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The performance of photodetectors fabricated from emerging semiconductors such as perovskites, quantum dots, two-dimensional materials or organics, for example, can be prone to misinterpretation. This Comment exposes the problems and proposes some guidelines for accurate characterization.

To construct large-scale silicon electro-optical switches for optical interconnections, we developed a method using a limited number of power monitors inserted at certain positions to detect and determine the optimum operating points of all switch units to eliminate non-uniform effects arising from fabrication errors. We also introduced an optical phase bias to one phase-shifter arm of a Mach–Zehnder interferometer (MZI)-type switch unit to balance the two operation statuses of a silicon electro-optical switch during push–pull operation. With these methods, a 32 × 32 MZI-based silicon electro-optical switch was successfully fabricated with 180-nm complementary metal–oxide–semiconductor (CMOS) process technology, which is the largest scale silicon electro-optical switch to the best of our knowledge. At a wavelength of 1520 nm, the on-chip insertion losses were 12.9 to 16.5 dB, and the crosstalk ranged from −17.9 to −24.8 dB when all units were set to the ‘Cross’ status. The losses were 14.4 to 18.5 dB, and the crosstalk ranged from −15.1 to −19.0 dB when all units were in the ‘Bar’ status. The total power consumptions of the 32 × 32 switch were 247.4 and 542.3 mW when all units were set to the ‘Cross’ and ‘Bar’ statuses, respectively.

Photodetectors based on two-dimensional (2D) materials have been the focus of intensive research and development over the past decade. However, a gap has long persisted between fundamental research and mature applications. One of the main reasons behind this gap has been the lack of a practical and unified approach for the characterization of their figures of merit, which should be compatible with the traditional performance evaluation system of photodetectors. This is essential to determine the degree of compatibility of laboratory prototypes with industrial technologies. Here we propose general guidelines for the characterization of the figures of merit of 2D photodetectors and analyze common situations when the specific detectivity, responsivity, dark current, and speed can be misestimated. Our guidelines should help improve the standardization and industrial compatibility of 2D photodetectors. The lack of a standardized approach for the characterization of the performance of 2D photodetectors represents an important obstacle towards their industrialization. Here, the authors propose practical guidelines to characterize their figures of merit and analyse common situations where their performance can be misestimated.

Photonic components with adjustable parameters, such as variable-focal-length lenses or spectral filters, which can change functionality upon optical stimulation, could offer numerous useful applications. Tuning of such components is conventionally achieved by either micro- or nanomechanical actuation of their constituent parts, by stretching or by heating. Here, we report a novel approach for making reconfigurable optical components that are created with light in a non-volatile and reversible fashion. Such components are written, erased and rewritten as two-dimensional binary or greyscale patterns into a nanoscale film of phase-change material by inducing a refractive-index-changing phase transition with tailored trains of femtosecond pulses. We combine germanium–antimony–tellurium-based films with a diffraction-limited resolution optical writing process to demonstrate a variety of devices: visible-range reconfigurable bichromatic and multi-focus Fresnel zone plates, a super-oscillatory lens with subwavelength focus, a greyscale hologram, and a dielectric metamaterial with on-demand reflection and transmission resonances. A metasurface composed of pixels of optically switchable phase change material yields a photonic platform that can be configured on demand to perform a variety of optical tasks.

Early works 1 and recent advances in thin-film lithium niobate (LiNbO 3 ) on insulator have enabled low-loss photonic integrated circuits 2 , 3 , modulators with improved half-wave voltage 4 , 5 , electro-optic frequency combs 6 and on-chip electro-optic devices, with applications ranging from microwave photonics to microwave-to-optical quantum interfaces 7 . Although recent advances have demonstrated tunable integrated lasers based on LiNbO 3 (refs. 8 , 9 ), the full potential of this platform to demonstrate frequency-agile, narrow-linewidth integrated lasers has not been achieved. Here we report such a laser with a fast tuning rate based on a hybrid silicon nitride (Si 3 N 4 )–LiNbO 3 photonic platform and demonstrate its use for coherent laser ranging. Our platform is based on heterogeneous integration of ultralow-loss Si 3 N 4 photonic integrated circuits with thin-film LiNbO 3 through direct bonding at the wafer level, in contrast to previously demonstrated chiplet-level integration 10 , featuring low propagation loss of 8.5 decibels per metre, enabling narrow-linewidth lasing (intrinsic linewidth of 3 kilohertz) by self-injection locking to a laser diode. The hybrid mode of the resonator allows electro-optic laser frequency tuning at a speed of 12 × 10 15  hertz per second with high linearity and low hysteresis while retaining the narrow linewidth. Using a hybrid integrated laser, we perform a proof-of-concept coherent optical ranging (FMCW LiDAR) experiment. Endowing Si 3 N 4 photonic integrated circuits with LiNbO 3 creates a platform that combines the individual advantages of thin-film LiNbO 3 with those of Si 3 N 4 , which show precise lithographic control, mature manufacturing and ultralow loss 11 , 12 . A frequency-tunable laser based on a hybrid silicon nitride and lithium niobate integrated photonic platform has a fast tuning rate and could be used for optical ranging applications.

Research in photonic computing has flourished due to the proliferation of optoelectronic components on photonic integration platforms. Photonic integrated circuits have enabled ultrafast artificial neural networks, providing a framework for a new class of information processing machines. Algorithms running on such hardware have the potential to address the growing demand for machine learning and artificial intelligence in areas such as medical diagnosis, telecommunications, and high-performance and scientific computing. In parallel, the development of neuromorphic electronics has highlighted challenges in that domain, particularly related to processor latency. Neuromorphic photonics offers sub-nanosecond latencies, providing a complementary opportunity to extend the domain of artificial intelligence. Here, we review recent advances in integrated photonic neuromorphic systems, discuss current and future challenges, and outline the advances in science and technology needed to meet those challenges.Photonics offers an attractive platform for implementing neuromorphic computing due to its low latency, multiplexing capabilities and integrated on-chip technology.

Holography has been identified as a vital platform for three-dimensional displays, optical encryption, microscopy and artificial intelligence through different physical dimensions. However, unlike the wavelength and polarization divisions, orbital angular momentum (OAM) of light, despite its helical wavefront being an independent physical dimension, has not been implemented as an information carrier for holography due to the lack of helical mode index selectivity in the Bragg diffraction formula. Here, we demonstrate OAM holography by discovering strong OAM selectivity in the spatial-frequency domain without a theoretical helical mode index limit. As such, OAM holography allows the multiplexing of a wide range of OAM-dependent holographic images with a helical mode index spanning from −50 to 50, leading to a 10 bit OAM-encoded hologram for high-security optical encryption. Our results showing up to 210 OAM-dependent distinctive holographic images mark a new path to achieving ultrahigh-capacity holographic information systems harnessing the previously inaccessible OAM division.The orbital angular momentum degree of freedom is used to demonstrate 10 bit holographic images with a helical mode index spanning from −50 to 50.

Graphene and transition metal dichalcogenides (TMDCs) are the two major types of layered materials under intensive investigation. However, the zero-bandgap nature of graphene and the relatively low mobility in TMDCs limit their applications. Here we reintroduce black phosphorus (BP), the most stable allotrope of phosphorus with strong intrinsic in-plane anisotropy, to the layered-material family. For 15-nm-thick BP, we measure a Hall mobility of 1,000 and 600 cm 2 V −1 s −1 for holes along the light ( x ) and heavy ( y ) effective mass directions at 120 K. BP thin films also exhibit large and anisotropic in-plane optical conductivity from 2 to 5 μm. Field-effect transistors using 5 nm BP along x direction exhibit an on–off current ratio exceeding 10 5 , a field-effect mobility of 205 cm 2 V −1 s −1 , and good current saturation characteristics all at room temperature. BP shows great potential for thin-film electronics, infrared optoelectronics and novel devices in which anisotropic properties are desirable. The applications of graphene and transition metal dichalcogenides in electronics are limited by their zero-bandgap and low mobility, respectively. Here, the authors demonstrate the potential of an emerging layered material—black phosphorous—for thin film electronics and infrared optoelectronics.

Modern advanced photonic integrated circuits require dense integration of high-speed electro-optic functional elements on a compact chip that consumes only moderate power. Energy efficiency, operation speed, and device dimension are thus crucial metrics underlying almost all current developments of photonic signal processing units. Recently, thin-film lithium niobate (LN) emerges as a promising platform for photonic integrated circuits. Here, we make an important step towards miniaturizing functional components on this platform, reporting high-speed LN electro-optic modulators, based upon photonic crystal nanobeam resonators. The devices exhibit a significant tuning efficiency up to 1.98 GHz V −1 , a broad modulation bandwidth of 17.5 GHz, while with a tiny electro-optic modal volume of only 0.58 μ m 3 . The modulators enable efficient electro-optic driving of high-Q photonic cavity modes in both adiabatic and non-adiabatic regimes, and allow us to achieve electro-optic switching at 11 Gb s −1 with a bit-switching energy as low as 22 fJ. The demonstration of energy efficient and high-speed electro-optic modulation at the wavelength scale paves a crucial foundation for realizing large-scale LN photonic integrated circuits that are of immense importance for broad applications in data communication, microwave photonics, and quantum photonics. Lithium niobate (LN) devices are promising for future photonic integrated circuits. Here, the authors demonstrate an electro-optic LN modulator with a very small modal volume based on photonic crystal resonator architecture.

Reverse intersystem crossing (RISC), originally considered forbidden in purely organic materials, has recently become possible by minimizing the energy gap between the lowest excited singlet state (S1) and lowest triplet state (T1) in thermally activated delayed fluorescence systems. However, direct spin-inversion from T1 to S1 is still inefficient when both states are of the same charge transfer (CT) nature (that is, 3CT and 1CT, respectively). Intervention of locally excited triplet states (3LE) between 3CT and 1CT is expected to trigger fast spin-flipping. Here, we report the systematic design of ideal thermally activated delayed fluorescence molecules with near-degenerate 1CT, 3CT and 3LE states by controlling the distance between the donor and acceptor segments in a molecule with tilted intersegment angles. This system realizes very fast RISC with a rate constant (kRISC) of 1.2 × 107 s−1, resulting in organic light-emitting diodes with excellent performance, particularly at high brightness.An organic molecule, TpAT-tFFO, which is designed to support rapid reverse intersystem crossing allows the fabrication of efficient organic light-emitting diodes.