Showing papers in "Nature Photonics in 2020"

Integrated photonic quantum technologies

Abstract: Quantum technologies comprise an emerging class of devices capable of controlling superposition and entanglement of quantum states of light or matter, to realize fundamental performance advantages over ordinary classical machines. The technology of integrated quantum photonics has enabled the generation, processing and detection of quantum states of light at a steadily increasing scale and level of complexity, progressing from few-component circuitry occupying centimetre-scale footprints and operating on two photons, to programmable devices approaching 1,000 components occupying millimetre-scale footprints with integrated generation of multiphoton states. This Review summarizes the advances in integrated photonic quantum technologies and its demonstrated applications, including quantum communications, simulations of quantum chemical and physical systems, sampling algorithms, and linear-optic quantum information processing. This Review covers recent progress in integrated quantum photonics (IQP) technologies and their applications. The challenges and opportunities of realizing large-scale, monolithic IQP circuits for future quantum applications are discussed.

High-efficiency organic solar cells with low non-radiative recombination loss and low energetic disorder

Abstract: Energy loss within organic solar cells (OSCs) is undesirable as it reduces cell efficiency1–4. In particular, non-radiative recombination loss3 and energetic disorder5, which are closely related to the tail states below the band edge and the overall photon energy loss, need to be minimized to improve cell performance. Here, we report how the use of a small-molecule acceptor with torsion-free molecular conformation can achieve a very low degree of energetic disorder and mitigate energy loss in OSCs. The resulting single-junction OSC has an energy loss due to non-radiative recombination of just 0.17 eV and a high power conversion efficiency of up to 16.54% (certified as 15.89% by the National Renewable Energy Laboratory). The findings take studies of organic photovoltaics deeper into a new regime, beyond the limits of energetic disorder and large energy offset for charge generation. An organic solar cell designed with minimal energetic disorder exhibits very low energy loss due to non-radiative recombination and highly efficient operation.

Hybrid integrated quantum photonic circuits.

Abstract: Recent developments in chip-based photonic quantum circuits have radically impacted quantum information processing. However, it is challenging for monolithic photonic platforms to meet the stringent demands of most quantum applications. Hybrid platforms combining different photonic technologies in a single functional unit have great potential to overcome the limitations of monolithic photonic circuits. Our Review summarizes the progress of hybrid quantum photonics integration, discusses important design considerations, including optical connectivity and operation conditions, and highlights several successful realizations of key physical resources for building a quantum teleporter. We conclude by discussing the roadmap for realizing future advanced large-scale hybrid devices, beyond the solid-state platform, which hold great potential for quantum information applications. The Review summarizes the progress of hybrid quantum photonics integration in terms of its important design considerations and fabrication approaches, and highlights some successful realizations of key physical resources for building integrated quantum devices, such as quantum teleporters, quantum repeaters and quantum simulators.

Efficient and stable Ruddlesden–Popper perovskite solar cell with tailored interlayer molecular interaction

Abstract: Two-dimensional Ruddlesden–Popper phase (2DRP) perovskites are known to exhibit improved photostability and environmental stability compared with their three-dimensional (3D) counterparts. However, fundamental questions remain over the interaction between the bulky alkylammoniums and the 2DRP perovskite framework. Here, we unambiguously demonstrate that a sulfur–sulfur interaction is present for a new bulky alkylammonium, 2-(methylthio)ethylamine hydrochloride (MTEACl). In addition to a weaker van der Waals interaction, the interaction between sulfur atoms in two MTEA molecules enables a (MTEA)2(MA)4Pb5I16 (n = 5) perovskite framework with enhanced charge transport and stabilization. The result is 2DRP perovskite solar cells with significantly improved efficiency and stability. Cells with a power conversion efficiency as high as 18.06% (17.8% certified) are achieved, along with moisture tolerance for up to 1,512 h (under 70% humidity conditions), thermal stability for 375 h (at 85 °C) and stability under continuous light stress (85% of the initial efficiency retained over 1,000 h of operation at the maximum power point). Two-dimensional perovskite solar cells have been engineered to be robust against moisture, high temperatures and light stress.

Terahertz topological photonics for on-chip communication

Abstract: The realization of integrated, low-cost and efficient solutions for high-speed, on-chip communication requires terahertz-frequency waveguides and has great potential for information and communication technologies, including sixth-generation (6G) wireless communication, terahertz integrated circuits, and interconnects for intrachip and interchip communication. However, conventional approaches to terahertz waveguiding suffer from sensitivity to defects and sharp bends. Here, building on the topological phase of light, we experimentally demonstrate robust terahertz topological valley transport through several sharp bends on the all-silicon chip. The valley kink states are excellent information carriers owing to their robustness, single-mode propagation and linear dispersion. By leveraging such states, we demonstrate error-free communication through a highly twisted domain wall at an unprecedented data transfer rate (exceeding ten gigabits per second) that enables real-time transmission of uncompressed 4K high-definition video (that is, with a horizontal display resolution of approximately 4,000 pixels). Terahertz communication with topological devices opens a route towards terabit-per-second datalinks that could enable artificial intelligence and cloud-based technologies, including autonomous driving, healthcare, precision manufacturing and holographic communication. Robust terahertz wave transport is demonstrated on a silicon chip using the valley Hall topological phase. Error-free communication is achieved at a data rate of 11 Gbit s−1, enabling real-time transmission of uncompressed 4K high-definition video.

Orbital angular momentum holography for high-security encryption

Abstract: 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.

Demonstration of sub-3 ps temporal resolution with a superconducting nanowire single-photon detector

Abstract: Improvements in temporal resolution of single-photon detectors enable increased data rates and transmission distances for both classical and quantum optical communication systems, higher spatial resolution in laser ranging, and observation of shorter-lived fluorophores in biomedical imaging. In recent years, superconducting nanowire single-photon detectors (SNSPDs) have emerged as the most efficient time-resolving single-photon-counting detectors available in the near-infrared, but understanding of the fundamental limits of timing resolution in these devices has been limited due to a lack of investigations into the timescales involved in the detection process. We introduce an experimental technique to probe the detection latency in SNSPDs and show that the key to achieving low timing jitter is the use of materials with low latency. By using a specialized niobium nitride SNSPD we demonstrate that the system temporal resolution can be as good as 2.6 ± 0.2 ps for visible wavelengths and 4.3 ± 0.2 ps at 1,550 nm. Knowledge about detection latency provides a guideline to reduce the timing jitter of niobium nitride superconducting nanowire single-photon detectors. A timing jitter of 2.6 ps at visible wavelength and 4.3 ps at 1,550 nm is achieved.

A MHz-repetition-rate hard X-ray free-electron laser driven by a superconducting linear accelerator

Abstract: The European XFEL is a hard X-ray free-electron laser (FEL) based on a high-electron-energy superconducting linear accelerator. The superconducting technology allows for the acceleration of many electron bunches within one radio-frequency pulse of the accelerating voltage and, in turn, for the generation of a large number of hard X-ray pulses. We report on the performance of the European XFEL accelerator with up to 5,000 electron bunches per second and demonstrating a full energy of 17.5 GeV. Feedback mechanisms enable stabilization of the electron beam delivery at the FEL undulator in space and time. The measured FEL gain curve at 9.3 keV is in good agreement with predictions for saturated FEL radiation. Hard X-ray lasing was achieved between 7 keV and 14 keV with pulse energies of up to 2.0 mJ. Using the high repetition rate, an FEL beam with 6 W average power was created.

Higher-order topological states in photonic kagome crystals with long-range interactions

Abstract: Photonic topological insulators enable topological boundary modes that are resilient to defects and disorder, irrespective of manufacturing precision. This property is known as topological protection. Although originally limited to dimensionality of modes one lower than that of topological insulators, the recently discovered higher-order topological insulators (HOTIs) offer topological protection over an extended range of dimensionalities. Here, we introduce a photonic HOTI with kagome lattice that exhibits topological bulk polarization, leading to the emergence of one-dimensional edge states, as well as higher-order zero-dimensional states confined to the corners of the structure. Interestingly, in addition to the corner states due to nearest-neighbour interactions, we discover a new class of topological corner states induced by long-range interactions and specific to photonic systems. Our findings demonstrate that photonic HOTIs possess richer physics than their condensed-matter counterparts, offering opportunities for engineering novel designer electromagnetic states with unique topological robustness. One- and zero-dimensional optical states are revealed in photonic higher-order topological insulators.

Reversible 3D laser printing of perovskite quantum dots inside a transparent medium

Abstract: The three-dimensional (3D) patterning of semiconductors is potentially important for exploring new functionalities and applications in optoelectronics1,2. Here, we show that it is possible to write on demand 3D patterns of perovskite quantum dots (QDs) inside a transparent glass material using a femtosecond laser. By utilizing the inherent ionic nature and low formation energy of perovskite, highly luminescent CsPbBr3 QDs can be reversibly fabricated in situ and decomposed through femtosecond laser irradiation and thermal annealing. This pattern of writing and erasing can be repeated for many cycles, and the luminescent QDs are well protected by the inorganic glass matrix, resulting in stable perovskite QDs with potential applications such as high-capacity optical data storage, information encryption and 3D artwork. Luminescent CsPbBr3 quantum dots can be written into glass using femtosecond laser pulses and thermal annealing, and erased by further femtosecond laser irradiation. The resulting quantum dot patterns could prove useful for data storage, decoration or security purposes.

Fast spin-flip enables efficient and stable organic electroluminescence from charge-transfer states

Abstract: A spin-flip from a triplet to a singlet excited state, that is, reverse intersystem crossing (RISC), is an attractive route for improving light emission in organic light-emitting diodes, as shown by devices using thermally activated delayed fluorescence (TADF). However, device stability and efficiency roll-off remain challenging issues that originate from a slow RISC rate (kRISC). Here, we report a TADF molecule with multiple donor units that form charge-resonance-type hybrid triplet states leading to a small singlet–triplet energy splitting, large spin–orbit couplings, and a dense manifold of triplet states energetically close to the singlets. The kRISC in our TADF molecule is as fast as 1.5 × 107 s−1, a value some two orders of magnitude higher than typical TADF emitters. Organic light-emitting diodes based on this molecule exhibit good stability (estimated T90 about 600 h for 1,000 cd m−2), high maximum external quantum efficiency (>29.3%) and low efficiency roll-off (<2.3% at 1,000 cd m−2). An organic molecule, 5Cz-TRZ, with multiple donor units supports fast reverse intersystem crossing, allowing fabrication of high-performance organic light-emitting diodes.

Organic light emitters exhibiting very fast reverse intersystem crossing

Abstract: 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.

Deep-learning-enabled self-adaptive microwave cloak without human intervention

Abstract: Becoming invisible at will has fascinated humanity for centuries and in the past decade it has attracted a great deal of attention owing to the advent of metamaterials. However, state-of-the-art invisibility cloaks typically work in a deterministic system or in conjunction with outside help to achieve active cloaking. Here, we propose the concept of an intelligent (that is, self-adaptive) cloak driven by deep learning and present a metasurface cloak as an example implementation. In the experiment, the metasurface cloak exhibits a millisecond response time to an ever-changing incident wave and the surrounding environment, without any human intervention. Our work brings the available cloaking strategies closer to a wide range of real-time, in situ applications, such as moving stealth vehicles. The approach opens the way to facilitating other intelligent metadevices in the microwave regime and across the wider electromagnetic spectrum and, more generally, enables automatic solutions of electromagnetic inverse design problems. A deep-learning-enabled metasurface cloak actively self-adapts to take into account changing microwave illumination and varying physical surroundings.

Bright high-colour-purity deep-blue carbon dot light-emitting diodes via efficient edge amination

Abstract: Deep-blue light-emitting diodes (LEDs) (emitting at wavelengths of less than 450 nm) are important for solid-state lighting, vivid displays and high-density information storage. Colloidal quantum dots, typically based on heavy metals such as cadmium and lead, are promising candidates for deep-blue LEDs, but these have so far had external quantum efficiencies lower than 1.7%. Here we present deep-blue light-emitting materials and devices based on carbon dots. The carbon dots produce emission with a narrow full-width at half-maximum (about 35 nm) with high photoluminescence quantum yield (70% ± 10%) and a colour coordinate (0.15, 0.05) closely approaching the standard colour Rec. 2020 (0.131, 0.046) specification. Structural and optical characterization, together with computational studies, reveal that amine-based passivation accounts for the efficient and high-colour-purity emission. Deep-blue LEDs based on these carbon dots display high performance with a maximum luminance of 5,240 cd m−2 and an external quantum efficiency of 4%, notably exceeding that of previously reported quantum-tuned solution-processed deep-blue LEDs. Deep-blue high-colour-purity light-emitting materials are developed by using amine-based edge passivation. The light-emitting diodes based on the carbon dots exhibit a maximum luminance of 5,240 cd m–2 and an external quantum efficiency of 4%.

4H-silicon-carbide-on-insulator for integrated quantum and nonlinear photonics

Abstract: Optical quantum information processing will require highly efficient photonic circuits to connect quantum nodes on-chip and across long distances. This entails the efficient integration of optically addressable qubits into photonic circuits, as well as quantum frequency conversion to the telecommunications band. 4H-silicon carbide (4H-SiC) offers unique potential for on-chip quantum photonics, as it hosts a variety of promising colour centres and has a strong second-order optical nonlinearity. Here, we demonstrate within a single, monolithic platform the strong enhancement of emission from a colour centre and efficient optical frequency conversion. We develop a fabrication process for thin films of 4H-SiC, which are compatible with industry-standard, CMOS nanofabrication. This work provides a viable route towards industry-compatible, scalable colour-centre-based quantum technologies, including the monolithic generation and frequency conversion of quantum light on-chip. Monolithic photonics devices based on SiC are fabricated by a wafer bonding and thinning technique. The strong enhancement of single-photon emission from a colour centre and optical frequency conversion with an efficiency of 360% W−1 are demonstrated.

Flat optics for image differentiation

Abstract: Image processing has become a critical technology in a variety of science and engineering disciplines. Although most image processing is performed digitally, optical analog processing has the advantages of being low-power and high-speed, but it requires a large volume. Here, we demonstrate flat optics for direct image differentiation, allowing us to significantly shrink the required optical system size. We first demonstrate how the differentiator can be combined with traditional imaging systems such as a commercial optical microscope and camera sensor for edge detection with a numerical aperture up to 0.32. We next demonstrate how the entire processing system can be realized as a monolithic compound flat optic by integrating the differentiator with a metalens. The compound nanophotonic system manifests the advantage of thin form factor as well as the ability to implement complex transfer functions, and could open new opportunities in applications such as biological imaging and computer vision. Vertical integration of a metalens to realize compound nanophotonic systems for optical analog image processing is realized, significantly reducing the size and complexity of conventional optical systems.

Test of general relativity by a pair of transportable optical lattice clocks

Abstract: A clock at a higher altitude ticks faster than one at a lower altitude, in accordance with Einstein’s theory of general relativity. The outstanding stability and accuracy of optical clocks, at 10−18 levels1–5, allows height differences6 of a centimetre to be measured. However, such state-of-the-art clocks have been demonstrated only in well-conditioned laboratories. Here, we demonstrate an 18-digit-precision frequency comparison in a broadcasting tower, Tokyo Skytree, by developing transportable optical lattice clocks. The tower provides the clocks with adverse conditions to test the robustness and a 450 m height difference to test the gravitational redshift at (1.4 ± 9.1) × 10−5. The result improves ground-based clock comparisons7–9 by an order of magnitude and is comparable with space experiments10,11. Our demonstration shows that optical clocks resolving centimetres are technically ready for field applications, such as monitoring spatiotemporal changes of geopotentials caused by active volcanoes or crustal deformation12 and for defining the geoid13,14, which will have an immense impact on future society. A pair of transportable optical lattice clocks with 10−18 uncertainty is developed. The relativistic redshift predicted by the theory of general relativity has been tested at the 10–5 level by the two optical clocks with a height difference of 450 m on the ground.

Tunable isolated attosecond X-ray pulses with gigawatt peak power from a free-electron laser

Abstract: The quantum-mechanical motion of electrons in molecules and solids occurs on the sub-femtosecond timescale. Consequently, the study of ultrafast electronic phenomena requires the generation of laser pulses shorter than 1 fs and of sufficient intensity to interact with their target with high probability. Probing these dynamics with atomic-site specificity requires the extension of sub-femtosecond pulses to the soft X-ray spectral region. Here, we report the generation of isolated soft X-ray attosecond pulses with an X-ray free-electron laser. Our source has a pulse energy that is millions of times larger than any other source of isolated attosecond pulses in the soft X-ray spectral region, with a peak power exceeding 100 GW. This unique combination of high intensity, high photon energy and short pulse duration enables the investigation of electron dynamics with X-ray nonlinear spectroscopy and single-particle imaging, unlocking a path towards a new era of attosecond science. The generation of ultrashort X-ray pulses with a peak power exceeding 100 GW offers new opportunities for studying electron dynamics with nonlinear spectroscopy and single-particle imaging.

Large-area near-infrared perovskite light-emitting diodes

Abstract: The performance of perovskite light-emitting diodes (PeLEDs) has progressed rapidly in recent years, with electroluminescence efficiency now reaching 20%1–12. However, devices, so far, have featured small areas and usually show notable variation in device-to-device performance. Here, we show that the origin of suboptimal device performance stems from inadequate hole injection, and that the use of a hole-transporting polymer with a shallower ionization potential can improve device charge balance, efficiency and reproducibility. Using an ITO/ZnO/PEIE/FAPbI3/poly-TPD/MoO3/Al device structure, we report a 799 nm near-infrared PeLED that operates with an external quantum efficiency (EQE) of 20.2%, at a current density of 57 mA cm−2 and a radiance of 57 W sr−1 m−2. The standard deviation in the device EQE is only 1.2%, demonstrating high reproducibility. Large-area devices measuring 900 mm2 operate with a high EQE of 12.1%, and are shown to suit medical applications such as subcutaneous deep-tissue illumination and heart rate monitoring. Near-infrared LEDs as large as 30 × 30 mm2 in size are fabricated on both rigid and flexible substrates.

Photonic microwave generation in the X- and K-band using integrated soliton microcombs

Abstract: Microwave photonic technologies, which upshift the carrier into the optical domain, have facilitated the generation and processing of ultra-wideband electronic signals at vastly reduced fractional bandwidths. For microwave photonic applications such as radars, optical communications and low-noise microwave generation, optical frequency combs are useful building blocks. By virtue of soliton microcombs, frequency combs can now be built using CMOS-compatible photonic integrated circuits. Yet, currently developed integrated soliton microcombs all operate with repetition rates significantly beyond those that conventional electronics can detect, preventing their use in microwave photonics. Access to this regime is challenging due to the required ultra-low waveguide loss and large dimensions of the nanophotonic resonators. Here, we demonstrate soliton microcombs operating in two widely employed microwave bands, the X-band (~10 GHz, for radar) and the K-band (~20 GHz, for 5G). Driven by a low-noise fibre laser, these devices produce more than 300 frequency lines within the 3 dB bandwidth, and generate microwave signals featuring phase noise levels comparable to modern electronic microwave oscillators. Our results establish integrated microcombs as viable low-noise microwave generators. Furthermore, the low soliton repetition rates are critical for future dense wavelength-division multiplexing channel generation schemes and could significantly reduce the system complexity of soliton-based integrated frequency synthesizers and atomic clocks. Nanophotonic microwave synthesizers in the X-band (10 GHz, for radar) and K-band (20 GHz, for 5G), based on integrated soliton microcombs driven by a low-noise fibre laser, link the fields of microwave photonics and integrated microcombs.

High-purity orbital angular momentum states from a visible metasurface laser

Abstract: Orbital angular momentum (OAM) from lasers holds promise for compact, at-source solutions for applications ranging from imaging to communications. However, conjugate symmetry between circular spin and opposite helicity OAM states (±l) from conventional spin–orbit approaches has meant that complete control of light’s angular momentum from lasers has remained elusive. Here, we report a metasurface-enhanced laser that overcomes this limitation. We demonstrate new high-purity OAM states with quantum numbers reaching l = 100 and non-symmetric vector vortex beams that lase simultaneously on independent OAM states as much as Δl = 90 apart, an extreme violation of previous symmetric spin–orbit lasing devices. Our laser conveniently outputs in the visible, producing new OAM states of light as well as all previously reported OAM modes from lasers, offering a compact and power-scalable source that harnesses intracavity structured matter for the creation of arbitrary chiral states of structured light. A metasurface laser generates orbital angular momentum states with quantum numbers reaching l = 100. Simultaneous output vortex beams, with Δl as great as 90, are demonstrated in the visible regime.

Ultrafast and energy-efficient all-optical switching with graphene-loaded deep-subwavelength plasmonic waveguides

Abstract: All-optical switches have attracted attention because they can potentially overcome the speed limitation of electric switches. However, ultrafast, energy-efficient all-optical switches have been challenging to realize owing to the intrinsically small optical nonlinearity in existing materials. As a solution, we propose the use of graphene-loaded deep-subwavelength plasmonic waveguides (30 × 20 nm2). Thanks to extreme light confinement, we have greatly enhanced optical nonlinear absorption in graphene, and achieved ultrafast all-optical switching with a switching energy of 35 fJ and a switching time of 260 fs. The switching energy is four orders of magnitude smaller than that in previous graphene-based devices and is the smallest value reported for any all-optical switch operating at a few picoseconds or less. This device can be efficiently connected to conventional silicon waveguides and used in silicon photonic integrated circuits. We believe that this graphene-based device will pave the way towards on-chip ultrafast and energy-efficient photonic processing. All-optical switching with a switching energy of 35 fJ and a switching time of 260 fs is reported in a nanoscale integrated optical circuit.

Perovskite-filled membranes for flexible and large-area direct-conversion X-ray detector arrays

Abstract: The soft nature of metal halide perovskites makes them potentially applicable as flexible X-ray detectors. Here we report a structure of perovskite-filled membranes (PFMs) for highly sensitive, flexible and large-area X-ray detectors. PFMs with areas up to 400 cm2 are formed by infiltrating saturated perovskite solution through porous polymer membranes followed by hot lamination. The good connectivity and crystallization of perovskite crystals in the membranes enable a large mobility–lifetime product. The sensitivity of the X-ray detectors under a field of 0.05 V µm−1 reaches 8,696 ± 228 µC Gyair−1 cm−2 and shows no degradation after storage for over six months and exposure to a dose of 376.8 Gyair, equivalent to 1.88 million chest X-ray scans. The flexible PFMs can be bent at radii down to 2 mm without losing performance. The stand-alone detector array is curved and put inside metal pipes for the detection of material defects with imaging quality superior to flat-panel detectors. Perovskite-filled-membranes enable flexible, sensitive and large-area X-ray detectors. The structures are made by infiltrating perovskite solution into porous polymer membranes.

Overcoming the energy gap law in near-infrared OLEDs by exciton–vibration decoupling

Abstract: The development of high-performance near-infrared organic light-emitting diodes is hindered by strong non-radiative processes as governed by the energy gap law Here, we show that exciton delocalization, which serves to decouple the exciton band from highly vibrational ladders in the S0 ground state, can bring substantial enhancements in the photoluminescence quantum yield of emitters, bypassing the energy gap law Experimental proof is provided by the design and synthesis of a series of new Pt(ii) complexes with a delocalization length of 5–9 molecules that emit at 866–960 nm with a photoluminescence quantum yield of 5–12% in solid films The corresponding near-infrared organic light-emitting diodes emit light with a 930 nm peak wavelength and a high external quantum efficiency up to 214% and a radiance of 416 W sr−1 m−2 Both theoretical and experimental results confirm the exciton–vibration decoupling strategy, which should be broadly applicable to other well-aligned molecular solids Pt(ii) complexes allow the fabrication of efficient near-infrared organic light-emitting diodes that operate beyond the 900 nm region

NIR II-responsive photon upconversion through energy migration in an ytterbium sublattice

Abstract: Smart control of photon upconversion is a key strategy for lanthanide-based materials used in biological and photonic applications. However, this has remained a challenge for the upconversion luminescence of lanthanides under excitation in the second near-infrared (NIR II) biowindow instead of at the conventional 980 and 808 nm wavelengths. Here, we report a conceptual design for an energy-migratory ytterbium sublattice in an erbium-sensitized multilayer core–shell nanostructure that is able to achieve photon upconversion from a broad range of lanthanide ions (Yb3+, Tm3+, Ho3+, Gd3+, Eu3+ and Tb3+) under 1,530 nm irradiation. The quasi-single-band upconversion in the first near-infrared (NIR I) biowindow is also realized through fine manipulation of the introduced cross-relaxations. By establishing an interfacial energy-transfer-mediated nanostructure, we also gain a deep insight into the mechanistic features of the energy migration. These results open new opportunities in a variety of frontier applications, such as information security. An ytterbium sublattice in an erbium-sensitized multilayer core–shell structure enables photon upconversion from lanthanide ions under 1,530 nm irradiation.

Generating optical vortex beams by momentum-space polarization vortices centred at bound states in the continuum

Abstract: Optical vortices, beams with spiral wavefronts and screw phase dislocations, have been attracting increasing interest in various fields. Here, we theoretically propose and experimentally realize an easy approach to generating optical vortices. We leverage the inherent momentum-space topological vortex-like response of polarization (strong polarization anisotropy) around bound states in the continuum of two-dimensional periodic structures, for example photonic crystal slabs, to induce Pancharatnam–Berry phases and spin–orbit interaction in the beams. This new class of optical vortex generators operates in momentum space, meaning that the structure is almost homogeneous without a real-space centre. In principle, any even-order optical vortex that is a diffraction-resistant high-order quasi-Bessel beam can be achieved at any desired working wavelength. The proposed approach expands the application of bound states in the continuum and topological photonics. Optical vortices can be generated by applying the winding behaviour of resonances in the momentum space of a photonic crystal slab, which naturally exists and is associated with bound states in the continuum, to modify the phase front of a beam.

Triplet management for efficient perovskite light-emitting diodes

Abstract: Perovskite light-emitting diodes are promising for next-generation lighting and displays because of their high colour purity and performance1. Although the management of singlet and triplet excitons is fundamental to the design of efficient organic light-emitting diodes, the nature of how excitons affect performance is still not clear in perovskite2–4 and quasi-two-dimensional (2D) perovskite-based devices5–9. Here, we show that triplet excitons are key to efficient emission in green quasi-2D perovskite devices and that quenching of triplets by the organic cation is a major loss path. Employing an organic cation with a high triplet energy level (phenylethylammonium) in a quasi-2D perovskite based on formamidinium lead bromide yields efficient harvesting of triplets. Furthermore, we show that upconversion of triplets to singlets can occur, making 100% harvesting of electrically generated excitons potentially possible. The external quantum and current efficiencies of our green (527 nm) devices reached 12.4% and 52.1 cd A−1, respectively. Careful harvesting of triplet excitons allows the realization of efficient green-emitting quasi-2D perovskite LEDs.

Generation of spatiotemporal optical vortices with controllable transverse orbital angular momentum

Abstract: Today, it is well known that light possesses a linear momentum that is along the propagation direction. Besides, scientists also discovered that light can possess an angular momentum, a spin angular momentum (SAM) associated with circular polarization and an orbital angular momentum (OAM) owing to the azimuthally dependent phase. Even though such angular momenta are longitudinal in general, an SAM transverse to the propagation direction has opened up a variety of key applications1. In contrast, investigations of the transverse OAM are rare due to its complex nature. Here, we demonstrate a three-dimensional wave packet that is a spatiotemporal (ST) optical vortex with a controllable purely transverse OAM. Contrary to the transverse SAM, the magnitude of the transverse OAM carried by the ST vortex is scalable to a larger value by simple adjustments. Since the ST vortex carries a controllable OAM uniquely in the transverse dimension, it has strong potential for novel applications that may not be possible otherwise. The scheme reported here can be readily adapted for other spectral regimes and different wave fields, opening opportunities for the study and applications of ST vortices in a wide range of areas. By applying a spiral phase in a pulse shaper, a three-dimensional wave packet, which is a spatiotemporal optical vortex with a controllable purely transverse orbital angular momentum, is demonstrated.

Strain-engineered high-responsivity MoTe2 photodetector for silicon photonic integrated circuits

Abstract: In integrated photonics, specific wavelengths such as 1,550 nm are preferred due to low-loss transmission and the availability of optical gain in this spectral region. For chip-based photodetectors, two-dimensional materials bear scientifically and technologically relevant properties such as electrostatic tunability and strong light–matter interactions. However, no efficient photodetector in the telecommunication C-band has been realized with two-dimensional transition metal dichalcogenide materials due to their large optical bandgaps. Here we demonstrate a MoTe2-based photodetector featuring a strong photoresponse (responsivity 0.5 A W–1) operating at 1,550 nm in silicon photonics enabled by strain engineering the two-dimensional material. Non-planarized waveguide structures show a bandgap modulation of 0.2 eV, resulting in a large photoresponse in an otherwise photoinactive medium when unstrained. Unlike graphene-based photodetectors that rely on a gapless band structure, this photodetector shows an approximately 100-fold reduction in dark current, enabling an efficient noise-equivalent power of 90 pW Hz–0.5. Such a strain-engineered integrated photodetector provides new opportunities for integrated optoelectronic systems. A strain-induced absorption-enhanced MoTe2-based silicon photonic microring-integrated photodetector is demonstrated, featuring high responsivity of ~0.5 A W–1 at 1,550 nm, with a low noise-equivalent power of 90 pW Hz–0.5.

Implementation of quantum key distribution surpassing the linear rate-transmittance bound

Abstract: Quantum key distribution (QKD)1,2 offers a long-term solution to secure key exchange. Due to photon loss in transmission, it was believed that the repeaterless key rate is bounded by a linear function of the transmittance, O(η) (refs. 3,4), limiting the maximal secure transmission distance5,6. Recently, a novel type of QKD scheme has been shown to beat the linear bound and achieve a key rate performance of $$O(\sqrt{\eta })$$ (refs. 7–9). Here, by employing the laser injection technique and the phase post-compensation method, we match the modes of two independent lasers and overcome the phase fluctuation. As a result, the key rate surpasses the linear bound via 302 km and 402 km commercial-fibre channels, over four orders of magnitude higher than existing results5. Furthermore, our system yields a secret key rate of 0.118 bps with a 502 km ultralow-loss fibre. This new type of QKD pushes forward long-distance quantum communication for the future quantum internet. Phase-matching quantum key distribution is implemented with a 502 km ultralow-loss optical fibre. The fluctuations of the laser initial phases and frequencies are suppressed by the laser injection technique and the phase post-compensation method.