Frontiers of Optoelectronics

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Review of fabrication methods of large-area transparent graphene electrodes for industry
Petri MUSTONEN, David M. A. MACKENZIE, Harri LIPSANEN
Front. Optoelectron.    https://doi.org/10.1007/s12200-020-1011-5
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Graphene is a two-dimensional material showing excellent properties for utilization in transparent electrodes; it has low sheet resistance, high optical transmission and is flexible. Whereas the most common transparent electrode material, tin-doped indium-oxide (ITO) is brittle, less transparent and expensive, which limit its compatibility in flexible electronics as well as in low-cost devices. Here we review two large-area fabrication methods for graphene based transparent electrodes for industry: liquid exfoliation and low-pressure chemical vapor deposition (CVD). We discuss the basic methodologies behind the technologies with an emphasis on optical and electrical properties of recent results. State-of-the-art methods for liquid exfoliation have as a figure of merit an electrical and optical conductivity ratio of 43.5, slightly over the minimum required for industry of 35, while CVD reaches as high as 419.

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Rapid and simultaneous analysis of multiple wine quality indicators through near-infrared spectroscopy with twice optimization for wavelength model
Jiemei CHEN, Sixia LIAO, Lijun YAO, Tao PAN
Front. Optoelectron.    https://doi.org/10.1007/s12200-020-1005-3
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Alcohol, total sugar, total acid, and total phenol contents are the main indicators of wine quality detection. This study aims to establish simultaneous analysis models for the four indicators through near-infrared (NIR) spectroscopy with wavelength optimization. A Norris derivative filter (NDF) platform with multiparameter optimization was established for spectral pretreatment. The optimal parameters (i.e., derivative order, number of smoothing points, and number of differential gaps) were (2, 9, 3) for alcohol, (1, 19, 5) for total sugar, (1, 17, 11) for total acid, and (1, 1, 1) for total phenol. The equidistant combination-partial least squares (EC-PLS) was used for large-scale wavelength screening. The wavelength step-by-step phase-out PLS (WSP-PLS) and exhaustive methods were used for secondary optimization. The final optimization models for the four indicators included 7, 10, 15, and 13 wavelengths located in the overtone or combination regions, respectively. In an independent validation, the root mean square errors, correlation coefficient for prediction (i.e., SEP and RP), and ratio of performance-to-deviation (RPD) were 0.41 v/v, 0.947, and 3.2 for alcohol; 1.48 g/L, 0.992, and 6.8 for total sugar; 0.68 g/L, 0.981, and 5.1 for total acid; and 0.181 g/L, 0.948, and 2.9 for total phenol. The results indicate high correlation, low error, and good overall prediction performance. Consequently, the established reagent-free NIR analytical models are important in the rapid and real-time quality detection of the wine fermentation process and finished products. The proposed wavelength models provide a valuable reference for designing small dedicated instruments.

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Highly stable and repeatable femtosecond soliton pulse generation from saturable absorbers based on two-dimensional Cu3−xP nanocrystals
Haoran MU, Zeke LIU, Xiaozhi BAO, Zhichen WAN, Guanyu LIU, Xiangping LI, Huaiyu SHAO, Guichuan XING, Babar SHABBIR, Lei LI, Tian SUN, Shaojuan LI, Wanli MA, Qiaoliang BAO
Front. Optoelectron.    https://doi.org/10.1007/s12200-020-1018-y
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Heavily doped colloidal plasmonic nanocrystals have attracted great attention because of their lower and adjustable free carrier densities and tunable localized surface plasmonic resonance bands in the spectral range from near-infra to mid-infra wavelengths. With its plasmon-enhanced optical nonlinearity, this new family of plasmonic materials shows a huge potential for nonlinear optical applications, such as ultrafast switching, nonlinear sensing, and pulse laser generation. Cu3xP nanocrystals were previously shown to have a strong saturable absorption at the plasmonic resonance, which enabled high-energy Q-switched fiber lasers with 6.1 µs pulse duration. This work demonstrates that both high-quality mode-locked and Q-switched pulses at 1560 nm can be generated by evanescently incorporating two-dimensional (2D) Cu3xP nanocrystals onto a D-shaped optical fiber as an effective saturable absorber. The 3 dB bandwidth of the mode-locking optical spectrum is as broad as 7.3 nm, and the corresponding pulse duration can reach 423 fs. The repetition rate of the Q-switching pulses is higher than 80 kHz. Moreover, the largest pulse energy is more than 120 µJ. Note that laser characteristics are highly stable and repeatable based on the results of over 20 devices. This work may trigger further investigations on heavily doped plasmonic 2D nanocrystals as a next-generation, inexpensive, and solution-processed element for fascinating photonics and optoelectronics applications.

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In vivo volumetric monitoring of revascularization of traumatized skin using extended depth-of-field photoacoustic microscopy
Zhongwen CHENG, Haigang MA, Zhiyang WANG, Sihua YANG
Front. Optoelectron.    https://doi.org/10.1007/s12200-020-1040-0
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Faster and better wound healing is a critical medical issue. Because the repair process of wounds is closely related to revascularization, accurate early assessment and postoperative monitoring are very important for establishing an optimal treatment plan. Herein, we present an extended depth-of-field photoacoustic microscopy system (E-DOF-PAM) that can achieve a constant spatial resolution and relatively uniform excitation efficiency over a long axial range. The superior performance of the system was verified by phantom and in vivo experiments. Furthermore, the system was applied to the imaging of normal and trauma sites of volunteers, and the experimental results accurately revealed the morphological differences between the normal and traumatized skin of the epidermis and dermis. These results demonstrated that the E-DOF-PAM is a powerful tool for observing and understanding the pathophysiology of cutaneous wound healing.

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Fe3O4 nanoparticle-enabled mode-locking in an erbium-doped fiber laser
Xiaohui LI, Jiajun PENG, Ruisheng LIU, Jishu LIU, Tianci FENG, Abdul Qyyum, Cunxiao GAO, Mingyuan XUE, Jian ZHANG
Front. Optoelectron.    https://doi.org/10.1007/s12200-020-1057-4
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In this paper, we have proposed and demonstrated the generation of passively mode-locked pulses and dissipative soliton resonance in an erbium-doped fiber laser based on Fe3O4 nanoparticles as saturable absorbers. We obtained self-starting mode-locked pulses with fundamental repetition frequency of 7.69 MHz and center wavelength of 1561 nm. The output of a pulsed laser has spectral width of 0.69 nm and pulse duration of 14 ns with rectangular pulse profile at the pump power of 190 mW. As far as we know, this is the first time that Fe3O4 nanoparticles have been developed as low-dimensional materials for passive mode-locking with rectangular pulse. Our experiments have confirmed that Fe3O4 has a wide prospect as a nonlinear photonics device for ultrafast fiber laser applications.

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Performance of integrated optical switches based on 2D materials and beyond
Yuhan YAO, Zhao CHENG, Jianji DONG, Xinliang ZHANG
Front. Optoelectron.    https://doi.org/10.1007/s12200-020-1058-3
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Applications of optical switches, such as signal routing and data-intensive computing, are critical in optical interconnects and optical computing. Integrated optical switches enabled by two-dimensional (2D) materials and beyond, such as graphene and black phosphorus, have demonstrated many advantages in terms of speed and energy consumption compared to their conventional silicon-based counterparts. Here we review the state-of-the-art of optical switches enabled by 2D materials and beyond and organize them into several tables. The performance tables and future projections show the frontiers of optical switches fabricated from 2D materials and beyond, providing researchers with an overview of this field and enabling them to identify existing challenges and predict promising research directions.

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3D printing of glass by additive manufacturing techniques: a review
Dao ZHANG, Xiaofeng LIU, Jianrong QIU
Front. Optoelectron.    https://doi.org/10.1007/s12200-020-1009-z
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Additive manufacturing (AM), which is also known as three-dimensional (3D) printing, uses computer-aided design to build objects layer by layer. Here, we focus on the recent progress in the development of techniques for 3D printing of glass, an important optoelectronic material, including fused deposition modeling, selective laser sintering/melting, stereolithography (SLA) and direct ink writing. We compare these 3D printing methods and analyze their benefits and problems for the manufacturing of functional glass objects. In addition, we discuss the technological principles of 3D glass printing and applications of 3D printed glass objects. This review is finalized by a summary of the current achievements and perspectives for the future development of the 3D glass printing technique.

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Experimental Hong–Ou–Mandel interference using two independent heralded single-photon sources
Meng YE, Yong WANG, Peng GAO, Likun XU, Guanjin HUANG
Front. Optoelectron.    https://doi.org/10.1007/s12200-020-0986-2
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Hong–Ou–Mandel (HOM) interference is one of the most important experimental phenomena in quantum optics. It has drawn considerable attention with respect to quantum cryptography and quantum communication because of the advent of the measurement device independent (MDI) quantum key distribution (QKD) protocol. Here, we realize HOM interference, having a visibility of approximately 38.1%, using two independent heralded single-photon sources (HSPSs). The HOM interference between two independent HSPSs is a core technology for realizing the long-distance MDI QKD protocol, the quantum coin-tossing protocol, and other quantum cryptography protocols.

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Preparation of shape-controlling VO2(M/R) nanoparticles via one-step hydrothermal synthesis
Yuchao LI, Fengyu KONG, Bin WANG, Yanhua ZHAO, Zuankai WANG
Front. Optoelectron.    https://doi.org/10.1007/s12200-020-1006-2
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In this study, we developed a facile one-step hydrothermal process that allows to synthesize high-purity VO2(M/R) nanoparticles with various morphologies such as nanorods, nanogranules, nanoblocks, and nanospheres. W dopants are successfully implanted in VO2(M/R) unit cells with high doping efficiency, which allows to regulate the size, morphology, and phase of obtained nanoparticles. The underlying regulation mechanism is presented in detail to reveal how hydrothermal products vary with W doping contents, which provides a synthetic strategy for the preparation of shape-controlling VO2(M/R) nanoparticles with high purity to satisfy different specific demands for corresponding applications in the field of thermochromic smart windows.

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