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Van der Waals (vdW) heterostructures based on transition metal dichalcogenides (TMDs) generally possess a type-II band alignment that facilitates the formation of interlayer excitons between constituent monolayers. Manipulation of the interlayer excitons in TMD vdW heterostructures holds great promise for the development of excitonic integrated circuits that serve as the counterpart of electronic integrated circuits, which allows the photons and excitons to transform into each other and thus bridges optical communication and signal processing at the integrated circuit. As a consequence, numerous studies have been carried out to obtain deep insight into the physical properties of interlayer excitons, including revealing their ultrafast formation, long population recombination lifetimes, and intriguing spin-valley dynamics. These outstanding properties ensure interlayer excitons with good transport characteristics, and may pave the way for their potential applications in efficient excitonic devices based on TMD vdW heterostructures. At present, a systematic and comprehensive overview of interlayer exciton formation, relaxation, transport, and potential applications is still lacking. In this review, we give a comprehensive description and discussion of these frontier topics for interlayer excitons in TMD vdW heterostructures to provide valuable guidance for researchers in this field.

Electromagnetic metasurface cloaks provide an alternative paradigm toward rendering arbitrarily shaped scatterers invisible. Most transformation-optics (TO) cloaks intrinsically need wavelength-scale volume/thickness, such that the incoming waves could have enough long paths to interact with structured meta-atoms in the cloak region and consequently restore the wavefront. Other challenges of TO cloaks include the polarization-dependent operation to avoid singular parameters of composite cloaking materials and limitations of canonical geometries, e.g., circular, elliptical, trapezoidal, and triangular shapes. Here, we report for the first time a conformal-skin metasurface carpet cloak, enabling to work under arbitrary states of polarization (SOP) at Poincaré sphere for the incident light and arbitrary conformal platform of the object to be cloaked. By exploiting the foundry three-dimensional (3D) printing techniques to fabricate judiciously designed meta-atoms on the external surface of a conformal object, the spatial distributions of intensity and polarization of its scattered lights can be reconstructed exactly the same as if the scattering wavefront were deflected from a flat ground at any SOP, concealing targets under polarization-scanning detections. Two conformal-skin carpet cloaks working for partial- and full-azimuth plane operation are respectively fabricated on trapezoid and pyramid platforms via 3D printing. Experimental results are in good agreement with numerical simulations and both demonstrate the polarization-insensitive cloaking within a desirable bandwidth. Our approach paves a deterministic and robust step forward to the realization of interfacial, free-form, and full-polarization cloaking for a realistic arbitrary-shape target in real-world applications.

Vector beams, non-separable in spatial mode and polarisation, have emerged as enabling tools in many diverse applications, from communication to imaging. This applicability has been achieved by sophisticated laser designs controlling the spin and orbital angular momentum, but so far is restricted to only two-dimensional states. Here we demonstrate the first vectorially structured light created and fully controlled in eight dimensions, a new state-of-the-art. We externally modulate our beam to control, for the first time, the complete set of classical Greenberger–Horne–Zeilinger (GHZ) states in paraxial structured light beams, in analogy with high-dimensional multi-partite quantum entangled states, and introduce a new tomography method to verify their fidelity. Our complete theoretical framework reveals a rich parameter space for further extending the dimensionality and degrees of freedom, opening new pathways for vectorially structured light in the classical and quantum regimes.

Optical tweezers and associated manipulation tools in the far field have had a major impact on scientific and engineering research by offering precise manipulation of small objects. More recently, the possibility of performing manipulation with surface plasmons has opened opportunities not feasible with conventional far-field optical methods. The use of surface plasmon techniques enables excitation of hotspots much smaller than the free-space wavelength; with this confinement, the plasmonic field facilitates trapping of various nanostructures and materials with higher precision. The successful manipulation of small particles has fostered numerous and expanding applications. In this paper, we review the principles of and developments in plasmonic tweezers techniques, including both nanostructure-assisted platforms and structureless systems. Construction methods and evaluation criteria of the techniques are presented, aiming to provide a guide for the design and optimization of the systems. The most common novel applications of plasmonic tweezers, namely, sorting and transport, sensing and imaging, and especially those in a biological context, are critically discussed. Finally, we consider the future of the development and new potential applications of this technique and discuss prospects for its impact on science., Optical manipulation: plasmonic tweezers Plasmonic tweezers exploit sub-wavelength confinement of light to allow trapping and manipulation of small particles with far greater precision than usual. Yuquan Zhang and coworkers from Shenzhen University in China and Delft University of Technology in The Netherlands have now reviewed the principles of operation, benefits and potential applications of such tweezers. They document the popular designs of plasmonic nanostructures that have been used to create tweezers to date and the theories behind the generation of surface plasmon polaritons and the forces that they induce. They also discuss the opportunities for improving performance of the tweezers in the future and their applications in the areas of manipulation, sorting, characterization and sensing of objects, especially biological entities such as viruses, DNA, biomolecules and cells.

For several centuries, far-field optical microscopy has remained a key instrument in many scientific disciplines, including physical, chemical, and biomedical research. Nonetheless, far-field imaging has many limitations: the spatial resolution is controlled by the diffraction of light, and the imaging speed follows the Nyquist–Shannon sampling theorem. The recent development of super-resolution techniques has pushed the limits of spatial resolution. However, these methods typically require complicated setups and long acquisition times and are still not applicable to deep-tissue bioimaging. Here, we report imaging through an ultra-thin fibre probe with a spatial resolution beyond the Abbe limit and a temporal resolution beyond the Nyquist limit simultaneously in a simple and compact setup. We use the random nature of mode coupling in a multimode fibre, the sparsity constraint and compressive sensing reconstruction. The new approach of super-resolution endo-microscopy does not use any specific properties of the fluorescent label, such as depletion or stochastic activation of the molecular fluorescent state, and therefore can be used for label-free imaging. We demonstrate a spatial resolution more than 2 times better than the diffraction limit and an imaging speed 20 times faster than the Nyquist limit. The proposed approach can significantly expand the realm of the application of nanoscopy for bioimaging., Microscopy: Bio-imaging beyond limits A new procedure for imaging living cells and tissues using an ultra-fine optical fibre to detect light from fluorescent dyes overcomes fundamental limits restricting existing methods. Lyubov Amitonova and Johannes de Boer developed and demonstrated the technology at the Free University of Amsterdam in the Netherlands. Optical imaging has previously been subject to constraints known as the Abbe and Nyquist limits. The Abbe limit sets the smallest distance that can be resolved at about half the wavelength of the light. The Nyquist limit determines the fastest rate at which a light signal can be sampled. The researchers have achieved fluorescence microscopy more than three times better than the Abbe limit and 20 times faster than the Nyquist limit. Their breakthrough is based on innovative light manipulation and sampling procedures in a simple and compact set-up.

Ovaj je rad uvodnik u specijalno izdanje časopisa Light Science and Applications na temu "“Topological photonics and beyond: novel concepts and recent advances”"

A crystal structure has a profound influence on the physical properties of the corresponding material. By synthesizing crystals with particular symmetries, one can strongly tune their properties, even for the same chemical configuration (compare graphite and diamond, for instance). Even more interesting opportunities arise when the structural phases of crystals can be changed dynamically through external stimulations. Such abilities, though rare, lead to a number of exciting phenomena, such as phase-change memory effects. In the case of trilayer graphene, there are two common stacking configurations (ABA and ABC) that have distinct electronic band structures and exhibit very different behaviors. Domain walls exist in the trilayer graphene with both stacking orders, showing fascinating new physics such as the quantum valley Hall effect. Extensive efforts have been dedicated to the phase engineering of trilayer graphene. However, the manipulation of domain walls to achieve precise control of local structures and properties remains a considerable challenge. Here, we experimentally demonstrate that we can switch from one structural phase to another by laser irradiation, creating domains of different shapes in trilayer graphene. The ability to control the position and orientation of the domain walls leads to fine control of the local structural phases and properties of graphene, offering a simple but effective approach to create artificial two-dimensional materials with designed atomic structures and electronic and optical properties., Laser on trilayer graphene: stack switch with a difference Heat from a laser changes how atoms stack up within trilayer graphene, providing a simple and effective approach for fabricating new materials with interesting optical and electronic properties. Mengjian Zhu of China’s National University of Defense Technology and colleagues used laser light to switch regions within trilayer graphene from one type of atomic layering to another. This is interesting, as the properties of trilayer graphene vary depending on how its atoms are stacked up. Zhu and his colleagues found that heat from the laser manipulated the dividing walls separating differently stacked atomic regions within the graphene flakes. This led to a gradual switch from one type of atomic stacking to another, changing the graphene’s properties. The finding could lead to applications for optical storage media and photonic devices.

Across optics and photonics, excess intensity noise is often considered a liability. Here, we show that excess noise in broadband supercontinuum and superluminescent diode light sources encodes each spectral channel with unique intensity fluctuations, which actually serve a useful purpose. Specifically, we report that excess noise correlations can both characterize the spectral resolution of spectrometers and enable cross-calibration of their wavelengths across a broad bandwidth. Relative to previous methods that use broadband interferometry and narrow linewidth lasers to characterize and calibrate spectrometers, our approach is simple, comprehensive, and rapid enough to be deployed during spectrometer alignment. First, we employ this approach to aid alignment and reduce the depth-dependent degradation of the sensitivity and axial resolution in a spectrometer-based optical coherence tomography (OCT) system, revealing a new outer retinal band. Second, we achieve a pixel-to-pixel correspondence between two otherwise disparate spectrometers, enabling a robust comparison of their respective measurements. Thus, excess intensity noise has useful applications in optics and photonics.

Coupled photonic systems exhibit intriguing optical responses attracting intensive attention, but available theoretical tools either cannot reveal the underlying physics or are empirical in nature. Here, we derive a rigorous theoretical framework from first principles (i.e., Maxwell’s equations), with all parameters directly computable via wave function integrations, to study coupled photonic systems containing multiple resonators. Benchmark calculations against Mie theory reveal the physical meanings of the parameters defined in our theory and their mutual relations. After testing our theory numerically and experimentally on a realistic plasmonic system, we show how to utilize it to freely tailor the lineshape of a coupled system, involving two plasmonic resonators exhibiting arbitrary radiative losses, particularly how to create a completely “dark” mode with vanishing radiative loss (e.g., a bound state in continuum). All theoretical predictions are quantitatively verified by our experiments at near-infrared frequencies. Our results not only help understand the profound physics in such coupled photonic systems, but also offer a powerful tool for fast designing functional devices to meet diversified application requests., Plasmonics: developing a theory for coupled photonic systems A new theoretical framework can help clarify the underlying physics of coupled photonic systems whose interacting oscillating particles lead to interesting applications in nanolasing and information transport. The theory, developed by Lei Zhou of Fudan University and colleagues in China, is based on foundational Maxwell’s equations and capable to predict the optical lineshapes of arbitrarily coupled systems. The team tested their theory in simulations and an experiment where near-infrared light was shone on specially placed and shaped nanostructures, obtaining the expected spectroscopic results. They also used it to choose a specific coupling between two nanostructures that achieves a bound state in continuum: waves that remain localized despite coexisting with radiating waves that can carry their energy away. The scientists hope the theory can help improve the theoretical understanding of coupled photonic systems, which has so far been lacking.

We identify and characterize a novel type of quantum emitter formed from InGaN monolayer islands grown using molecular beam epitaxy and further isolated via the fabrication of an array of nanopillar structures. Detailed optical analysis of the characteristic emission spectrum from the monolayer islands is performed, and the main transmission is shown to act as a bright, stable, and fast single-photon emitter with a wavelength of ~400 nm., Quantum emission: a better way to release single photons Monolayers of indium gallium nitride (InGaN) sandwiched within isolated nano-pillars composed of gallium nitride (GaN) can be stimulated to emit light in the near-ultraviolet region of the spectrum, one photon at a time. This achievement, by researchers in China, Japan and Germany led by Xinqiang Wang at Peking University, opens a novel route to single-photon emission for research and potential commercial applications. Single-photon emitters are keenly sought for developing new optical quantum computing technologies, including secure communication systems known as quantum key technologies. Several single-photon emission methods already exist, but each is associated with drawbacks, including difficulties with control and stability. The researchers believe their InGaN system could have advantages including wide tunability of the emitted wavelength and compatibility with silicon substrates for manufacturing optoelectronic and power handling devices.

Tuneable microlasers that span the full visible spectrum, particularly red, green, and blue (RGB) colors, are of crucial importance for various optical devices. However, RGB microlasers usually operate in multimode because the mode selection strategy cannot be applied to the entire visible spectrum simultaneously, which has severely restricted their applications in on-chip optical processing and communication. Here, an approach for the generation of tuneable multicolor single-mode lasers in heterogeneously coupled microresonators composed of distinct spherical microcavities is proposed. With each microcavity serving as both a whispering-gallery-mode (WGM) resonator and a modulator for the other microcavities, a single-mode laser has been achieved. The colors of the single-mode lasers can be freely designed by changing the optical gain in coupled cavities owing to the flexibility of the organic materials. Benefiting from the excellent compatibility, distinct color-emissive microspheres can be integrated to form a heterogeneously coupled system, where tuneable RGB single-mode lasing is realized owing to the capability for optical coupling between multiple resonators. Our findings provide a comprehensive understanding of the lasing modulation that might lead to innovation in structure designs for photonic integration., Lasers: Generating all colors in a better mode A system for improved control over laser light emission generates any frequency in the visible spectrum, creating what is termed a red, green and blue (RGB) laser, in the advantageous form known as ‘single-mode’ lasing. Most RGB lasers deliver ‘multimode’ signals, in which the light is distributed among several wavelengths around the central most intense and desired wavelength. The preferable single-mode RGB lasing has been developed by researchers in China, led by Yong Sheng Zhao at the Institute of Chemistry of the Chinese Academy of Sciences in Beijing. It uses closely coupled organic (carbon compound-based) microcavities as the regions in which the light is generated and amplified, delivering single frequencies of tunable laser light. This innovation will open new possibilities for using RGB lasing in optical processing and communications systems.

We present Floquet fractal topological insulators: photonic topological insulators in a fractal-dimensional lattice consisting of helical waveguides. The helical modulation induces an artificial gauge field and leads to a trivial-to-topological phase transition. The quasi-energy spectrum shows the existence of topological edge states corresponding to real-space Chern number 1. We study the propagation of light along the outer edges of the fractal lattice and find that wavepackets move along the edges without penetrating into the bulk or backscattering even in the presence of disorder. In a similar vein, we find that the inner edges of the fractal lattice also exhibit robust transport when the fractal is of sufficiently high generation. Finally, we find topological edge states that span the circumference of a hybrid half-fractal, half-honeycomb lattice, passing from the edge of the honeycomb lattice to the edge of the fractal structure virtually without scattering, despite the transition from two dimensions to a fractal dimension. Our system offers a realizable experimental platform to study topological fractals and provides new directions for exploring topological physics., Light: science and applications topological photonics: fractal lattices Photonic topological insulators are currently a subject of great interest because they support edge states that can propagate without being affected by defects and disorder. All topological insulators discovered thus far have a bulk surrounded by edges. Now, Zhaoju Yang and coworkers from Technion in Israel, found theoretically that photonic topological insulators can also exist in fractal lattices, comprising only edges—with no bulk at all. They studied fractal lattices structured as Sierpinski gasket composed of an array of evanescently coupled helical waveguides. Despite the lack periodicity in such structures, tight-binding simulations and quasienergy analysis predict the existence of topological edge states, residing either on outer or on inner edges, exhibiting a Chern number of 1 and displaying scattering-free propagation. The fractal symmetries of such lattices are found to be crucial for the existence of the topological properties. Such fractal lattices could be fabricated by femtosecond laser writing technology.

Over the past decade, topology has emerged as a major branch in broad areas of physics, from atomic lattices to condensed matter. In particular, topology has received significant attention in photonics because light waves can serve as a platform to investigate nontrivial bulk and edge physics with the aid of carefully engineered photonic crystals and metamaterials. Simultaneously, photonics provides enriched physics that arises from spin-1 vectorial electromagnetic fields. Here, we review recent progress in the growing field of topological photonics in three parts. The first part is dedicated to the basics of topological band theory and introduces various two-dimensional topological phases. The second part reviews three-dimensional topological phases and numerous approaches to achieve them in photonics. Last, we present recently emerging fields in topological photonics that have not yet been reviewed. This part includes topological degeneracies in nonzero dimensions, unidirectional Maxwellian spin waves, higher-order photonic topological phases, and stacking of photonic crystals to attain layer pseudospin. In addition to the various approaches for realizing photonic topological phases, we also discuss the interaction between light and topological matter and the efforts towards practical applications of topological photonics., Topological photonics: Light and topology linked in theory and practice The structural property of materials known as their topology can be exploited to control the behavior of light in the emerging field of topological photonics, offering potential applications in areas including topology theory, optical communication and optical computing. Researchers in South Korea and the USA, led by Junsuk Rho at Pohang University of Science and Technology (POSTECH) in South Korea, review recent progress in this rapidly developing field. They consider detailed theoretical and practical aspects of the interaction of light with materials whose topology influences the behavior of light waves. These materials include ‘photonic crystals’ and ‘metamaterials’, which have structural features on scales shorter than the wavelength of light. The authors also discuss progress towards developing practical applications for topological photonics. The ability to achieve highly efficient light transmission without dissipation is particularly promising.

Efficient calculation of the light diffraction in free space is of great significance for tracing electromagnetic field propagation and predicting the performance of optical systems such as microscopy, photolithography, and manipulation. However, existing calculation methods suffer from low computational efficiency and poor flexibility. Here, we present a fast and flexible calculation method for computing scalar and vector diffraction in the corresponding optical regimes using the Bluestein method. The computation time can be substantially reduced to the sub-second level, which is 105 faster than that achieved by the direct integration approach (~hours level) and 102 faster than that achieved by the fast Fourier transform method (~minutes level). The high efficiency facilitates the ultrafast evaluation of light propagation in diverse optical systems. Furthermore, the region of interest and the sampling numbers can be arbitrarily chosen, endowing the proposed method with superior flexibility. Based on these results, full-path calculation of a complex optical system is readily demonstrated and verified by experimental results, laying a foundation for real-time light field analysis for realistic optical implementation such as imaging, laser processing, and optical manipulation., Optics: Calculating diffraction effects in optical technologies A fast and flexible procedure for evaluating the propagation of light in optical systems is achieved by calculating the diffraction of the light using a computational process called the Bluestein method. It yields information along the entire optical path length on both ‘scalar’ variations in the general magnitude of light waves and ‘vector’ – directional – variations. Researchers led by Jiawen Li and Dong Wu at the University of Science and Technology of China, developed the process and demonstrated that it can deliver results between 100 and 100,000 times faster than two existing alternative methods. Understanding the influence of diffraction in prevailing conditions is important for predicting the fine behaviuor of light waves. The new procedure should lead to greatly improved real-time analyses that will assist in microscopy, laser-based fabrication and optical manipulation technologies.

Deflecting and changing the direction of propagation of electromagnetic waves are needed in multiple applications, such as in lens–antenna systems, point-to-point communications and radars. In this realm, metamaterials have been demonstrated to be great candidates for controlling wave propagation and wave–matter interactions by offering manipulation of their electromagnetic properties at will. They have been studied mainly in the frequency domain, but their temporal manipulation has become a topic of great interest during the past few years in the design of spatiotemporally modulated artificial media. In this work, we propose an idea for changing the direction of the energy propagation of electromagnetic waves by using time-dependent metamaterials, the permittivity of which is rapidly changed from isotropic to anisotropic values, an approach that we call temporal aiming. In so doing, here, we show how the direction of the Poynting vector becomes different from that of the wavenumber. Several scenarios are analytically and numerically evaluated, such as plane waves under oblique incidence and Gaussian beams, demonstrating how proper engineering of the isotropic—anisotropic temporal function of εr(t) can lead to a redirection of waves to different spatial locations in real time., Metamaterials: temporal beam steering Theoretical analysis shows that beam-steering of electromagnetic waves can be accomplished by temporally changing the permittivity of metamaterials between isotropic to anisotropic values. The approach, called “temporal aiming”, has been formulated by Victor Pacheco-Peña from Newcastle University, UK and Nader Engheta from University of Pennsylvania in the US. In principle, it could open new opportunities for the flow of information around integrated photonic circuits, with flat metamaterial elements deflecting electromagnetic waves to specific targets or receivers on an optical chip as desired. Simulations performed with the software COMSOL with both plane, monochromatic waves and more complex Gaussian beams confirm the feasibility of the approach. It is proposed that the required temporal changes in the metamaterial’s relative permittivity could be achieved by the use of tunable metasurfaces or transmission lines with time-varying circuit elements.

Perfect lenses, superlenses and time-reversal mirrors can support and spatially separate evanescent waves, which is the basis for detecting subwavelength information in the far field. However, the inherent limitations of these methods have prevented the development of systems to dynamically distinguish subdiffraction-limited signals. Utilizing the physical merits of spoof surface plasmon polaritons (SPPs), we demonstrate that subdiffraction-limited signals can be transmitted on planar integrated SPP channels with low loss, low channel interference, and high gain and can be radiated with a very low environmental sensitivity. Furthermore, we show how deep subdiffraction-limited signals that are spatially coupled can be distinguished after line-of-sight wireless transmission. For a visualized demonstration, we realize the high-quality wireless communication of two movies on subwavelength channels over the line of sight in real time using our plasmonic scheme, showing significant advantages over the conventional methods., Surface plasmons open new channels for communication The unique properties of surface plasmons enable wireless transmission of signals separated by less than one wavelength. Until recently, it was considered impossible to distinguish signals with sub-wavelength separation, due to the so-called diffraction limit. This limit can be overcome using artificial structures called metamaterials, but it is difficult to integrate these new components with conventional electronics. Now, Tie Jun Cui at Southeast University in Nanjing, China, and co-workers have shown that sub-wavelength signals can be transmitted using surface plasmon polaritons (SPPs)—combinations of electromagnetic waves and charge motion that travel on the surface of a metal. By encoding signals in ‘spoof’ SPPs that mimic natural SPPs, the team were able to wirelessly transmit two high-definition movies on channels just one-tenth of a wavelength apart, even with a concrete wall in the way.

Owing to the tremendous demands for high-resolution pixel-scale thin lenses in displays, we developed a graphene-based ultrathin square subpixel lens (USSL) capable of electrically tuneable focusing (ETF) with a performance competitive with that of a typical mechanical refractive lens. The fringe field due to a voltage bias in the graphene proves that our ETF-USSL can focus light onto a single point regardless of the wavelength of the visible light—by controlling the carriers at the Dirac point using radially patterned graphene layers, the focal length of the planar structure can be adjusted without changing the curvature or position of the lens. A high focusing efficiency of over 60% at a visible wavelength of 405 nm was achieved with a lens thickness of, Graphene: ultrathin tuneable lens Graphene-based ultrathin lenses with an electrically tuneable focal length could prove useful for providing displays with autostereoscopic 3D functionality, multiview or privacy protection. Developed by scientists in South Korea, the UK and the US, the lenses are just 13 nm thick and are based on a Fresnel Zone Plate (FZP) design made from a series of concentric rings of graphene on a glass substrate. Consisting of up to 5 layers of graphene that is patterned by focused ion-beam milling, the lenses offer a 60% transmission in the visible region and a focal length of 200–300 µm that can be tuned within ~20% range by applying a DC bias voltage. The applied voltage changes the charge carrier density in the graphene, modifying the topology of the FZP and thus its focusing behaviour.

Defect density is one of the most significant characteristics of perovskite single crystals (PSCs) that determines their optical and electrical properties, but few strategies are available to tune this property. Here, we demonstrate that voltage regulation is an efficient method to tune defect density, as well as the optical and electrical properties of PSCs. A three-step carrier transport model of MAPbBr3 PSCs is proposed to explore the defect regulation mechanism and carrier transport dynamics via an applied bias. Dynamic and steady-state photoluminescence measurements subsequently show that the surface defect density, average carrier lifetime, and photoluminescence intensity can be efficiently tuned by the applied bias. In particular, when the regulation voltage is 20 V (electrical poling intensity is 0.167 V μm−1), the surface defect density of MAPbBr3 PSCs is reduced by 24.27%, the carrier lifetime is prolonged by 32.04%, and the PL intensity is increased by 112.96%. Furthermore, a voltage-regulated MAPbBr3 PSC memristor device shows an adjustable multiresistance, weak ion migration effect and greatly enhanced device stability. Voltage regulation is a promising engineering technique for developing advanced perovskite optoelectronic devices., Perovskite devices: Digital memories thrive by tuning out defects Innovative materials gaining favor as replacements for silicon solar cells can also be transformed into memory logic circuits using a new fabrication procedure. Lead halide perovskites can be turned into optoelectronic devices through low-cost solution depositions, but these approaches often leave numerous charge-trapping defects in the perovskite. Weili Yu from China’s Changchun Institute of Optics and colleagues have now developed a technique for modifying the defect population of perovskite crystals without requiring chemical additives. The team used probes to apply an electric field to the surface of a perovskite sample for helping move injected charges into defect sites with a high degree of control, which further modulated the optical and electrical properties of perovskite sample. Optimized defect populations enabled the perovskite to act as memristor device, capable of activating multiple resistance states.

Oscillators are one of the key elements in various applications as a signal source to generate periodic oscillations. Among them, an optical parametric oscillator (OPO) is a driven harmonic oscillator based on parametric frequency conversion in an optical cavity, which has been widely investigated as a coherent light source with an extremely wide wavelength tuning range. However, steady oscillation in an OPO is confined by the cavity delay, which leads to difficulty in frequency tuning, and the frequency tuning is discrete with the minimum tuning step determined by the cavity delay. Here, we propose and demonstrate a counterpart of an OPO in the optoelectronic domain, i.e., an optoelectronic parametric oscillator (OEPO) based on parametric frequency conversion in an optoelectronic cavity to generate microwave signals. Owing to the unique energy-transition process in the optoelectronic cavity, the phase evolution in the OEPO is not linear, leading to steady single-mode oscillation or multimode oscillation that is not bounded by the cavity delay. Furthermore, the multimode oscillation in the OEPO is stable and easy to realize owing to the phase control of the parametric frequency-conversion process in the optoelectronic cavity, while stable multimode oscillation is difficult to achieve in conventional oscillators such as an optoelectronic oscillator (OEO) or an OPO due to the mode-hopping and mode-competition effect. The proposed OEPO has great potential in applications such as microwave signal generation, oscillator-based computation, and radio-frequency phase-stable transfer., A new oscillator featuring phase-controlled oscillation Parametric oscillators are driven harmonic oscillators that widely used in various areas of applications. In the past, parametric oscillators have been designed in the pure optical domain or the electrical domain, which are both delay-controlled oscillators with a steady oscillation confined by the cavity delay. Ming Li from the Chinese Academy of Sciences in Beijing and his colleagues have now developed a brand-new parametric oscillator in the microwave photonics domain, i.e., a hybrid optical-electrical oscillator. Owing to the unique parametric process in the optoelectronics cavity, the oscillation in the optoelectronic parametric oscillator is a phase-controlled operation, leading to a steady oscillation that is not bounded by the cavity delay. Continuously tuneable single frequency oscillation and stable multimode oscillation are produced by the new optoelectronic parametric oscillator, which are hard or even impossible to achieve in traditional delay-controlled oscillators.

The remarkable achievements in the area of integrated optical memories and optical random access memories (RAMs) together with the rapid adoption of optical interconnects in the Datacom and Computercom industries introduce a new perspective for information storage directly in the optical domain, enabling fast access times, increased bandwidth and transparent cooperation with optical interconnect lines. This article reviews state-of-the-art integrated optical memory technologies and optical RAM cell demonstrations describing the physical mechanisms of several key devices along with their performance metrics in terms of their energy, speed and footprint. Novel applications are outlined, concluding with the scaling challenges to be addressed toward allowing light to serve as both a data-carrying and data-storage medium., Optical memory: integration overview Integrated optical memory technologies may in the future become an attractive option for storing data in an energy efficient and compact manner. The progress that has been made in the field has now been reviewed by three Greek researchers. Theoni Alexoudi and Nikos Pleros from Aristotle University of Thessaloniki and George Kanellos from University of Bristol describe how the energy consumption, speed and size of optical Random Access Memory (RAM) and flip-flop memory has been transformed over the past 25 years. They report how the footprint of optical memory cells has shrunk from the meter to the sub-micrometer scale, while the energy per bit has now reached 1 fJ/bit and access times have approached a few tens of picoseconds. Schemes based on semiconductor optical amplifiers, micro-ring and micro-disk lasers and photonic crystals are all discussed.

In vivo fundus imaging offers non-invasive access to neuron structures and biochemical processes in the retina. However, optical aberrations of the eye degrade the imaging resolution and prevent visualization of subcellular retinal structures. We developed an adaptive optics two-photon excitation fluorescence microscopy (AO-TPEFM) system to correct ocular aberrations based on a nonlinear fluorescent guide star and achieved subcellular resolution for in vivo fluorescence imaging of the mouse retina. With accurate wavefront sensing and rapid aberration correction, AO-TPEFM permits structural and functional imaging of the mouse retina with submicron resolution. Specifically, simultaneous functional calcium imaging of neuronal somas and dendrites was demonstrated. Moreover, the time-lapse morphological alteration and dynamics of microglia were characterized in a mouse model of retinal disorder. In addition, precise laser axotomy was achieved, and degeneration of retinal nerve fibres was studied. This high-resolution AO-TPEFM is a promising tool for non-invasive retinal imaging and can facilitate the understanding of a variety of eye diseases as well as neurodegenerative disorders in the central nervous system., Adaptive-Optics Fluorescence Microscopy: Seeing clearly into eyes An advance in Two-Photon Excitation Fluorescence Microscopy (TPEFM) allows greatly improved subcellular imaging of the retina, promising better understanding of the eye and the central nervous system in health and disease. Researchers led by Jianan Y. Qu at Hong Kong University of Science and Technology used adaptive optics techniques to correct the optical aberrations that have limited application of TPEFM in living eyes. They demonstrated their innovation using fluorescent detector molecules to image the activity of calcium ions in mouse retinal neurons. They also monitored subcellular structures in microglial cells, and guided laser light to cut the axons that transmit nerve signals. This technology could help investigate the development of neurodegenerative diseases, since the eye offers a window into nerves of the central nervous system that link the eye with the brain.

Numerous valuable studies on electron dynamics have focussed on the extraordinary properties of molybdenum disulfide (MoS2); however, most of them were confined to the level below the damage threshold. Here the electron dynamics of MoS2 under intense ultrafast laser irradiation was investigated by experiments and simulations. Two kinds of ablation mechanisms were revealed, which led to two distinct types of electron dynamics and final ablation morphology. At a higher fluence, the emergence of superheated liquid induced a dramatic change in the transient reflectivity and micro-honeycomb structures. At a lower fluence, the material was just removed by sublimation, and the ablation structure was relatively flat. X-ray photoelectron spectroscopic (XPS) measurements demonstrated that thermal decomposition only occurred at the higher fluence. Furthermore, a theoretical model was developed to deeply reveal the ultrafast dynamics of MoS2 ablation. The simulation results were in good agreement with the temporal and spatial reflectivity distribution obtained from the experiment. The electron and lattice temperature evolution was also obtained to prove the ablation mechanism. Our results revealed ultrafast dynamics of MoS2 above the damage threshold and are helpful for understanding the interaction mechanism between MoS2 and intense ultrafast lasers, as well as for MoS2 processing applications., Optoelectronics: Digging into molybdenum disulfide Ultrafast damage-causing laser pulses affect bulk molybdenum disulfide differently depending on their energy per area, revealing further insight into the ultrafast electron dynamics. The research group of Lan Jiang from Beijing Institute of Technology, in cooperation with Tianhong Cui of University of Minnesota, has predicted and examined what happened to molybdenum disulfide on electron and lattice level when intense femtosecond laser pulses irradiate. Lower damage-causing energies per area, known as fluences, removed material through inducing a solid-to-gas transition. Higher fluences caused superheating, liquidation, and the formation of nanocracks and nanoridges. Tests on electron dynamics obtained by the materials’ reflectivity with different fluences were followed by theoretical simulations, which further helped to reveal the electron and lattice temperature evolution. The findings further understandings for laser processing of molybdenum disulfide and for its use in optoelectronics.

Two-dimensional (2D) materials such as graphene and transition-metal chalcogenides have been extensively studied because of their superior electronic and optical properties. Recently, 2D materials have shown great practical application in position-sensitive detectors (PSDs), originating from the lateral photoelectrical effect of the materials or junctions. The high position sensitivity and ultrafast photoresponse of PSDs based on 2D materials, especially compatibility with Si technology, may enable diverse optoelectronic applications. In this review, recent studies of PSDs based on 2D materials are summarized, providing a promising route for high-performance PSDs., 2D materials: position-sensitive detectors Photodetectors that can monitor the location of an incident laser spot are useful for applications such as laser beam alignment and stabilization. While silicon p-n or p-i-n detectors are commonly used, Chang Hu, Xianjie Wang and Bo Song from Harbin Institute of Technology in China, have now reviewed the advantages and capabilities of using a variety of 2D materials for performing this task. They report how junctions based on pairing graphene or molybdenum disulfide with silicon can result in positive-sensitive detectors can be highly sensitive and offer an ultrafast response. The good performance is thought to be due to the high charge mobility and strong absorption of the 2D materials. Future challenges include extending the wavelength range of operation of such detectors beyond the visible to the near-infrared and ultraviolet.

The Environmental Trace Gases Monitoring Instrument (EMI) is the first Chinese satellite-borne UV–Vis spectrometer aiming to measure the distribution of atmospheric trace gases on a global scale. The EMI instrument onboard the GaoFen-5 satellite was launched on 9 May 2018. In this paper, we present the tropospheric nitrogen dioxide (NO2) vertical column density (VCD) retrieval algorithm dedicated to EMI measurement. We report the first successful retrieval of tropospheric NO2 VCD from the EMI instrument. Our retrieval improved the original EMI NO2 prototype algorithm by modifying the settings of the spectral fit and air mass factor calculations to account for the on-orbit instrumental performance changes. The retrieved EMI NO2 VCDs generally show good spatiotemporal agreement with the satellite-borne Ozone Monitoring Instrument and TROPOspheric Monitoring Instrument (correlation coefficient R of ~0.9, bias, Remote sensing: Tuned-up satellite identifies pollutant hot spots Improvements to a high-resolution spectrometer onboard China’s recently-launched GaoFen-5 (GF-5) satellite can give researchers a new tool to assess atmospheric trace gases. Cheng Liu from the University of Science and Technology of China and his colleagues reported the first successful retrieval of nitrogen dioxide from the GF-5 orbiter thanks to crucial adjustments to calibration algorithms. The satellite’s payload Environmental Trace Gases Monitoring Instrument (EMI) uses visible spectroscopic measurements to detect nitrogen dioxide in the atmosphere, which can be used to identify the spatial distribution of pollution emissions on the Earth. To overcome on-orbit issues including detector saturation, the team has optimized the spectral retrieval settings by spectral precalibration, fitting wavelength adjustment, reference spectrum selection, etc. Comparisons to other satellite and ground measurements revealed the optimization effort yielded reliable results for monitoring pollution levels.

Femtosecond mode-locked laser frequency combs have served as the cornerstone in precision spectroscopy, all-optical atomic clocks, and measurements of ultrafast dynamics. Recently frequency microcombs based on nonlinear microresonators have been examined, exhibiting remarkable precision approaching that of laser frequency combs, on a solid-state chip-scale platform and from a fundamentally different physical origin. Despite recent successes, to date, the real-time dynamical origins and high-power stabilities of such frequency microcombs have not been fully addressed. Here, we unravel the transitional dynamics of frequency microcombs from chaotic background routes to femtosecond mode-locking in real time, enabled by our ultrafast temporal magnifier metrology and improved stability of dispersion-managed dissipative solitons. Through our dispersion-managed oscillator, we further report a stability zone that is more than an order-of-magnitude larger than its prior static homogeneous counterparts, providing a novel platform for understanding ultrafast dissipative dynamics and offering a new path towards high-power frequency microcombs.

Lensless imaging eliminates the need for geometric isomorphism between a scene and an image while allowing the construction of compact, lightweight imaging systems. However, a challenging inverse problem remains due to the low reconstructed signal-to-noise ratio. Current implementations require multiple masks or multiple shots to denoise the reconstruction. We propose single-shot lensless imaging with a Fresnel zone aperture and incoherent illumination. By using the Fresnel zone aperture to encode the incoherent rays in wavefront-like form, the captured pattern has the same form as the inline hologram. Since conventional backpropagation reconstruction is troubled by the twin-image problem, we show that the compressive sensing algorithm is effective in removing this twin-image artifact due to the sparsity in natural scenes. The reconstruction with a significantly improved signal-to-noise ratio from a single-shot image promotes a camera architecture that is flat and reliable in its structure and free of the need for strict calibration., Lensless imaging: reduced noise Two simple innovations have been found to significantly improve the quality of lensless imaging. Jiachen Wu and coworkers from Tsinghua University in China and MIT in the US report that introducing a Fresnel optical element and applying a compressive sensing algorithm can significantly reduce the level of noise in reconstructed images. Their simple lensless system consists of a Fresnel Zone Aperture (FZA) placed 3 mm in front of a CMOS image sensor. The FZA is a mask of patterned chrome that consists of a series of concentric rings that alternatingly transmit and block light. Images on an LCD screen placed ~30 cm from the mask are used as a test object. The signal recorded by the CMOS sensor is then reconstructed by a compressive sensing algorithm with total variation denoising to generate an improved image of the object.

Mode-locked fibre lasers (MLFLs) are fundamental building blocks of many photonic systems used in industrial, scientific and biomedical applications. To date, 1–2 μm MLFLs have been well developed; however, passively mode-locked fibre lasers in the visible region (380–760 nm) have never been reported. Here, we address this challenge by demonstrating an all-fibre visible-wavelength passively mode-locked picosecond laser at 635 nm. The 635 nm mode-locked laser with an all-fibre figure-eight cavity uses a Pr/Yb codoped ZBLAN fibre as the visible gain medium and a nonlinear amplifying loop mirror as the mode-locking element. First, we theoretically predict and analyse the formation and evolution of 635 nm mode-locked pulses in the dissipative soliton resonance (DSR) regime by solving the Ginzburg-Landau equation. Then, we experimentally demonstrate the stable generation of 635 nm DSR mode-locked pulses with a pulse duration as short as ~96 ps, a radio-frequency signal-to-noise ratio of 67 dB and a narrow spectral bandwidth of 1 nm) and modulated optical spectrum. This work represents an important step towards miniaturized ultrafast fibre lasers in the visible spectral region., Photonics: Lighting the way ultrafast compact visible light lasers Chinese scientists have demonstrated a low-cost and compact ultrafast passively mode-locked laser that operates in the visible light spectrum and could see use in a range of industrial, scientific, and biomedical applications. Although Mode-locked fibre lasers (MLFLs) are the fundamental building blocks of many photonic systems, ultrafast lasers in the visible light spectral region are costly and challenging to make. For the first time, Zhengqian Luo and colleagues from Xiamen University in China have demonstrated a visible-wavelength passively mode-locked all-fibre laser that operates in the dissipative soliton resonance regime. The laser generates picosecond pulses of light at 635 nanometres and represents an essential step towards miniaturized ultrafast fibre lasers in the visible light range. The work lays the foundations for photonic devices for use in applications such as material processing, medicine, spectroscopy, and optical communications.

The sun and outer space are two of the most important fundamental thermodynamic resources for renewable energy harvesting. A significant amount of work has focused on understanding the fundamental limit of energy harvesting from the sun. More recently, there have been several theoretical analyses of the fundamental limit of energy harvesting from outer space. However, far less is understood about the fundamental limits of simultaneous energy harvesting from both the sun and outer space. Here, we consider and introduce various schemes that are capable of simultaneous energy harvesting and elucidate the fundamental thermodynamic limits of these schemes. We show that the theoretical limits can far exceed the previously established limit associated with utilizing only one thermodynamic resource. Our results highlight the significant potential of simultaneous energy harvesting and indicate new fundamental opportunities for improving the efficiency of energy harvesting systems., Energy harvesting: trapping the two-way flow Energy harvesting systems that combine capturing incoming solar energy with exploiting the emission of thermal energy outwards into space offer possibilities for improving our ability to generate power from the natural and freely available energy flows on Earth. Shanhui Fan and colleagues at Stanford University, USA, explore the theoretical limits of such simultaneous energy harvesting processes. While the theoretical efficiency of trapping incoming solar energy has been extensively studied, much less attention has been given to ‘radiative cooling’ systems, such as devices that extract energy from the dissipation of heat outwards towards the night sky. Interest in harnessing that previously neglected energy flow, however, is steadily increasing. The research team’s theoretical calculations suggest that schemes exploiting energy flow in both directions should be explored to enhance our use of the energy gradients available to us.

Optical logic operations lie at the heart of optical computing, and they enable many applications such as ultrahigh-speed information processing. However, the reported optical logic gates rely heavily on the precise control of input light signals, including their phase difference, polarization, and intensity and the size of the incident beams. Due to the complexity and difficulty in these precise controls, the two output optical logic states may suffer from an inherent instability and a low contrast ratio of intensity. Moreover, the miniaturization of optical logic gates becomes difficult if the extra bulky apparatus for these controls is considered. As such, it is desirable to get rid of these complicated controls and to achieve full logic functionality in a compact photonic system. Such a goal remains challenging. Here, we introduce a simple yet universal design strategy, capable of using plane waves as the incident signal, to perform optical logic operations via a diffractive neural network. Physically, the incident plane wave is first spatially encoded by a specific logic operation at the input layer and further decoded through the hidden layers, namely, a compound Huygens’ metasurface. That is, the judiciously designed metasurface scatters the encoded light into one of two small designated areas at the output layer, which provides the information of output logic states. Importantly, after training of the diffractive neural network, all seven basic types of optical logic operations can be realized by the same metasurface. As a conceptual illustration, three logic operations (NOT, OR, and AND) are experimentally demonstrated at microwave frequencies., Metasurface provides clarity for optical logic Carefully-designed diffractive metasurfaces enable accurate logical operations for optical computing. Using photons instead of electrons to convey information could reduce energy consumption and increase computing speed. However, the light properties must be controlled with very high precision in order to clearly distinguish between ‘0’ and ‘1’ logical states, and this is difficult to do without adding extra bulky components. Now, Hongsheng Chen at Zhejiang University in China and co-workers have used diffractive metasurfaces – carefully structured optical components designed from the atomic scale upwards – to realize all seven of the basic logic operations used in computation. Their components, which can be shrunk to the chip scale, clearly direct light into one of two designated areas indicating ‘0’ or ‘1’, even when the properties of the input light have not been optimally controlled.

Nonlinear frequency conversion is a ubiquitous technique that is used to obtain broad-range lasers and supercontinuum coherent sources. The phase-matching condition (momentum conservation relation) is the key criterion but a challenging bottleneck in highly efficient conversion. Birefringent phase matching (BPM) and quasi-phase matching (QPM) are two feasible routes but are strongly limited in natural anisotropic crystals or ferroelectric crystals. Therefore, it is in urgent demand for a general technique that can compensate for the phase mismatching in universal nonlinear materials and in broad wavelength ranges. Here, an additional periodic phase (APP) from order/disorder alignment is proposed to meet the phase-matching condition in arbitrary nonlinear crystals and demonstrated from the visible region to the deep-ultraviolet region (e.g., LiNbO3 and quartz). Remarkably, pioneering 177.3-nm coherent output is first obtained in commercial quartz crystal with an unprecedented conversion efficiency above 1‰. This study not only opens a new roadmap to resuscitate those long-neglected nonlinear optical crystals for wavelength extension, but also may revolutionize next-generation nonlinear photonics and their further applications.

Continuous room temperature nanowire lasing from silicon-integrated optoelectronic elements requires careful optimisation of both the lasing cavity Q-factor and population inversion conditions. We apply time-gated optical interferometry to the lasing emission from high-quality GaAsP/GaAs quantum well nanowire laser structures, revealing high Q-factors of 1250 ± 90 corresponding to end-facet reflectivities of R = 0.73 ± 0.02. By using optimised direct–indirect band alignment in the active region, we demonstrate a well-refilling mechanism providing a quasi-four-level system leading to multi-nanosecond lasing and record low room temperature lasing thresholds (~6 μJ cm−2 pulse−1) for III–V nanowire lasers. Our findings demonstrate a highly promising new route towards continuously operating silicon-integrated nanolaser elements., Lasers: Record-breaking lasing nanowires Persistent short pulses of near-infrared laser light can be produced at record low thresholds using tiny nanowire lasers, suitable for small-scale optoelectronic devices, including optical computing chips. The technology has been developed and demonstrated by researchers in the UK, led by Patrick Parkinson at the University of Manchester. Their system is based on semiconducting nanowires composed of gallium arsenide phosphide and gallium arsenide. It combines both key functions of a laser in one material, as the gain medium that amplifies the light emission and the cavity medium for containing and manipulating the light. The researchers engineered the precise behavior of the nanowires to demonstrate highly optimized nano-lasing performance at room temperature lasing thresholds. Their work addresses and overcomes significant challenges that have been limiting developments in this promising field.

Organic-inorganic hybrid perovskite (OIHP) photodetectors that simultaneously achieve an ultrafast response and high sensitivity in the near-infrared (NIR) region are prerequisites for expanding current monitoring, imaging, and optical communication capbilities. Herein, we demonstrate photodetectors constructed by OIHP and an organic bulk heterojunction (BHJ) consisting of a low-bandgap nonfullerene and polymer, which achieve broadband response spectra up to 1 μm with a highest external quantum efficiency of approximately 54% at 850 nm, an ultrafast response speed of 5.6 ns and a linear dynamic range (LDR) of 191 dB. High sensitivity, ultrafast speed and a large LDR are preeminent prerequisites for the practical application of photodetectors. Encouragingly, due to the high-dynamic-range imaging capacity, high-quality visible-NIR actual imaging is achieved by employing the OIHP photodetectors. We believe that state-of-the-art OIHP photodetectors can accelerate the translation of solution-processed photodetector applications from the laboratory to the imaging market., Perovskites peer into near-infrared Novel photodetectors developed by researchers in China provide imaging in the near-infrared (NIR) region with record-breaking efficiency and speed. A new class of semiconducting materials called organic-inorganic hybrid perovskites (OIHPs) display excellent optical and electrical properties for thin-film solar cells, LEDs and light detectors. To expand their detection range to NIR, which is useful for biomedical applications, OIHPs can be combined with structures called organic bulk-heterojunctions (BHJs). Now, Weida Hu, Liang Shen and co-workers at Jilin University, Chinese Academy of Sciences and Peking University have designed new OIHP-BHJ photodetectors that efficiently detect a wide range of both visible and NIR radiation. Their prototype sensors have ultra-fast response times of just 5.6 nanoseconds and remain sensitive even in low brightness, suggesting that they could accelerate the movement of OIHP devices from lab tests to commercial imaging.

By integrating a free-standing cadmium sulfide (CdS) nanowire onto a silicon nitride (SiN) photonic chip, we demonstrate a highly compact on-chip single-mode CdS nanowire laser. The mode selection is realized using a Mach-Zehnder interferometer (MZI) structure. When the pumping intensity exceeds the lasing threshold of 4.9 kW/cm2, on-chip single-mode lasing at ~518.9 nm is achieved with a linewidth of 0.1 nm and a side-mode suppression ratio of up to a factor of 20 (13 dB). The output of the nanowire laser is channelled into an on-chip SiN waveguide with high efficiency (up to 58%) by evanescent coupling, and the directional coupling ratio between the two output ports can be varied from 90 to 10% by predesigning the coupling length of the SiN waveguide. Our results open new opportunities for both nanowire photonic devices and on-chip light sources and may pave the way towards a new category of hybrid nanolasers for chip-integrated applications.

Dirac semimetals, the materials featured with discrete linearly crossing points (called Dirac points) between four bands, are critical states of topologically distinct phases. Such gapless topological states have been accomplished by a band-inversion mechanism, in which the Dirac points can be annihilated pairwise by perturbations without changing the symmetry of the system. Here, we report an experimental observation of Dirac points that are enforced completely by the crystal symmetry, using a nonsymmorphic three-dimensional phononic crystal. Intriguingly, our Dirac phononic crystal hosts four spiral topological surface states, in which the surface states of opposite helicities intersect gaplessly along certain momentum lines, as confirmed by our further surface measurements. The novel Dirac system may release new opportunities for studying the elusive (pseudo)relativistic physics, and also offer a unique prototype platform for acoustic applications.

Semiconductors that can provide optical gain at extremely low carrier density levels are critically important for applications such as energy efficient nanolasers. However, all current semiconductor lasers are based on traditional semiconductor materials that require extremely high density levels above the so-called Mott transition to realize optical gain. The new emerging 2D materials provide unprecedented opportunities for studying new excitonic physics and exploring new optical gain mechanisms at much lower density levels due to the strong Coulomb interaction and co-existence and mutual conversion of excitonic complexes. Here, we report a new gain mechanism involving charged excitons or trions in electrically gated 2D molybdenum ditelluride well below the Mott density. Our combined experimental and modelling study not only reveals the complex interplay of excitonic complexes well below the Mott transition but also establishes 2D materials as a new class of gain materials at densities 4–5 orders of magnitude lower than those of conventional semiconductors and provides a foundation for lasing at ultralow injection levels for future energy efficient photonic devices. Additionally, our study could help reconcile recent conflicting results on 2D materials: While 2D material-based lasers have been demonstrated at extremely low densities with spectral features dominated by various excitonic complexes, optical gain was only observed in experiments at densities several orders of magnitude higher, beyond the Mott density. We believe that our results could lead to more systematic studies on the relationship between the mutual conversion of excitonic species and the existence of optical gain well below the Mott transition., Trion tricks give optical gain A new mechanism for amplifying optical signals in two-dimensional semiconductors could be used to develop nano-sized lasers with little input power. When high-energy photons strike a semiconductor they produce excitons – pairs comprising an excited electron and the positively-charged ‘hole’ that it leaves behind, or even trions - states comprising two electrons and one hole, or one electron and two holes. However, scientists have so far limited understanding of the many exotic exciton states that can exist and mutually convert. Cun-Zheng Ning at Tsinghua University in Beijing and co-workers studied ultra-thin single and dual layers of molybdenum ditelluride, and observed a complex interplay between excitons and trions. The trion states enabled the system to show optical gain – a vital property for lasers - despite being orders of magnitude below the so-called Mott transition density, a minimum condition for optical gain in conventional semiconductors.

Light 9, 1-7 (2020). doi:https://doi.org/10.1038/s41377-020-0254-7, Silicon is well known for its strong third-order optical nonlinearity, exhibiting efficient supercontinuum and four-wavemixing processes. A strong second-order effect that is naturally inhibited in silicon can also be observed, for example,by electrically breaking the inversion symmetry and quasi-phase matching the pump and the signal. To generate anefficient broadband second-harmonic signal, however, the most promising technique requires matching the groupvelocities of the pump and the signal. In this work, we utilize dispersion engineering of a silicon waveguide to achievegroup velocity matching between the pump and the signal, along with an additional degree of freedom to broadenthe second harmonic through the strong third-order nonlinearity. We demonstrate that the strong self-phasemodulation and cross-phase modulation in silicon help broaden the second harmonic by 200 nm in the O-band.Furthermore, we show a waveguide design that can be used to generate a second-harmonic signal in the entire nearinfrared region. Our work paves the way for various applications, such as efficient and broadband complementarymetal oxide semiconductor based on—chip frequency synthesizers, entangled photon pair generators, and opticalparametric oscillators, Published by Nature Publishing Group, London

Graphene has attracted much attention for the realization of high-speed photodetection for silicon photonics over a wide wavelength range. However, the reported fast graphene photodetectors mainly operate in the 1.55 μm wavelength band. In this work, we propose and realize high-performance waveguide photodetectors based on bolometric/photoconductive effects by introducing an ultrathin wide silicon−graphene hybrid plasmonic waveguide, which enables efficient light absorption in graphene at 1.55 μm and beyond. When operating at 2 μm, the present photodetector has a responsivity of ~70 mA/W and a setup-limited 3 dB bandwidth of >20 GHz. When operating at 1.55 μm, the present photodetector also works very well with a broad 3 dB bandwidth of >40 GHz (setup-limited) and a high responsivity of ~0.4 A/W even with a low bias voltage of −0.3 V. This work paves the way for achieving high-responsivity and high-speed silicon–graphene waveguide photodetection in the near/mid-infrared ranges, which has applications in optical communications, nonlinear photonics, and on-chip sensing., 2 µm photodetectors: silicon−graphene−metal hybrid plasmonics The use of a silicon−graphene plasmonic waveguide has enabled the realization of fast and sensitive photodetectors that operate at the wavelength of 2 µm. In order to satisfy the demands for the applications in optical communication and optical sensing, there is the need to extend silicon photonics to wavelengths beyond 1.55 µm. However, it is a challenge to create high-performance photodetectors at these wavelengths. Now, Daoxin Dai and coworkers from Zhejiang University and Southeast University in China have proposed and realized a silicon−graphene hybrid plasmonic waveguide photodetector that operates at 2 µm with a responsivity of ~70 mA/W and a 3-dB bandwidth over 20 GHz. In this design, efficient light absorption in graphene is enabled by using a hybrid plasmonic waveguide with a wide thin silicon ridge core and a metal cap that serves as a signal electrode.

The launch of the big data era puts forward challenges for information preservation technology, both in storage capacity and security. Herein, a brand new optical storage medium, transparent glass ceramic (TGC) embedded with photostimulated LiGa5O8: Mn2+ nanocrystals, capable of achieving bit-by-bit optical data write-in and read-out in a photon trapping/detrapping mode, is developed. The highly ordered nanostructure enables light–matter interaction with high encoding/decoding resolution and low bit error rate. Importantly, going beyond traditional 2D optical storage, the high transparency of the studied bulk medium makes 3D volumetric optical data storage (ODS) possible, which brings about the merits of expanded storage capacity and improved information security. Demonstration application confirmed the erasable–rewritable 3D storage of binary data and display items in TGC with intensity/wavelength multiplexing. The present work highlights a great leap in photostimulated material for ODS application and hopefully stimulates the development of new multi-dimensional ODS media. An optical data storage (ODS) system that is cost-effective to make and easily scalable could lay the foundations for data storage technologies with higher capacity and improved security. The era of big data is driving the need for data storage systems with larger capacity, better security and improved performance. However, even the latest two-dimensional optical disk technologies cannot store more than one terabyte of data, leading scientists to explore alternative technologies. Now, a team of Chinese researchers, led by Yuansheng Wang from the Chinese Academy of Sciences, has developed a three-dimensional ODS system that uses light-emitting nanocrystals (NCs) to store and retrieve information. Made from a transparent glass ceramic embedded with photostimulated luminescent rare-earth-doped metallic NCs, the new ODS system can store 3D optical data with increased capacity and improved security compared with current technologies.

Direct femtosecond (fs) laser processing is a maskless fabrication technique that can effectively modify the optical, electrical, mechanical, and tribological properties of materials for a wide range of potential applications. However, the eventual implementation of fs-laser-treated surfaces in actual devices remains challenging because it is difficult to precisely control the surface properties. Previous studies of the morphological control of fs-laser-processed surfaces mostly focused on enhancing the uniformity of periodic microstructures. Here, guided by the plasmon hybridisation model, we control the morphology of surface nanostructures to obtain more control over spectral light absorption. We experimentally demonstrate spectral control of a variety of metals [copper (Cu), aluminium (Al), steel and tungsten (W)], resulting in the creation of broadband light absorbers and selective solar absorbers (SSAs). For the first time, we demonstrate that fs-laser-produced surfaces can be used as high-temperature SSAs. We show that a tungsten selective solar absorber (W-SSA) exhibits excellent performance as a high-temperature solar receiver. When integrated into a solar thermoelectric generation (TEG) device, W-SSA provides a 130% increase in solar TEG efficiency compared to untreated W, which is commonly used as an intrinsic selective light absorber., Materials processing: laser modified metals Femtosecond-laser processing of materials has many potential applications including high-efficiency incandescent lamps and solar-thermal receivers. A longstanding challenge is the ability to control the emerging properties of the treated surfaces. A research team at Chunlei Guo’s lab from the Institute of Optics at the University of Rochester showed that the spectral absorption properties of treated metals can be controlled systematically by controlling the laser processing parameters. They showed that the formed structures act as interacting nano-antennas which absorbs light based on the excitation of electromagnetic resonances. As a direct application, they converted a Tungsten surface to a selective solar-absorber and used it to operate a thermoelectric generator which resulted in a 130% increase in efficiency compared to an untreated tungsten receiver.

With ever-increasing laser power, the requirements for ultraviolet (UV) coatings increase continuously. The fundamental challenge for UV laser-resistant mirror coatings is to simultaneously exhibit a high reflectivity with a large bandwidth and high laser resistance. These characteristics are traditionally achieved by the deposition of laser-resistant layers on highly reflective layers. We propose a “reflectivity and laser resistance in one” design by using tunable nanolaminate layers that serve as an effective layer with a high refractive index and a large optical bandgap. An Al2O3–HfO2 nanolaminate-based mirror coating for UV laser applications is experimentally demonstrated using e-beam deposition. The bandwidth, over which the reflectance is >99.5%, is more than twice that of a traditional mirror with a comparable overall thickness. The laser-induced damage threshold is increased by a factor of ~1.3 for 7.6 ns pulses at a wavelength of 355 nm. This tunable, nanolaminate-based new design strategy paves the way toward a new generation of UV coatings for high-power laser applications.

The orbital angular momentum (OAM) of light has been shown to be useful in diverse fields ranging from astronomy and optical trapping to optical communications and data storage. However, one of the primary impediments preventing such applications from widespread adoption is the lack of a straightforward and dynamic method to sort incident OAM states without altering the states. Here, we report a technique that can dynamically filter individual OAM states and preserve the incident OAM states for subsequent processing. Although the working principle of this technique is based on resonance, the device operation is not limited to a particular wavelength. OAM states with different wavelengths can resonate in the resonator without any additional modulation other than changing the length of the cavity. Consequently, we are able to demonstrate a reconfigurable OAM sorter that is constructed by cascading such optical resonators. This approach does not require specially designed components and is readily amenable to integration into potential applications., Orbital angular momentum states: dynamic sorting The sorting of light of different states of orbital angular momentum (OAM) is often required for applications wishing to exploit twisted light beams. Now, Shibiao Wei and coworkers from China and Australia have developed a convenient solution to the task which is reconfigurable and dynamically tunable. The scheme makes use of a Fabry Perot (FP) cavity with a tunable length that is adjusted by a piezoelectric transducer. When the length of the cavity is tuned to be resonant with the desired OAM state, it is transmitted while all the others are rejected. By cascading several such cavities and actively tuning their lengths it is possible to create a dynamically reconfigurable OAM sorter. The team tested the approach with twisted vortex light beams generated by passing 632.8 nm HeNe laser light through a spatial light modulator.

The emergence of super-resolution (SR) fluorescence microscopy has rejuvenated the search for new cellular sub-structures. However, SR fluorescence microscopy achieves high contrast at the expense of a holistic view of the interacting partners and surrounding environment. Thus, we developed SR fluorescence-assisted diffraction computational tomography (SR-FACT), which combines label-free three-dimensional optical diffraction tomography (ODT) with two-dimensional fluorescence Hessian structured illumination microscopy. The ODT module is capable of resolving the mitochondria, lipid droplets, the nuclear membrane, chromosomes, the tubular endoplasmic reticulum, and lysosomes. Using dual-mode correlated live-cell imaging for a prolonged period of time, we observed novel subcellular structures named dark-vacuole bodies, the majority of which originate from densely populated perinuclear regions, and intensively interact with organelles such as the mitochondria and the nuclear membrane before ultimately collapsing into the plasma membrane. This work demonstrates the unique capabilities of SR-FACT, which suggests its wide applicability in cell biology in general., Microscopy: New tool furthers our understanding of living cells Scientists from China have developed a new microscopy tool that could help to further expand our understanding of cell biological processes in live cells. Although the emergence of superresolution (SR) fluorescence microscopy is revolutionising the life sciences, the technique can only be used to observe a handful of biomolecules simultaneously and cannot provide a holistic map of the cellular landscape. Furthermore, the imaging of live cells in three dimensions (3D) over time also remains very challenging. By combining two different techniques, a team of Chinese researchers led by Liangyi Chen and Kebin Shi from Peking University has created a novel dual-mode high-speed SR microscopy technique. The tool is capable of revealing previously unseen sub-cellular structures and dynamic processes, allowing both the cellular landscape and molecular identity in live cells to be observed in 3D over prolonged periods.

Organic light-emitting diodes (OLEDs) have become a major pixel technology in the display sector, with products spanning the entire range of current panel sizes. The ability to freely scale the active area to large and random surfaces paired with flexible substrates provides additional application scenarios for OLEDs in the general lighting, automotive, and signage sectors. These applications require higher brightness and, thus, current density operation compared to the specifications needed for general displays. As extended transparent electrodes pose a significant ohmic resistance, OLEDs suffering from Joule self-heating exhibit spatial inhomogeneities in electrical potential, current density, and hence luminance. In this article, we provide experimental proof of the theoretical prediction that OLEDs will display regions of decreasing luminance with increasing driving current. With a two-dimensional OLED model, we can conclude that these regions are switched back locally in voltage as well as current due to insufficient lateral thermal coupling. Experimentally, we demonstrate this effect in lab-scale devices and derive that it becomes more severe with increasing pixel size, which implies its significance for large-area, high-brightness use cases of OLEDs. Equally, these non-linear switching effects cannot be ignored with respect to the long-term operation and stability of OLEDs; in particular, they might be important for the understanding of sudden-death scenarios., Lighting: Modelling the electrothermal performance of organic light-emitting diodes Modelling the impact of electrically induced heat on the performance of organic light-emitting diodes (OLEDs) could pave the way for their use in large-area lighting applications, including general lighting, signage and automobiles. In recent years, OLEDs have become ubiquitous in lighting and display technology because of their high power efficiency and sharp colours. However, their luminance diminishes with the increased current density required for high-brightness large-area lighting applications, an effect that is related to Joule self-heating. Now, Sebastian Reineke and colleagues from the Technische Universität Dresden and Weierstrass Institute Berlin in Germany have, for the first time, demonstrated so-called ‘switched-back’ regions of decreased luminance on a lab-sized OLED display. By modelling the electrothermal characteristics of OLEDs, the researchers could lay the foundations for handling these effects in large-area lighting applications, such as car lights and flexible lighting panels.

Parallelized fluorescence imaging has been a long-standing pursuit that can address the unmet need for a comprehensive three-dimensional (3D) visualization of dynamical biological processes with minimal photodamage. However, the available approaches are limited to incomplete parallelization in only two dimensions or sparse sampling in three dimensions. We hereby develop a novel fluorescence imaging approach, called coded light-sheet array microscopy (CLAM), which allows complete parallelized 3D imaging without mechanical scanning. Harnessing the concept of an “infinity mirror”, CLAM generates a light-sheet array with controllable sheet density and degree of coherence. Thus, CLAM circumvents the common complications of multiple coherent light-sheet generation in terms of dedicated wavefront engineering and mechanical dithering/scanning. Moreover, the encoding of multiplexed optical sections in CLAM allows the synchronous capture of all sectioned images within the imaged volume. We demonstrate the utility of CLAM in different imaging scenarios, including a light-scattering medium, an optically cleared tissue, and microparticles in fluidic flow. CLAM can maximize the signal-to-noise ratio and the spatial duty cycle, and also provides a further reduction in photobleaching compared to the major scanning-based 3D imaging systems. The flexible implementation of CLAM regarding both hardware and software ensures compatibility with any light-sheet imaging modality and could thus be instrumental in a multitude of areas in biological research., Microscopy: parallel light sheets give a gentler view A new imaging technique called coded light-sheet array microscopy (CLAM) brings an improved ability to visualize biological samples in three dimensions while minimizing the damage. A research team, led by Kevin Tsia at the University of Hong Kong, developed the technique and demonstrated its use on a variety of biological imaging scenarios. CLAM is a new form of 3D fluorescence microscopy, which collects all the fluorescence signals from the entire illuminated volume of a sample in parallel. Its key innovation is the way it uses a concept called “infinity mirror” to generate an array of parallel slices of laser light (“light sheets”) for three-dimensional imaging without the need for repetitive scanning and with much gentler illumination than the state-of-the-art techniques. CLAM could be applied to existing light sheet fluorescence microscopes, with minimal system modifications.

Transparent solar cells (TSCs) are emerging devices that combine the advantages of visible transparency and light-to-electricity conversion. Currently, existing TSCs are based predominantly on organics, dyes, and perovskites; however, the rigidity and color-tinted transparent nature of those devices strongly limit the utility of the resulting TSCs for real-world applications. Here, we demonstrate a flexible, color-neutral, and high-efficiency TSC based on a freestanding form of n-silicon microwires (SiMWs). Flat-tip SiMWs with controllable spacing are fabricated via deep-reactive ion etching and embedded in a freestanding transparent polymer matrix. The light transmittance can be tuned from ~10 to 55% by adjusting the spacing between the microwires. For TSCs, a heterojunction is formed with a p-type polymer in the top portion of the n-type flat-tip SiMWs. Ohmic contact with an indium-doped ZnO film occurs at the bottom, and the side surface has an Al2O3 passivation layer. Furthermore, slanted-tip SiMWs are developed by a novel solvent-assisted wet etching method to manipulate light absorption. Finite-difference time-domain simulation revealed that the reflected light from slanted-tip SiMWs helps light-matter interactions in adjacent microwires. The TSC based on the slanted-tip SiMWs demonstrates 8% efficiency at a visible transparency of 10% with flexibility. This efficiency is the highest among Si-based TSCs and comparable with that of state-of-the-art neutral-color TSCs based on organic–inorganic hybrid perovskite and organics. Moreover, unlike others, the stretchable and transparent platform in this study is promising for future TSCs., Transparent solar cells: Microwire-array shows potential A flexible, stretchable and fully transparent solar cell shows promise for harvesting sunlight as it hits windows. In designing transparent solar cells, there is a trade-off between efficiency and transparency – many existing cells are rigid and tinted because of the way they absorb certain wavelengths of light, and this limits their use in electronic devices and windows. Now, Kyoung Jin Choi at the Ulsan National Institute of Science and Technology, Ulsan, Korea, and co-workers have used a freestanding form of microwire-arrays to create a flexible, fully transparent solar cell. Their design comprises a crystalline silicon microwire array embedded in a transparent polymer matrix. The conducting junction occurs where the n-type-Si microwire tips meet the p-type polymer. The team also manipulated the shape of the microwire tips so that this enhances efficiency whilst maintaining transparency.

A novel concept of the heterophase optics-electronics synergistic effect has been demonstrated in a single-layer α/δ-heterophase perovskite CsPbI3 in order to realize white LEDs featuring only one broadband emissive layer.

Exciton diffusion lengths reaching the micrometer length scale have long been desired in solution-processed semiconductors but have remained unattainable using conventional materials to date. Now halide perovskite nanocrystal films show unprecedented exciton migration with diffusion lengths approaching 1 µm owing to the efficient combination of radiative and nonradiative energy transfer.

Abundances of a range of air pollutants can be inferred from satellite UV-Vis spectroscopy measurements by using the unique absorption signatures of gas species. Here, we implemented several spectral fitting methods to retrieve tropospheric NO2, SO2, and HCHO from the ozone monitoring instrument (OMI), with radiative simulations providing necessary information on the interactions of scattered solar light within the atmosphere. We analyzed the spatial distribution and temporal trends of satellite-observed air pollutants over eastern China during 2005–2017, especially in heavily polluted regions. We found significant decreasing trends in NO2 and SO2 since 2011 over most regions, despite varying temporal features and turning points. In contrast, an overall increasing trend was identified for tropospheric HCHO over these regions in recent years. Furthermore, generalized additive models were implemented to understand the driving forces of air quality trends in China and assess the effectiveness of emission controls. Our results indicated that although meteorological parameters, such as wind, water vapor, solar radiation and temperature, mainly dominated the day-to-day and seasonal fluctuations in air pollutants, anthropogenic emissions played a unique role in the long-term variation in the ambient concentrations of NO2, SO2, and HCHO in the past 13 years. Generally, recent declines in NO2 and SO2 could be attributed to emission reductions due to effective air quality policies, and the opposite trends in HCHO may urge the need to control anthropogenic volatile organic compound (VOC) emissions., Air pollution: satellite sees significant trends in China Satellite-based observations of air pollution in China reveal a recent declining trend in the levels of nitrogen dioxide (NO2) and sulfur dioxide (SO2), but rising levels of formaldehyde (HCHO). Chengxin Zhang and colleagues at the University of Science and Technology of China, with co-workers at other Chinese research centers, analyzed ultraviolet-visible spectroscopy data from the Ozone Monitoring Instrument of NASA’s EOS-Aura satellite. Meteorological effects such as winds, temperature and humidity caused day-to-day and seasonal fluctuations, but human activity appears responsible for long term trends. Declines in NO2 and SO2 levels occurred from 2011, across most regions, and are attributed to effective air quality policies. The rising trend in HCHO levels suggests that new measures may be needed to control the release by human activities of that and other volatile organic compounds.

Tomography is an informative imaging modality that is usually implemented by mechanical scanning, owing to the limited depth-of-field (DOF) in conventional systems. However, recent imaging systems are working towards more compact and stable architectures; therefore, developing nonmotion tomography is highly desirable. Here, we propose a metalens-based spectral imaging system with an aplanatic GaN metalens (NA = 0.78), in which large chromatic dispersion is used to access spectral focus tuning and optical zooming in the visible spectrum. After the function of wavelength-switched tomography was confirmed on cascaded samples, this aplanatic metalens is utilized to image microscopic frog egg cells and shows excellent tomographic images with distinct DOF features of the cell membrane and nucleus. Our approach makes good use of the large diffractive dispersion of the metalens and develops a new imaging technique that advances recent informative optical devices., Bioimaging: metalenses make 3D analysis effortless An ultrathin “metalens” that uses semiconductor nanostructures to focus light can help microscopes resolve detailed images of live cells without complex mechanical components. In conventional tomographic imaging, a scanning instrument passes over the sample and takes multiple cross-sectional images, which are then reconstructed into a 3D picture. Tao Li from Nanjing University in China and colleagues have now simplified this technique by fabricating lenses patterned with concentric rings of gallium nitride nanopillars. The lens made of nanopillars has wavelength-dependent properties that enable focusing on different regions of a target simply by sweeping through the visible light spectrum. Experiments with frog egg cells demonstrated this approach could zoom in and obtain detailed, high quality images of the inner membrane and nucleus without harming the sample or moving the metalens.

Intelligence at either the material or metamaterial level is a goal that researchers have been pursuing. From passive to active, metasurfaces have been developed to be programmable to dynamically and arbitrarily manipulate electromagnetic (EM) wavefields. However, the programmable metasurfaces require manual control to switch among different functionalities. Here, we put forth a smart metasurface that has self-adaptively reprogrammable functionalities without human participation. The smart metasurface is capable of sensing ambient environments by integrating an additional sensor(s) and can adaptively adjust its EM operational functionality through an unmanned sensing feedback system. As an illustrative example, we experimentally develop a motion-sensitive smart metasurface integrated with a three-axis gyroscope, which can adjust self-adaptively the EM radiation beams via different rotations of the metasurface. We develop an online feedback algorithm as the control software to make the smart metasurface achieve single-beam and multibeam steering and other dynamic reactions adaptively. The proposed metasurface is extendable to other physical sensors to detect the humidity, temperature, illuminating light, and so on. Our strategy will open up a new avenue for future unmanned devices that are consistent with the ambient environment., Metamaterials: Sensing surfaces that program themselves A metamaterial surface developed by researchers in China automatically adjusts its electromagnetic properties in response to changes in its environment. Metasurfaces are designed with tiny structures arranged in unique ways to manipulate electromagnetic radiation, and efforts are being made to integrate them with digital coding to produce adaptive, programmable systems. Tie Jun Cui at Southeast University in Nanjing and co-workers have designed smart metasurfaces with integrated sensors that detect features such as movement, light or temperature, linked to a microcontroller which responds by changing the configuration of a metasurface. A potential application of the system would be to help moving airplanes maintain contact with satellites; a gyroscope sensor detects changes in orientation, and the metasurface adapts its electromagnetic properties to ensure that the communication beam will always point in the satellite’s direction.