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Frontiers of Optoelectronics

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, Volume 10 Issue 2 Previous Issue   
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RESEARCH ARTICLE
Hole-transporting layer-free inverted planar mixed lead-tin perovskite-based solar cells
Yuqin LIAO, Xianyuan JIANG, Wenjia ZHOU, Zhifang SHI, Binghan LI, Qixi MI, Zhijun NING
Front. Optoelectron.. 2017, 10 (2): 103-110.   DOI: 10.1007/s12200-017-0716-6
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Mixed lead-tin (Pb-Sn) perovskites present a promising strategy to extend the light-harvesting range of perovskite-based solar cells (PSCs). The use of electron-transporting layer or hole-transporting layer (HTL) is critical to achieve high device efficiency. This strategy, however, requires tedious layer-by-layer fabrication as well as high-temperature annealing for certain oxides. In this work, we fabricated HTL-free planar FAPb0.5Sn0.5I3 PSCs with the highest efficiency of 7.94%. High short-circuit current density of 23.13 mA/cm2 was attained, indicating effective charge extraction at the ITO/FAPb0.5Sn0.5I3 interface. This finding provides an alternative strategy to simplify the manufacture of single-junction or tandem PSCs.

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Basic properties of a new Nd-doped laser crystal: Nd:GdNbO4
Shoujun DING, Qingli ZHANG, Wenpeng LIU, Jianqiao LUO, Dunlu SUN
Front. Optoelectron.. 2017, 10 (2): 111-116.   DOI: 10.1007/s12200-017-0715-7
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A Nd-doped GdNbO4 single crystals have been grown successfully using the Czochralski technique. The chemical etching method was employed to study the defects in the structural morphology of Nd:GdNbO4 crystal with phosphoric acid etchant. Mechanical properties (such as hardness, yield strength, fracture toughness, and brittle index) of the as-grown crystal were systematically estimated on the basis of the Vickers hardness test for the first time. The transmission spectrum of Nd:GdNbO4 was measured in the wavelength range of 320–2400 nm at room temperature, and the absorption peaks were assigned. Results hold great significance for further research on Nd:GdNbO4.

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Fabricate organic thermoelectric modules use modified PCBM and PEDOT:PSS materials
Feng GAO, Yuchun LIU, Yan XIONG, Ping WU, Bin HU, Ling XU
Front. Optoelectron.. 2017, 10 (2): 117-123.   DOI: 10.1007/s12200-017-0712-x
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In this paper, we fabricated an organic thermoelectric (TE) device with modified [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS); the device showed good stability in air condition. For n-leg, PCBM were doped with acridine orange base (3,6-bis(dimethylamino)acridine) (AOB) and 1,3-dimethyl-2,3-dihydro-1H-benzoimidazole (N-DMBI). Co-doped PCBM utilizes synergistic effects of AOB and N-DMBI, resulting in excellent electrical conductivity and Seebeck coefficient values reaching 2 S/cm and -500 mV/K, respectively, at room temperature with dopant molar ratio of 0.11. P-type leg used modified PEDOT:PSS. Based on modified PCBM and PEDOT:PSS materials, we fabricated a TE module device with 48 p-type and n-type thermocouple and tested their output voltage, short current, and power. Output voltage measured ~0.82 V, and generated power reached almost 945 mW with 75 K temperature gradient at 453 K hot-side temperature. These promising results showed potential of modified PEDOT and PCBM as TE materials for application in device optimization.

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Broadband and conformal metamaterial absorber
Xiangkun KONG, Junyi XU, Jin-jun MO, Shaobin LIU
Front. Optoelectron.. 2017, 10 (2): 124-131.   DOI: 10.1007/s12200-017-0682-z
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In this study, a new broadband and conformal metamaterial absorber using two flexible substrates was proposed. Simulation results showed that the proposed absorber exhibited an absorption band from 6.08 to 13.04 GHz and a high absorption of 90%, because it was planar. The absorber was broadband as its relative absorption bandwidth was 72.8%. Moreover, the proposed absorber was insensitive to the polarization of the TE and TM waves. The absorber was ultra-thin; its total thickness was only 0.07λ at the lowest operating frequency. Furthermore, different regions of absorption can be adjusted by lumping and loading two resistors onto the polyimide film, respectively. Moreover, compared with the conventional microwave absorber, the absorption bandwidth of the proposed absorber can be broadened and enhanced when it was bent and conformed to the surface of objects. Experimental and simulation results were in agreement. The proposed absorber is a promising absorbing element in scientific and technical applications because of its broadband absorption, polarization insensitivity, and flexible substrates.

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Characterization of the cooling process of solid n-alkanes via terahertz spectroscopy
Chen JIANG, Honglei ZHAN, Kun ZHAO, Cheng FU
Front. Optoelectron.. 2017, 10 (2): 132-137.   DOI: 10.1007/s12200-017-0681-0
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The terahertz (THz) time-domain spectroscopy technique was used to characterize the cooling process of solid n-alkanes. The THz waveforms of n-octadecane, n-nonadecane, n-eicosane, n-heneicosane, n-docosane, and n-pentacosane were obtained with the cooling time using the aforementioned noncontact optical method. The peak values of the THz signal were found to be related to the cooling temperature of n-alkanes. The THz wave was sensitive to the size and structure of particles in the liquid; therefore, the crystallization process of n-alkanes was monitored. An empirical equation based on signal attenuation was proposed to quantitatively distinguish the content change of structural order in the samples. Results present a new noncontact optical approach for characterizing wax crystallization via THz time-domain spectroscopy.

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Coupled two aluminum nanorod antennas for near-field enhancement
Yan DENG, Jian OU, Jiangying YU, Min ZHANG, Li ZHANG
Front. Optoelectron.. 2017, 10 (2): 138-143.   DOI: 10.1007/s12200-017-0663-2
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Aluminum (Al) plasmonic nanoantennas possess many tunabilities in the ultraviolet (UV) region and have a variety of new applications, such as in sensitive UV photodetection and UV photolithography. Using discrete dipole approximation (DDA), the resonant optical properties and enhanced local field distribution of coupled Al nanorod antennas were investigated. The effects of gap distance on the extinction spectra were analyzed to obtain the surface plasmon modes of these nanostructures across the visible and in the UV spectral range, which can be attributed to the coupling of the surface plasmon modes from each Al nanorod. In addition, the enhanced local field factors plotted as a function of gap distance were simulated under transverse and longitudinal polarizations to achieve maximum near-field enhancement for the optical antennas. When the gap distance was decreased to 5 nm, the maximum value of the enhanced factor was 18.04 at the transverse mode peak of 424 nm. This could be explained by the combination of the interaction between the charges distributed at the opposite ends of two Al nanorods and the interaction between the charges distributed at the lateral sides of each Al nanorod. Results showed that the coupled Al nanorod antennas with enhanced local field show promise for UV plasmonics.

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Nested microring resonator with a doubled free spectral range for sensing application
Xin ZHANG, Jiawen JIAN, Han JIN, Peipeng XU
Front. Optoelectron.. 2017, 10 (2): 144-150.   DOI: 10.1007/s12200-017-0670-3
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The microring resonator has received increasing attention in the optical sensing application because of its micro-size, optical property, and high sensitivity. An additional waveguide is commonly used to change the output spectra in the early research on microring resonators. In this study, we proposed a nested microring resonator that doubles the free spectral range (FSR) compared with the conventional single microring. This structure improved the sensing property as the FSR in the filter output spectra could be considered as a measurement range in the microring sensor. Moreover, the parameters including the coupling coefficient of the three coupling sections, length of the U-bend waveguide, and effective index of a waveguide were tested and carefully selected to optimize the sensing properties. The relationship between these parameters and the output spectra was demonstrated. With linear sensitivity, the structure has a good potential in sensing application.

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Defect detection on button surfaces with the weighted least-squares model
Yu HAN, Yubin WU, Danhua CAO, Peng YUN
Front. Optoelectron.. 2017, 10 (2): 151-159.   DOI: 10.1007/s12200-017-0687-7
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Defect detection is important in quality assurance on production lines. This paper presents a fast machine-vision-based surface defect detection method using the weighted least-squares model. We assume that an inspection image can be regarded as a combination of a defect-free template image and a residual image. The defect-free template image is generated from training samples adaptively, and the residual image is the result of the subtraction between each inspection image and corresponding defect-free template image. In the weighted least-squares model, the residual error near the edge is suppressed to reduce the false alarms caused by spatial misalignment. Experiment results on different types of buttons show that the proposed method is robust to illumination vibration and rotation deviation and produces results that are better than those of two other methods.

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Generation and transmission analysis of 4-ary frequency shift keying based on dual-parallel Mach-Zehnder modulator
Liu YANG, Fengguang LUO
Front. Optoelectron.. 2017, 10 (2): 160-165.   DOI: 10.1007/s12200-016-0637-9
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We proposed an optical 4-ary frequency shift keying (FSK) modulation scheme applying dual-parallel Mach-Zehnder (MZ) modulator. The 4-ary FSK based on the single-side-band modulation scheme can greatly lower the transmission speed in each sub-carriers and increase the transmission performance, comparing with the 2-FSK signal. The transmission performance of the 4-ary FSK was demonstrated after a 50 km single mode fiber. The results showed that the 4-ary FSK can realize error-free transmission. Moreover, we analyzed the influence of factors (such as disperse compensation and demodulation bandwidth) on the transmission performance in this paper. The analysis of the influenced factors can provide a theoretical basic for experiment.

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Off-axis two-mirror laser communication antenna designed using differential equations
Chunqiu XIA, Xing ZHONG
Front. Optoelectron.. 2017, 10 (2): 166-173.   DOI: 10.1007/s12200-017-0710-z
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In satellite laser communication technology, which is built between planets and between a planet and the Earth, the optical antenna is the key point. Thus, research on optical system design is important. The off-axis reflective system has no obscuration and hence possesses a high efficiency for energy transfer. This study proposes a novel method for designing a free-form off-axis reflective imaging system. This study also introduces differential equations that depend on Fermat’s principle and sine condition. Finally, a free-form off-axis two-mirror optical system was designed by using the differential equation method. This system includes one intermediate image plane, in which the field of view (FOV) is −5° to −4° in the y-axis and −1° to 0° in the x-axis. The modulation transfer function was greater than 0.7 at 50 lp/mm, and the efficiency of energy transmission was high. The free-form off-axis two-mirror optical system involves a wide range of application prospects in the optical antenna used in the satellite laser communication systems. Moreover, the design method that used differential equations was proven simple and effective.

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Dynamic spot tracking system based on 2D galvanometer in free space optical communication for short distance
Qingshan JIANG, Ciling ZENG, Fengqiang GU, Ming ZHAO
Front. Optoelectron.. 2017, 10 (2): 174-179.   DOI: 10.1007/s12200-016-0638-8
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Dynamic tracking of laser spot is a key process in the establishment of free space optical communication. In this paper, a dynamic tracking system was presented. In this system, a two-dimensional (2D) galvanometer was used to change the angle of the optical axis of the incident beam at a certain scanning frequency as optical signal jitter simulator, and another galvanometer was used to track the jitter with quadrant detector (QD) and data processing module to acquire the position information of laser spot. Results indicated that the tracking accuracy of this system mainly composed of 2D galvanometer was as high as 27.8 μrad, and its linear deviation was less than 0.013. The system could still keep the dynamic tracking of the spot stable when the jitter frequency of the optical signal was less than 1000 Hz. Those results suggested that this system could be suitable for the short distance in free space communication due to its simple structure, easy to control and low cost compared with conventional system.

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Theoretical demonstration of all-optical switchable and tunable UWB doublet pulse train generator utilizing SOA wavelength conversion and tunable time delay
Zhefeng HU, Jianhui XU, Min HOU
Front. Optoelectron.. 2017, 10 (2): 180-188.   DOI: 10.1007/s12200-017-0711-y
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An all-optical ultrawide band (UWB) doublet pulse train signal generator is proposed and theoretically simulated by utilizing an inverse wavelength conversion base on the cross-gain modulation (XGM) effect in a semiconductor optical amplifier (SOA) and controllable time delay in two optical delay lines (ODLs). The proposed scheme is not only optically switchable in the polarity of pulse by switching the polarity of input pulse but also tunable in signal pulse width and radiofrequency (RF) spectrum by tuning the ODLs.

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Edge preserving super-resolution infrared image reconstruction based on L1- and L2-norms
Shaosheng DAI, Dezhou ZHANG, Junjie CUI, Xiaoxiao ZHANG, Jinsong LIU
Front. Optoelectron.. 2017, 10 (2): 189-194.   DOI: 10.1007/s12200-016-0659-3
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Super-resolution (SR) is a widely used technology that increases image resolution using algorithmic methods. However, preserving the local edge structure and visual quality in infrared (IR) SR images is challenging because of their disadvantages, such as lack of detail, poor contrast, and blurry edges. Traditional and advanced methods maintain the quantitative measures, but they mostly fail to preserve edge and visual quality. This paper proposes an algorithm based on high frequency layer features. This algorithm focuses on the IR image edge texture in the reconstruction process. Experimental results show that the mean gradient of the IR image reconstructed by the proposed algorithm increased by 1.5, 1.4, and 1.2 times than that of the traditional algorithm based on L1-norm, L2-norm, and traditional mixed norm, respectively. The peak signal-to-noise ratio, structural similarity index, and visual effect of the reconstructed image also improved.

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