Frontiers of Optoelectronics

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Enabling technologies and challenges for transmission of 400 Gb/s signals in 50 GHz channel grid
Xiang ZHOU
Front Optoelec.  2013, 6 (1): 30-45.   https://doi.org/10.1007/s12200-012-0298-2
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This paper reviewed the recent progress in transmission of 400 Gb/s, wavelength-division-multiplexed (WDM) channels for optical networks based on the standard 50 GHz grid. We discussed the enabling modulation, coding, and line system technologies, as well as the existing challenges. It is shown that, 400 Gb/s per channel signal can be transmitted on the standard 50 GHz ITU-T grid at 8.4 b/ds/Hz net spectral efficiency (SE) over meaningful transmission reach for regional and metropolitan applications. However, further studies are needed to fully understand the potential for meeting the requirements of long-haul transmission applications.

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Key technologies and system proposals of TWDM-PON
Zhengxuan LI, Lilin YI, Weisheng HU
Front Optoelec.  2013, 6 (1): 46-56.   https://doi.org/10.1007/s12200-012-0305-7
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In this paper, key technologies, system proposals and future directions of next generation passive optical networks stage 2 (NG-PON2) are reviewed. We first discuss the potential solutions for NG-PON2 standardization. Then we focus on time and wavelength division multiplexed PON (TWDM-PON), which is the primary solution selected by Full Service Access Network (FSAN). The key technologies in TWDM-PON configuration are analyzed, including how to improve the bandwidth capacity and power budget of the system, and choose upstream tunable transceiver, etc. Several system proposals are illustrated as candidates for NG-PON2 configuration.

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A review of multiple optical vortices generation: methods and applications
Long ZHU, Jian WANG
Front. Optoelectron..  2019, 12 (1): 52-68.   https://doi.org/10.1007/s12200-019-0910-9
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Optical vortices carrying orbital angular momentum (OAM) have attracted increasing interest in recent years. Optical vortices have seen a variety of emerging applications in optical manipulation, optical trapping, optical tweezers, optical vortex knots, imaging, microscopy, sensing, metrology, quantum information processing, and optical communications. In various optical vortices enabled applications, the generation of multiple optical vortices is of great importance. In this review article, we focus on the methods of multiple optical vortices generation and its applications. We review the methods for generating multiple optical vortices in three cases, i.e., 1-to-N collinear OAM modes, 1-to-N OAM mode array and N-to-N collinear OAM modes. Diverse applications of multiple OAM modes in optical communications and non-communication areas are presented. Future trends, perspectives and opportunities are also discussed.

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Challenges in the ambient Raman spectroscopy characterization of methylammonium lead triiodide perovskite thin films
Yuanyuan ZHOU,Hector F. GARCES,Nitin P. PADTURE
Front. Optoelectron..  2016, 9 (1): 81-86.   https://doi.org/10.1007/s12200-016-0573-8
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The importance of methylammonium lead triiodide (CH3NH3PbI3 or MAPbI3) organic-inorganic hybrid perovskites has shot up dramatically since their use in highly efficient thin-film perovskite solar cells (PSCs). However, the basic structural characterization of these fascinating materials remains sparse. In particular, Raman spectroscopy, which is a powerful vibrational spectroscopy characterization tool and complements other characterization methods, of MAPbI3 under ambient conditions is plagued with difficulties. Here, a systematic ambient Raman spectroscopy characterization study of MAPbI3 thin films is conducted under different conditions (excitation laser wavelength, integration time, filter characteristic). The results from this study help elucidate the possible sources of artifacts in the Raman spectra, and raise the awareness of the challenges in the ambient Raman spectroscopy of MAPbI3 perovskites. Approaches to overcome these challenges are suggested.

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Nonlinear optical response of graphene in terahertz and near-infrared frequency regime
Yee Sin ANG,Qinjun CHEN,Chao ZHANG
Front. Optoelectron..  2015, 8 (1): 3-26.   https://doi.org/10.1007/s12200-014-0428-0
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In this review, we discuss our recent theoretical work on the nonlinear optical response of graphene and its sister structure in terahertz (THz) and near-infrared frequency regime. Due to Dirac-like linear energy-momentum dispersion, the third-order nonlinear current in graphene is much stronger than that in conventional semiconductors. The nonlinear current grows rapidly with increasing temperature and decreasing frequency. The third-order nonlinear current can be as strong as the linear current under moderate electric field strength of 104 V/cm. In bilayer graphene (BLG) with low energy trigonal warping effect, not only the optical response is strongly nonlinear, the optical nonlinearity is well-preserved at elevated temperature. In the presence of a bandgap (such as semihydrogenated graphene (SHG)), there exists two well separated linear response and nonlinear response peaks. This suggests that SHG can have a unique potential as a two-color nonlinear material in the THz frequency regime where the relative intensity of the two colors can be tuned with the electric field. In a graphene superlattice structure of Kronig-Penney type periodic potential, the Dirac cone is elliptically deformed. We found that not only the optical nonlinearity is preserved in such a system, the total optical response is further enhanced by a factor proportional to the band anisotropy. This suggests that graphene superlattice is another potential candidate in THz device application.

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Recent advances in microwave photonics
Ming LI,Ninghua ZHU
Front. Optoelectron..  2016, 9 (2): 160-185.   https://doi.org/10.1007/s12200-016-0633-0
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Microwave photonics (MWP) is an interdisciplinary field that combines two different areas of microwave engineering and photonics. It has several key features by transferring signals between the optical domain and microwave domain, which leads to the advantages of broad operation bandwidth for generation, processing and distribution of microwave signals and high resolution for optical spectrum measurement. In this paper, we comprehensively review past and current status of MWP in China by introducing the representative works from most of the active MWP research groups. Future prospective is also discussed from the national strategy to key enabling technology that we have developed.

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Photonic crystal fibers, devices, and applications
Wei JIN, Jian JU, Hoi Lut HO, Yeuk Lai HOO, Ailing ZHANG
Front Optoelec.  2013, 6 (1): 3-24.   https://doi.org/10.1007/s12200-012-0301-y
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This paper reviews different types of air-silica photonic crystal fibers (PCFs), discusses their novel properties, and reports recent advances in PCF components and sensors as well as techniques for splicing PCFs to standard telecomm fibers.

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Generation and detection of pulsed terahertz waves in gas: from elongated plasmas to microplasmas
Fabrizio BUCCHERI, Pingjie HUANG, Xi-Cheng ZHANG
Front. Optoelectron..  2018, 11 (3): 209-244.   https://doi.org/10.1007/s12200-018-0819-8
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The past two decades have seen an exponential growth of interest in one of the least explored region of the electromagnetic spectrum, the terahertz (THz) frequency band, ranging from to 0.1 to 10 THz. Once only the realm of astrophysicists studying the background radiation of the universe, THz waves have become little by little relevant in the most diverse fields, such as medical imaging, industrial inspection, remote sensing, fundamental science, and so on. Remarkably, THz wave radiation can be generated and detected by using ambient air as the source and the sensor. This is accomplished by creating plasma under the illumination of intense femtosecond laser fields. The integration of such a plasma source and sensor in THz time-domain techniques allows spectral measurements covering the whole THz gap (0.1 to 10 THz), further increasing the impact of this scientific tool in the study of the four states of matter.

In this review, the authors introduce a new paradigm for implementing THz plasma techniques. Specifically, we replaced the use of elongated plasmas, ranging from few mm to several cm, with sub-mm plasmas, which will be referred to as microplasmas, obtained by focusing ultrafast laser pulses with high numerical aperture optics (NA from 0.1 to 0.9).

The experimental study of the THz emission and detection from laser-induced plasmas of submillimeter size are presented. Regarding the microplasma source, one of the interesting phenomena is that the main direction of THz wave emission is almost orthogonal to the laser propagation direction, unlike that of elongated plasmas. Perhaps the most important achievement is the demonstration that laser pulse energies lower than 1 mJ are sufficient to generate measurable THz pulses from ambient air, thus reducing the required laser energy requirement of two orders of magnitude compared to the state of art. This significant decrease in the required laser energy will make plasma-based THz techniques more accessible to the scientific community, as well as opening new potential industrial applications.

Finally, experimental observations of THz radiation detection with microplasmas are also presented. As fully coherent detection was not achieved in this work, the results presented herein are to be considered a first step to understand the peculiarities involved in using the microplasma as a THz sensor.

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Semiconductor activated terahertz metamaterials
Hou-Tong CHEN
Front. Optoelectron..  2015, 8 (1): 27-43.   https://doi.org/10.1007/s12200-014-0436-0
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Metamaterials have been developed as a new class of artificial effective media realizing many exotic phenomena and unique properties not normally found in nature. Metamaterials enable functionality through structure design, facilitating applications by addressing the severe material issues in the terahertz frequency range. Consequently, prototype functional terahertz devices have been demonstrated, including filters, antireflection coatings, perfect absorbers, polarization converters, and arbitrary wavefront shaping devices. Further integration of functional materials into metamaterial structures have enabled actively and dynamically switchable and frequency tunable terahertz metamaterials through the application of external stimuli. The enhanced light-matter interactions in active terahertz metamaterials may result in unprecedented control and manipulation of terahertz radiation, forming the foundation of many terahertz applications. In this paper, we review the progress during the past few years in this rapidly growing research field. We particularly focus on the design principles and realization of functionalities using single-layer and few-layer terahertz planar metamaterials, and active terahertz metamaterials through the integration of semiconductors to achieve switchable and frequency-tunable response.

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Detection of photonic orbital angular momentum with micro- and nano-optical structures
Chenhao WAN, Guanghao RUI, Jian CHEN, Qiwen ZHAN
Front. Optoelectron..  2019, 12 (1): 88-96.   https://doi.org/10.1007/s12200-017-0730-8
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Light with an optical orbital angular momentum (OAM) has attracted an increasing amount of interest and has found its way into many disciplines ranging from optical trapping, edge-enhanced microscopy, high-speed optical communication, and secure quantum teleportation to spin-orbital coupling. In a variety of OAM-involved applications, it is crucial to discern different OAM states with high fidelity. In the current paper, we review the latest research progress on OAM detection with micro- and nano-optical structures that are based on plasmonics, photonic integrated circuits (PICs), and liquid crystal devices. These innovative OAM sorters are promising to ultimately achieve the miniaturization and integration of high-fidelity OAM detectors and inspire numerous applications that harness the intriguing properties of the twisted light.

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Research on multi-kilowatts level tapered fiber bundle N×1 pumping combiner for high power fiber laser
Qirong XIAO,Yusheng HUANG,Junyi SUN,Xuejiao WANG,Dan LI,Mali GONG,Ping YAN
Front. Optoelectron..  2016, 9 (2): 301-305.   https://doi.org/10.1007/s12200-016-0609-0
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Pumping combiner is a kernel component of high power fiber laser (HPFL). We demonstrate two types of tapered fiber bundle (TFB) end-pumping combiner able to combining multi-kilowatts of pumping laser. After the experimental test of coupling performance, the 3×1 coupler is proved to have a power handling capacity of 2.11 kW with a coupling efficiency of 95.1%, and the 7×1 coupler is capable of handling pumping power of 4.72 kW with a coupling efficiency of 99.4%. These two coupler have obtained the ability to be used in laser diodes (LDs) direct beam combining and the pumping coupling of multi-kilowatts level fiber lasers.

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High-performance, stable and low-cost mesoscopic perovskite (CH3NH3PbI3) solar cells based on poly(3-hexylthiophene)-modified carbon nanotube cathodes
Xiaoli ZHENG,Haining CHEN,Zhanhua WEI,Yinglong YANG,He LIN,Shihe YANG
Front. Optoelectron..  2016, 9 (1): 71-80.   https://doi.org/10.1007/s12200-016-0566-7
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This work explores the use of poly(3-hexylthiophene) (P3HT) modified carbon nanotubes (CNTs@P3HT) for the cathodes of hole transporter free, mesoscopic perovskite (CH3NH3PbI3) solar cells (PSCs), simultaneously achieving high-performance, high stability and low-cost PSCs. Here the thin P3HT modifier acts as an electron blocker to inhibit electron transfer into CNTs and a hydrophobic polymer binder to tightly cross-link the CNTs together to compact the carbon electrode film and greatly stabilize the solar cell. On the other hand, the presence of CNTs greatly improve the conductivity of P3HT. By optimizing the concentration of the P3HT modifier (2 mg/mL), we have improved the power conversion efficiencies (PCEs) of CNTs@P3HT based PSCs up to 13.43% with an average efficiency of 12.54%, which is much higher than the pure CNTs based PSCs (best PCE 10.59%) and the sandwich-type P3HT/CNTs based PSCs (best PCE 9.50%). In addition, the hysteresis of the CNTs@P3HT based PSCs is remarkably reduced due to the intimate interface between the perovskite and CNTs@P3HT electrodes. Degradation of the CNTs@ P3HT based PSCs is also strongly retarded as compared to cells employing the pure CNTs electrode when exposed to the ambient condition of 20%-40% humidity.

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Fourier domain optical coherence tomography with ultralong depth range
Zhihua DING,Yi SHEN,Wen BAO,Peng LI
Front. Optoelectron..  2015, 8 (2): 163-169.   https://doi.org/10.1007/s12200-015-0463-5
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The depth ranges of typical implementations of Fourier domain optical coherence tomography (FDOCT), including spectral domain OCT (SDOCT) and swept source OCT (SSOCT), are limited to several millimeters. To extend the depth range of current OCT systems, two novel systems with ultralong depth range were developed in this study. One is the orthogonal dispersive SDOCT (OD-SDOCT), and the other is the recirculated swept source (R-SS) interferometer/OCT. No compromise between depth range and depth resolution is required in both systems. The developed OD-SDOCT system realized the longest depth range (over 100 mm) ever achieved by SDOCT, which is ready to be modified for depth-encoded parallel imaging on multiple sites. The developed R-SS interferometer achieved submicron precision within a depth range of 30 mm, holding potential in real-time contact-free on-axis metrology of complex optical systems.

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Topological photonic crystals: a review
Hongfei WANG, Samit Kumar GUPTA, Biye XIE, Minghui LU
Front. Optoelectron..  2020, 13 (1): 50-72.   https://doi.org/10.1007/s12200-019-0949-7
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The field of topological photonic crystals has attracted growing interest since the inception of optical analog of quantum Hall effect proposed in 2008. Photonic band structures embraced topological phases of matter, have spawned a novel platform for studying topological phase transitions and designing topological optical devices. Here, we present a brief review of topological photonic crystals based on different material platforms, including all-dielectric systems, metallic materials, optical resonators, coupled waveguide systems, and other platforms. Furthermore, this review summarizes recent progress on topological photonic crystals, such as higher-order topological photonic crystals, non-Hermitian photonic crystals, and nonlinear photonic crystals. These studies indicate that topological photonic crystals as versatile platforms have enormous potential applications in maneuvering the flow of light.

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Technology developments and biomedical applications of polarization-sensitive optical coherence tomography
Zhenyang DING,Chia-Pin LIANG,Yu CHEN
Front. Optoelectron..  2015, 8 (2): 128-140.   https://doi.org/10.1007/s12200-015-0475-1
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Polarization-sensitive optical coherence tomography (PS-OCT) enables depth-resolved mapping of sample polarization information, such as phase-retardation and optical axis orientation, which is particularly useful when the nano-scale organization of tissue that are difficult to be observed in the intensity images of a regular optical coherence tomography (OCT). In this review, we survey two types of methods and systems of PS-OCT. The first type is PS-OCT with single input polarization state, which contain bulk optics or polarization maintaining fiber (PMF) based systems and single-mode fiber (SMF) based systems. The second type is PS-OCT with two different input polarization states, which contain SMF based systems and PMF based systems, through either time, frequency, or depth multiplexing. In addition, representative biomedical applications using PS-OCT, such as retinal imaging, skin cancer detection, and brain mapping, are demonstrated.

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Detection of small ship targets from an optical remote sensing image
Mingzhu SONG, Hongsong QU, Guixiang ZHANG, Guang JIN
Front. Optoelectron..  2018, 11 (3): 275-284.   https://doi.org/10.1007/s12200-018-0744-x
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Detection of small ships from an optical remote sensing image plays an essential role in military and civilian fields. However, it becomes more difficult if noise dominates. To solve this issue, a method based on a low-level vision model is proposed in this paper. A global channel, high-frequency channel, and low-frequency channel are introduced before applying discrete wavelet transform, and the improved extended contrast sensitivity function is constructed by self-adaptive center-surround contrast energy and a proposed function. The saliency image is achieved by the three-channel process after inverse discrete wavelet transform, whose coefficients are weighted by the improved extended contrast sensitivity function. Experimental results show that the proposed method outperforms all competing methods with higher precision, higher recall, and higher F-score, which are 100.00%, 90.59%, and 97.96%, respectively. Furthermore, our method is robust against noise and has great potential for providing more accurate target detection in engineering applications.

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In vivo imaging of a single erythrocyte with high-resolution photoacoustic microscopy
Guo HE,Bingbing LI,Sihua YANG
Front. Optoelectron..  2015, 8 (2): 122-127.   https://doi.org/10.1007/s12200-014-0461-z
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In this letter, we reported a high-resolution photoacoustic microscopy (PAM) to image erythrocytes and blood vessels. The developed system had the ability to provide a lateral resolution of 1.0 μm at the wavelength of 532 nm with a × 10 objective. First, we used a sharp edge to measure the lateral resolution of the PAM and testified the stability with carbon fibers. Then, using this system, in vivo blood vessels and capillaries of a mouse ear, even a single erythrocyte can be clearly imaged. There was a pair of accompanying venule and arteriole, whose detailed and further complicated branches can be clearly identified. And likely red blood cells (RBCs) arrayed one by one in microvasculature was also shown. The experimental results demonstrate that the high-resolution PAM has potential clinical applications for imaging of erythrocytes and blood vessels.

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Identifying PM2.5 samples collected in different environment by using terahertz time-domain spectroscopy
Chenghong WU, Xinyang MIAO, Kun ZHAO
Front. Optoelectron..  2018, 11 (3): 256-260.   https://doi.org/10.1007/s12200-018-0805-1
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Particulate matter with the diameter of less than 2.5 mm (PM2.5) is the most important causation of air pollution. In this study, PM2.5 samples were collected in three different environment including ordinary atmospheric environment, lampblack environment and the environment with an air conditioning exhaust fan, and analyzed by using terahertz time-domain spectroscopy (THz-TDS). The linear regression analysis and the principal component analysis (PCA) are used to identify PM2.5 samples collected in different environment. The results indicate that combining THz-TDS with statistical methods can serve as a contactless and efficient approach to identify air pollutants in different environment.

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Fiber-based optical trapping and manipulation
Hongbao XIN, Baojun LI
Front. Optoelectron..  2019, 12 (1): 97-110.   https://doi.org/10.1007/s12200-017-0755-z
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An optical fiber serves as a versatile tool for optical trapping and manipulation owing to its many advantages over conventional optical tweezers, including ease of fabrication, compact configurations, flexible manipulation capabilities, ease of integration, and wide applicability. Here, we review recent progress in fiber-based optical trapping and manipulation, which includes mainly photothermal-based and optical-force-based trapping and manipulation. We focus on five topics in our review of progress in this area: massive photothermal trapping and manipulation, evanescent-field-based trapping and manipulation, dual-fiber tweezers for single-nanoparticle trapping and manipulation, single-fiber tweezers for single-particle trapping and manipulation, and single-fiber tweezers for multiple-particle/cell trapping and assembly.

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De-noising research on terahertz holographic reconstructed image based on weighted nuclear norm minimization method
Wenshu MA, Qi LI, Jianye LU, Liyu SUN
Front. Optoelectron..  2018, 11 (3): 267-274.   https://doi.org/10.1007/s12200-018-0829-6
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Terahertz imaging is one of the forefront topics of imaging technology today. Denoising process is the key for improving the resolution of the terahertz holographic reconstructed image. Based on the fact that the weighted nuclear norm minimization (WNNM) method preserves the details of the reconstructed image well and the non-local mean (NLM) algorithm performs better in the removal of background noise, this paper proposes a new method in which the NLM algorithm is used to improve the WNNM method. The experimental observation and quantitative analysis of the denoising results prove that the new method has better denoising effect for the terahertz holographic reconstructed image.

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Universal numerical calculation method for the Berry curvature and Chern numbers of typical topological photonic crystals
Chenyang WANG, Hongyu ZHANG, Hongyi YUAN, Jinrui ZHONG, Cuicui LU
Front. Optoelectron..  2020, 13 (1): 73-88.   https://doi.org/10.1007/s12200-019-0963-9
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Chern number is one of the most important criteria by which the existence of a topological photonic state among various photonic crystals can be judged; however, few reports have presented a universal numerical calculation method to directly calculate the Chern numbers of different topological photonic crystals and have denoted the influence of different structural parameters. Herein, we demonstrate a direct and universal method based on the finite element method to calculate the Chern number of the typical topological photonic crystals by dividing the Brillouin zone into small zones, establishing new properties to obtain the discrete Chern number, and simultaneously drawing the Berry curvature of the first Brillouin zone. We also explore the manner in which the topological properties are influenced by the different structure types, air duty ratios, and rotating operations of the unit cells; meanwhile, we obtain large Chern numbers from −2 to 4. Furthermore, we can tune the topological phase change via different rotation operations of triangular dielectric pillars. This study provides a highly efficient and simple method for calculating the Chern numbers and plays a major role in the prediction of novel topological photonic states.

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Discrete combination method based on equidistant wavelength screening and its application to near-infrared analysis of hemoglobin
Tao Pan, Bingren Yan, Jiemei Chen, Lijun Yao
Front. Optoelectron..  2018, 11 (3): 296-305.   https://doi.org/10.1007/s12200-018-0804-2
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A wavelength selection method for discrete wavelength combinations was developed based on equidistant combination-partial least squares (EC-PLS) and applied to a near-infrared (NIR) spectroscopic analysis of hemoglobin (Hb) in human peripheral blood samples. An allowable model set was established through EC-PLS on the basis of the sequence of the predicted error values. Then, the wavelengths that appeared in the allowable models were sorted, combined, and utilized for modeling, and the optimal number of wavelengths in the combinations was determined. The ideal discrete combination models were obtained by traversing the number of allowable models. The obtained optimal EC-PLS and discrete wavelength models contained 71 and 42 wavelengths, respectively. A simple and high-performance discrete model with 35 wavelengths was also established. The validation samples excluded from modeling were used to validate the three models. The root-mean-square errors for the NIR-predicted and clinically measured Hb values were 3.29, 2.86, and 2.90 g·L−1, respectively; the correlation coefficients, relative RMSEP, and ratios of performance to deviation were 0.980, 0.983, and 0.981; 2.7%, 2.3%, and 2.4%; and 4.6, 5.3, and 5.2, respectively. The three models achieved high prediction accuracy. Among them, the optimal discrete combination model performed the best and was the most effective in enhancing prediction performance and removing redundant wavelengths. The proposed optimization method for discrete wavelength combinations is applicable to NIR spectroscopic analyses of complex samples and can improve prediction performance. The proposed wavelength models can be utilized to design dedicated spectrometers for Hb and can provide a valuable reference for non-invasive Hb detection.

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Longitudinal twinning α-In2Se3 nanowires for UV-visible-NIR photodetectors with high sensitivity
Zidong ZHANG, Juehan YANG, Fuhong MEI, Guozhen SHEN
Front. Optoelectron..  2018, 11 (3): 245-255.   https://doi.org/10.1007/s12200-018-0820-2
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Longitudinal twinning α-In2Se3 nanowires with the (101¯8) twin plane were synthesized to fabricate high performance single nanowire based photodetectors. As-synthesized α-In2Se3 nanowire exhibited typical n-type semiconducting behavior with an electron mobility of 23.1 cm2·V1·S1 and a broadband spectral response from 300 to 1100 nm, covering the ultraviolet-visible-near-infrared (UV-visible-NIR) region. Besides, the fabricated device showed a high responsivity of 8.57 × 105 A·W1, high external quantum efficiency up to 8.8 × 107% and a high detectivity of 1.58 × 1012 Jones under 600 nm light illumination at a basis of 3 V, which are much higher than previously reported In2Se3 nanostructures due to the interface defect effect of the twin plane. The results indicated that the longitudinal twinning α-In2Se3 nanowires have immense potential for further applications in highly performance broadband photodetectors and other optoelectronic devices.

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Retinal projection head-mounted display
Junguo LIN,Dewen CHENG,Cheng YAO,Yongtian WANG
Front. Optoelectron..  2017, 10 (1): 1-8.   https://doi.org/10.1007/s12200-016-0662-8
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Retinal projection displays (RPDs) are an important development direction for head-mounted displays (HMDs). This paper reviews the literature on optical engineering aspects based on the data on advanced technology in RPD design and development. The review includes the principles and applications of four theories, e.g., the Maxwellian view and its modified modality and the monocular and binocular depth cues of stereoscopic objects in the physiology of the human visual system. To support the Maxwellian view and achieve retinal projection systems with depth cues, results of previous design works were summarized using different methods and their advantages and disadvantages are analyzed. With an extremely long focal depth, a prototype of a full-color stereoscopic see-through RPD system was discussed. Finally, a brief outlook of the future development trends and applications of the RPDs was presented

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Tailoring electromagnetic responses in terahertz superconducting metamaterials
Xiaoling ZHANG,Jianqiang GU,Jiaguang HAN,Weili ZHANG
Front. Optoelectron..  2015, 8 (1): 44-56.   https://doi.org/10.1007/s12200-014-0439-x
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Superconducting terahertz metamaterials have attracted significant interest due to low loss, efficient resonance switching and large-range frequency tunability. The super conductivity in the metamaterials dramatically reduces ohmic loss and absorption to levels suitable for novel devices over a broad range of electromagnetic spectrum. Most metamaterials utilize subwavelength-scale split-ring resonators as unit building blocks, which are proved to support fundamental inductive-capacitive resonance, to achieve unique resonance performance. We presented a review of terahertz superconducting metamaterials and their implementation in multifunctional devices. We began with the recent development of superconducting metamaterials and their potential applications in controlling and manipulating terahertz waves. Then we explored the tuning behaviors of resonance properties in several typical, actively controllable metamaterials through integrating active components. Finally, the ultrafast dynamic nonlinear response to high intensity terahertz field in the superconducting metamaterials was presented.

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Near-infrared carbon-implanted Er3+/Yb3+ co-doped phosphate glass waveguides
Xiaoliang SHEN, Yue WANG, Haitao GUO, Chunxiao LIU
Front. Optoelectron..  2018, 11 (3): 291-295.   https://doi.org/10.1007/s12200-018-0803-3
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The Er3+/Yb3+ co-doped phosphate (EYDP) glass waveguides operated at 1539 nm have been manufactured by using the implantation technique of carbon ions under the condition of 6.0 MeV energy and 5.0 × 1013 ions/cm2 fluence in this work. The ion implantation process was computed by means of the stopping and range of ions in matter. The dark-mode spectrum at 1539 nm of the waveguide was recorded by the method of the prism coupling measurement. The microscopic image of the fabricated structure was photographed by an optical microscope. It is the first step for the application of the waveguides on the base of EYDP glasses in optical-integrated photonic devices at near-infrared band.

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Antimony doped Cs2SnCl6 with bright and stable emission
Jinghui LI, Zhifang TAN, Manchen HU, Chao CHEN, Jiajun LUO, Shunran LI, Liang GAO, Zewen XIAO, Guangda NIU, Jiang TANG
Front. Optoelectron..  2019, 12 (4): 352-364.   https://doi.org/10.1007/s12200-019-0907-4
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Lead halide perovskites, with high photoluminescence efficiency and narrow-band emission, are promising materials for display and lighting. However, the lead toxicity and environmental sensitivity hinder their potential applications. Herein, a new antimony-doped lead-free inorganic perovskites variant Cs2SnCl6:xSb is designed and synthesized. The perovskite variant Cs2SnCl6:xSb exhibits a broadband orange-red emission, with a photoluminescence quantum yield (PLQY) of 37%. The photoluminescence of Cs2SnCl6:xSb is caused by the ionoluminescence of Sb3+ within Cs2SnCl6 matrix, which is verified by temperature dependent photoluminescence (PL) and PL decay measurements. In addition, the all inorganic structure renders Cs2SnCl6:xSb with excellent thermal and water stability. Finally, a white light-emitting diode (white-LED) is fabricated by assembling Cs2SnCl6:0.59%Sb, Cs2SnCl6:2.75%Bi and Ba2Sr2SiO4:Eu2+ onto the commercial UV LED chips, and the color rendering index (CRI) reaches 81.

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Optical fiber amplifiers for space-division multiplexing
Dagong JIA, Haiwei ZHANG, Zhe JI, Neng BAI, Guifang LI
Front Optoelec.  2012, 5 (4): 351-357.   https://doi.org/10.1007/s12200-012-0294-6
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Recently, space-division multiplexing (SDM) techniques using multi-core fiber (MCF) and few-mode fiber (FMF) have been introduced into optical fiber communication to increase transmission capacity. Two main types of optical fiber amplifiers based on the Erbium-doped fiber (EDF) and the Raman effect have been developed to amplify signals in the MCF and FMF. In this paper, we reviewed the principles and configurations of these amplifiers.

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Time behavior of field screening effects in small-size GaAs photoconductive terahertz antenna
Tianyi WANG,Zhengang YANG,Si ZOU,Kejia WANG,Shenglie WANG,Jinsong LIU
Front. Optoelectron..  2015, 8 (1): 98-103.   https://doi.org/10.1007/s12200-015-0488-9
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The field screening effects in small-size GaAs photoconductive (PC) antenna are investigated via the well-known pump and probe terahertz (THz) generation technique. The peak amplitude of the THz pulses excited by the probe laser pulse as a function of the pump-probe time delay was measured. An equivalent-circuit model was used to simulate the experimental data. Based on the good agreement between the results of simulation and experiment, the time behavior of the radiation and space-charge fields was simulated. The results show that the space-charge screening dominantly determines the device response in the whole time, while the radiation filed screening plays a key role in initial time which strongly affects the peak THz field. The parameter analysis was performed, which may be valuable on the optimum design for the antenna as a THz emitter.

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ZnO/Nb2O5 core/shell nanorod array photoanode for dye-sensitized solar cells
Xiaoyan HU, Heng WANG
Front. Optoelectron..  2018, 11 (3): 285-290.   https://doi.org/10.1007/s12200-018-0758-4
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In this paper, ZnO/Nb2O5 core/shell nanorod arrays were synthesized and used as photoanodes for dye-sensitized solar cells (DSSCs). We first synthesized ZnO nanorod array on fluorine-doped tin oxide (FTO) glasses by a hydrothermal method, and then ZnO/Nb2O5 core/shell nanorod array was directly obtained via solvothermal reaction in NbCl5 solution. The scanning electron microscope (SEM) and transmission electron microscope (TEM) images revealed that the ZnO nanorods were uniformly wrapped by Nb2O5 shell layers with a thickness of 30–40 nm. Photovoltaic characterization showed that the device based on ZnO/Nb2O5 core/shell nanorod photoanode exhibited an improved efficiency of 1.995%, which was much higher than the efficiency of 0.856% for the DSSC based on bare ZnO nanorod photoanode. This proved that the photovoltaic performance of ZnO nanorods could be improved by wrapping with Nb2O5 shells.

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