Frontiers of Optoelectronics

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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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Spectrally efficient single carrier 400G optical signal transmission
Jianjun YU
Front. Optoelectron.    2019, 12 (1): 15-23.   https://doi.org/10.1007/s12200-018-0833-x
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In this paper, the recent progress on spectrally efficient single carrier (SC) 400G optical signal transmission was summarized. By using quadrature phase shift keying (QPSK), 16 quadrature amplitude modulation (16QAM) and 64QAM, we can realize transmission distance over 10000, 6000 and 3000 km, respectively, with large area fiber and all-Raman amplification. To improve the system performance and generate high-order QAM, advanced digital signal processing algorithms such as probabilistic shaping and look-up table pre-distortion are employed to improve the transmission performance.

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Photonic crystals and topological photonics
C.T. CHAN
Front. Optoelectron.    2020, 13 (1): 2-3.   https://doi.org/10.1007/s12200-020-1022-2
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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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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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Modulation of orbital angular momentum on the propagation dynamics of light fields
Peng LI, Sheng LIU, Yi ZHANG, Lei HAN, Dongjing WU, Huachao CHENG, Shuxia QI, Xuyue GUO, Jianlin ZHAO
Front. Optoelectron.    2019, 12 (1): 69-87.   https://doi.org/10.1007/s12200-017-0743-3
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Optical vortices carrying orbital angular momentum (OAM) have attracted extensive attention in recent decades because of their interesting applications in optical trapping, optical machining, optical communication, quantum information, and optical microscopy. Intriguing effects induced by OAMs, such as angular momentum conversion, spin Hall effect of light (SHEL), and spin–orbital interaction, have also gained increasing interest. In this article, we provide an overview of the modulations of OAMs on the propagation dynamics of scalar and vector fields in free space. First, we introduce the evolution of canonical and noncanonical optical vortices and analyze the modulations by means of local spatial frequency. Second, we review the Pancharatnam–Berry (PB) phases arising from spin–orbital interaction and reveal the control of beam evolution referring to novel behavior such as spin-dependent splitting and polarization singularity conversion. Finally, we discuss the propagation and focusing properties of azimuthally broken vector vortex beams.

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Research development on fabrication and optical properties of nonlinear photonic crystals
Huangjia LI, Boqin MA
Front. Optoelectron.    2020, 13 (1): 35-49.   https://doi.org/10.1007/s12200-019-0946-x
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Since the lasers at fixed wavelengths are unable to meet the requirements of the development of modern science and technology, nonlinear optics is significant for overcoming the obstacle. Investigation on frequency conversion in ferroelectric nonlinear photonic crystals with different superlattices has been being one of the popular research directions in this field. In this paper, some mature fabrication methods of nonlinear photonic crystals are concluded, for example, the electric poling method at room temperature and the femtosecond direct laser writing technique. Then the development of nonlinear photonic crystals with one-dimensional, two-dimensional and three-dimensional superlattices which are used in quasi-phase matching and nonlinear diffraction harmonic generation is introduced. In the meantime, several creative applications of nonlinear photonic crystals are summarized, showing the great value of them in an extensive practical area, such as communication, detection, imaging, and so on.

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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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On-chip programmable pulse processor employing cascaded MZI-MRR structure
Yuhe ZHAO, Xu WANG, Dingshan GAO, Jianji DONG, Xinliang ZHANG
Front. Optoelectron.    2019, 12 (2): 148-156.   https://doi.org/10.1007/s12200-018-0846-5
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Optical pulse processor meets the urgent demand for high-speed, ultra wideband devices, which can avoid electrical confinements in various fields, e.g., all-optical communication, optical computing technology, coherent control and microwave fields. To date, great efforts have been made particularly in on-chip programmable pulse processing. Here, we experimentally demonstrate a programmable pulse processor employing 16-cascaded Mach-Zehnder interferometer coupled microring resonator (MZI-MRR) structure based on silicon-on-insulator wafer. With micro-heaters loaded to the device, both amplitude and frequency tunings can be realized in each MZI-MRR unit. Thanks to its reconfigurability and integration ability, the pulse processor has exhibited versatile functions. First, it can serve as a fractional differentiator whose tuning range is 0.51−2.23 with deviation no more than 7%. Second, the device can be tuned into a programmable optical filter whose bandwidth varies from 0.15 to 0.97 nm. The optical filter is also shape tunable. Especially, 15-channel wavelength selective switches are generated.

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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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Multi-channel phase regeneration of QPSK signals based on phase sensitive amplification
Hongxiang WANG, Tiantian LUO, Yuefeng JI
Front. Optoelectron.    2019, 12 (1): 24-30.   https://doi.org/10.1007/s12200-018-0754-8
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In this paper, we propose and demonstrate simultaneous phase regeneration of four different channels of QPSK signal based on phase sensitive amplification. The configuration can be divided into two parts. The first one uses four wave mixing in high nonlinear fiber (HNLF) to generate the corresponding three harmonic conjugates precisely at the frequency of the original signals. The other one uses optical combiner to realize coherent addition which is aimed at completely removing the interaction in phase regeneration stage. The simulation results suggest that this scheme can optimize signal constellation to a large extend especially in high noise environment. Besides, optical signal to noise ratio (OSNR) can improve more than 3 dB while the bit-error-rate (BER) reaches 103 with a constant white noise and 15° phase noise.

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Terahertz wave generation from ring-Airy beam induced plasmas and remote detection by terahertz-radiation-enhanced-emission-of-fluorescence: a review
Kang LIU, Pingjie HUANG, Xi-Cheng ZHANG
Front. Optoelectron.    2019, 12 (2): 117-147.   https://doi.org/10.1007/s12200-018-0860-7
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With the increasing demands for remote spectroscopy in many fields ranging from homeland security to environmental monitoring, terahertz (THz) spectroscopy has drawn a significant amount of attention because of its capability to acquire chemical spectral signatures non-invasively. However, advanced THz remote sensing techniques are obstructed by quite a few factors, such as THz waves being strongly absorbed by water vapor in the ambient air, difficulty to generate intense broadband coherent THz source remotely, and hard to transmit THz waveform information remotely without losing the signal to noise ratio, etc. In this review, after introducing different THz air-photonics techniques to overcome the difficulties of THz remote sensing, we focus mainly on theoretical and experimental methods to improve THz generation and detection performance for the purpose of remote sensing through tailoring the generation and detection media, air-plasma.

For the THz generation part, auto-focusing ring-Airy beam was introduced to enhance the THz wave generation yield from two-color laser induced air plasma. By artificially modulated exotic wave packets, it is exhibited that abruptly auto-focusing beam induced air-plasma can give an up to 5.3-time-enhanced THz wave pulse energy compared to normal Gaussian beam induced plasma under the same conditions. At the same time, a red shift on the THz emission spectrum is also observed. A simulation using an interference model to qualitatively describe these behaviors has be developed.

For the THz detection part, the results of THz remote sensing at 30 m using THz-radiation-enhanced-emission-of-fluorescence (THz-REEF) technique are demonstrated, which greatly improved from the 10 m demonstration last reported. The THz-REEF technique in the counter-propagation geometry was explored, which is proved to be more practical for stand-off detections than co-propagation geometry. We found that in the counter-propagating geometry the maximum amplitude of the REEF signal is comparable to that in the co-propagating case, whereas the time resolved REEF trace significantly changes. By performing the study with different plasmas, we observed that in the counter-propagating geometry the shape of the REEF trace depends strongly on the plasma length and electron density. A new theoretical model suggesting that the densest volume of the plasma does not contribute to the fluorescence enhancement is proposed to reproduce the experimental measurements.

Our results further the understanding of the THz-plasma interaction and highlight the potential of THz-REEF technique in the plasma detection applications.

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Exciton polaritons based on planar dielectric Si asymmetric nanogratings coupled with J-aggregated dyes film
Zhen CHAI, Xiaoyong HU, Qihuang GONG
Front. Optoelectron.    2020, 13 (1): 4-11.   https://doi.org/10.1007/s12200-019-0940-3
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Optical cavity polaritons, originated from strong coupling between the excitons in materials and photons in the confined cavities field, have recently emerged as their applications in the high-speed low-power polaritons devices, low-threshold lasing and so on. However, the traditional exciton polaritons based on metal plasmonic structures or Fabry-Perot cavities suffer from the disadvantages of large intrinsic losses or hard to integrate and nanofabricate. This greatly limits the applications of exciton poalritons. Thus, here we implement a compact low-loss dielectric photonic – organic nanostructure by placing a 2-nm-thick PVA doped with TDBC film on top of a planar Si asymmetric nanogratings to reveal the exciton polaritons modes. We find a distinct anti-crossing dispersion behavior appears with a 117.16 meV Rabi splitting when varying the period of Si nanogratings. Polaritons dispersion and mode anti-crossing behaviors are also observed when considering the independence of the height of Si, width of Si nanowire B, and distance between the two Si nanowires in one period. This work offers an opportunity to realize low-loss novel polaritons applications.

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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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Origin of peculiar inerratic diffraction patterns recorded by charge-coupled device cameras
Kuanhong XU, Xiaonong ZHU, Peng HUANG, Zhiqiang Yu, Nan ZHANG
Front. Optoelectron.    2019, 12 (2): 174-179.   https://doi.org/10.1007/s12200-018-0840-y
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A peculiar and regular diffraction pattern is recorded while using either a color or a monochrome charge-coupled device (CCD) camera to capture the image of the micro air plasma produced by femtosecond laser pulses. The diffraction pattern strongly disturbs the observation of the air plasma, so the origin and eliminating method of these diffraction patterns must be investigated. It is found that the Fourier transform of the periodic surface structure of either the mask mosaic of the color CCD or the pixel array of the monochrome CCD is responsible for the formation of the observed pattern. The residual surface reflection from the protection window of a CCD camera plays the essential role in forming the interesting two-dimensional diffraction spots on the same CCD sensor. Both experimental data and theoretical analyses confirm our understanding of this phenomenon. Therefore removing the protection window of the CCD camera can eliminate these diffraction patterns.

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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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Holographic fabrication of octagon graded photonic super-crystal and potential applications in topological photonics
Oliver SALE, Safaa HASSAN, Noah HURLEY, Khadijah ALNASSER, Usha PHILIPOSE, Hualiang ZHANG, Yuankun LIN
Front. Optoelectron.    2020, 13 (1): 12-17.   https://doi.org/10.1007/s12200-019-0941-2
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Novel optical properties in graded photonic super-crystals can be further explored if new types of graded photonic super-crystals are fabricated. In this paper, we report holographic fabrication of graded photonic super-crystal with eight graded lattice clusters surrounding the central non-gradient lattices through pixel-by-pixel phase engineering in a spatial light modulator. The prospect of applications of octagon graded photonic super-crystal in topological photonics is discussed through photonic band gap engineering and coupled ring resonators.

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Review on partially coherent vortex beams
Jun ZENG, Rong LIN, Xianlong LIU, Chengliang ZHAO, Yangjian CAI
Front. Optoelectron.    2019, 12 (3): 229-248.   https://doi.org/10.1007/s12200-019-0901-x
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Ever since vortex beams were proposed, they are known for owning phase singularity and carrying orbital angular momentum (OAM). In the past decades, coherent optics developed rapidly. Vortex beams have been extended from fully coherent light to partially coherent light, from scalar light to vector light, from integral topological charge (TC) to fractional TC. Partially coherent vortex beams have attracted tremendous interest due to their hidden correlation singularity and unique propagation properties (e.g., beam shaping, beam rotation and self-reconstruction). Based on the sufficient condition for devising a genuine correlation function of partially coherent beam, partially coherent vortex beams with nonconventional correlation functions (i.e., non-Gaussian correlated Schell-model functions) were introduced recently. This timely review summarizes basic concepts, theoretical models, generation and propagation of partially coherent vortex beams.

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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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Distributed feedback organic lasing in photonic crystals
Yulan FU, Tianrui ZHAI
Front. Optoelectron.    2020, 13 (1): 18-34.   https://doi.org/10.1007/s12200-019-0942-1
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Considerable research efforts have been devoted to the investigation of distributed feedback (DFB) organic lasing in photonic crystals in recent decades. It is still a big challenge to realize DFB lasing in complex photonic crystals. This review discusses the recent progress on the DFB organic laser based on one-, two-, and three-dimensional photonic crystals. The photophysics of gain materials and the fabrication of laser cavities are also introduced. At last, future development trends of the lasers are prospected.

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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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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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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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Plasma characteristics of energetic liquid polymer ablated by nanosecond laser pulses
Jing QI, Siqi ZHANG, Tian LIANG, Weichong TANG, Ke XIAO, Lu GAO, Hua GAO, Zili ZHANG, Zhiyuan ZHENG
Front. Optoelectron.    2018, 11 (3): 261-266.   https://doi.org/10.1007/s12200-018-0752-x
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The plasma characteristics of carbon-doped glycidyl azide polymer (GAP) are investigated ablation by nanosecond laser pulses. For the GAP energetic liquid, a specific impulse of 840 s and an ablation efficiency up to 98% are obtained, which can be attributed to the low mass loss owing to the carbon doping. A comparison between the chemical energies shows that the carbon-doped GAP provides better propulsion than pure GAP. This indicates that even for an energetic liquid, an efficient approach to enhance the thrust performance is to reduce the splashing. High ablation thrust could be achieved at a low laser fluence and high carbon content.

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Screen printing process control for coating high throughput titanium dioxide films toward printable mesoscopic perovskite solar cells
Zhining WAN, Mi XU, Zhengyang FU, Da LI, Anyi MEI, Yue HU, Yaoguang RONG, Hongwei HAN
Front. Optoelectron.    2019, 12 (4): 344-351.   https://doi.org/10.1007/s12200-019-0904-7
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Screen printing technique has been widely applied for the manufacturing of both traditional silicon solar cells and emerging photovoltaics such as dye-sensitized solar cells (DSSCs) and perovskite solar cells (PSCs). Particularly, we have developed a printable mesoscopic PSC based on a triple layer scaffold of TiO2/ZrO2/carbon. The deposition of the scaffold is entirely based on screen printing process, which provides a promising prospect for low-cost photovoltaics. However, the optimal thickness of the TiO2 layer for fabricating efficient printable PSCs is much smaller than the typical thickness of screen printed films. Here, we tune the concentration of the pastes and the printing parameters for coating TiO2 films, and successfully print TiO2 films with the thickness of 500−550 nm. The correlation between the thickness of the films and printing parameters such as the solid content and viscosity of the pastes, the printing speed and pressure, and the temperature has been investigated. Besides, the edge effect that the edge of the TiO2 films possesses a much larger thickness and printing positional accuracy have been studied. This work will significantly benefit the further development of printable mesoscopic PSCs.

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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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Robot visual guide with Fourier-Mellin based visual tracking
Chao PENG, Danhua CAO, Yubin WU, Qun YANG
Front. Optoelectron.    2019, 12 (4): 413-421.   https://doi.org/10.1007/s12200-019-0862-0
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Robot vision guide is an important research area in industrial automation, and image-based target pose estimation is one of the most challenging problems. We focus on target pose estimation and present a solution based on the binocular stereo vision in this paper. To improve the robustness and speed of pose estimation, we propose a novel visual tracking algorithm based on Fourier-Mellin transform to extract the target region. We evaluate the proposed tracking algorithm on online tracking benchmark-50 (OTB-50) and the results show that it outperforms other lightweight trackers, especially when the target is rotated or scaled. The final experiment proves that the improved pose estimation approach can achieve a position accuracy of 1.84 mm and a speed of 7 FPS (frames per second). Besides, this approach is robust to the variances of illumination and can work well in the range of 250-700 lux.

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On-chip silicon polarization and mode handling devices
Yong ZHANG, Yu HE, Qingming ZHU, Xinhong JIANG, Xuhan Guo, Ciyuan QIU, Yikai SU
Front. Optoelectron.    2018, 11 (1): 77-91.   https://doi.org/10.1007/s12200-018-0772-6
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Mode- and polarization-division multiplexing are new promising options to increase the transmission capacity of optical communications. On-chip silicon polarization and mode handling devices are key components in integrated mode- and polarization-division multiplexed photonic circuits. In this paper, we review our recent progresses on silicon-based polarization beam splitters, polarization splitters and rotators, mode (de)multiplexers, and mode and polarization selective switches. Silicon polarization beam splitters and rotators are demonstrated with high extinction ratio, compact footprint and high fabrication tolerance. For on-chip mode multiplexing, we introduce a low loss and fabrication tolerant three-mode (de)multiplexer employing sub-wavelength grating structure. In analogy to a conventional wavelength selective switch in wavelength-division multiplexing, we demonstrate a selective switch that can route mode- and polarization-multiplexed signals.

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Structure formation dynamics in drawing silica photonic crystal fibres
Wenyu WANG, Ghazal Fallah TAFTI, Mingjie DING, Yanhua LUO, Yuan TIAN, Shuai WANG, Tomasz KARPISZ, John CANNING, Kevin COOK, Gang-Ding PENG
Front. Optoelectron.    2018, 11 (1): 69-76.   https://doi.org/10.1007/s12200-018-0775-3
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The special features of photonic crystal fibres (PCFs) are achieved by their air hole structures. PCF structure is determined and formed by its origin preform design and drawing process. Therefore, structure formation dynamics in drawing PCF is important for the fabrication of PCF achieving desirable structure and thus the intended feature. This paper will investigate structure formation dynamics of PCF drawing in relation to key parameters and conditions, such as hole dimension, temperature, pressure, etc.

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