Over the past 5 years, digital coding and programmable metamaterials have been developed rapidly since their first exhibition in 2014. The iconic feature of the digital coding metamaterial is using digital codes like “0” and “1” to represent the distinct electromagnetic (EM) responses. This seemingly trivial progress has successfully reform the design theory from the effective medium to coding patterns, bridging the physical world and digital information world. More interestingly, beyond the simple coding on the parameters or patterns, the digital coding metamaterials are more intend to introduce the concept of direct interactions and operations of digital information within EM fields, to realize information processing, transmission or recognition. To accurately exhibit the informational specialties, we classify the coding metamaterials, digital metamaterials and programmable metamaterials, as well as other information-operating metamaterials, as information metamaterials. In this review article, we firstly introduce the digital coding concept, working mechanism, and related design methods. Then, three important theories including the scattering pattern calculation, convolution operation, and entropy of digital coding metamaterials, are discussed in details. Finally we introduce several system-level works based on the information metamaterials, such as the new-architecture wireless communication systems and reprogrammable imaging systems, to show the powerful manipulation capabilities of information metamaterials. As the next generation of information metamaterials, two proof-of-concept smart metamaterials and their advanced architectures are discussed. In the summary, the development track of information metamaterials and future trends are presented.

Metalens, a prominent application of two-dimensional metasurfaces, has demonstrated powerful abilities even beyond traditional optical lenses. By manipulating the phase distribution of metalens composed of appropriately arranged nanoscale building blocks, the wavefront of incident wave can be controlled based on Huygens principle, thus achieving the desired reflected and transmitted wave for many different purposes. Metalenses will lead a revolution in optical imaging due to its flat nature and compact size, multispectral acquisition and even off-axis focusing. Here, we review the recent progress of metalenses presenting excellent properties, with a focus on the imaging application using these metalenses. We firstly discuss the mechanism for achieving metalenses with high efficiency, large numerical aperture, controlling the chromatic dispersion or monochromatic aberrations and large area fabrication. Then, we review several important imaging applications including wide-band focusing imaging, polarization dependent imaging, light field imaging and some other significant imaging systems in different areas. Finally, we make a conclusion with an outlook on the future development and challenges of this developing research field.

Neuromorphic computing applies concepts extracted from neuroscience to develop devices shaped like neural systems and achieve brain-like capacity and efficiency. In this way, neuromorphic machines, able to learn from the surrounding environment to deduce abstract concepts and to make decisions, promise to start a technological revolution transforming our society and our life. Current electronic implementations of neuromorphic architectures are still far from competing with their biological counterparts in terms of real-time information-processing capabilities, packing density and energy efficiency. A solution to this impasse is represented by the application of photonic principles to the neuromorphic domain creating in this way the field of neuromorphic photonics. This new field combines the advantages of photonics and neuromorphic architectures to build systems with high efficiency, high interconnectivity and high information density, and paves the way to ultrafast, power efficient and low cost and complex signal processing. In this Perspective, we review the rapid development of the neuromorphic computing field both in the electronic and in the photonic domain focusing on the role and the applications of memristors. We discuss the need and the possibility to conceive a photonic memristor and we offer a positive outlook on the challenges and opportunities for the ambitious goal of realising the next generation of full-optical neuromorphic hardware.

InP technology is the principal enabler for implementing fully monolithic photonic integrated circuits (PIC), uniquely including transmitter elements. In this article we present an overview of recent achievements on ultra-high speed electro-absorption modulated lasers (EML) which represent a simple transmitter PIC comprising a single-mode laser diode and an electro-absorption modulator. Using a so-called identical-layer approach single-wavelength modulation rates up to 100 Gb/s have been accomplished. By additionally integrating an optical amplifier section modulated optical output power of > 10 dBm has been achieved. Multi-level amplitude modulation was successfully demonstrated. Extended EML chips designed for wavelength-division and space-division multiplexing, respectively, will be presented. For dual-polarization transmission a novel EML related transmitter as well as a corresponding receiver PIC have been introduced. The latter devices were made on a generic PIC platform that is available for open-access foundry service.

As an emerging channel resource for modern optics, big data, internet traffic and quantum technologies, twisted photons carrying orbital angular momentum (OAM) have been extended their applicable boundary in different media, such as optical fiber and atmosphere. Due to the extreme condition of loss and pressure, underwater transmission of twisted photons has not been well investigated yet. Especially, single-photon tests were all limited at a level of a few meters, and it is in practice unclear what will happen for longer transmission distances. Here we experimentally demonstrate the transmission of single-photon twisted light over an underwater channel up to 55 m, which reach a distance allowing potential real applications. For different order OAM states and their superposition, a good preservation of modal structure and topological charge are observed. Our results for the first time reveal the real transmission performance of twisted photons in a long-distance regime, representing a step further towards OAM-based underwater quantum communication.

In this paper, we propose a holographic capture and projection system of real objects based on tunable zoom lenses. Different from the traditional holographic system, a liquid lens-based zoom camera and a digital conical lens are used as key parts to reach the functions of holographic capture and projection, respectively. The zoom camera is produced by combing liquid lenses and solid lenses, which has the advantages of fast response and light weight. By electrically controlling the curvature of the liquid-liquid surface, the focal length of the zoom camera can be changed easily. As another tunable zoom lens, the digital conical lens has a large focal depth and the optical property is perfectly used in the holographic system for adaptive projection, especially for multilayer imaging. By loading the phase of the conical lens on the spatial light modulator, the reconstructed image can be projected with large depths. With the proposed system, holographic zoom capture and color reproduction of real objects can be achieved based on a simple structure. Experimental results verify the feasibility of the proposed system. The proposed system is expected to be applied to micro-projection and three-dimensional display technology.

Dual-comb spectroscopy is a powerful spectroscopic tool with ultrahigh-resolution, high-sensitivity properties, which opens up opportunities for the parallel detection of multi-species molecules. However, in its conventional form, highly stable laser combs with sophisticated control systems are required to perform dual-comb spectroscopy. Here, a passive mutually coherent dual-comb spectroscopy system via an optical-optical modulation method is addressed, where all fast phase-locking electronics are retired. Without post computer-based phase-correction, a high degree of mutual coherence between the two combs with a relative comb-tooth linewidth of 10 mHz is achieved, corresponding to a coherent time of 100 s. To demonstrate the performance and versatility of the system, the dual comb spectrometer is applied to record the mode-resolved single molecular spectra as well as parallel detected spectra of mixed gases including CO2, CO and C2H2 that well agree with the established spectral parameters. Our technique exhibits flexible wavelength tuning capability in the near-infrared region and can be potentially extended to the mid-infrared region for more applications.

The effect of transverse mode instability (TMI) is currently the main limitation for the further average-power scaling of fiber laser systems with diffraction-limited beam quality. In this work a main driving force for TMI in fiber amplifiers is identified. Our experiments and simulations illustrate that the performance of fiber laser systems in terms of their diffraction-limited output power can be significantly reduced when the pump or seed radiation exhibit intensity noise. This finding emphasizes the fact that the TMI threshold is not only determined by the active fiber but, rather, by the whole system. In the experiment an artificially applied pump intensity-noise of 2.9% led to a reduction of the TMI threshold of 63%, whereas a similar seed intensity-noise decreased it by just 13%. Thus, even though both noise sources have an impact on the TMI threshold, the pump intensity-noise can be considered as the main driver for TMI in saturated fiber amplifiers. Additionally, the work unveils that the physical origin of this behavior is linked to the noise transfer function in saturated fiber amplifiers. With the gained knowledge and the experimental and theoretical results, it can be concluded that a suppression of pump-noise frequencies below 20 kHz could strongly increase the TMI threshold in high-power fiber laser systems.

Noninvasive tomographic imaging of cellular processes in vivo may provide valuable cytological and histological information for disease diagnosis. However, such strategies are usually hampered by optical aberrations caused by the imaging system and tissue turbidity. State-of-the-art aberration correction methods require that the light signal be phase stable over the full-field data acquisition period, which is difficult to maintain during dynamic cellular processes in vivo. Here we show that any optical aberrations in the path length difference (OPD) domain can be corrected without the phase stability requirement based on maximum intensity assumption. Specifically, we demonstrate a novel optical tomographic technique, termed amplitude division aperture synthesis optical coherence tomography (ADAS-OCT), which corrects aberrations induced by turbid tissues by physical aperture synthesis and simultaneously data acquisition from sub-apertures. Even with just two sub-apertures, ADAS-OCT enabled in vivo visualization of red blood cells in human labial mucosa. We further demonstrated that adding sub-apertures could significantly scale up the aberration correction capability. This technology has the potential to impact a number of clinical areas where noninvasive examinations are preferred, such as blood count and cancers detection.

Conventional stereoscopic three-dimensional displays suffer from vergence- accommodation conflict because the stimulus to accommodation is fixed by the display panel and viewing optics, but that to vergence changes with image contents. With the recent rapid development of head-mounted displays, several methods have been proposed to offer the accommodation cues, among which multifocal display technology is an effective and practical solution. The first two decades of this century has witnessed the fast growth of multifocal displays from basic concept to mature implementations. This review systematically presents the state-of-the-art multifocal display design and development. Firstly, a comprehensive classification of numerous potential optical architectures to provide the multiplanar functionality is introduced, based on how the information is multiplexed and how the focal planes are generated. Next, the strengths and obstacles of reported or potential designs in each category are analyzed and compared with each other. In addition to enabling optics, the image rendering approaches for the multifocal planes are also described. This review presents a sufficient collection of past designs and is expected to offer a roadmap for future research and development of multifocal displays.

Since the first report of aggregation-induced emission (AIE) concept in 2001, it has received intense attentions from academy and industry because of its important applications in diverse research fronts. Up to now, the luminogens with AIE property (AIEgens) have been widely used in optoelectronic devices, fluorescent bioprobes and chemosensors, and researchers have also committed to exploring the potentials of AIEgens in other cross-cutting areas. The AIEgens have shown superior advantages such as highly efficient emissions in the aggregated state and thus exhibited better performances in comparison with traditional luminescent materials whose emissions are usually quenched upon aggregate formation. In view of the significant achievements of AIEgens in recent years, this review presents representative advancements of AIEgens for the applications in organic optoelectronic devices, mainly including organic light-emitting diodes (OLEDs), circularly polarized luminescence (CPL) devices, electrofluorochromic (EFC) devices, luminescent solar concentrators (LSCs), and liquid crystal displays (LCDs). Not only the design strategies of AIEgens for these optoelectronic devices are analyzed, but also their structure-property relationship and working mechanism are elucidated. It is foreseeable that robust AIEgens with specific functionalities will find more and more applications in various research fields and play an increasingly important role in high-tech devices.

With the non-ionizing, non-invasive, high penetration, high resolution and spectral fingerprinting features of terahertz (THz) wave, THz spectroscopy has great potential for the qualitative and quantitative identification of key substances in biomedical field, such as the early diagnosis of cancer, the accurate boundary determination of pathological tissue and non-destructive detection of superficial tissue. However, biological samples usually contain various of substances (such as water, proteins, fat and fiber), resulting in the signal-to-noise ratio (SNR) for the absorption peaks of target substances are very small and then the target substances are hard to be identified. Here, we present recent works for the SNR improvement of THz signal. These works include the usage of attenuated total reflection (ATR) spectroscopy, the fabrication of sample-sensitive metamaterials, the utilization of different agents (including contrast agents, optical clearing agents and aptamers), the application of reconstruction algorithms and the optimization of THz spectroscopy system. These methods have been proven to be effective theoretically, but only few of them have been applied into actual usage. We also analyze the reasons and summarize the advantages and disadvantages of each method. At last, we present the prospective application of THz spectroscopy in biomedical field.

Optical vortex is a promising candidate for capacity scaling in next-generation optical communications. The generation of multi-vortex beams is of great importance for vortex-based optical communications. Traditional approaches for generating multi-vortex beams are passive, unscalable and cumbersome. Here, we propose and demonstrate a multi-vortex laser, an active approach for creating multi-vortex beams directly at the source. By printing a specially-designed concentric-rings pattern on the cavity mirror, multi-vortex beams are generated directly from the laser. Spatially, the generated multi-vortex beams are decomposable and coaxial. Temporally, the multi-vortex beams can be simultaneously self-mode-locked, and each vortex component carries pulses with GHz-level repetition rate. Utilizing these distinct spatial-temporal characteristics, we demonstrate that the multi-vortex laser can be spatially and temporally encoded for data transmission, showing the potential of the developed multi-vortex laser in optical communications. The demonstrations may open up new perspectives for diverse applications enabled by the multi-vortex laser.

This paper presents a short review for phase measuring deflectometry (PMD). PMD is a phase calculation based technique for three-dimensional (3D) measurement of specular surfaces. PMD can achieve nano-scale form measurement accuracy with the advantages of high dynamic range, non-contact, full field measurement which makes it a competitive method for specular surface measurement. With the development of computer science, display and imaging technology, there has been an advancement in speed for PMD in recent years. This paper discusses PMD focusing on the difference on its system configuration. Measurement principles, progress, advantages and problems are discussed for each category. The challenges and future development of PMD are also discussed.

For a practical photodetector, fast switching speed and high on-off ratio are essential, and more importantly, the integration capability of the device finally determines its application level. In this work, the judiciously engineered Si3N4/Si detector with an open-circuit voltage of 0.41 V is fabricated by chemical vapor deposition methods, and exhibits good performance with repeatability. The advanced integration technology of Si3N4 and Si is the foundation for imaging functions in the near future. Compare to the current commercial Si p-i-n photodiodes, the detector cuts off the long-wavelength UV light over 260 nm, realizing the spectrum selectivity without filters or complexed accessories. The stability of this detector is further characterized by cycling response, temperature and light intensity dependence tests. In addition, we also analyze and explain the inherent mechanisms that govern the different operations of two types of Si3N4/Si photodetectors.

Fourier transform, mapping the information in one domain to its reciprocal space, is of fundamental significance in real-time and parallel processing of massive data for sound and image manipulation. As a powerful platform of high-efficiency wave control, Huygens’ metasurface may offer to bridge the electromagnetic signal processing and analog Fourier transform at the hardware level and with remarkably improved performance. We here demonstrate a Huygens’ metasurface hologram, where the image pattern can be self-rotated or projected in free space by modulating the phase distribution based on the rotational invariance, time-shifting and scaling properties of Fourier transform. Our proof-of-concept experiment shows high-efficiency imaging operation in accordance with theoretical predictions, validating the proposed scheme as an ideal way to perform largely parallel spatial-domain mathematical operations in the analog domain using electromagnetic fields.

Black phosphorus has a strong Raman anisotropy on the basal and cross planes due to its orthorhombic crystal structure. However, almost all the studies on black phosphorus’ anisotropy focus on basal plane with the cross plane neglected. Here, we performed a systematic angle-resolved polarized Raman scattering on both the basal and cross planes of black phosphorus and obtained its integral Raman tensors. It is discovered that when the polarization direction of excitation light is along different crystal axes, the Raman intensity ratio (Ixx : Iyy: Izz) of $$ {A}_g^1 $$ mode is 256:1:5. Besides, via calculation, it is confirmed that the strong Raman anisotropy mainly comes from different differential polarizability alone different directions. This phenomenon is also observed when it comes to the $$ {A}_g^2 $$ mode.

Double-pass polarimetry measures the polarization properties of a sample over a range of polar angles and all azimuths. Here, we present a tolerance analysis of all the optical elements in both the calibration and measurement procedures to predict the sensitivities of the double-pass polarimeter. The calibration procedure is described by a Mueller matrix based on the eigenvalue calibration method (ECM) [1]. Our numerical results from the calibration and measurement in the Mueller matrix description with tolerances limited by systematic and stochastic noise from specifications of commercially available hardware components are in good agreement with previous experimental observations. Furthermore, by using the orientation Zernike polynomials (OZP) which are an extension of the Jones matrix formalism, similar to the Zernike polynomials wavefront expansion, the pupil distribution of the polarization properties of non-depolarizing samples under test are expanded. Using polar angles ranging up to 25∘, we predict a sensitivity of 0.5% for diattenuation and 0.3∘ for retardance using the root mean square (RMS) of the corresponding OZP coefficients as a measure of the error. This numerical tool provides an approach for further improving the sensitivities of polarimeters via error budgeting and replacing sensitive components with those having better precision.

The orbital angular momentum (OAM) of beams provides a new dimension, and have already found lots of applications in various domains. Among such applications, the precisely and quantitatively diagnostic of intensity distributions among different OAM modes, namely the OAM spectrum of a beam, is of great significance. In this paper we propose and experimentally validate a simple interferential method to achieve this goal. By analyzing the interference pattern formed by the beam and a reference field, the OAM spectrum can be obtained instantaneously. Furthermore, the proposed method is also available for more complex light fields, for instance, the multi-ring optical vortices. In the proof-of-concept experiment, the OAM spectra of both single-mode and N-fold multiplexed OAM modes with various intensity distributions are well detected. Our work offers a new way to precisely measure the OAM spectra of beams and will advance the development of many applications ranging from classical to quantum physics as the OAM based large-capacity data transmissions, rotation detection, quantum manipulation and so on.

Holography has attracted tremendous interest due to its capability of storing both the amplitude and phase of light field and reproducing vivid three-dimensional scenes. However, the large pixel size, low resolution, small field-of-view (FOV) and limited space-bandwidth of traditional spatial light modulator (SLM) devices restrict the possibility of improving the quality of reconstructed images. With the development of nanofabrication technologies, metasurfaces have shown great potential in manipulating the amplitude, phase, polarization, frequency or simultaneously multiple parameters of output light in ultrashort distance with subwavelength resolution by tailoring the scattering behaviour of consisted nanostructures. Such flexibilities make metasurface a promising candidate for holographic related applications. Here, we review recent progresses in the field of metasurface holography. From the perspective of the fundamental properties of light, we classify the metasurface holography into several categories such as phase-only holography, amplitude-only holography, complex amplitude holography and so on. Then, we introduce the corresponding working principles and design strategies. Meanwhile, some emerging types of metasurface holography such as tunable holography, nonlinear holography, Janus (or directional related) and bilayer metasurfaces holography are also discussed. At last, we make our outlook on metasurface holography and discuss the challenges we may face in the future.

The new record efficiency in Thermophotovoltaics relies upon a highly reflective rear mirror. The excellent rear mirror boosts voltage by enhancing the luminescence extraction, and separately also reflects low energy photons, which would otherwise be useless in thermophotovoltaics. The reflected low energy photons reheat the thermal emitter, and regenerate above-bandgap energy photons. The efficiency calibration for such regenerative thermophotovoltaics depends on several factors, yet predominantly on the accurate measurement of the rear mirror reflectivity. Here, we report on the technique for accurate measurement of mirror reflectivity, and of record thermophotovoltaic efficiency 29.1 ± 0.6%, at 1207 °C.

High-performance vacuum-ultraviolet (VUV) photodetectors are of great significance to space science, radiation monitoring, electronic industry and basic science. Due to the absolute advantages in VUV selective response and radiation resistance, ultra-wide bandgap semiconductors such as diamond, BN and AlN attract wide interest from researchers, and thus the researches on VUV photodetectors based on these emerging semiconductor materials have made considerable progress in the past 20 years. This paper takes ultra-wide bandgap semiconductor filterless VUV photodetectors with different working mechanisms as the object and gives a systematic review in the aspects of figures of merit, performance evaluation methods and research progress. These miniaturized and easily-integrated photodetectors with low power consumption are expected to achieve efficient VUV dynamic imaging and single photon detection in the future.