Efficiency droop suppression in GaN-based light-emitting diodes by chirped multiple quantum well structure at high current injection

Gallium nitride （GaN） based light-emitting diodes （LEDs） with chirped multiple quantum well （MQW） structures have been investigated experimentally and numerically in this paper. Compared to conventional LEDs with uniform quantum wells （QWs）, LEDs with chirped MQW structures have better internal quantum efficiency （IQE） and carrier injection efficiency. The droop ratios of LEDs with chirped MQW structures show a remarkable improvement at 600 mA/mm2, reduced down from 28.6% （conventional uniform LEDs） to 23.7% （chirped MQWs-a） and 18.6% （chirped MQWs-b）, respectively. Meanwhile, the peak IQE increases from 76.9% （uniform LEDs） to 83.7% （chirped MQWs-a） and 88.6% （chirped MQWs-b）. The reservoir effect of chirped MQW structures is the significant reason as it could increase hole injection efficiency and radiative recombination. The leakage current and Auger recombination of chirped MQW structures can also be suppressed. Furthermore, the chirped MQWs-b structure with lower potential barriers can enhance the reservoir effect and obtain further improvement of the carrier injection efficiency and radiative recombination, as well as further suppressing efficiency droop.

Measurement of Diffusion Coefficient of Liquids by Using an Asymmetric Liquid-Core Cylindrical Lens： Observing the Diffusion Process Directly

We report a method for measuring diffusion coefficient D of liquids by using an aplanatic and asymmetric cylinder lens with a liquid core, which is designed as both a diffusion pool and the main imaging element. The precision is better than 10-4 RIU in measuring refractive index. The D values of ethylene glycol （EG） in water are measured for various EG concentrations at 25℃, and Dinf = 1.043 × 10^-5 cm^2/s under the condition of infinite dilution is obtained. The method is characterized by observing the diffusion process directly, faster measurement and obtaining the D value under the condition of infinite dilution.

Current spreading in GaN-based light-emitting diodes

We have investigated the factors affecting the current spreading length（CSL） in GaN-based light-emitting diodes（LEDs） by deriving theoretical expressions and performing simulations with APSYS.For mesa-structure LEDs,the effects of both indium tin oxide（ITO） and n-GaN are taken into account for the first time,and a new Q factor is introduced to explain the effects of different current flow paths on the CSL.The calculations and simulations show that the CSL can be enhanced by increasing the thickness of the ITO layer and resistivity of the n-GaN layer,or by reducing the resistivity of the ITO layer and thickness of the n-GaN layer.The results provide theoretical support for calculating the CSL clearly and directly.For vertical-structure LEDs,the effects of resistivity and thickness of the CSL on the internal quantum efficiency（IQE） have been analyzed.The theoretical expression relating current density and the parameters（resistivity and thickness）of the CSL is obtained,and the results are then verified by simulation.The IQE under different current injection conditions is discussed.The effects of CSL resistivity play a key role at high current injection,and there is an optimal thickness for the largest IQE only at a low current injection.

Tailoring chaotic motion of microcavity photons in ray and wave dynamics by tuning the curvature of space

Microcavity photon dynamics in curved space is an emerging interesting area at the crossing point of nanophotonics,chaotic science,and non-Euclidean geometry.We report the sharp difference between the regular and chaotic motions of cavity photons subjected to the varying space curvature.While the island modes of regular motion rise in the phase diagram in the curved space,the chaotic modes show special mechanisms to adapt to the space curvature,including the fast diffusion of ray dynamics,and the localization and hybridization of the Husimi wavepackets among different periodic orbits.These observations are unique effects enabled by the combination of the chaotic trajectory,the wave nature of light,and the non-Euclidean orbital motion,and therefore make the system a versatile optical simulator for chaotic science under quantum mechanics in curved space-time.

A High-Throughput Phenotyping System Using Machine Vision to Quantify Severity of Grapevine Powdery Mildew

Powdery mildews present specific challenges to phenotyping systems that are based on imaging.Having previously developed lowthroughput,quantitative microscopy approaches for phenotyping resistance to Erysiphe necator on thousands of grape leaf disk samples for genetic analysis,here we developed automated imaging and analysis methods for E.necator severity on leaf disks.By pairing a 46-megapixel CMOS sensor camera,a long-working distance lens providing 3.5×magnification,X-Y sample positioning,and Z-axis focusing movement,the system captured 78%of the area of a 1-cm diameter leaf disk in 3 to 10 focus-stacked images within 13.5 to 26 seconds.Each image pixel represented 1.44 m2 of the leaf disk.A convolutional neural network(CNN)based on GoogLeNet determined the presence or absence of E.necator hyphae in approximately 800 subimages per leaf disk as an assessment of severity,with a training validation accuracy of 94.3%.For an independent image set the CNN was in agreement with human experts for 89.3%to 91.7%of subimages.This live-imaging approach was nondestructive,and a repeated measures time course of infection showed differentiation among susceptible,moderate,and resistant samples.Processing over one thousand samples per day with good accuracy,the system can assess host resistance,chemical or biological efficacy,or other phenotypic responses of grapevine to E.necator.In addition,new CNNs could be readily developed for phenotyping within diverse pathosystems or for diverse traits amenable to leaf disk assays.

Analysis of TM/TE mode enhancement and droop reduction by a nanoporous n-AIGaN underlayer in a 290 nm UV-LED

A full structure 290 nm ultraviolet light emitting diode(UV-LED)with a nanoporous n-AlGaN underlayer was fabricated by top via hole formation followed by high-voltage electrochemical etching.The 20 to 120 nm nanopores were prepared in regular doped n-AlGaN by adjusting the etching voltage.The comparison between the Raman spectrum and the photoluminescence wavelength shows that the biaxial stress in the nanoporous material is obviously relaxed.The photoluminescence enhancement was found to be highly dependent on the size of the pores.It not only improves the extraction efficiency of top-emitting transverse-electric(TE)-mode photons but also greatly improves the efficiency of side emitting tran sverse-magnetic(TM)-mode photons.This leads to the polarization change of the side-emitting light from-0.08 to-0.242.The intensity of the electroluminescence was increased by 36.5%at 100 mA,and the efficiency droop at high current was found to decrease from 61%to 31%.

Realization of directional single-mode lasing by a GaN-based warped microring

Multimode and random directionalities are major issues restricting the application of whispering gallery mode microcavity lasers.We demonstrated a 40μm diameter microring with an off-centered embedded hole and warped geometry from strained III-nitride quantum well multilayers.Single-mode directional whispering gallery mode lasing was achieved by the warped structure and high-order mode suppression induced by the off-centered hole.In addition,the introduction of the off-centered hole reduced the lasing threshold from 3.24 to 2.79 MW/cm^(2) compared with the warped microdisk without an embedded hole while maintaining a high-quality factor of more than 4000.Directional light emission in 3D was achieved and attributed to the warped structure,which provides a vertical component of the light emission,making it promising for building multifunctional coherent light sources in optoelectronic integration.

Broadband high-efficiency polymerized liquid crystal metasurfaces with spin-multiplexed functionalities in the visible

Traditional optical components are usually designed for a single functionality and narrow operation band,leading to the limited practical applications.To date,it is still quite challenging to efficiently achieve multifunctional performances within broadband operating bandwidth via a single planar optical element.Here,a broadband high-efficiency polarization-multiplexing method based on a geometric phase polymerized liquid crystal metasurface is proposed to yield the polarization-switchable functionalities in the visible.As proofs of the concept,two broadband high-efficiency polymerized liquid crystal metalenses are designed to obtain the spin-controlled behavior from diffraction-limited focusing to sub-diffraction focusing or focusing vortex beams.The experimental results within a broadband range indicate the stable and excellent optical performance of the planar liquid crystal metalenses.In addition,low-cost polymerized liquid crystal metasurfaces possess unique superiority in large-scale patterning due to the straightforward processing technique rather than the point-by-point nanopatterning method with high cost and low throughput.The high-efficiency liquid crystal metasurfaces also have unrivalled advantages benefiting from the characteristic with low waveguide absorption.The proposed strategy paves the way toward multifunctional and high-integrity optical systems,showing great potential in mobile devices,optical imaging,robotics,chiral materials,and optical interconnections.

Phosphor-free microLEDs with ultrafast and broadband features for visible light communications

Modulation bandwidth and the emission region are essential features for the widespread use of visible light communications(VLC).This paper addresses the contradictory requirements to achieve broadband and proposes ultrafast,asymmetric pyramids grown on adjacent deep concave holes via lateral overgrowth.Multicolor emission with an emission region between 420 nm and 600 nm is obtained by controlling the growth rate at different positions on the same face,which also can provide multiple subcarrier frequency points for the employment of wavelength division multiplexing technology.The spontaneous emission rate distinction is narrowed by lowering the number of the crystal plane,ensuring a high modulation bandwidth over broadband.More importantly,the residual stress and dislocation density were minimized by employing a patterned substrate,and lateral overgrowth resulted in a further enhancement of the recombination rate.Finally,the total modulation bandwidth of multiple subcarriers of the asymmetric pyramids is beyond GHz.These ultrafast,multicolor microLEDs are viable for application in VLC systems and may also enable applications for intelligent lighting and display.

LED和HID园艺灯具测试报告(上)

伦斯勒理工学院照明研究中心(LRC)通过与高压钠灯(HPS)和金属卤化物灯(MH)园艺灯具进行对比测试,对发光二极管(LED)园艺灯具的能耗和经济性进行了评估。依据照明研究中心2016年进行的在线调查和文献综述的结果,该中心项目团队开发了评估和对比园艺灯具的测试框架,框架包括测试、评估和报告方法等内容。基于等量光合光子通量密度(PPFD)条件下实现了对灯具的比较测试。植物的PPFD类似于建筑应用中工作表面明视照度指标。该框架包括11种灯具的指标和5种特定应用指标的分析,这些指标为种植户提供了评估园艺灯具性能的参考信息。照明研究中心使用该框架测试和评估了13只园艺灯具,包括10只LED灯、2只高压钠灯和1只金属卤化物灯产品。(本报告结果均是对每种模型的单只灯具样品进行基于电学和光度学的测试。该项目未进行寿命测试,也未进行作物种植中灯具的评估)。首先,照明研究中心对单只灯具进行光度学测试。然后,以建模的方式建立灯具在模拟的温室中应用的模型,估测要达到最低的PPFD和均匀性标准所需的灯具数量以及照明系统能耗。照明研究中心发现,要保持原有PPFD,LED园艺灯具不能以一对一的形式替代高压钠灯。通常,获得与典型高压钠灯布局相同的PPFD,需要更多LED园艺灯具才能实现。结果表明,强度分布起着重要作用,其中2只被测试的LED灯具有高于高压钠灯的灯具光效,但是在温室应用中仍然有更高的总功率要求。照明研究中心发现,与高压钠灯相比,LED灯具阴影增加是由于其尺寸相对较大造成的,而且温室中为获得相同PPFD需要更多的LED灯具。LED灯具的阴影使温室中的日光减少了13%～55%,而高压钠灯灯具仅使日光减少了5%,因此,有日光时LED系统光源可能需要更多的电能。通常,更多数量的LED灯具以及同等应用效能的电力需求会影响方案的全生命周期成本。照明研究中心发现,有3只被测试的LED园艺灯照明系统的生命周期成本较低,其余7只LED园艺灯的生命周期成本都高于2只被测试的1 000 W高压钠灯照明系统。评估结果表明,单独考虑灯具效率可能会影响评估结果。在达到PPFD标准时,灯具光强分布和布局对进行精确的寿命周期成本分析十分必要。该报告提供了一个技术中立的分析框架,相关各方可用其对照明系统进行评估。

LED和HID园艺灯具测试报告(下)

(6)光合光子强度分布缩写/符号:Ip单位:μmol/(sr·s)光合光强分布是在最大光强的两个二维平面上显示光合光子强度的空间分布。“蝙蝠翼”型的光合光子强度分布可以在更宽区域范围提供更均匀的PPFD(垂直角较高强度较高),这是照明布局的典型目标。