Frontiers of Optoelectronics

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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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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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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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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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2D materials as a new platform for photonic applications
Jianji DONG, Zhipei SUN
Front. Optoelectron..  2020, 13 (2): 89-90.   https://doi.org/10.1007/s12200-020-1059-2
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Two-dimensional material functional devices enabled by direct laser fabrication
Tieshan YANG, Han LIN, Baohua JIA
Front. Optoelectron..  2018, 11 (1): 2-22.   https://doi.org/10.1007/s12200-017-0753-1
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During the past decades, atomically thin, two-dimensional (2D) layered materials have attracted tremendous research interest on both fundamental properties and practical applications because of their extraordinary mechanical, thermal, electrical and optical properties, which are distinct from their counterparts in the bulk format. Various fabrication methods, such as soft-lithography, screen-printing, colloidal-templating and chemical/dry etching have been developed to fabricate micro/nanostructures in 2D materials. Direct laser fabrication with the advantages of unique three-dimensional (3D) processing capability, arbitrary-shape designability and high fabrication accuracy up to tens of nanometers, which is far beyond the optical diffraction limit, has been widely studied and applied in the fabrication of various micro/nanostructures of 2D materials for functional devices. This timely review summarizes the laser-matter interaction on 2D materials and the significant advances on laser-assisted 2D materials fabrication toward diverse functional photonics, optoelectronics, and electrochemical energy storage devices. The perspectives and challenges in designing and improving laser fabricated 2D materials devices are discussed as well.

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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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Review of fabrication methods of large-area transparent graphene electrodes for industry
Petri MUSTONEN, David M. A. MACKENZIE, Harri LIPSANEN
Front. Optoelectron..  2020, 13 (2): 91-113.   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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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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Halide perovskites: from materials to optoelectronic devices
Jiang TANG, Dehui LI
Front. Optoelectron..  2020, 13 (3): 191-192.   https://doi.org/10.1007/s12200-020-1092-1
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Non-thermal plasma fixing of nitrogen into nitrate: solution for renewable electricity storage?
Yi HE, Zhengwu CHEN, Zha LI, Guangda NIU, Jiang TANG
Front. Optoelectron..  2018, 11 (1): 92-96.   https://doi.org/10.1007/s12200-018-0807-z
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The rapid deployment of solar and wind technology produces significant amount of low-quality electricity that calls for a better storage or usage instead of being discarded by the grid. Instead of electrochemical CO2 reduction and/or NH3 production, here we propose that non-thermal plasma oxidation of N2 into nitrate or other valuable nitrogen containing compounds deserve more research attention because it uses free air as the reactant and avoids the solubility difficulty, and also because its energy consumption is merely 0.2 MJ/mol, even lower than the industrially very successful Haber–Bosch process (0.48 MJ/mol) for NH3 production. We advocate that researchers from the plasma community and chemistry community should work together to build energy efficient non-thermal plasma setup, identify robust, active and low-cost catalyst, and understand the catalyzing mechanism in a plasma environment. We are confident that free production of nitrate with zero CO2 emission will come true in the near future.

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Focus on performance of perovskite light-emitting diodes
Peipei DU, Liang GAO, Jiang TANG
Front. Optoelectron..  2020, 13 (3): 235-245.   https://doi.org/10.1007/s12200-020-1042-y
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Perovskite-based optoelectronic devices, especially perovskite light-emitting diodes (PeLEDs) and perovskite solar cells, have recently attracted considerable attention. The National Renewable Energy Laboratory (NREL) chart inspires us to develop a counterpart for PeLEDs. In this study, we collect the record performance of PeLEDs including several new entries to address their latest external quantum efficiency (EQE), highest luminance, and stability status. We hope that these performance tables and future updated versions will show the frontiers of PeLEDs, assist researchers in capturing the overview of this field, identify the remaining challenges, and predict the promising research directions.

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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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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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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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Design and analysis of high birefringence and nonlinearity with small confinement loss photonic crystal fiber
Rekha SAHA, Md. Mahbub HOSSAIN, Md. Ekhlasur RAHAMAN, Himadri Shekhar MONDAL
Front. Optoelectron..  2019, 12 (2): 165-173.   https://doi.org/10.1007/s12200-018-0837-6
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High birefringence with low confinement loss photonic crystal fiber (PCF) has significant advantages in the field of sensing, dispersion compensation devices, nonlinear applications, and polarization filter. In this report, two different models of PCFs are presented and compared. Both the models contain five air holes rings with combination of circular and elliptical air holes arrangement. Moreover, the elliptical shaped air holes polarization and the third ring air holes rotational angle are varied. To examine different guiding characteristics, finite element method (FEM) with perfectly matched layer (PML) absorbing boundary condition is applied from 1.2 to 1.8 µm wavelength range. High birefringence, low confinement loss, high nonlinearity, and moderate dispersion values are successfully achieved in both the PCFs models. Numeric analysis shows that model-1 gives higher birefringence (2.75 × 102) and negative dispersion (−540.67 ps/(nm·km)) at 1.55 µm wavelength. However, model-2 gives more small confinement loss than model-1 at the same wavelength. In addition, the proposed design demonstrates the variation of rotation angle has great impact to enhance guiding properties especially the birefringence.

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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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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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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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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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Vector mode based optical direct detection orthogonal frequency division multiplexing transmission in short-reach optical link
Jianping LI, Zhaohui LI
Front. Optoelectron..  2019, 12 (1): 41-51.   https://doi.org/10.1007/s12200-018-0836-7
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As one solution to implement the large-capacity space division multiplexing (SDM) transmission systems, the mode division multiplexing (MDM) has gained much attention recently. The vector mode (VM), which is the eigenmode of the optical fiber, has also been adopted to realize the optical communications including the transmission over free-space optical (FSO) and optical fiber links. Considering the concerns on the short-reach optical interconnects, the low cost and high integration technologies should be developed. Direct detection (DD) with higher-order modulation formats in combination of MDM technologies could offer an available trade-off in system performance and complexity. We review demonstrations of FSO and fiber high-speed data transmission based on the VM MDM (VMDM) technologies. The special VMs, cylindrical vector beams (CVB), have been generated by the q-plate (QP) and characterized accordingly. And then they were used to implement the VMDM transmission with direct-detection orthogonal frequency division multiplexing (DD-OFDM). These demonstrations show the potential of VMDM-DD-OFDM technology in the large-capacity short-reach transmission links.

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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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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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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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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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Influence of precursor concentration on printable mesoscopic perovskite solar cells
Shuangquan JIANG, Yusong SHENG, Yue HU, Yaoguang RONG, Anyi MEI, Hongwei HAN
Front. Optoelectron..  2020, 13 (3): 256-264.   https://doi.org/10.1007/s12200-020-1013-3
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Over the last decade, the power conversion efficiency of hybrid organic–inorganic perovskite solar cells (PSCs) has increased dramatically from 3.8% to 25.2%. This rapid progress has been possible due to the accurate control of the morphology and crystallinity of solution-processed perovskites, which are significantly affected by the concentration of the precursor used. This study explores the influence of precursor concentrations on the performance of printable hole-conductor-free mesoscopic PSCs via a simple one-step drop-coating method. The results reveal that lower concentrations lead to larger grains with inferior pore filling, while higher concentrations result in smaller grains with improved pore filling. Among concentrations ranging from 0.24–1.20 M, devices based on a moderate strength of 0.70 M were confirmed to exhibit the best efficiency at 16.32%.

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Dimensionality engineering of metal halide perovskites
Rashad F. KAHWAGI, Sean T. THORNTON, Ben SMITH, Ghada I. KOLEILAT
Front. Optoelectron..  2020, 13 (3): 196-224.   https://doi.org/10.1007/s12200-020-1039-6
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Metal halide perovskites are a class of materials that are ideal for photodetectors and solar cells due to their excellent optoelectronic properties. Their low-cost and low temperature synthesis have made them attractive for extensive research aimed at revolutionizing the semiconductor industry. The rich chemistry of metal halide perovskites allows compositional engineering resulting in facile tuning of the desired optoelectronic properties. Moreover, using different experimental synthesis and deposition techniques such as solution processing, chemical vapor deposition and hot-injection methods, the dimensionality of the perovskites can be altered from 3D to 0D, each structure opening a new realm of applications due to their unique properties. Dimensionality engineering includes both morphological engineering–reducing the thickness of 3D perovskite into atomically thin films–and molecular engineering–incorporating long-chain organic cations into the perovskite mixture and changing the composition at the molecular level. The optoelectronic properties of the perovskite structure including its band gap, binding energy and carrier mobility depend on both its composition and dimensionality. The plethora of different photodetectors and solar cells that have been made with different compositions and dimensions of perovskite will be reviewed here. We will conclude our review by discussing the kinetics and dynamics of different dimensionalities, their inherent stability and toxicity issues, and how reaching similar performance to 3D in lower dimensionalities and their large-scale deployment can be achieved.

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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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Silicon waveguide cantilever displacement sensor for potential application for on-chip high speed AFM
Peng WANG, Aron MICHAEL, Chee Yee KWOK
Front. Optoelectron..  2018, 11 (1): 53-59.   https://doi.org/10.1007/s12200-018-0774-4
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This paper reviews an initial achievement of our group toward the development of on-chip parallel high-speed atomic force microscopy (HS-AFM). A novel AFM approach based on silicon waveguide cantilever displacement sensor is proposed. The displacement sensing approach uniquely allows the use of nano-scale wide cantilever that has a high resonance frequency and low spring constant desired for on-chip parallel HS-AFM. The approach consists of low loss silicon waveguide with nano-gap, highly efficient misalignment tolerant coupler, novel high aspect ratio (HAR) sharp nano-tips that can be integrated with nano-scale wide cantilevers and electrostatically driven nano-cantilever actuators. The simulation results show that the displacement sensor with optical power responsivity of 0.31%/nm and AFM cantilever with resonance frequency of 5.4 MHz and spring constant of 0.21 N/m are achievable with the proposed approach. The developed silicon waveguide fabrication method enables silicon waveguide with 6 and 7.5 dB/cm transmission loss for TE and TM modes, respectively, and formation of 13 nm wide nano-gaps between silicon waveguides. The coupler demonstrates misalignment tolerance of ±1.8 µm for 5 µm spot size lensed fiber and coupling loss of 2.12 dB/facet for standard cleaved single mode fiber without compromising other performance. The nano-tips with apex radius as small as 2.5 nm and aspect ratio of more than 50 has been enabled by the development of novel HAR nano-tip fabrication technique. Integration of the HAR tips onto an array of 460 nm wide cantilever beam has also been demonstrated.

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