In the year of 2000, the Nobel prize in chemistry was awarded to three professors, Alan Heeger, Alan MacDiarmid, and Hideki Shirakawa, for their contributions to discovery and development of conducting polymers. Their works altered the previously-held idea that organic and polymer materials are electrically insulating. Instead, conjugated organic or polymeric materials can be conductors and semiconductors. Compared with traditional inorganic counterparts, organic conductors and semiconductors have advantages of excellent mechanical flexibility, easy processing, potentially low cost, recyclability and easy tuning of their optoelectronic properties by molecular tailoring. In the past years, various organic optoelectronic devices have been demonstrated, including organic light-emitting diodes (OLED), organic photovoltaics (OPV), organic photodiodes (OPD), organic field-effect transistors (OFET), organic energy storage devices, etc. OLEDs have become mature technology for displays and lighting. Other organic optoelectronic devices are still being researched for practical applications. Novel organic optoelectronic materials, physics, devices and applications are ripe for investigation and exploration for a more sustainable society.

Fiber is one of the most fundamental material forms seen in human life. Befitting from their long and bendable shape, fibers with different specialties and different dimensions are used in a multitude of applications, ranging from fabrics [1] to telecommunications [2], from generating laser [3] to sensing and actuating, etc. [4–6]. In recent years, major breakthroughs were made, demonstrating that fibers have novel optical [7–9], electronic [10], acoustic [11, 12] and cell interfacing [13, 14] properties that enable new functionalities. Functional fibers and related application research are at the crossroads of many disciplines, including optics, materials science, device physics, nanotechnology, and fluid dynamics.

Optical metasurfaces, composed of planar arrays of sub-wavelength dielectric or metallic structures that collectively mimic the operation of conventional bulk optical elements, have revolutionized the field of optics by their potential in constructing high-efficiency and multi-functional optoelectronic systems with a compact form factor. By engineering the geometry, placement, and alignment of its constituent elements, an optical metasurface arbitrarily controls the magnitude, polarization, phase, angular momentum, or dispersion of incident light. The study of metasurface now spans various multidisciplinary fields in both fundamental research on light-matter interaction [1–3], and emerging applications from solid-state LiDAR [4,5] to compact imaging, spectroscopy, and quantum optical devices [6–11]. High-performance metasurface devices have been experimentally demonstrated over the entire optical spectrum from the deep ultraviolet to the terahertz (THz) [12–16], and have been employed to manipulate optical waves in both spatial and temporal domains [17–21].This special issue on “Recent Advances in Optical Metasurfaces” includes five review articles and five research articles, covering various topics ranging from metasurface design to practical applications. Qiu et al. [22] provide a comprehensive review of the fundamentals and applications of spin-decoupled Pancharatnam – Berry (PB) metasurfaces. Different from traditional PB-phase-based metasurfaces which impinge phase modulations with opposite signs onto the left-handed and right-handed circularly polarized light, the spin-decoupled PB metasurfaces release the above spin-locked limitation and allow independent and arbitrary control over orthogonal circular polarizations. The recent development of bianisotropic metasurfaces has allowed versatile control over the state of polarization and propagation direction of light. Xiong et al. [23] discuss the electromagnetic properties of photonic bianisotropic structures using the finite element method. The authors show that the vector wave equation with the presence of bianisotropy is self-adjoint under the scalar inner product and propose a balanced formulation of weak form in the practical implementation that outperforms the standard formulation in finite element modeling. Realizing active devices with adjustable functionalities is of great interest to the metasurface research community. Bi et al. [24] review the physical mechanisms and device applications of magnetically controllable metasurfaces. Magnetic field manipulation has advantages of ultra-fast response, non-contact and continuous adjustment, thus paving the way toward realizing multi-functional and dynamic metasurface-based devices and systems.Several typical as well as emerging applications of the metasurface technology, are covered by this special issue. Fu et al. [25] give a comprehensive review of metalenses, tiny planar imaging devices enabled by metasurface technology. The article covers the basic phase modulation techniques, design principles, characterization methods, and functional applications of metalenses. Although a metalens might not fully compete with a conventional lens in terms of imaging quality at the current stage, it possesses unique advantages in terms of multi-dimensional and multi-degree-of-freedom control over an incident light, thus facilitating novel functionalities that are extremely difficult or even impossible to implement using conventional technology. The electromagnetic absorber is another typical application of metasurface technology. Gandhi et al. [26] propose a polarization-insensitive metasurface absorber operating in the THz regime. The device consists of metal-dielectric-metal resonators and exhibits absorption greater than 90% over the 2.54 to 5.54 THz range. In recent years, edge detection using metasurfaces has raised a significant interest and could find promising applications in all-optical computing and artificial intelligence. Wan et al. [27] review the development of dielectric metasurfaces for spatial differentiation and edge detection. The article focuses on the underlying principles of dielectric metasurfaces as first- or second-order spatial differentiators and their applications in biological imaging and machine vision.Non-diffractive beams are highly desired for a number of applications, including biomedical imaging, particle manipulation, and material processing. Liu et al. [28] investigate dual non-diffractive THz beam generation using dielectric metasurfaces. The authors design and experimentally implement Bessel beams and abruptly autofocusing beams, two representative kinds of non-diffractive beams with dramatically opposite focusing properties. With its compact footprint and multiple functionalities, metasurface offers new possibilities in constructing high-performance optical sensors. Ye et al. [29] introduce an ultra-sensitive optical sensing platform based on the parity-time-reciprocal scaling (PTX)-symmetric non-Hermitian metasurfaces. Such devices leverage exotic singularities, such as the exceptional point and the coherent perfect absorber-laser point, to significantly enhance the sensitivity and detectability of photonic sensors. Ren et al. [30] propose a U-shaped THz metamaterial with polarization-sensitive and actively-controllable electromagnetically induced transparency, which could find useful applications in tunable integrated devices such as biosensors, filters, and THz modulators. Realizing large-scale and low-cost fabrication of metasurface could greatly facilitate the technology’s practical applications. Oh et al. [31] review the development of nanoimprint lithography for high-throughput fabrication of optical metasurfaces. The authors elaborate various imprint methods for scalable fabrication of metasurfaces and share their perspectives on the technology’s future development.We hope that this special issue on “Recent Advances in Optical Metasurfaces” could provide useful information for metasurface researchers and inspire new ideas for their future exploration. We thank all authors for their contribution to this special issue, and reviewers for their valuable comments. In the end, we would like to express sincere gratitude to the editors of Frontiers of Optoelectronics for providing us such an excellent opportunity to put together this special issue and their invaluable assistance along the way.

Terahertz (THz) waves, with the spectrum located between microwaves/millimeter-waves and infrared radiation, were historically difficult to be generated and detected and known as “terahertz gap”. However, Since the 1980s, the rapid development of semiconductor optoelectronics, ultrafast optics and ultrafast electronics has greatly promoted the research on generation, detection and application of THz waves. Many breakthroughs were achieved in the past decades, including high performance sources, sensitive detectors and interaction of THz wave with matters. THz waves have low photon energy and are non-ionizing. They can penetrate through most non-conductive materials. Many complex molecules have intra-/inter-molecule rotational or vibrational modes in THz frequencies. Due to these unique features, THz waves have great scientific value and have found an increasingly wide variety of applications in molecular fingerprint detection, diagnostic imaging, security and anti-terrorism, broadband communication, astronomical research, etc.

Biomedical optics is an interdisciplinary subject of optics and life sciences. In recent years, as researches of life science have entered microscopic scale at cellular level, the application of optics in biomedical researches has been extensively studied. Biomedical optics can be divided into two major branches according to research purposes. One is to use advanced optical methods to obtain biomedical information, whose recent trend of development is highspeed in vivo imaging, such as super-resolution imaging [1], fluorescence imaging [2], optical coherence tomography [3], and photoacoustic imaging [4]. The other is to use the interaction of light and biological tissues for disease treatment, such as photodynamic therapy [5], whose recent trend of development is precise treatment. This special issue on “Biomedical Optics” includes two reviews and six research articles, covering most of the topics mentioned.In this special issue, Cheng et al. [6] present an extended depth-of-field photoacoustic microscopy (E-DOF-PAM) system that can achieve a constant spatial resolution and relatively uniform excitation efficiency over a long axial range. Gong et al. [7] use stochastic optical reconstruction microscopy (STORM) to observe the tube-like structures of tunneling nanotubes (TNTs) linking live cells with an easily prepared fluorescent dye. The cleavage process of TNTs is observed with high spatial resolution. Huang et al. [8] summarize the preparation and performance of black phosphorus. Thereafter, black phosphorene-based multifunctional platforms employed for the diagnosis and treatment of diseases, including cancer, bone injuries, brain diseases, progressive oxidative diseases, and kidney injury, are reviewed in detail. Xu et al. [9] give a summary on the recent advances in cognitive functions in the optical and multimodal neuroimaging fields, including the processing of brain cognitive functions foundation during the circadian rhythm phase, how circadian rhythms affect the cognition component, as well as the brain circuit supporting the cognition. Yakovlev et al. [10] show the fluorescence diagnostics (FD) results of cholangiocellular cancer, which are obtained by modified optical fiber, and the results of photodynamic therapy (PDT) using a therapeutic laser. The authors find that it is possible to use a therapeutic laser with a wavelength of 660 nm for both diagnosis and treatment of bile ducts cancer, which significantly reduces the operation time without decreasing its effectiveness. Savelieva et al. [11] propose a spectrum-processing algorithm, which combines empirical and theorybased approaches, to correct the influence of optical properties on the photosensitizer concentration analysis by fluorescence spectroscopy. Maklygina et al. [12] find that, due to the specific metabolism in the glioma tissue and normal brain tissue, the crosstalk between tumor cells and immune cells of different geneses can be prevented by using 5-aminolevulinic acid (5 ALA)-induced protoporphyrin IX (PpIX) and methylene blue (MB) in fluorescent diagnostics. Ogien et al. [13] present the latest advances in line-field confocal optical coherence tomography (LCOCT) which allow the generation of either horizontal (x × y) section images at an adjustable depth or vertical (x × z) section images at an adjustable lateral position, as well as three-dimensional images.These eight articles appeared in this special issue only cover a rather small portion of the recent advances in biomedical optics. We hope this special issue will provide useful references for biomedical optics community and motivate more investigations in these research fields.

Halide perovskites have been extensively studied in last decade partially due to the unprecedentedly rapid increase of power conversion efficiency of perovskite based solar cells. In addition to the solar cells, perovskite based optoelectronic devices such as photodetectors and light emitting devices have also been demonstrated with impressive performance, benefited from the large absorption coefficient, tunable band gap, defect tolerance and long carrier diffusion length. Although significant progress has been made in these fields, a few challenges including long-term stability and toxicity of lead greatly limit their commercialization. Great effort has been put into those fields to address the long-last issues from aspects of fundamental understanding of their photophysics, material engineering and performance optimization.This special issue on “Halide perovskites: from materials to optoelectronic devices” including one comment, four reviews, and five original research articles covers all topics mentioned. In this special issue, Xiong et al. [1] from Nanyang Technological University (Singapore) give an in-depth comment on the current development and future research direction of Bose-Einstein condensation of exciton polariton based on perovskites. Koleilat et al. [2] give a thorough summary on how the dimensionality engineering including morphological engineering and molecular engineering could affect their band gap, binding energy and carrier mobility and subsequently the performance of photodetectors and solar cells. Li et al. [3] review the research process of self-trapped excitons in two-dimensional perovskite including the origins of self-trapped excitons, how to detect and control the self-trapped excitons and how the presence of self-trapped excitons influences the performance of perovskite based optoelectronic devices. Tang et al. [4] collect the performance matrix including external quantum efficiency, luminance, and stability status of perovskite based light emitting diodes, presenting a brief yet comprehensive overview of this field to readers. Chen et al. [5] summarize the possible top cells for next-generation Si-based tandem solar cells and further propose the promising candidate top cells. Mei et al. [6] explore how the precursor concentrations affects the performance of printable hole-conductor-free mesoscopic perovskite solar cells via a simple one-step drop-coating method while You et al. [7] investigate the performance and thermal stability of inorganic perovskite solar cells by using dopant-free polymer, poly(3-hexylthiophene-2,5-diyl) (P3HT), as the holetransport layer. Zhong et al. [8] fabricate a wide-bandgap formamidinium lead bromide film using a doctor-bladecoating method and study the effect of the species of surfactants on the performance of the solar cells based on the as-fabricated film. Wei et al. [9] demonstrate how to fabricate efficient perovskite based light emitting diodes via composite engineering. Mu et al. [10] propose a corona modulation device structure to achieve random lasing in perovskite quantum dots under electron beam excitation.These ten articles appeared in this special issue only cover a rather small portion of the recent advances in this rapid growing perovskite community. We hope this special issue will provide useful references for halide perovskite community and motivate more investigations in these research fields.

Two-dimensional (2D) layered materials possess sheet-like structures with single-atom or few-atom thicknesses. They exhibit exceptional electronic and optical properties due to quantum confinement in the direction perpendicular to the 2D plane. Besides being the basis of various modulators and photodetectors, 2D materials have realized light sources with femtosecond pulse duration utilizing their ultrahigh optical nonlinearity. Recent developments and explorations of various 2D materials and their heterostructures have prompted intense research on various photonic devices with superior performance and functionalities. These developments can pave the way for realistic applications of 2D materials, and boost the fundamental study of various physical effects and phenomena.This special issue on “2D materials for photonic applications” covers the most recent progresses in photonic applications of 2D materials. In the three reviews and two original research articles included, it introduces light sources, modulators, and fabrication methods of 2D materials. The significant benefits of 2D materials for electronics and photonics are also presented, providing a roadmap for a broad range of application fields.Mustonen et al. [1] summarized the fabrication methods of large-area transparent graphene electrodes for industry, including liquid exfoliation and chemical vapor deposition. Zhong et al. [2] reviewed different graphenebased all-optical modulators, and comprehensively discussed their performances in detail. Yao et al. [3] focused on integrated optical switches enabled by 2D materials and beyond. They summarized state-of-the-art optical switches (e.g., all-optical, thermo-optical, and electro-optical modulators) in terms of their energy consumption and response time, showing several stimulating charts. Mu et al. [4] and Li et al. [5] demonstrated pulse generation based on 2D nanoparticles, which can potentially generate stable and rectangular pulses. These researches are highly promising for practical applications.The five articles selected for this special issue cover only a portion of the recent advances and applications in this rapidly growing domain of 2D materials. In the future, significant developments in 2D materials for photonic applications are expected. We hope that this special issue on “2D materials for photonic applications” will interest readers, and provide useful references that will inspire future research in this exciting field.In addition, we would like to express sincere gratitude to the editors of Frontiers of Optoelectronics for providing a valuable opportunity to organize this special issue. We also thank all authors who have contributed their articles, and the many reviewers who have generously offered their valuable time to provide high-quality reviews of all papers.

Photonic crystals offer a platform for manipulating light at the mesoscopic scale owing to the unique photonic bandgap properties originating from spatially periodic dielectric distributions. Not only various integrated photonic devices have been realized based on photonic crystals, such as photonic crystal laser and logic devices, but also many physical effects including negative refractive and optical cloak have been realized in photonic crystals. Based on the photonic band structure, topological photonics has been an emerging research hotspot nowadays. Topological photonics provide two new modulation degrees of freedom, i.e., topological state degrees of freedom and energy valley degrees of freedom. It can be expected that topological photonics will not only boost the fundamental study of physical effects and phenomena, but also improve the research of high-performance photonic devices. Following the research trend in the field of photonics, the journal of Frontiers of Optoelectronics produces a special issue on Photonic Crystal and Topological Photonics in order to promote the research in the area of topological photonics and development of photonic devices based on topological features. In this special issue, Prof. Chan from The Hong Kong University of Science and Technology gives an in-depth comment on the development and research direction of photonic crystal and topological photonics. There are also three research articles and three review articles in this special issue. This special issue will improve the fundamental and application research of the field of photonic crystal and topological photonics.

Optoelectronics is based on the quantum mechanical effects of light on electronic materials, especially semiconductors, sometimes in the presence of electric field. Solar cell, light emitting diodes, photodetectors and fiber lasers all belong to optoelectronics devices. For these devices, the semiconductor electrode interface and materials play a vital role in improving charge carrier separation, transport and recombination pathways. Solar energy is an important clean, renewable and abundant source to meet the sustainable development of human society. In comparison to the conventional fluorescent devices, the discovery of phosphorescent organic lightemitting diodes (OLEDs) is a major breakthrough. Nowadays, OLED and LED could complement each other with their own advantages and meet different commercial requirements. It is our intention to bring the research community’s attention to these above mentioned hot topics. In this Special Issue on “Optoelectronics for Energy”, 2 review articles and 7 research articles focusing on relevant subjects are specially presented.

Canada has a long standing history of contributions to the field of photonics. Photonics research occurs at a number of universities across the country as well as in government research labs. The Canadian photonics industry includes small start-ups to large scale corporations, many of which are involved in research. It is estimated that photonics is a $6 billion industry in Canada that employs more than 24000 people. Photonics clusters can be found in Vancouver, Toronto, Ottawa, Montreal, and Quebec City—cities that are also home to universities where world renowned photonics research takes place. In this special issue, we present a highlight of recent research activities in Canada. Prominent researchers across the country have contributed papers on topics ranging from optical and wireless communications, fiber and integrated technologies, and quantum photonics.

Biophotonics is an emerging multidisciplinary research area, embracing all light-based technologies applied to the life sciences and medicine [1–6]. The expression itself is the combination of the Greek syllables “bios” standing for life and “phos” standing for light. Photonics is the technical term for all methodologies and technologies utilizing photons over the whole spectrum from X-ray over the ultraviolet, visible and the infrared to the terahertz region, and its interaction with any matter. Beyond this definition, biophotonics is a scientific discipline of remarkable societal importance. As a part of photonics, it has proved to be an important enabling technology for accelerated progress in medicine and biotechnology. It can do so, because it originated at the interface of the most innovative academic disciplines of the last century, i.e., photonics, biotechnology and nanotechnologies. This field of research brings together scientists from different professions, such as physicists, biologists, pharmacists, medical doctors, etc. To work on common projects, they need to understand the ideas and the techniques of their counterparts.

Prof. Dexiu Huang is a well-known expert in the field of semiconductor optoelectronics, especially semiconductor optical amplifier (SOA) and its applications. He is also famous for presenting the first proposal to establish hightechnology zone of optoelectronic industry in Wuhan City, which is known as Wuhan Optics Valley of China. Thanks for the support from the editorial board of the Frontiers of Optoelectronics, we are honored to organize this special issue with emphasis on the current hot topics and future trends in the field of semiconductor optoelectronics to celebrate Prof. Dexiu Huang’s 80th birthday. Started from 1980s, Prof. Dexiu Huang has been dedicated himself to develop SOAs for about thirty years. Collaborated with his colleagues, he invented a novel reflected SOA with bulk materials in the late 1980s, then successfully developed a strained multiple quantum well (MQW) SOA with more than 23 dB small signal gain and 1 dB polarization dependence in the mid-1990s. After that, he also exploited SOAs to realize some signal processing functions, such as wavelength conversion. He edited a textbook on Semiconductor Optoelectronics, which has been widely used in the major of Optoelectronics all over the China. Acted as founding associate editor-in-chief of the Frontiers of Optoelectronics, he dedicated himself to improve the academic quality of this journal through inviting well-known scholars to submit papers and organize special issues. This special issue includes 21 excellent scientific reviews, original research papers and tutorial paper with authors from China, Denmark, Singapore, France, Ireland, USA and UK. Regarding SOA related topics, Prof. Connelly, which is a well-known expert of SOA, reported a 40 Gb/s non-return-to-zero differential quadrature phase shift keying (NRZ-DQPSK) wavelength conversion with quantum dash SOA; Prof. Xuelin Yang numerically analyzed the SOAbased turbo-switches with time-domain and frequency-domain SOA models; Tong Cao, Prof. Xinliang Zhang’s PhD student, presented a scheme for performance improvement of SOA by enhancing the well-barrier hole burning. Regarding all-optical signal processing topics, Yunhong Ding from Denmark presented a review paper about linear signal processing functions with silicon micro-ring resonators; Prof. Xinliang Zhang and his PhD student presented a theoretical analysis paper in which a completed model was developed to analysis linear signal processing functions with different optical filters; Xiong Meng from Denmark compared wavelength conversion efficiency between silicon waveguide and micro-ring resonators, while Yi Yu from Denmark analysis switching dynamics of InP photonic-crystal nanocavity; Prof. Jianji Dong demonstrated a large range tunable fractional-order differentiator and theoretical analyzed an optomechanical all-optical transistor. Regarding key optoelectronic devices, Prof. Yongzhen Huang’s group presented a paper about mode characteristics of rectangular microresonators; Prof. Hongbo Sun’s group reviewed the progress of microcavity lasers with femtosecond laser processing method; Prof. Damin Zhang presented a research article about cross-cascaded arrayed waveguide gratings (AWG)-based wavelength selective optical switching optical cross-connect (OXC) modules; Prof. Daoxin Dai from Zhejiang University gave two excellent review articles about silicon photonics; and Prof. Wei Lei reported an interesting work on liquid crystal photonic bandgap fibers. Regarding applications of optoelectronic devices in the field of optical fiber communication and microwave photonics, Prof. Jianping Chen reported microwave photonic phase shifters with microring coupling modulation; Prof. Jianji Dong reported a fiber-chip-fiber optomechanical system for isolator; Prof. Shilong Pan presented a review paper about GaAs-based polarization modulators; Prof. Lilin Yi gave a review paper about key technologies in chaotic optical communications; Prof. Xinlun Cai reviewed photonic integrated devices for exploiting orbital angular momentum (OAM) of light in optical communications; and Prof. Zhaohui Li from Jinan University reported an in-band optical signal-to-noise ratio (OSNR) monitoring technique with high resolution and large measurement range. It is a good way for us, Prof. Dexiu Huang’s students and friends, to express our admiration and thanks to Prof. Dexiu Huang through reporting our research progress. We hope this would be a valuable present to greet Prof. Dexiu Huang’s 80th birthday and publication of this special issue could also promote academic exchange and cooperation. We would sincerely appreciate all the authors for their excellent contributions and the managing editors and other editorial office members of the Frontiers of Optoelectronics for their valuable efforts in publishing this special issue.

Prof. Bingkun Zhou is a famous expert in optoelectronics and has made the outstanding contribution for the development of optoelectronic technology in China. Thanks for the guidance and support from Prof. Dexiu Huang, Associate Editor-in-Chief of the Frontiers of Optoelectronics, we are honored to edit this special issue with emphasis on the current hot topics and future trends in the area of optoelectronics and applications to celebrate Prof. Bingkun Zhou’s 80th Birthday. Optoelectronics have made enormous progress in the past several decades, during which Prof. Bingkun Zhou dedicated himself to this area. Prof. Bingkun Zhou invented in 1984 a laser-diode-pumped monolithic Nd:YAG laser featuring highest efficiency, narrowest line-width, and most frequency-stability of that time, which starts a brand-new researching field–Diode Pump Solid State Laser. In the years that followed, he has made outstanding contributions to information optoelectronics area, in the meanwhile mentoring a large number of young researchers. His textbook on Laser Principles, first edited in the early 1980s and currently 7th edition, has been widely used as a classic one in Chinese universities and Institutes by many generations of students. In 2008, he was the main founding editor of the Journal Frontiers of Optoelectronics which seeks to provide a multidisciplinary forum for a broad mix of peerreviewed academic papers in order to promote rapid communication and exchange between researchers. This special issue includes 21 excellent scientific reviews and original research papers from China, USA, Germany, UK, Australia, Japan, etc. These papers over the research area of optical communications and networks, microwave photonics, fiber lasers and semiconductor lasers, integrated optics and nano-photonics, etc. with the world’s leading research level. As a present to greet Prof. Bingkun Zhou’s 80th birthday, this special issue is a valuable opportunity for us to express our admiration to Prof. Binghun Zhou through reporting our research progress and sharing our research finding. We sincerely wish that publication of this special issue can promote academic exchange and cooperation, stimulate more innovative achievements. We would sincerely appreciate all the authors for their excellent contributions and the managing editors and other editorial office members of the Frontiers of Optoelectronics for their valuable efforts in the publication of this special issue.

Energy crisis and environment pollution are the main challenges of human beings in the 21th century. Developing solar cells with high efficiency and stability at low production cost is an effective way to achieve “price parity” with fossil-fuel-based grid power and large-scale use of solar energy. Since the breakthrough of dye-sensitized solar cells (DSSCs) in 1991, mesoscopic solar cells have been developing fast. Especially in the past three years, the power conversion efficiency (PCE) of perovskite solar cells based on mesoporous structure increased at an amazing rate. Recently, a certificated PCE of 21.0% was reached, which is higher than the certificated record PCE of polycrystalline silicon solar cells, indicating a competitive advantage. However, as new type of photovoltaics, solar cells based on dyes, quantum dots or perovskite pigments as light harvesters, still have many unknowns to be explored. I am delighted to be invited by Frontiers of Optoelectronics, as the guest editor of the first issue in 2016, to arrange the special edition on Mesoscopic Solar Cells. This issue contains 7 review and research papers, all invited written by international well known scientists who are active in the field of mesoscopic solar cells. It covers research frontiers from DSSCs, quantum-dot-sensitized solar cells to mesoscopic perovskite solar cells, from fabrication, characterization to optimization of different components of mesoscopic solar cells. This special issue offers a general view of the development of mesoscopic solar cells. One of the most important components in DSSCs is the sensitizing dye. The donor-π bridge-acceptor (D-π-A) structure is the mainstream in the design of organic sensitizers. Hua et al. discussed the donor design and modification strategies of metal-free sensitizers for highly-efficient DSSCs, however, some sensitizers that do not follow D-π-A structure also present good efficiency when applied in DSSC devices. Robertson and Hu summarized the atypical dyes in order to inspire more diverse designs towards highly efficient DSSCs. The effect of the perovskite morphology on the photovoltaic performance is a critical factor. Etgar and Cohen reviewed various parameters influence and control the organo-metal halide perovskite crystallization and morphology, such as the annealing process, the precursor solvent, anti-solvent treatment and additives to the perovskite solution. Except the comprehensive reviews of organic sensitizers in DSSC, some research results of mesoscopic solar cells with novel photoelectrode, sensitizers, and cathode layers were presented as well. Zhao et al. proposed a new architecture design, micro-scale hierarchical TiO2 nanowires, for nanowire-based quantum-dot-sensitized solar cells to improve the photovoltaic performance. Li et al. designed and synthesized a series of metal-free organic sensitizers with D-A′- π-A configurations, in which quninoxaline or benzoxadiazole moiety was incorporated into the conjugated bridge as the auxiliary electron acceptor (A′) to extend the absorption spectra and broaden the light-harvesting region. Yang et al. explored the use of P3HT modified carbon nanotubes (CNTs@P3HT) (P3HT = poly(3-hexylthiophene)) for the cathodes of hole transporter free, mesoscopic perovskite solar cells, simultaneously achieving high-performance, high stability and low-cost perovskite solar cells. Despite of the increasing importance of CH3NH3PbI3based organicinorganic hybrid perovskites, studies relating to basic structural characterization of these powerful light-harvesting materials remains sparse. Padture et al. conducted a systematic ambient Raman spectroscopy characterization study of MAPbI3 thin films to elucidate the possible sources of artifacts in the Raman spectra, and raised the awareness of the challenges in the ambient Raman spectroscopy of MAPbI3 perovskites. I would like to thank the authors for their excellent works, which set a new landmark in this rapidly evolving and fascinating field. I am particularly grateful to the strong support from Prof. Dexiu Huang and the assistance from Prof. Hongwei Han for this special issue.

The use of optics for data storage could trace back to 40 years ago when the researchers explored the possibilities of optical technology to record and playback audio signals. Optical storage devices have great advantages in low cost, high reliability and good compatibility, which make them suitable for data backup, digital publications, audio & visual products, and so on. To meet with the increasing demand for information storage in the era of big data, optical storage technologies are actively involved in continuous improvement of ultra-high densities and data transfer rates. Several promising technologies have grained growing interest, such as holographic data storage (HDS), multi-layer optical storage, multi-dimensional optical storage and super-resolution near-field structure (super-RENS) optical storage, they are with great expectations because they utilize the volume of optical materials to store information and show great potentials in capacity, density and speed. These technologies, emerging with the revival of optical storage, are now providing a platform for people from both academic and industrial fields to overcome the challenges and realize the practical applications for next generation optical data storage in near future. In this “Special Issue on Optical Storage”, 5 review articles and 3 research articles are specially presented, which covers the research frontiers in relevant subjects.

Information technology has entered a big-data era. The every-increasing data traffic has driven the rapid development of information optoelectronics, which is related to key devices, crucial technologies, and important applications for the generation, transmission, processing, and detection of optical data information. Recent progresses on devices, technologies and applications of information optoelectronics have led to reduced footprint, improved efficiency, enhanced flexibility and extended functionality generating, delivering/manipulating, and detecting optical data information. Lots of integrated photonic devices, such as silicon-, graphene- and plasmonassisted waveguides, lasers, modulators and detectors, speciality optical fiber, and coherent optical communications, have been widely reported, showing significant advances in devices, technologies and applications of information optoelectronics. It is believed that information optoelectronics will continue its rapid development towards integrated nanophotonic devices, more advanced technologies, flexible and multi-functional applications. It is our intention to draw great attention of the research community to those hot topics in devices, technologies and applications of information optoelectronics. In this “Special Issue on Information Optoelectronics: Devices, Technologies and Applications”, we present 8 review articles and 4 research articles focusing on relevant subjects by internationally active groups in the related fields.

I am delighted to introduce the first special issue on Terahertz (THz) Wave Science, Technology, and Application. This issue contains five review and research papers, all written by former Ph.D students, Dr. Xiaofei Lu, Dr. Jingle Liu, Dr. Benjamin Clough, Dr. I-Chen Ho, and Dr. Yuting W. Chen, at the Center of THz Research, Rensselaer Polytechnic Institute. These gifted scientists and engineers are among the most brilliant students that have graduated in the THz community in the past four years. This issue covers research frontiers from THz antireflection silicon structure (Dr. Chen), remote sensing (Dr. Liu), and THz wave air photonics (Dr. Lu) to nonlinear THz wavematter interaction (Dr. Ho), and THz enhanced acoustics (Dr. Clough).

With the gradual reduction of fossil fuel and the growing environmental concerns over the climate change associated with the use of fossil fuel, renewable energy sources such as solar, wind, biomass, geothermal and hydroelectric are becoming increasingly important. In particular, solar power generation has emerged as the most rapidly growing renewable source. As a result, solar cells of different types have been intensively studied. In this special issue, 2 review articles and 2 research articles focusing on solar cells are presented. Dye-sensitized solar cells (DSSCs) are regarded as one of the most promising types due to their low-cost, transparency and relatively high conversion efficiency. Dr. Hongwei Han et al. reviewed the recent progress of materials and achievements for all-solid-state DSSCs and highlighted some representative examples. Also, in a research article, they reported a monobasal solidstate DSSCs with mesoporous TiO2 beads, and as high as 4% efficiency is achieved under air mass (AM) 1.5 illumination. Dr. Mingkui Wang et al. introduced the design and understanding of sensitizers, which are extremely important in determining the performance of DSSCs. The advances in the conception and performance of various sensitizers including ruthenium complexes, organic dyes and porphyrins are discussed. Dr. Guoli Tu et al. synthesized five 4,7-dithien-2-yl-2,1,3-benzothiadiazole (DTBT)-based conjugated copolymers with controlled molecular weight. Compared with the fluorene-based polymer, the carbazole-DTBTcopolymer showed higher short circuit current density (Jsc) and power conversion efficiency (PCE) value that was due to its better intermolecular stacking.