Jon. A. Schuller

Red-emitting indium gallium nitride (InGaN) micro-light-emitting diodes (μLEDs) remain challenging due to reduced radiative efficiency at the high indium compositions required for long-wavelength emission. Here, we demonstrate InGaN/GaN red μLEDs emitting at 619 nm that are laterally enclosed by an annular distributed Bragg reflector (DBR) to enhance light extraction. Fully packaged devices exhibit peak external quantum efficiencies as high as 20.8% and wall-plug efficiencies of 13.2%, placing them as the highest-performing planar (non-V-defect engineered) red GaN μLEDs on sapphire substrates reported to date. In addition to high efficiency, the devices demonstrate strong thermal robustness, maintaining >74% of their room-temperature light output at 200 °C under high injection conditions with <8 nm peak wavelength shift. These overall results highlight laterally enclosed reflector architectures as a scalable approach for achieving efficient LED emission and, in conjunction with the natural thermal resilience of GaN devices, enable thermally resilient red μLEDs for applications in optical interconnects, visible-light communication, and next-generation displays.

In bulk semiconductors, luminescence is treated under the electric dipole approximation, where magnetic dipole (MD) and higher-order multipolar transitions are assumed negligible under parity selection rules. However, two-dimensional (2D) metal-halide perovskites frequently exhibit anomalous low-energy emission signatures arising from out-of-plane oriented MD transitions. As these transitions are nominally disallowed due to parity selection rules, we aim to identify the symmetry-breaking mechanisms that activate these forbidden emissive pathways. Here, we demonstrate that anomalous MD emission in 2D HOIPs is correlated with the deviation of interoctahedral bond angles from 180°. Beyond establishing a fundamental structure–property link, this finding equips researchers with a synthetic design rule for harvesting multipolar emission from 2D perovskites for next-generation optoelectronic devices. Our findings also reveal that modular metal cation substitution─specifically replacing Pb with Sn─provides a robust mechanism for tuning forbidden emission intensity through lone-pair-driven dynamic structural distortions.

Group III-nitride light emitting diodes (LEDs) suffer from poor efficiency for longer wavelength emission. This is partly due to increased polarization-induced barriers to vertical carrier injection at InGaN/GaN interfaces in the polar c-plane, where higher In-content is required for long-wavelength emission. Polarization-induced barriers can be bypassed by lateral carrier injection through the semipolar sidewalls of V-defects, which form at the apex of threading dislocations (TDs) during kinetically limited growth. This increases wall-plug efficiency (WPE) through the reduction of forward voltage (VF). TD and resulting V-defect density can be controlled through the formation of edge dislocation half-loops prior to V-defect opening. In this work, we demonstrate a green V-defect LED with optimized AlGaN caps with high external quantum efficiency (EQE) and WPE. The green V-defect LED demonstrated here with such a device structure achieves a peak EQE and a peak WPE of 43.9% and 37.0%, respectively.

Enhancing light-matter interactions at the nanoscale is foundational to nanophotonics, with epsilon-near-zero (ENZ) materials demonstrating significant potential. High-quality factor (Q) resonances that maximize these interactions are typically realized in photonic crystals requiring sub-50 nm precision nanofabrication over large areas, limiting scalability and increasing complexity. Mie resonances offer an alternative but are constrained by low Q-factors due to the scarcity of high-refractive index materials, necessitating large refractive index changes for effective resonance switching and limiting dynamic reconfigurability. We overcome these limitations by embedding Mie resonators within ENZ media, thereby enhancing Q-factors, mitigating geometric dispersion and fabrication challenges, and maximizing optical reconfigurability. We introduce three resonator-ENZ configurations: voids in AlN, Ge in SiO₂, and intrinsic InSb in doped InSb─spanning from low-loss phononic to lossy plasmonic ENZ modes. Using novel epitaxial regrowth techniques, we achieve significant Q-factor improvements over nonembedded resonators. An air-based Mie resonator embedded in AlN supports resonant Q-factors exceeding 100, with negligible geometric dispersion across sizes from 800 to 2800 nm. Additionally, we demonstrate dynamic reconfigurability of intrinsic InSb resonators by thermally tuning the ENZ wavelength over a 2 μm range in the mid-infrared (11-16 μm) wavelength regime. These results showcase the potential of Mie reonators embedded in ENZ media for high-fidelity sensors, thermal emitters, and reconfigurable metasurfaces, bridging theoretical predictions with practical applications and advancing the development of dynamic, high-Q optical devices.

Recently, layered 2D hybrid organic-inorganic perovskites (HOIPs) like butylammonium lead iodide (BA₂PbI₄) have been shown to exhibit ultrabright out-of-plane-oriented magnetic dipole (MD_OP) photoluminescence (PL) arising from self-trapped excitons (STEs). The MD_OP emission, however, has considerable spectral overlap with the dominant in-plane-oriented electric dipole (ED_IP) transitions, making it difficult to interrogate STE properties. Here, we theoretically investigate opportunities to use Mie resonator dimers to selectively enhance the MD_OP emission through the Purcell effect. We calculate relative MD and ED Purcell enhancements at dimer center as well as average values across the dimer geometry. We show that the selective enhancement is excellent at the dimer center enabling nearly pure MD_OP emission (96%) at the MD emission peak (540 nm) as well as predominant MD_OP emission (up to 77%) across the entire integrated spectrum (500-600 nm). We subsequently show, however, that away from the dimer center, Purcell enhancement of the relatively weak out-of-plane ED_OP transitions competes with MD_OP enhancements, reducing the branching ratio (73% at the MD emission peak, 39% spectrally integrated). Lastly, we calculate how the Mie resonator dimer modifies the PL spectra and emitter radiation pattern. Notably, for volume-averaged dipoles, both MD and ED emissions are mediated via the dimer, producing a single donut-beam-like radiation pattern across the entire emission spectrum. Our results clarify the potential for achieving "pure" MD emission from 2D HOIPs via simple Mie resonator Purcell enhancements and highlight the importance of designing nanophotonic structures that can maintain desired selective enhancements away from high-symmetry points.

Three-dimensional magneto-optical traps (3D-MOTs) are an integral component of atomic clocks, quantum computers, and other cold-atom science applications. Due to the dependence on bulk optics and lasers, conventional 3D-MOTs occupy a large volume, limiting their portability. Efforts to build 3D-MOTs using integrated photonics promise to reduce the size and weight of these systems allowing applications beyond the lab. However, the need for counterpropagating beams to facilitate 4- and 6-beam geometries necessitates free-space mirrors and quarter wave plates (QWPs) that limit integration. Replacing these mirrors and QWPs with planar retroreflecting metasurfaces provides a route to achieving a complete 3D-MOT within an integrated package. Here, we report on the design and demonstration of a retroreflecting metasurface for 3D-MOTs that operates at large angles and preserves circular polarization. Specifically, we utilize Bayesian optimization to design an amorphous silicon (a-Si) on gold metasurface for high efficiency polarization-insensitive retroreflection of 780 nm circularly polarized light at 54.7°. Numerical simulations demonstrate maintenance of circular polarization after highly efficient retroreflection (ϵ−1=1.10, R−1=0.86). Experimentally, we demonstrate similarly excellent performance at 736 nm at 50.3° (ϵ−1=1.04, R−1=0.73) and show that deviation from the target design is due to oxidation of the a-Si metaelements. We conclude by discussing mitigation strategies for future devices and propose a corrective optic for the currently fabricated device. This work represents a step toward the miniaturization of 3D-MOTs and expansion of cold-atom science beyond the laboratory.

Metasurfaces have recently risen to prominence in optical research, providing unique functionalities that can be used for imaging, beam forming, holography, polarimetry, and many more, while keeping device dimensions small. Despite the fact that a vast range of basic metasurface designs has already been thoroughly studied in the literature, the number of metasurface-related papers is still growing at a rapid pace, as metasurface research is now spreading to adjacent fields, including computational imaging, augmented and virtual reality, automotive, display, biosensing, nonlinear, quantum and topological optics, optical computing, and more. At the same time, the ability of metasurfaces to perform optical functions in much more compact optical systems has triggered strong and constantly growing interest from various industries that greatly benefit from the availability of miniaturized, highly functional, and efficient optical components that can be integrated in optoelectronic systems at low cost. This creates a truly unique opportunity for the field of metasurfaces to make both a scientific and an industrial impact. The goal of this Roadmap is to mark this “golden age” of metasurface research and define future directions to encourage scientists and engineers to drive research and development in the field of metasurfaces toward both scientific excellence and broad industrial adoption.

Metasurface‐based optical elements offer a wide design space for miniature and lightweight optical applications. Typically, metasurface optical elements transform an incident light beam into a desired output waveform. Recent demonstrations of light‐emitting metasurfaces highlight the potential for directly producing desired output waveforms via metasurface‐mediated spontaneous emission. In this work, reciprocal finite‐difference time‐domain (FDTD) simulations and machine learning are used to enable the inverse design of highly unidirectional photoluminescent III‐Nitride quantum well metasurfaces capable of directive p‐, s‐, or combined p‐ and s‐ polarized emission at arbitrary angles. In comparison with previous intuition‐guided designs using the same quantum well architectures, the inverse design approach enables new polarization capabilities and experimentally demonstrated improvements in directivity of 54%. An analysis of ways in which the inverse design both validates and contradicts previous intuition‐guided design heuristics is presented. Ultimately, the combination of reciprocal simulations and efficient global optimization (EGO) grants remarkable improvements in emission directivity and results in full control over the polarization and momentum of emitted light, including simultaneous directional emission of s‐ and p‐polarized light.

Phased-array metasurfaces enable the imprinting of complex beam structures onto coherent incident light. Recent demonstrations of photoluminescent phased-array metasurfaces highlight possibilities for achieving similar control in electroluminescent light-emitting diodes (LEDs). However, phased-array metasurface LEDs have not yet been demonstrated owing to the complexities of integrating device stacks and electrodes within nanopatterned metasurfaces. Here, we demonstrate metasurface LEDs that emit directional or focused light. We first design nanoribbon elements that achieve the requisite phase control within typical LED device constraints. Subsequently, we demonstrate unidirectional emission that can be engineered at will via phased-array concepts. This control is further exhibited in metasurface LEDs that directly emit focused beams. Finally, we show that these metasurface LEDs exhibit external quantum efficiencies (EQEs) superior to those of unpatterned LEDs. These results demonstrate metasurface designs that are compatible with high-EQE metal-free LED devices and portend opportunities for new classes of metasurface LEDs that directly produce complex beam structures.

Phased-array metasurfaces grant the ability to arbitrarily shape the wavefront of light. As such, they have been used as various optical elements including waveplates, lenses, and beam deflectors. Luminescent metasurfaces, on the other hand, have largely comprised uniform arrays and are therefore unable to provide the same control over the wavefront of emitted light. Recently, phased-array control of the wavefront of spontaneous emission has been experimentally demonstrated in luminescent phased-array metalenses and beam deflectors. However, current luminescent metasurface beam deflectors exhibit unidirectional emission for only p-polarized light. In this paper, we use a reciprocal simulation strategy to explain the polarization disparity and improve the directionality of incoherent emission from current quantum-well emitting phased-array metasurfaces. We also design complementary metasurfaces to direct emission from systems where emission originates from alternate quantum mechanical processes.