Metasurfaces consisting of subwavelength elements exhibit unparalleled flexibility in light manipulation in terms of phase, amplitude, and/or polarization at ultrathin dimensions. Typically, a continuous and complete phase distribution covering a full 2π range is required in metasurface design to produce the performance of conventional optical components, such as gratings, lenses, and beam splitters. However, an incomplete phase, i.e., with phase change less or larger than 2π, can provide additional degrees of freedom for optical wavefront reconstruction. This article shows that designed metagratings, which unlocked the 2π phase constraint in supercell designs, achieved arbitrary control of the intensity ratio between any adjacent diffraction orders, while keeping the diffraction directions consistent with those of conventional gratings. Four metagratings, as representatives, with different phase ranges in the supercell, i.e., π, 2π, 3π, and 4π, have been designed and fabricated to demonstrate the diffraction intensity redistribution capability of metagratings. The 0th- and the 1st-order splitting ratios measured in experiments can reach 0.07 to 24.8, which is a hard task for traditional grating devices. Using a simple design methodology, incomplete phase metasurfaces hold great promise for developing various functional ultrathin nanophotonic devices, such as controllable beam splitters, spectrometers, and multifoci metalens.

Magnetic monopoles are hypothetical particles which, similar to the electric monopoles that generate electric fields, are at the origin of magnetic fields. Despite many efforts, to date, these theoretical particles have yet to be observed. Nevertheless, many systems or physical phenomena mimic the behavior of magnetic monopoles. Here, we propose a new type of photonic nanoantenna behaving as a radiating magnetic monopole. We demonstrate that a half-nanoslit in a semi-infinite gold layer generates a single pole of enhanced magnetic field at the nanoscale and that this single pole radiates efficiently in the far field. We also introduce an effective magnetic charge using Gauss’s law of magnetism, in analogy to the electric charge, which further highlights the monopolar behavior of this new antenna. Finally, we show that different plasmonic and metallic materials can provide magnetic monopole antennas covering the visible-to-near infrared range, even down to GHz frequencies. This original antenna concept opens the way to a new model system to study magnetic monopoles and a new optical magnetic field source to study “magnetic light–matter coupling.” Furthermore, it shows potential applications at lower frequencies, such as in magnetic resonance imaging.

When light passes through a multimode fiber, two-dimensional random intensity patterns are formed due to complex interference within the fiber. The extreme sensitivity of speckle patterns to the frequency of light paved the way for high-resolution multimode fiber spectrometers. However, this approach requires expensive IR cameras and impedes the integration of spectrometers on-chip. In this study, we propose a single-pixel multimode fiber spectrometer by exploiting wavefront shaping. The input light is structured with the help of a spatial light modulator, and optimal phase masks, focusing light at the distal end of the fiber, are stored for each wavelength. Variation of the intensity in the focused region is recorded by scanning all wavelengths under fixed optimal masks. Based on the intensity measurements, we show that an arbitrary input spectrum having two wavelengths 20 pm apart from each other can be reconstructed successfully (with a reconstruction error of ∼3%) in the near-infrared regime, corresponding to a resolving power of R ≈ 105. We also demonstrate the reconstruction of broadband continuous spectra with varying bandwidths. With the installation of a single-pixel detector, our method provides compact detection and a lower budget alternative to conventional systems, with potential promise to operate at low-signal levels.

As solution-processed hybrid quantum dot light-emitting diodes (QLEDs), they may undergo a positive ageing process to improve their performance. It is highly desirable to investigate the ageing treatment and further use this positive effect to regulate the performance of the device. Under different ageing periods, we analyze how CdSe/ZnxCd1–xSe/ZnSeyS1–y core/shell quantum dot (QD)–ZnMgO interface and possible interface reactions between the QD, ZnMgO, and the Al electrode can affect device performance via positive ageing. The Kelvin probe measurements indicate a reduction in the energy level difference between ZnMgO and Al, leading to a relatively large built-in potential. By simply adjusting the annealing temperature of ZnMgO, the degree of positive ageing can be adjusted to optimize the device performance. By comparing the work function change of ZnMgO at different annealing temperatures, the change of surface electron affinity becomes more obvious, which may affect the degree of positive ageing. With an about 22 times improvement in operational lifetime, the peak external quantum efficiency of aged QLEDs can be optimized from 14.7 to 26.7%. This work presents an entirely new perspective on positive ageing and can serve as an important scientific guideline to further improve device performance.

Next-generation integrated nanophotonic device designs leverage advanced optimization techniques such as inverse design and topology optimization, which achieve high performance and extreme miniaturization by optimizing a massively complex design space enabled by small feature sizes. However, unless the optimization is heavily constrained, the generated small features are not reliably fabricated, leading to optical performance degradation. Even for simpler, conventional designs, fabrication-induced performance degradation still occurs. The degree of deviation from the original design depends not only on the size and shape of its features but also on the distribution of features and the surrounding environment, presenting a complex, proximity-dependent behavior. Without proprietary fabrication process specifications, design corrections can only be made after calibrating fabrication runs take place. In this work, we introduce a general deep machine learning model that automatically corrects photonic device design layouts prior to first fabrication. Only a small set of scanning electron microscopy images of engineered training features are required to create the deep learning model. By making corrections to the design layout, the fabricated structure more closely aligns with the original intended design and therefore results in improved optical performance. Without modifying the nanofabrication process, adding significant computation in design, or requiring proprietary process specifications, we believe that our model opens the door to new levels of reliability and performance in next-generation photonic circuits.

Room-temperature strong coupling between plasmonic nanocavities and monolayer semiconductors is a prominent path toward efficient, integrated devices. However, designing such systems is challenging due to the nontrivial dependence of the strong coupling on the properties of both the cavity and the emitter, as well as the subwavelength scale of interaction. In this work, we develop a general methodology for obtaining strongly coupled hybrid metasurfaces consisting of plasmonic nanocavities coupled to atomically thin semiconductor layers, exhibiting extreme values of Rabi splitting, by inverse design of the near-field plasmonic response. We experimentally demonstrate large values of Rabi splitting in a nanoantenna design while providing theoretically optimal configurations for additional types of nanostructures. Our results open a path to maximizing light–matter interactions in integrated platforms for classical and quantum-optical applications.

We report on the coupling of a reconfigurable high Q fiber micro-cavity to an organic color center grafted to a carbon nanotube for telecom wavelength emission of single photons in the Purcell regime. Using three complementary approaches, we assess various figures of merit of this tunable single photon source and of the cavity quantum electrodynamical effects: the brightening of the emitter is obtained by comparison of the count rates of the very same emitter in free-space and cavity coupled regimes. We demonstrate a fiber coupled single-photon output rate up to 20 MHz at 1275 nm. Using time-resolved and saturation measurements, we determine independently the radiative quantum yield and the Purcell factor of the system with values up to 30 for the smallest mode volumes. Finally, we take advantage of the tuning capability of the cavity to measure the spectral profile of the brightness of the source which gives access to the vacuum Rabi splitting g with values up to 25 μeV.

The degradation mechanism of blue-fluorescent organic-light-emitting diodes (OLEDs) has been continuously studied to overcome their limited operational lifetime. However, studies thus far have focused primarily on only one critical factor after a sufficiently long progression of degradation, such as the time to reach 50% of initial luminance (LT50). However, due to the sensitivity of human eyes, only 3–5% of luminance changes can be recognized. Thus, research on early-stage degradation is needed from a more practical viewpoint. Herein, we propose a new methodology to analyze the factors affecting early-stage degradation, such as LT97 in blue-fluorescent OLEDs. The result is expressed numerically (or quantitively) and subdivided into three factors in total, including bleaching of organic materials, quenching in excited states, and variations in charge balance. It was found that throughout early-stage degradation, the bleaching of emissive materials and the quenching of excitons adversely affect a device’s operational stability. However, interestingly, the degree of charge balance slightly enhances up to LT97 and thereafter deteriorates. Degradation factors are further investigated up to LT88. The methodology presented here can be further expanded to analyze degradation factors quantitatively across various OLED stages and colors.

We report on a new approach of a low phase noise electro-optomechanical oscillator directly working in the GHz frequency range. The developed nanoscale oscillator is a one-dimensional photonic crystal made of gallium phosphide (GaP), heterogeneously integrated on silicon-on-insulator circuitry. Based on the strong interaction between the optical mode at the telecommunication wavelength and the mechanical mode in GHz, ultra-pure mechanical oscillations are enabled and directly imprinted on an optical carrier. Further stabilization is achieved with a delayed optoelectronic feedback loop using integrated electro-mechanical self-injection. We achieve a short-term stability of 0.7 Hz linewidth and a long-term stability with an Allan deviation below 10–7 Hz/Hz at 10 s averaging time, which represents an important step toward fully integrated optomechanical oscillators. Integrability and the low phase noise of this oscillator address some of the most important needs of optoelectronic oscillators and pave the way toward on-chip integrated microwave oscillators for microwave applications such as RADARs.

Passive daytime radiative cooling (PDRC) devices have enabled subambient cooling of terrestrial objects without any energy input, offering great potential to future clean energy technology. Among various PDRC structures, random dielectric particles in a polymer matrix or paint-like coatings have displayed powerful radiative cooling performances with excellent scalability and easy fabrication. While modeling and analyzing such a system is nontrivial to enhance the cooling effect and engineer the structures to be utilized in various applications, it is essential to understand its complex physical relations and determine the optimal design conditions. In this work, we have thoroughly analyzed the optical properties and radiative cooling performances of PDRC paints composed of two-material particles (SiO2 and Al2O3) using 2D FDTD simulation and investigated the optimal design conditions. Specifically, we have studied the effects of design parameters, such as particle size, size distribution, binder volume ratio, and coating thickness. Subsequently, we have conducted an outdoor cooling measurement of the fabricated PDRC paints to demonstrate their radiative cooling potential and to analyze and understand their performance based on our numerical investigations. The fabricated PDRC paints exhibited high solar reflectance (0.958) and strong long-wave infrared emission (0.937) in the atmospheric transparency window, achieving a maximum temperature drop of 9.1 °C. This comprehensive study provides a detailed characterization of the structure and material parameters of the multimaterial PDRC paint system.

In recent years, quantum dots (QDs) have emerged as bright, color-tunable light sources for various applications such as light-emitting devices, lasing, and bioimaging. One important next step to advance their applicability is to reduce particle-to-particle variations of the emission properties as well as fluctuations of a single QD’s emission spectrum, also known as spectral diffusion (SD). Characterizing SD is typically inefficient as it requires time-consuming measurements at the single-particle level. Here, however, we demonstrate multiparticle spectroscopy (MPS) as a high-throughput method to acquire statistically relevant information about both fluctuations at the single-particle level and variations at the level of a synthesis batch. In MPS, we simultaneously measure emission spectra of many (20–100) QDs with a high time resolution. We obtain statistics on single-particle emission line broadening for a batch of traditional CdSe-based core–shell QDs and a batch of the less toxic InP-based core–shell QDs. The CdSe-based QDs show significantly narrower homogeneous line widths, less SD, and less inhomogeneous broadening than the InP-based QDs. The time scales of SD are longer in the InP-based QDs than in the CdSe-based QDs. Based on the distributions and correlations in single-particle properties, we discuss the possible origins of line-width broadening of the two types of QDs. Our experiments pave the way to large-scale, high-throughput characterization of single-QD emission properties and will ultimately contribute to facilitating rational design of future QD structures.

High-performance spectrally distinctive photodetectors (PDs) are of great importance in sensing and information processing. PDs based on photoelectrochemical (PEC) principles are of particular interest due to their simple fabrication process and tunable photoresponse through both physical and chemical processes. Despite the recent advancement in PEC-PDs, they are far less ideal. For example, most of them are either not stable or not compatible with existing epitaxial semiconductor device platforms, whereas although III-nitride nanowire-based PEC-PDs can largely mitigate these drawbacks, dual-band photodetection is limited to the ultraviolet (UV) range. Herein, we show that by using fully epitaxial n-GaN/p-InGaN p–n heterojunction photoelectrodes on the Si substrate, the dual-band photodetection of III-nitride nanowire-based PEC-PDs can be extended to the visible (VIS) range. Moreover, the present photoelectrodes can also exhibit dual-polarity photocurrent under a fixed illumination condition by tuning the applied potential, extending their functionality. This study represents the first achievement of UV–VIS dual-band photodetection with simple, fully epitaxial semiconductor nanowire p–n heterojunctions. The discussion on the photocarrier dynamics further sheds light on the design of dual-band PEC-PDs based on emerging semiconductor nanowire p–n heterojunctions.

Label-free detection of single biomolecules in solution has been achieved using a variety of experimental approaches over the past decade. Yet, our understanding of the magnitude of the optical contrast and its relationship with the underlying atomic structure as well as the achievable measurement sensitivity and precision remain poorly defined. Here, we use a Fourier optics approach combined with an atomic structure-based molecular polarizability model to simulate mass photometry experiments from first principles. We find excellent agreement between several key experimentally determined parameters such as optical contrast-to-mass conversion, achievable mass accuracy, and molecular shape and orientation dependence. This allows us to determine detection sensitivity and measurement precision mostly independent of the optical detection approach chosen, resulting in a general framework for light-based single-molecule detection and quantification.

The internal quantum efficiency of (In,Ga)N/GaN quantum wells can surpass 90% for blue-emitting structures at moderate drive current densities but decreases significantly for longer emission wavelengths and at higher excitation rates. This latter effect is known as efficiency “droop” and limits the brightness of light-emitting diodes (LEDs) based on such quantum wells. Several mechanisms have been proposed to explain efficiency droop including Auger recombination, both intrinsic and defect-assisted, carrier escape, and the saturation of localized states. However, it remains unclear which of these mechanisms is most important because it has proven difficult to reconcile theoretical calculations of droop with measurements. Here, we first present experimental photoluminescence measurements extending over three orders of magnitude of excitation for three samples grown at different temperatures that indicate that droop behavior is not dependent on the point defect density in the quantum wells studied. Second, we use an atomistic tight-binding electronic structure model to calculate localization-enhanced radiative and Auger rates and show that both the corresponding carrier density-dependent internal quantum efficiency and the carrier density decay dynamics are in excellent agreement with our experimental measurements. Moreover, we show that point defect density, Auger recombination, and the effect of the polarization field on recombination rates only limit the peak internal quantum efficiency to about 70% in the resonantly excited green-emitting quantum wells studied. This suggests that factors external to the quantum wells, such as carrier injection efficiency and homogeneity, contribute appreciably to the significantly lower peak external quantum efficiency of green LEDs.

Establishing high-precision patterning technology for high-quality perovskite films is critical for practical applications requiring optoelectronic device arrays. Here, we report a femtosecond laser printing strategy to realize high-precision and high-quality patterned perovskite films by integrating in situ crystallization and patterning in one printing step. Compared with a thermal-annealed CsPbBr3 film, the crystallization quality of a laser-printed film has been improved because of localized nucleation and growth at the laser-irradiated area, avoiding the incomplete film coverage and poor crystallization quality due to rapid solvent evaporation in the large area during the thermal annealing process. Also, high-resolution pixel arrays and designed patterns with a minimum line width of 2 μm have been demonstrated by means of the high precision of femtosecond laser printing. More importantly, we have fabricated a prototype of perovskite light-emitting diodes (LEDs) demonstrating the potential of femtosecond laser printing technology for the fabrication of high-precision patterned perovskite film optoelectronic devices for display application.

Multimode fiber (MMF) imaging is an emerging field of fiber imaging technology in the last few decades. However, its high sensitivity to dynamic perturbance limits its practical applications. In this study, we propose an anti-perturbation scheme for MMF imaging based on the active measurement of the fiber configuration. We fabricate an imaging device composed of the MMF and fiber Bragg grating array to measure the MMF configuration parameters in real time and record the object–speckle pairs in different configurations for neural network training. Image reconstruction subjected to dynamic perturbations can be realized using deep learning, and the experimental results show that the introduction of fiber configuration parameters can improve the quality of anti-perturbation imaging. In addition, we realize speckle prediction using the configuration parameters and a trained neural network. The predicted speckle can be applied to flexible MMF compressive imaging. Our work proposes a new scheme for flexible MMF imaging and provides an important reference for the practical application of MMF imaging.

We study lateral light scattering on fiber Bragg gratings (FBG) with the goal of creating an optical fiber-based linear light source with controllable emission angles. The scattering from two FBGs was measured for polar angles (measured from the fiber axis) in the range θ = 27.1–152.9° and for all azimuth angles around the fiber. The observed light emission is strongly concentrated in one or more scattering cones around the fiber axis, showing four intensity peaks on opposite sides. These scattering phenomena are described and explained using the volume current method. This method shows a novel and simple way to understand the side scattering on FBGs as a combined effect of the grating’s longitudinal period, the grating harmonics, and its transversal shape. Further, this work contributes to a better understanding of the azimuth FBG-scattering caused by an interplay of the transversal grating shape and the fiber modes. The presented method can generate tailored side emissions for fiber light source applications, create light power sensors, or suppress unwanted scattering on FBGs for low optical loss applications.

This paper presents the high-resolution (>2000 PPI) multicolor patterning of InP quantum dot films using a conventional photolithography process with a positive photoresist (PR). The solvent resistance of the quantum dot (QD) film is achieved by depositing an ultrathin ZnO layer through atomic layer deposition. This is different from previous studies, which lack high-resolution patterning or compatibility with indium phosphide (InP) QDs owing to chemical weaknesses. By employing a positive PR with a photoacid generator, the side-by-side patterning process yields multicolor patterns of red- and green-colored InP-based QDs. Additionally, the stacking of each color QD film is achieved. The patterning process can be used to fabricate QD light-emitting diode devices without degrading their performance. This process can be used not only for thin (1 μm) QD films, which can be used in the color-conversion layer with a backlight.

Shape optimization of microstructures such as microlenses and diffractive optical elements fabricated by multi-photon laser printing is routinely performed by optical characterization after completing printing and development (ex situ). It is, however, highly desirable, instead of or in addition, to optically characterize the samples during the printing process before development (in situ). Here, we successfully demonstrate the integration of in-situ quantitative phase imaging into a commercial multi-photon laser printer. In terms of hardware, this integration merely requires adding illumination by a collimated LED and a small aperture to the existing beam path. In terms of software, we use well-established reconstruction algorithms based on a stack of through-focus, wide-field optical images acquired within a few seconds. We verify this approach by inspecting the topography of various microoptical elements printed with different photoresists and comparing the results with ex-situ measurements obtained by using a spinning-disk confocal optical microscope.

Reconfigurable multichannel optical filters (MOFs) with a high resolution are key devices for high-spectral-efficiency multi-carrier optical transmission systems. However, very few on-chip MOFs reported so far can meet the demands of high spectral resolution, device complexity, and scalable input/output ports. Here, we present a 10 GHz-bandwidth self-configurable silicon MOF using a resonator-assisted discrete-Fourier-transform interferometer. With the help of ultra-low-loss silicon photonic waveguides and low-phase-error Mach–Zehnder switches, the present chip features a low excess loss, low power consumption, high port-count scalability, and sub-GHz spectral resolution. In particular, the present MOF is capable of performing some specific filtering responses with, e.g., near-rectangular or sinc shapes. Furthermore, an efficient self-configuring approach is also developed for the present chip to be programmable, so that different spectral responses can be achieved as desired by automatically optimizing the settings of all the tuning elements, showing great potential for high-capacity optical communication networks.