental demonstration of angle-independent gaps in one-dimensional photonic crystals containing layered hyperbolic metamaterials and dielectrics at visible wavelengths

Photonic gaps are the core of photonic crystals in the manipulation of light. However, in conventional photonic crystals composed of two types of dielectrics, the photonic gaps strongly depend on the angle of incidence, which limits their applications. In this Letter, we present the experimental demonstration of angle-independent gaps in the visible wavelength range in one-dimensional photonic crystals composed of layered hyperbolic metamaterials (HMMs) and dielectrics. HMMs with anomalous wave-vector dispersion can tune the propagating phase of light to an unprecedented extent that usual dielectrics cannot attain. The HMM is mimicked by the layered titanium dioxide and silver with a subwavelength unit cell. Based on the phase-variation compensation effect between the HMM and the dielectric, an angle-independent gap is experimentally realized, which agrees well with the simulated one. This angle-independent gap is very useful in the design of photonic devices such as omnidirectional reflectors with fixed bandwidth or all-angle filters.

Photonic Crystals as Omnidirectional Infrared Reflector

In the present work ,design and analysis of the complete omnidirectional Bragg reflector are described. Transfer matrix method (TMM) was adopted to study the unique direction spectral performance of this structure for application in the infrared spectral region (8-14μm).It was shown that omnidirectional 1D- photonic bandgap could be simply evaluated and broadened using graphical and flexible optimization methods ,named as needle optimization technique. Thus ,a complete wideband 1D- photonic omnidirectional reflector was achieved despite the fact that the periodicity was only in one direction

Study of one-dimensional nanolayered graded photonic crystal consisting of birefringent and dielectric materials

In the present paper, we have studied a comparison between dielectric-dielectric photonic crystal (PhC) and birefringent-dielectric PhC structure with, and without, gradation in the thickness of the layers of the proposed structure. Graded birefringent-dielectric PhC has a linear change in the thickness of the layers. With the help of the transfer matrix method, the proposed structures have been extensively investigated. It is found that photonic band gap (PBG) width and omnidirectional reflection band width has been widely enhanced, in graded birefringent-dielectric PhC structure as compared to the non-graded birefringent-dielectric PhC structure, for both TE and TM polarizations. We have also found that the gradation in the thickness of the birefringent layer is more effective, as compare the gradation in the thickness of the dielectric layer, for enhancement of reflection band width. The results obtained are quite good and thus, it may be widely used as broad band optical and omnidirectional reflector.

Periodic air-nanoplate-embedded AlGaN/air/Al omnidirectional reflector for high efficiency ultraviolet emitters

We report on the periodic air-nanoplate-embedded omnidirectional reflectors (ODRs) to enhance the efficiency of AlGaN-based ultraviolet (UV) emitters composed of periodic quarter-wavelength-thick air-nanoplates/Al reflective layer and indium tin oxide (ITO) mesh ohmic layer. To enhance the photon extraction efficiency (PEE) of UV emitter by increasing the volume of air in ODRs, the coverage of the air-nanoplates in ODRs could be increased to 70% per chip. The optical output of UV emitter with 70% coverage of air-nanoplate was significantly improved by the Lambertian enhancement of 28.7% at an injection current of 10mA, compared with that of ITO film/Al UV emitter.

Enhanced light extraction efficiency of micro-ring array AlGaN deep ultraviolet light-emitting diodes

An effective approach to overcome inherently poor light extraction efficiency of AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs) is presented. We demonstrated the 5 × 5 array micro-ring DUV LED having an inclined sidewall at the outer perimeter and a p-GaN-removed inner circle of the micro-ring, together with MgF2/Al omnidirectional reflectors. The micro-ring array DUV LED shows remarkably higher light output power by 70% than the reference, consistent with the calculated result, as well as comparable turn-on and operational voltages, which are attributed to the effective extraction of strong transverse-magnetic polarized anisotropic emission and the reduction of the absorption loss by the p-GaN contact layer, simultaneously.

Bioinspired Omnidirectional Self-Stable Reflectors with Multiscale Hierarchical Structures.

Structured surfaces, demonstrating various wondrous physicochemical performances, are ubiquitous phenomena in nature. Butterfly wings with impressive structural colors are an interesting example for multiscale hierarchical structures (MHSs). However, most natural structural colors are relatively unstable and highly sensitive to incident angles, which limit their potential practical applications to a certain extent. Here, we reported a bioinspired color reflector with omnidirectional reflective self-stable (ORS) properties, which is inspired by the wing scales of Papilio palinurus butterfly. Through experimental exploration and theoretical analysis, it was found that the vivid colors of such butterfly wings are structure-based and possess novel ORS properties, which attributes to the multiple optical actions between light and the complex structures coupling the inverse opal-like structures (IOSs) and stacked lamellar ridges (SLRs). On the basis of this, we designed and successfully fabricated the SiO2-based bioinspired color reflectors (BCRs) through a facile and effective biotemplate method. It was confirmed that the MHSs in biotemplate are inherited by the obtained SiO2-based BCRs. More importantly, the SiO2-based BCRs also demonstrated the similar ORS properties in a wide wavelength range. We forcefully anticipate that the reported MHS-based ORS performance discovered in butterfly wing scales here could offer new thoughts for scientists to solve unstable reflection issues in particular optical field. The involved biotemplate fabrication method offers a facile and effective strategy for fabricating functional nanomaterials or bioinspired nanodevices with 3D complex nanostructures, such as structured optical devices, displays, and optoelectronic equipment.

English

Much interest has been paid to omnidirectional dielectric mirrors based on the 1-D dielectric multilayers because these are better than metal mirrors owing to their low optical loss, high reflectivity and high mechanical robustness. In these applications, the required position and width of the reflection band can be obtained by adjusting the layer thickness and refractive indices of the constituent materials. Here, the omnidirectional reflection characteristics of a 1-D PC formed by TiO2/SiO2 multi-quantum wells (MQW) are investigated in the visible region. It is demonstrated that the omnidirectional high reflector, formed by the proper line-up of four TiO2/SiO2 MQWs, covers the visible band approximately when the number of periodic layers of each quantum well reaches 12. This study can provide guidance in the design of an omnidirectional high reflector in the visible band. Key words: Omnidirectional reflector, photonic crystals, multi-quantum wells.

Effect of standard deviation, strength of magnetic field and electron density on the photonic band gap of an extrinsic disorder plasma photonic structure

Transmission properties of electromagnetic waves within microwave region of the one-dimensional random extrinsic plasma photonic crystals were computed using the transfer matrix method. The layers thicknesses of the extrinsic random photonic structure follow a Gaussian distribution. Compared with the periodic extrinsic photonic crystal, wider photonic band gaps (PBGs) were found in case of random extrinsic plasma photonic crystals with few resonant peaks. The PBGs are much wider while the randomness was increased and the number and the strength of resonant peaks were enhanced. The above observations were confirmed through analysis of histogram of normalized average transmissions for four different values of standard deviation with one thousand random samples for each group. The normalized average transmission was controlled by changing the strength of external magnetic field and the electron density of magnetized cold plasma. These features of disordered extrinsic plasma photonic structures would have potential applications such as omnidirectional reflectors and random multichannel filters with lower and higher rate of disorder.

Improved light output power of GaN-based ultraviolet light-emitting diode using a mesh-type GaN/SiO2 /Al omnidirectional reflector

We investigated the effect of a mesh-type GaN/SiO2/Al omnidirectional reflector (ODR) on the light output power of AlGaN/InGaN-based ultraviolet (365 nm) LEDs and compared their performance with that of LEDs with a GaN/ITO/Al reflector. Using the scattering matrix method, the normal incidence reflectance was calculated to be 93.7% for the GaN/SiO2 (62 nm)/Al ODR and 79% for the GaN/ITO (30 nm)/Al reflector. The Ag/Ni/Al/Ni (52 nm/10 nm/200 nm/20 nm) contact showed a specific contact resistance of 3.2 × 10−5 Ω cm2 after annealing at 500 °C for 1 min. The forward-bias voltages at 20 mA of LEDs with ODR were in the range of 3.49–3.54 V, which were similar to that of LEDs with an ITO/Al reflector (3.51 V). The LEDs with ODR had series resistances in the range of 14.8–12.5 Ω, whereas the LED with an ITO/Al reflector showed 11.7 Ω. The LEDs with ODR yielded 9.3–19.9% higher light output power at 20 mA than the LED with an ITO/Al reflector. The improved light output power was attributed to the optimization of the high reflectance of the ODR (reflective area) and contact area.

Self-assembled indium tin oxide nanoball-embedded omnidirectional reflectors for high photon extraction efficiency in III-nitride ultraviolet emitters.

The control of the refractive index and electrical conductivity in the dielectric layer of omnidirectional reflectors (ODRs) is essential to improve the low efficiency of AlGaN-based UV emitters. Here, we report self-assembled indium tin oxide (ITO) nanoball-embedded omnidirectional reflectors (NODRs) for use in high-efficiency AlGaN-based UV emitters at 365 nm. These NODRs consisted of a reflective Al layer, a self-assembled conducting ITO nanoball layer for current injection and spreading, and nanovoids that provided a low refractive index to achieve highly efficient UV emitters. The NODR was able to realize both high electrical conductivity and reflectivity by decreasing the average refractive index of the ITO nanoball layers. We observed diffuse reflection as well as omnidirectional reflection from the NODR UV emitters because of the corrugated interfaces of the nanovoids, ITO nanoball layer, and Al layer. These structural and optical properties of the NODRs remarkably increased the output power of a UV emitter by a Lambertian enhancement factor of 57% at an injection current of 50 mA at all emission angles compared with that of an ITO film/Al UV emitter.

One-dimensional PMMA–V2O5 photonic crystals used as color indicators of chloroform vapors

The fabrication and characterization of one-dimensional photonic crystals (1D-PC) in the form of quarter-wavelength multilayered stack (Bragg stack) are presented. The 1D-PC comprises alternatively deposited spin-coated polymer PMMA (Poly(methyl methacrylate)) films as low-refractive index material that change thickness upon exposure to chloroform vapors, and both dense and porous V2O5 films providing both high optical contrast. The possibility of using PMMA–V2O5 structure as an omnidirectional reflector in both cases is confirmed experimentally. Besides, the prospect of utilizing the studied structures as optical indicators of chloroform vapors is demonstrated and discussed.

Omnidirectional photonic bandgap of the one-dimensional plasma photonic crystal based on a novel Fibonacci quasiperiodic structure

The binary one-dimensional plasma photonic crystal (1DPPC) based on Fibonacci quasiperiodic structure is studied systematically in this paper. We consider the two simplest cases. In one case, the initial sequences F0 and F1 are both of single layer structure. In another case, one initial sequence (F0 or F1) is of a single layer structure, while the other one (F1 or F0) is of a double layer structure. Thus there are ten different kinds of initial sequences in total. The photonic bandgap characteristics of the 1DPPC with these different initial sequences and numbers of period are analyzed. On these bases, a novel structure of one-dimensional plasma photonic crystal (F3)3 with initial sequence of F0=AP, F1=P and F0=PA, F1=P is proposed in this paper to enlarge the omnidirectional photonic bandgap (OPBG). Compared with previously reported structures in the literature, this structure is simple in configuration with fewer layers and materials, and its OPBG width is wide. The influences of the parameters of the plasma material, such as the thickness, plasma frequency and collision frequency, on the OPBG characteristics of this structure are also discussed. The OPBG width increases with the increase of the thickness and plasma frequency of the plasma layer. Compared with the structures in the literature, the change of OPBG width is the fastest for the proposed structure when the parameters are relatively small. And with the same parameters, the OPBG width for the proposed structure is the widest when the parameters are greater than a certain value. The plasma collision frequency has no effect on the OPBG width for all the structures. But the OPBG width for the proposed structure is the widest when this parameter has the same value. The reason why the proposed structure has an optimal OPBG width is explained by analyzing the dispersion properties of the plasma. The real and imaginary part of the dielectric constant of plasma change with frequency significantly only in the low frequency region. Since the imaginary part of dielectric constant is nearly zero when the frequency is larger than 2 GHz, only the dispersion effect of the real part of dielectric constant needs to be considered in the frequency range we investigate. And its value is much greater than that of conventional medium in the same frequency range. This makes the high-reflectance bands of the 1DPPC broader than those in the case of pure photonic interference phenomena with conventional medium. On the other hand, the corresponding highest proportion of plasma layers in the whole quasiperiodic structure can also be used to explain the broadest band gap of (F3)3. These results can provide important theoretical guidance for designing the novel omnidirectional reflectors.

Investigation on the properties of omnidirectional photonic band gaps in two-dimensional plasma photonic crystals

The properties of omnidirectional photonic band gaps (OBGs) in two-dimensional plasma photonic crystals (2D PPCs) are theoretically investigated by the modified plane wave expansion method. In the simulation, we consider the off-plane incident wave vector. The configuration of 2D PPCs is the triangular lattices filled with the nonmagnetized plasma cylinders in the homogeneous and isotropic dielectric background. The calculated results show that the proposed 2D PPCs possess a flatbands region and the OBGs. Compared with the OBGs in the conventional 2D dielectric-air PCs, it can be obtained more easily and enlarged in the 2D PPCs with a similar structure. The effects of configurational parameters of the PPCs on the OBGs also are studied. The simulated results demonstrate that the locations of OBGs can be tuned easily by manipulating those parameters except for changing plasma collision frequency. The achieved OBGs can be enlarged by optimizations. The OBGs of two novel configurations of PPCs with different cross sections are computed for a comparison. Both configurations have the advantages of obtaining the larger OBGs compared with the conventional configuration, since the symmetry of 2D PPCs is broken by different sizes of periodically inserted plasma cylinders or connected by the embedded plasma cylinders with thin veins. The analysis of the results shows that the bandwidths of OBGs can be tuned by changing geometric and physical parameters of such two PPCs structures. The theoretical results may open a new scope for designing the omnidirectional reflectors or mirrors based on the 2D PPCs.

Investigation of growth and characterization of nanostructured CuIn5S8 thin films produced by glancing angle deposition

Ternary chalcogenide of copper and indium (CuIn5S8) thin films were grown by thermal evaporation method using GLancing Angle Deposition (GLAD) technique. The samples were prepared under different incident angles (α=0°, 40°, 60° and 85° measured from the normal to the substrate surface) with a substrate rotation of 2rpm. X-ray diffraction, scanning electron microscopy, and ultraviolet–visible-infrared spectra are employed to characterize the microstructure and optical properties of the CuIn5S8 thin films deposited by this technique. Under the GLAD conditions, we demonstrate that with substrate rotation, the columns were grown vertically due to the shadowing symmetry. The optical constants of the deposited films were determined from the analysis of transmission and reflection data. The results show that the refractive index and the thickness were decreased as α rises from 0° to 85° while the porosity and the Urbach energy were increased with increasing of the incident angle. The minimum refractive index is found to be 2.03 for the helical CuIn5S8 film deposited at an angle of 85° and the Urbach energy was found to increase from 0.29 to 0.5eV as α rises from 0° to 85°. Such changes of the optical behaviors are correlated with changes of the microstructure, especially a porous architecture which is favored for high incident angle. These properties exhibit potential for use in applications such as photonic crystals, graded index optical filters, and birefrigent omnidirectional reflectors.

Light interaction in sapphire/MgF2/Al triple-layer omnidirectional reflectors in AlGaN-based near ultraviolet light-emitting diodes.

This study examined systematically the mechanism of light interaction in the sapphire/MgF2/Al triple-layer omnidirectional reflectors (ODR) and its effects on the light output power in near ultraviolet light emitting diodes (NUV-LEDs) with the ODR. The light output power of NUV-LEDs with the triple-layer ODR structure increased with decreasing surface roughness of the sapphire backside in the ODR. Theoretical modeling of the roughened surface suggests that the dependence of the reflectance of the triple-layer ODR structure on the surface roughness can be attributed mainly to light absorption by the Al nano-structures and the trapping of scattered light in the MgF2 layer. Furthermore, the ray tracing simulation based upon the theoretical modeling showed good agreement with the measured reflectance of the ODR structure in diffuse mode.

Omnidirectional reflector using linearly graded refractive index profile of 1D binary and ternary photonic crystal

The effect of the refractive index profile on the omnidirectional reflector (ODR) of one dimensional dielectric and semiconductor multilayer structure having linearly graded material has been simulated using transfer matrix method. The theoretical analysis shows that the proposed structure works as a perfect mirror within a certain wavelength range. These wavelength ranges can be completely controlled by the refractive index profile of a graded semiconductor layer and also observed that these wavelength ranges are also controlled by refractive index parameters. Reflectance characteristics of 1D binary and ternary photonic crystal (PC) structure have been studied. This property shows that the ODR range of structure increased when a third layer of semiconductor added in a binary photonic crystal.

The Tunable Omnidirectional Reflector Based on Two-Dimensional Photonic Crystals With Superconductor Constituents

The properties of omnidirectional band gap (OBG) in the two-dimensional superconductor photonic crystals (SPCs) with triangular lattices, that are composed of the superconductor (Nb) rods in the dielectric background, are theoretically studied in detail based on the modified plane wave expansion method, as the electromagnetic wave with a off-plane angle. The calculated results demonstrate that not only the complete photonic band gap (PBG) and a flatbands region can be achieved but also the OBG can be observed. The influences of the radius of inserted rod, the dielectric background, the plasma frequency of superconductor, the damping coefficient of the superconductor, and the ambient temperature on the OBG are investigated, respectively. The simulations show that the tunable OBG can be obtained except for changing the damping coefficient of the superconductor. The features of OBG for the complementary structure also are discussed as the anisotropic materials are introduced. The analysis results show that the complete PBG also can be achieved in the large incidence angle region. These values provide the theoretical instructions to realize the omnidirectional reflector by the proposed SPCs.

Optical Mode Splitting in Ring-Shaped Hollow Bragg Cavity

We investigate the optical modes of hollow ring resonator with omnidirectional reflector cladding by numerical method. We demonstrate that each TM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0ml</sub> mode (or TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0ml</sub> mode) can split into two modes whose wavelengths are very close. The doublet effect is caused by the symmetry breaking at circular cross section when a straight waveguide is curled into a ring shape. The ring-shaped hollow Bragg cavity provides a platform to research the interaction of Rydberg atoms and light mode.