Light Extraction Mechanism Research Articles

In-depth insights into polarization-dependent light extraction mechanisms of AlGaN-based deep ultraviolet light-emitting diodes.

The AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) dominated by transverse-magnetic (TM) polarized emission suffer from extremely poor light extraction efficiency (LEE) from their top surface, which severely limits the device performance. In this study, the underlying physics of polarization-dependent light extraction mechanisms of AlGaN-based DUV LEDs has been explored in depth via simple Monte Carlo ray-tracing simulations with Snell's law. It is especially worth noting that the structures of the p-type electron blocking layer (p-EBL) and multi-quantum wells (MQWs) have a significant impact on light extraction behavior, especially for TM-polarized emission. Thus, an artificial vertical escape channel (named GLRV) has been constructed to efficiently extract the TM-polarized light through the top surface, by adjusting the structures of the p-EBL, MQWs, sidewalls, and using the adverse total internal reflection in a positive manner. The results show that the enhancement times of the top-surface LEE is up to 18 for TM-polarized emission in the 300 × 300 µm2 chip comprising a single GLRV structure, and further increases to 25 by dividing this single GLRV structure into a 4 × 4 micro-GLRV array structure. This study provides a new perspective for understanding and modulating the extraction mechanisms of polarized light to overcome the inherently poor LEE for the TM-polarized light.

Boosted Ambient Contrast Ratio of Light-Emitting Diode Display Devices With High Light Output Using a Laminated Interlaced Microgroove Janus Structure

A high ambient contrast ratio (ACR) and brightness are critical for realizing high dynamic range (HDR) in advanced display applications. However, improving the ACR while maintaining high light output for display devices is still challenging. To solve this issue, we proposed a laminated interlaced microgroove (LIM) Janus structure, consisting of intersecting microgrooves cutting through the extinction and scattering layers, leading to highly improved ACR with less optical loss for display devices. Optical simulation was performed to optimize the LIM Janus structure, revealing the antireflection and light extraction mechanism with low reflectance and high light output. According to the high-speed dicing technique, the optimized LIM Janus film was prepared and assembled on light-emitting diode (LED) display devices. Compared with commercial display devices with graphite coating, the LIM display devices achieved a high ACR boosted from 2209 to 25 188 with an improvement of 1040% (ambient light 500 lux) while maintaining the high light output. The LIM Janus structure is facile and can be easily integrated into display devices to obtain excellent ACR and light output to satisfy the broad application of advanced displays.

Enhanced light output of scintillators by ZnO nanorod arrays

Scintillators are widely used in national security, industrial and medical fields. The light extraction efficiency of scintillator is quite low due to the total internal reflection. Therefore, enhancement the light output of scintillator is highly important. In this paper, we use ZnO nanorod (NR) arrays fabricated by a simple hydrothermal growth route to enhance the light extraction efficiency of Bi 4 Ge 3 O 12 (BGO) crystal scintillator. The light output results demonstrate that the light extraction efficiency of BGO with ZnO NRs is significantly enhanced, with an enhancement ratio of 4.1 and 3.5 at 510 nm wavelength under 266 nm laser and X-ray excitation, respectively. Our numerically calculated light transmissivity spectra and electric field intensity distributions of BGO with and without ZnO NRs by finite-difference time-domain (FDTD) simulation reveal that the ZnO NRs could reduce the total internal reflection and allow more light to be extracted, resulted in the enhancement of light output of BGO. • BGO with ZnO nanorods modifications yields a significant enhancement of light output. • The enhancement of light output in wide wavelength and angle ranges is demonstrated. • The mechanism of light extraction is revealed by numerical simulations.

Light extraction efficiency of AlGaN nanowire deep ultraviolet light-emitting diodes on Si with different photonic structures

The low light extraction efficiency (LEE) stemming from the transverse-magnetic dominant emission is one of the major factors limiting the performance of aluminum gallium nitride (AlGaN)-based deep ultraviolet (UV) light-emitting diodes (LEDs) when the light emission wavelengths are close to 200 nm. These wavelengths, nonetheless, are pivotal to applications including sensing and sterilization and are considered human safe. We investigate the LEE of AlGaN/AlN nanowire (NW) deep UV LEDs emitting at 225 nm, with a focus on the top-surface LEE by considering different NW arrangements, typical NW spacings and radii from AlGaN NWs grown by selective area epitaxy, the influence of Si on the LEE in comparison to an Al reflector, and graphene as the top electrode. Our results show that given the selected range of design parameters, the top-surface LEE for honeycomb, square, and hexagonal lattices can be up to around 42% for complete devices on Si with the graphene top electrode; and compared to using an Al reflector, the Si substrate does not reduce LEE considerably. In the end, the light extraction mechanism is discussed by simulating the two-dimensional photonic crystal band structures.

Radiant Efficacy Enhancement for Vertical-Cavity Surface-Emitting Laser Devices Using Enhanced Specular Reflection Structures

Vertical-cavity surface-emitting laser (VCSEL) devices operate using a variety of viewing angles to satisfy the demands of different applications. One simple and universal approach to accomplish this is by using scattering elements in the packaging process for VCSEL chips. However, inevitable backward scattering loss is introduced, which suppresses the radiant efficacy of VCSEL devices. Therefore, we have proposed an enhanced specular reflection (ESR) structure with an opening circle window (OCW) to recycle the backward scattered light. The ray-tracing method is used to optimize the diameter of the OCW, as well as the thickness of the ESR structure. Furthermore, the light extraction mechanism is discussed. According to theoretical results, VCSEL devices with optimized ESR structures are fabricated and compared with traditional VCSEL devices. The experiment results demonstrate that ESR structures can significantly enhance the radiant efficacy of VCSEL devices using scattering elements. This shows great potential for applications involving illumination and 3-D imaging.

Light extraction enhancement of BGO scintillator by monolayers of SiO2 periodic array

Utilizing the self-assembly method, we have successfully prepared a two-dimensional photonic crystal (PhC) of monolayer SiO2 spheres onto the Bismuth Germinate (Bi4Ge3O12, BGO) crystal and therefore supply another photonic crystal structure with the advantages of low-cost and large-area preparation to improve the light extraction efficiency of scintillator. A light extraction enhancement ratio up to 145.3% as compared the BGO coated by the PhC with the plain reference sample is observed and the underlying mechanism of scintillator light extraction is discussed in detail via a rigorous coupled-wave analysis calculation. The calculated transmission spectra and electric-field distribution exhibit the electric-field eigenmodes confined in SiO2 spheres resonating by the way of whispering gallery modes (WGMs) and becoming leaky modes due to the diffraction effect. Since almost no SiO2 nanospheres over the polystyrene spheres are immersed in preparation, we have demonstrated that the introduction of PhC structures using the SiO2 nanospheres onto the scintillator surface is an effective way to solve the light-trapping problem and therefore the design is very useful to be applied to the particle detection of radiation field.

Silica Nano-Structured Fiber for Illumination

This paper describes a novel fiber designed to produce uniform illumination in angular space along its length. The mechanism of light extraction from the silica glass guiding region is due to nano-sized gas filled voids formed during fiber preform consolidation and fiber draw process. The resulting effect creates an aesthetically pleasing line or string of light which provides a long, thin, and flexible illumination tool for a variety of designs or functional applications. The rate of light extraction is determined by size and number of the formed these elongated voids features. The scattering material in the secondary coating modifies the scattering distribution function to make it uniform in angular space. The rate of the scattering efficiency can be controlled by drawing conditions and light launch conditions from the light source. The spectral efficiency of scattering is very high due to use of pure silica and low-absorbance coating materials from UV through IR. The use of color conversion materials placed outside the secondary coating can be used to generate a wide range of colors using a blue wavelength light pump, including pure white colors with a color temperature ranging from 3000 to 10000 K. Finally, a range of aesthetic and functional illumination applications for this fiber are also discussed.

Investigation of Light-Extraction Mechanisms of Multiscale Patterned Arrays With Rough Morphology for GaN-Based Thin-Film LEDs

Micro-scale patterned arrays and nano-scale rough morphology are promising for improving the light-extraction performance of GaN-based thin film light-emitting diodes (TFLEDs), while the light-extraction mechanisms of the multiscale architectures combining these two structures have not been investigated yet. In this report, we have adopted a pattern transfer and wet etching combined method to fabricate multiscale patterned arrays with rough morphology (msPARM) on n-GaN layers for TFLEDs and investigated their light-extraction mechanisms by the finite-difference time domain and ray-tracing combined method. The results show that the TFLEDs achieve the maximum radiant efficacy using the msPARM with an etching time of 8 min, which is increased by 16.3% and 1.7% compared with that achieved using only the patterned arrays or only the rough morphology, respectively. Most importantly, optical simulation reveals that the msPARM can provide a high transmittance for light with large emission angles from the active region using the inclined surface of micro-scale concave cones, while effectively suppressing the reflection loss for light with small emission angles using the scattering effect of nano-scale rough morphology, resulting in enhancing the light-extraction of the TFLEDs. Consequently, this study can provide a better understanding to design the multiscale structures for achieving high efficiency LEDs.

Enhancement of luminous efficacy for LED lamps by introducing polyacrylonitrile electrospinning nanofiber film.

Polyacrylonitrile electrospinning nanofiber film was introduced into a light emitting diode (LED) lamp to exploit the strong reflective and scattering effects. The light extraction mechanism was studied systematically for three different electrospinning types in three different types of LED lamps. For the all-electrospinning types, the luminous efficacy increased for the white LED, outwards remote phosphor layer, and inwards remote phosphor layer lamps by 10.98, 16.97, and 18.35%, respectively, compared with the reference lamp. Lamps with stronger backscatter had larger luminous efficacy enhancements. The reflector-electrospinning type helped redirect lights with large emission angles. The substrate-electrospinning type was beneficial for recycling the total interior reflection lights and increasing the yellow to blue ratio. Additionally, the all-electrospinning white LED lamps remains 97.89% luminous flux after a 96-hour aging process. Electrospinning fiber films are favorable luminous efficacy enhancers for the future generation of LED lamps.

Enhancing Both TM- and TE-Polarized Light Extraction Efficiency of AlGaN-Based Deep Ultraviolet Light-Emitting Diode via Air Cavity Extractor With Vertical Sidewall

In this work, we investigate the effect of sidewall angle, height and space of an air cavity extractor (AC-Extractor) on the polarization-dependent light extraction efficiency (LEE) for AlGaN-based deep-ultraviolet light-emitting diodes (DUV LEDs) using three-dimensional finite difference time-domain method. Compared to the reported inclined sidewall metal reflector (SM-Reflector), DUV LED with AC-Extractor shows better light extraction at any height and inclined angle due to different light extraction mechanisms. The light in SM-Reflector DUV LED is extracted mainly through mirror reflection. However, total internal reflection from the sidewall dominants the contribution to LEE for the AC-Extractor with smaller inclined angle such that the space has little impact, while Fresnel scattering is more important for the AC-Extractor with larger inclined angle and that the additional out-light channel as well as the space play a critical role in enhancing the LEE. And the analysis is further developed into a three-escape-cones analytic model theory. In particular, the AC-Extractor with vertical sidewall is more beneficial to enhance the LEE of DUV LED than that with inclined sidewall, which can be rationally explained based on the three-escape-cones analytic model.

Study on the light extraction mechanism of organic light-emitting diodes with corrugated Ag cathode made by soft nanoimprint

We studied the optical out-coupling of green organic light-emitting diodes (OLEDs) with Ag cathodes having 265 nm periodic corrugated grating structure. The corrugated structure is fabricated by nanoimprint lithography using a poly(dimethylsiloxane) (PDMS) soft diffraction grating mold and by pressing the mold on spin-coated poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) hole-injection layer on indium–tin oxide (ITO) glass. The OLEDs’ current efficiency is enhanced by about 24% as compared to that of OLEDs with planar cathode structures. This enhanced is explained as due to a reduced optical loss from waveguide (WG) modes, total internal reflection (TIR), and electrode surface plasmon polaritons (SPPs). The dispersion relation of the SPP modes is computed theoretically and is found in good agreement with angle-resolved spectral measurements.

Thue-Morse nanostructures for tunable light extraction in the visible region

Controlling light propagation at the nanoscale is a fascinating opportunity offered by modern photonics, more than a challenge to face off. This study is aimed at investigating a particular kind of nanocomposite and reconfigurable optical metamaterials that can be exploited for the realization of a new class of switchable photonic devices, representing a breakthrough with respect to the state of the art. Existing photonic devices exhibit, in general, a drawback in the absence of tunability; this work aims to the design and characterization of metamaterials exploiting reconfigurable media, like LCs, which enable realization of a tunable, high quality, photonic quasi-crystal based switchable mode selector. It turned out that, starting from an unpolarized white light source, through a light extraction mechanism based on the diffraction of light, the high quality structure, combined with a uniformly aligned Photo-responsive Liquid Crystal (PLC), is able to give rise to an extremely narrow (FWHM ≈5 nm) and linearly polarized single mode peak of the extracted light intensity. Moreover, we have shown that the spectral properties (switching) of the samples can be finely controlled by using both an external applied voltage and a suitable pump light source with a maximum increase of 45% of the extracted light. Finally, both Scanning Electron Microscopy (SEM) and Far Field Diffraction (FFD) analysis have shown the high quality morphology of the realized structure.

Metal–Semiconductor Square Nanocavity and Light Extraction

We propose a design of metal–semiconductor square nanocavity on silicon that could be used for on-chip nanolaser application, together with an efficient mechanism of light extraction, which is important for practical application. The analysis of the spatial mode for the square nanocavity is carried out, and hybrid plasmonic–photonic mode profile is observed in the cavity. Optimized design of the square cavity gives a mode volume of 0.96( $\lambda $ /2n)3 and resonant wavelength of 1474 nm when the side length is 600 nm. The outcoupling mechanism for the nanocavity is based on a direct-joint plasmonic waveguide. The effects of width, position, and angle of the output plasmonic waveguide on the outcoupling efficiency are studied, and the difference in cavity mode with respect to circular nanocavity has contributed to the different outcoupling performances. The analysis shows that the output coupling efficiency can be tuned up to 60% by varying these parameters, and this offers an output coupling mechanism with great flexibility for ultra-compact on-chip light source.

Nanostructure surface patterning of GaN thin films and application to AlGaN/AlN multiple quantum wells: A way towards light extraction efficiency enhancement of III-nitride based light emitting diodes

Enhanced light extraction efficiency was demonstrated on nanostructure patterned GaN and AlGaN/AlN Multiple-Quantum-Well (MQW) structures using mass production techniques including natural lithography and interference lithography with feature size as small as 100 nm. Periodic nanostructures showed higher light extraction efficiency and modified emission profile compared to non-periodic structures based on integral reflection and angular-resolved transmission measurement. Light extraction mechanism of macroscopic and microscopic nanopatterning is discussed, and the advantage of using periodic nanostructure patterning is provided. An enhanced photoluminescence emission intensity was observed on nanostructure patterned AlGaN/AlN MQW compared to as-grown structure, demonstrating a large-scale and mass-producible pathway to higher light extraction efficiency in deep-ultra-violet light-emitting diodes.

Fabrication of high-brightness GaN-based light-emitting diodes via thermal nanoimprinting of ZnO-nanoparticle-dispersed resin

We fabricated high-brightness GaN-based light-emitting diodes (LEDs) with highly refractive patterned structures by using a thermal nanoimprint lithography (NIL). A highly refractive ZnO-nanoparticle-dispersed resin (ZNDR) was used in NIL, and a submicron hole, a submicron high-aspect-ratio pillar, and microconvex arrays were fabricated on the indium tin oxide (ITO) top electrode of GaN-based LED devices. We analyzed the light extraction mechanism for each of the three types of patterns by using a finite element method simulation, and found that the high-aspect-ratio pillar had a great ability to improve light extraction owing to its waveguide effect and prominent scattering effect. As a result, the light output power, which was measured in an integrating sphere, of the LED device was enhanced by up to 19.6% when the high-aspect-ratio pillar array was formed on the top ITO electrode of the device. Further, the electrical properties of none of the patterned LED devices fabricated using ZNDR degraded in comparison to those of bare LED devices.

Light transmission enhancement from hybrid ZnO micro-mesh and nanorod arrays with application to GaN-based light-emitting diodes

A hybrid ZnO micro-mesh and nanorod arrays (MMNR) was fabricated as a light output window for GaN-based light-emitting diodes (LEDs) to enhance the light extraction efficiency. The light output power of GaN-based LEDs with the ZnO MMNR is improved by 95% compared to the original planar LEDs. The ZnO MMNR is manufactured by photolithography techniques and a two-step wet chemical growth process. The incident angle-resolved light transmission of the ZnO MMNR beyond the critical angle of total internal reflection is greatly enhanced. The light diffraction pattern of the ZnO MMNR shows that it possesses both the two-dimensional diffraction grating effect of a ZnO micro-mesh and the light scattering effect of a ZnO nanorod array. LEDs with the ZnO MMNR have greater light extraction efficiency than those with only a ZnO micro-mesh or a ZnO nanorod array. The local optical field patterns of the ZnO micro-mesh and the ZnO MMNR are investigated using confocal scanning electroluminescence microscopy. The microscopic light extraction mechanism of the ZnO MMNR is analyzed in-depth.

Enhancing light extraction mechanisms of GaN-based light-emitting diodes through the integration of imprinting microstructures, patterned sapphire substrates, and surface roughness

Analysis of the various light extraction efficiency enhancement mechanisms for the GaN-based light emitting diodes (LEDs) was investigated. Experiments utilized the imprinting technique to fabricate pyramid and inverted pyramid microstructures. Roughness treatment was then integrated with these imprinting structures on patterned sapphire substrate (PSS) LEDs. An approximate 33% improvement in light output power was obtained using the pyramid profile when compared with the planar LED. This was nearly 15% higher than that of the inverted pyramid profile. The roughness effect provided an approximate 5% efficiency enhancement. The total light enhanced efficiency increased to 85.9% by integrating the imprinting pyramid structure, PSS, and surface roughness.

The contribution of sidewall light extraction to efficiencies of polygonal light-emitting diodes shaped with laser micromachining

The light extraction efficiencies of InGaN/GaN blue light-emitting diodes (LEDs) of different geometries ranging from a triangle to a decagon have been simulated by ray-tracing. The conventional rectangular LED was found to be the most inefficient among the investigated polygons, and light extraction through the device sidewalls was the key factor. The results were experimentally verified by fabricating LEDs shaped into polygons by nanosecond-pulsed laser micromachining, which proved the simulated results. The mechanism of light extrac-tion in polygonal LEDs is discussed in detail.

Mechanism of Bragg Diffraction-Assisted Light Extraction in GaN-based Light-Emitting Diodes Based on a Self-Consistent Model

By introducing the distribution of the light energy density in GaN-based light-emitting diode (LED), the LED model based on the incoherent regime and the light extraction efficiency are investigated. The energy density as a function of the angle of incidence is calculated to demonstrate the mechanism of the light extraction. The deviation between the tendencies of the transmissivity of the output layer and the extraction efficiency is also demonstrated.

High external emission efficiency in intentionally ordered GaInP/GaAs structures

The authors report on an additional effect of the intentionally ordered GaInP layers obtained by using the double tilt GaAs substrates, i.e., the misoriented (001) GaAs substrates toward the two directions of [110] and [−110]. In photoluminescence (PL) spectra at 7 K, it is found that the emission intensity is enhanced with the increase in the substrate misorientation angle toward [−110] direction, along with the redshift of the bandedge. However, the redshift of the bandedge means that the GaInP layers suffer from the ordering of group III atoms. The authors note that the surface morphology in the ordered GaInP layers becomes rapidly rough by the substrate misorientation, even though the linewidths of their PL spectra and x-ray rocking curves are almost similar, irrespective of the misorientation angle. As a result, the light extraction mechanism in the roughened surface structure and the surface roughening mechanism in the intentionally ordered GaInP layers are suggested.