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Frontiers of Optoelectronics

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, Volume 10 Issue 4 Previous Issue   
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Inkjet printing for electroluminescent devices:emissive materials, film formation, and display prototypes
Luhua LAN, Jianhua ZOU, Congbiao JIANG, Benchang LIU, Lei WANG, Junbiao PENG
Front. Optoelectron.. 2017, 10 (4): 329-352.
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Inkjet printing (IJP) is a versatile technique for realizinghigh-accuracy patterns in a cost-effective manner. It is consideredto be one of the most promising candidates to replace the expensivethermal evaporation technique, which is hindered by the difficultyof fabricating low-cost, large electroluminescent devices, such asorganic light-emitting diodes (OLEDs) and quantum dot light-emittingdiodes (QLEDs). In this invited review, we first introduce the recentprogress of some printable emissive materials, including polymers,small molecules, and inorganic colloidal quantum dot emitters in OLEDsand QLEDs. Subsequently, we focus on the key factors that influencefilm formation. By exploring stable ink formulation, selecting printparameters, and implementing droplet deposition control, a uniformfilm can be obtained, which in turn improves the device performance.Finally, a series of impressive inkjet-printed OLEDs and QLEDs prototypedisplay panels are summarized, suggesting a promising future for IJPin the fabrication of large and high-resolution flat panel displays.

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Optical design of rectangular illumination with freeform lenses for the application of LED road lighting
Chunyun XU, Haobo CHENG, Yunpeng FENG
Front. Optoelectron.. 2017, 10 (4): 353-362.
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We present a freeform lens for application to light-emitting diodes (LED) road lighting. We propose a simple source–target luminous intensity mapping method based on Snell’s law and geometric-optics analysis. We calculated different contours of cross-sections to construct a freeform lens with a smooth surface. The computer simulation results show that the lighting performance of a single freeform lens is not sufficient for road lighting. For the road lamp simulation, we adopted an oval arrangement of freeform lenses on a printed circuit board. In addition, we performed tolerance analysis to determine the tolerance limits of manufacturing and installation errors. A road lamp at a height of 12 m can create rectangular illumination with an area of 40 m × 12 m, 69.7% uniformity, and average illuminance of 24.6 lux. This lighting performance can fully comply with the urban road lighting design standard.

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Diluted magnetic characteristics of Ni-doped AlN films via ion implantation
Chong ZHAO, Qixin WAN, Jiangnan DAI, Jun ZHANG, Feng WU, Shuai WANG, Hanling LONG, Jingwen CHEN, Cheng CHEN, Changqing CHEN
Front. Optoelectron.. 2017, 10 (4): 363-369.
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The structural and magnetic properties, as well as the mechanism of magnetization, of Ni-implanted AlN films were studied. AlN was deposited on Al2O3 substrates by metalorganic chemical vapor deposition (MOCVD), and subsequently Ni ions were implanted into the AlN films by Metal Vapor Arc (MEVVA) sources at an energy of 100 keV for 3 h. The films were annealed at 900°C for 1 h in the furnace in order to transfer the Ni ions from interstitial sites to substitutional sites in AlN, thus activating the Ni3+ ions. Characterizations were performed in situ using X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), and vibrating sample magnetometry (VSM), which showed that the films have a wurtzite structure without the formation of a secondary phase after implanting and annealing. Ni ions were successfully implanted into substitutional sites of AlN films, and the chemical bonding states are Ni-N. The apparent hysteresis loops prove that the films exhibited magnetism at 300 K. The room temperature (RT) saturation magnetization moment (Ms) and coercivity (Hc) values were about 0.36 emu/g and 35.29 Oe, respectively. From the first-principles calculation, a total magnetic moment of 2.99 μB per supercell is expected, and the local magnetic moment of a NiN4 tetrahedron, 2.45 μB, makes the primary contribution. The doped Ni atom hybridizes with four nearby N atoms in a NiN4 tetrahedron; then the electrons of the N atoms are spin-polarized and couple with the electrons of the Ni atom with strong magnetization, which results in magnetism. Therefore, the p-d exchange mechanism between Ni-3d and N-2p can be the origin of the magnetism. It is expected that these room temperature, ferromagnetic, Ni-doped AlN films will have many potential applications as diluted magnetic semiconductors.

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Design and simulation to improve the structural efficiency of green light emission of GaN/InGaN/AlGaN light emitting diode
Sakhawat HUSSAIN, Tasnim ZERIN, Md. Ashik KHAN
Front. Optoelectron.. 2017, 10 (4): 370-377.
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This study considered the design of an efficient, high brightness polar InGaN/GaN light emitting diode (LED) structure with AlGaN capping layer for green light emission. The deposition of high In (>15%) composition within InGaN quantum well (QW) has limitations when providing intense green light. To design an effective model for a highly efficient InGaN green LEDs, this study considered the compositions of indium and aluminum for InxGa1−xN QW and AlyGa1−yN cap layers, along with different layer thicknesses of well, barrier and cap. These structural properties significantly affect different properties. For example, these properties affect electric fields of layers, polarization, overall elastic stress energy and lattice parameter of the structure, emission wavelength, and intensity of the emitted light. Three models with different composition and layer thicknesses are simulated and analyzed to obtain green light with in-plane equilibrium lattice parameter close to GaN (3.189 Å ) with the highest oscillator strength values. A structure model is obtained with an oscillator strength value of 1.18×10−1 and least in-plane equilibrium lattice constant of 3.218 Å. This emitter can emit at a wavelength of 540 nm, which is the expected design for the fabrication of highly efficient, bright green LEDs.

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Extracting the time delay signature of coupled optical chaotic systems by mutual statistical analysis
Xinhua ZHU, Mengfan CHENG, Lei DENG, Xingxing JIANG, Deming LIU
Front. Optoelectron.. 2017, 10 (4): 378-387.
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The time delay (TD) signature is a critical parameter in optical chaos-based applications. The feasibility of extracting the TD has been a crucial issue that significantly influences the performance of these applications. In this paper, statistical analyses have been conducted to extract the TD signatures from different types of coupled optical chaos systems. More specifically, a mutually coupled semiconductor laser chaotic system, an intensity-coupled electro-optic chaotic system, and a phase-coupled electro-optic chaotic system are studied in detail. These systems are proposed to resist the attack strategies against the TD signature. They are proved to be effective under statistical analyzes, such as the self-correlation function (SF) and mutual information (MI). However, only a single output has been considered for the attack process in the existing research. We demonstrated that the TD signature can still be extracted by analyzing the mutual statistical relationship between the different output signals which are generated simultaneously by the coupled system. Furthermore, we find that the extraction strategy is effective for a wide parameter range in these schemes.

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Waveform LiDAR signal denoising based on connected domains
Liyu SUN, Zhiwei DONG, Ruihuan ZHANG, Rongwei FAN, Deying CHEN
Front. Optoelectron.. 2017, 10 (4): 388-394.
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The streak tube imaging light detection and ranging (LiDAR) is a new type of waveform sampling laser imaging radar whose echo signals are stripe images with a high frame rate. In this study, the morphological and statistical characteristics of stripe signals are analyzed in detail. Based on the concept of mathematical morphology denoising, connected domains are constructed in a noise-containing stripe image, and the noise is removed using the difference in connected domains area between signals and noises. It is shown that, for stripe signals, the proposed denoising method is significantly more efficient than Wiener filtering.

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Synthesis of porous TiO2 nanowires and their photocatalytic properties
Yonglun TANG, Haibo REN, Jiarui HUANG
Front. Optoelectron.. 2017, 10 (4): 395-401.
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Porous titanium dioxide (TiO2) nanowires were synthesized via a surfactant-free hydrothermal method followed by acid-washing process and calcination. The structures and morphologies of products were characterized by field emission scanning electron microscopy (FESEM), Transmission electron microscopy (TEM), X-ray diffraction (XRD), and Brunauer-Emmett-Teller (BET) N2 adsorption-desorption analyses. The analysis of FESEM suggested the precursor was composed of a vast of uniform nanostructures like wires. The nanowire-like precursor was transformed into the porous nanowire after acid-treatment and calcination at 500°C for 2 h in air. The surface area of as-synthesized TiO2 nanowires calculated by BET is 86.4 m2/g. Furthermore, the photocatalytic properties of synthesized porous TiO2 nanowires were evaluated through the degradation of methylene blue (MB) and Rhodamine B (RhB). The results clearly suggested that the as-prepared porous TiO2 nanowires showed remarkable photocatalytic performance on the degradation of RhB and MB due to their small size of nanocrystallites and the porous naonstructure.

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Indium tin oxide-free inverted polymer solar cells with ultrathin metal transparent electrodes
Tao YUAN, Zhonghuan CAO, Guoli TU
Front. Optoelectron.. 2017, 10 (4): 402-408.
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Efficient indium tin oxide (ITO)-free inverted polymer solar cells (PSCs) were fabricated by applying ultrathin metal transparent electrodes as sunlight incident electrodes. Smooth and continuous Ag film of 4 nm thickness was developed through the introduction of a 2 nm Au seed layer. Ultrathin Ag transparent electrode with an average transmittance of up to 80% from 480 to 680 nm and a sheet resistance of 35.4 W/sq was obtained through the introduction of a ZnO anti-reflective layer. The ultrathin metal electrode could be directly used as cathode in polymer solar cells without oxygen plasma treatment. ITO-free inverted PSCs obtained a power conversion efficiency (PCE) of 5.2% by utilizing the ultrathin metal transparent electrodes. These results demonstrated a simple method of fabricating ITO-free inverted PSCs.

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Terahertz frequency characterization of anisotropicstructure of tourmaline
Weichong TANG, Zili ZHANG, Ke XIAO, Changchun ZHAO, Zhiyuan ZHENG
Front. Optoelectron.. 2017, 10 (4): 409-413.
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The absorption coefficient and refractive index of tourmalinein different directions were characterized for the first time usingterahertz time-domain spectroscopy. Results show that the absorptionand refractive index of terahertz frequency are related to the structureof tourmaline. Absorption along the optical axis direction is moresensitive than that along the vertical direction. This result indicatesthat the identification and characterization of crystals as well asminerals can be realized by the terahertz method.

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