Light-emitting metalenses and meta-axicons for focusing and beaming of spontaneous emission

Efficient Redirection of Trapped Broad-Band Fluorescence from Substrates into Free Space Using c-Si Metasurfaces

Fluorescent dye films on transparent substrates are essential for OLEDs, flexible displays, X-ray detection, and wireless optical communications. However, their efficiency is often hampered by fluorescence trapping due to total internal reflection (TIR) and waveguiding. This study tackles this longstanding challenge by reconceptualizing the integration of dye films with nanoantenna metasurfaces. Traditional methods involve directly spin-coating films onto c-Si metasurfaces on quartz substrates, resulting in edge luminescence and weak inner signals. We present a straightforward, adjustable approach by integrating dye films on the opposite side of quartz substrates, reaching a 2.5-fold photoluminescence enhancement and improving the uniformity of the emission compared to the conventional methods. These gains stem from redirecting a significant portion of leaked fluorescence light trapped inside the substrate into free space, surpassing TIR conditions through in-plane diffraction orders of the metasurfaces across the full RGB spectrum. Our findings facilitate the design of more efficient luminescent devices.

Local control of polarization and geometric phase in thermal metasurfaces

Thermal emission from a hot body is inherently challenging to control due to its incoherent nature. Recent advances have shown that patterned surfaces can transform thermal emission into partially coherent beams with tailored directionality and frequency selectivity. Here we experimentally demonstrate polarization-selective, unidirectional and narrowband thermal emission using single-layer metasurfaces. By implementing polarization gradients across the surface, we unveil a generalization of the photonic Rashba effect from circular polarizations to any pair of orthogonal polarizations and apply it to thermal emission. Leveraging pointwise specification of arbitrary elliptical polarization, we implement a thermal geometric phase and leverage it to prove previous theoretical predictions that asymmetric chiral emission is possible without violating reciprocity. This general platform can be extended to other frequency regimes in efforts to compactify metasurface optics technologies without the need for external coherent sources.

Switchable unidirectional emissions from hydrogel gratings with integrated carbon quantum dots

Directional emission of photoluminescence despite its incoherence is an attractive technique for light-emitting fields and nanophotonics. Optical metasurfaces provide a promising route for wavefront engineering at the subwavelength scale, enabling the feasibility of unidirectional emission. However, current directional emission strategies are mostly based on static metasurfaces, and it remains a challenge to achieve unidirectional emissions tuning with high performance. Here, we demonstrate quantum dots-hydrogel integrated gratings for actively switchable unidirectional emission with simultaneously a narrow divergence angle less than 1.5° and a large diffraction angle greater than 45°. We further demonstrate that the grating efficiency alteration leads to a more than 7-fold tuning of emission intensity at diffraction order due to the variation of hydrogel morphology subject to change in ambient humidity. Our proposed switchable emission strategy can promote technologies of active light-emitting devices for radiation control and optical imaging.

Multi-Dimensional Light-Emitting Meta-Display: Photoluminescence and Pumping Light Multiplexing

The advent of intelligent display devices has given rise to diverse and complex demands for miniature light-emitting devices. Light-emitting metasurfaces have emerged as a practical and efficient means of achieving precise light modulation. However, their practicality is limited by certain constraints. First, there is a need for further exploration of the ability to manipulate both pumping and emitting light simultaneously. Second, there is currently no encoding freedom in multi-dimensional emitting light. To address these concerns, using meta-atoms is proposed to encode both fluorescence and pumping light independently, and expand the encoding freedom with different incident wavevector directions. A light-emitting metasurface with quad-fold multiplex encoding meta-displays, including dual scattering images and dual fluorescence images, is further demonstrated. This design strategy not only manipulates both pumping and fluorescence light but also broadens encoding freedom for comprehensive multi-functionality. This can pave the way for multiplexing optical displays, information storage, and next-generation wearable displays.

Metasurface Light-Emitting Diodes with Directional and Focused Emission

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.

Revisiting the Design Strategies for Metasurfaces: Fundamental Physics, Optimization, and Beyond

Over the last two decades, the capabilities of metasurfaces in light modulation with subwavelength thickness have been proven, and metasurfaces are expected to miniaturize conventional optical components and add various functionalities. Herein, various metasurface design strategies are reviewed thoroughly. First, the scalar diffraction theory is revisited to provide the basic principle of light propagation. Then, widely used design methods based on the unit-cell approach are discussed. The methods include a set of simplified steps, including the phase-map retrieval and meta-atom unit-cell design. Then, recently emerging metasurfaces that may not be accurately designed using unit-cell approach are introduced. Unconventional metasurfaces are examined where the conventional design methods fail and finally potential design methods for such metasurfaces are discussed.

Hybrid Metasurfaces for Perfect Transmission and Customized Manipulation of Sound Across Water-Air Interface

Extreme impedance mismatch causes sound insulation at water-air interfaces, limiting numerous cross-media applications such as ocean-air wireless acoustic communication. Although quarter-wave impedance transformers can improve transmission, they are not readily available for acoustics and are restricted by the fixed phase shift at full transmission. Here, this limitation is broken through impedance-matched hybrid metasurfaces assisted by topology optimization. Sound transmission enhancement and phase modulation across the water-air interface are achieved independently. Compared to the bare water-air interface, it is experimentally observed that the average transmitted amplitude through an impedance-matched metasurface at the peak frequency is enhanced by ≈25.9 dB, close to the limit of the perfect transmission 30 dB. And nearly 42 dB amplitude enhancement is measured by the hybrid metasurfaces with axial focusing function. Various customized vortex beams are experimentally realized to promote applications in ocean-air communication. The physical mechanisms of sound transmission enhancement for broadband and wide-angle incidences are revealed. The proposed concept has potential applications in efficient transmission and free communication across dissimilar media.

Nonlinear two-photon pumped vortex lasing based on quasi-bound states in the continuum from perovskite metasurface

The experimental observation of nonlinear two-photon pumped vortex lasing from perovskite metasurfaces is demonstrated. The vortex lasing beam is based on symmetry-protected quasi-bound states in the continuum (QBICs). The topological charge is estimated to be +1 according to the simulation result. The quality factor and lasing threshold are around 1100 and 4.28 mJ/cm2, respectively. Theoretical analysis reveals that the QBIC mode originates from the magnetic dipole mode. The lasing wavelength can be experimentally designed within a broad spectral range by changing the diameter and periodicity of the metasurface. The finite array size effect of QBIC can affect the quality factor of the lasing and be used to modulate the lasing. Results shown in this study can lead to more complex vortex beam lasing from a single chip and previously unidentified ways to obtain ultrafast modulation of the QBIC lasing via the finite array size effect.

Polarization-Tunable Perovskite Light-Emitting Metatransistor

Emerging immersive visual communication technologies require light sources with complex functionality for dynamic control of polarization, directivity, wavefront, spectrum, and intensity of light. Currently, this is mostly achieved by free space bulk optic elements, limiting the adoption of these technologies. Flat optics based on artificially structured metasurfaces that operate at the sub-wavelength scale are a viable solution, however, their integration into electrically driven devices remains challenging. Here, a radically new approach to monolithic integration of a dielectric metasurface into a perovskite light-emitting transistor is demonstrated. It is shown that nanogratings directly structured on top of the transistor channel yield an 8-fold increase of electroluminescence intensity and dynamic tunability of polarization. This new light-emitting metatransistor device concept opens unlimited opportunities for light management strategies based on metasurface design and integration.

Topological beaming of light

Nanophotonic light emitters are key components in numerous application areas because of their compactness and versatility. Here, we propose a topological beam emitter structure that takes advantage of submicrometer footprint size, small divergence angle, high efficiency, and adaptable beam shaping capability. The proposed structure consists of a topological junction of two guided-mode resonance gratings inducing a leaky Jackiw-Rebbi state resonance. The leaky Jackiw-Rebbi state leads to in-plane optical confinement with funnel-like energy flow and enhanced emission probability, resulting in highly efficient optical beam emission. In addition, the structure allows adaptable beam shaping for any desired positive definite profiles by means of Dirac mass distribution control, which can be directly encoded in lattice geometry parameters. Therefore, the proposed approach provides highly desirable properties for efficient micro-light emitters and detectors in various applications including display, solid-state light detection and ranging, laser machining, label-free sensors, optical interconnects, and telecommunications.

Angle-resolved polarimetry of hybrid perovskite emission for photonic technologies

Coupling between light and matter strongly depends on the polarization of the electromagnetic field and the nature of excitations in a material. As hybrid perovskites emerge as a promising class of materials for light-based technologies such as LEDs, LASERs, and photodetectors, it is critical to understand how their microstructure changes the intrinsic properties of the photon emission process. While the majority of optical studies have focused on the spectral content, quantum efficiency and lifetimes of emission in various hybrid perovskite thin films and nanostructures, few studies have investigated other properties of the emitted photons such as polarization and emission angle. Here, we use angle-resolved cathodoluminescence microscopy to access the full polarization state of photons emitted from large-grain hybrid perovskite films with spatial resolution well below the optical diffraction limit. Mapping these Stokes parameters as a function of the angle at which the photons are emitted from the thin film surface, we reveal the effect of a grain boundary on the degree of polarization and angle at which the photons are emitted. Such studies of angle- and polarization-resolved emission at the single grain level are necessary for future development of perovskite-based flat optics, where effects of grain boundaries and interfaces need to be mitigated.

Robust Achromatic All-Dielectric Metalens for Infrared Detection in Intelligent Inspection

Metalens has the advantages of high design freedom, light weight and easy integration, thus provides a powerful platform for infrared detection. Here, we numerically demonstrated a broadband achromatic infrared all-dielectric metalens over a continuous 800 nm bandwidth, with strong environmental adaptability in air, water and oil. By building a database with multiple 2π phase coverage and anomalous dispersions, optimizing the corrected required phase profiles and designing the sizes and spatial distributions of silicon nanopillars, we numerically realized the design of broadband achromatic metalens. The simulation results of the designed metalens show nearly constant focal lengths and diffraction-limited focal spots over the continuous range of wavelengths from 4.0 to 4.8 μm, indicating the ability of the designed metalens to detect thermal signals over a temperature range from various fault points. Further simulation results show that the metalens maintains good focusing performance under the environment of water or oil. This work may facilitate the application of metalens in ultra-compact infrared detectors for power grid faults detection.

Beam manipulation for quantum dot light-emitting diode with an Ag grating and a phase-gradient metasurface

The quantum dot (QD) light-emitting diode (LED) is a robust scheme for single photon source. However, the spontaneous emission of a QD LED has arbitrary directions and polarizations, which is disadvantage for photon collection and manipulation. We propose a QD LED integrated with an Ag grating and a phase-gradient metasurface. The circular patterned Ag grating is adopted to collimate the emission beam with right phase and improve its spatial coherence, therefore a phase-gradient metasurface can work for beam manipulation. The 10°, 20°, and 30° angle deflection as well as doughnut-pattern generation are demonstrated by numerical simulation. A small metasurface with the width of 6 µm can provide a collection efficiency of 25.9% at the deflection angle of 10°. Furthermore, only one single QD can be selected from a QD assembly with a low-density.

Active multiband varifocal metalenses based on orbital angular momentum division multiplexing

Metalenses as miniature flat lenses exhibit a substantial potential in replacing traditional optical component. Although the metalenses have been intensively explored, their functions are limited by poor active ability, narrow operating band and small depth of field (DOF). Here, we show a dielectric metalens consisting of TiO₂ nanofins array with ultrahigh aspect ratio to realize active multiband varifocal function. Regulating the orbital angular momentum (OAM) by the phase assignment covering the 2π range, its focal lengths can be switched from 5 mm to 35 mm. This active optical multiplexing uses the physical properties of OAM channels to selectively address and decode the vortex beams. The multiband capability and large DOFs with conversion efficiency of 49% for this metalens are validated for both 532 nm and 633 nm, and the incidence wavelength can further change the focal lengths. This non-mechanical tunable metalens demonstrates the possibility of active varifocal metalenses.

A dielectric metalens consisting of ultrahigh aspect ratio TiO₂ nanofins array is demonstrated to realize active multiband varifocal functionality. By regulating the orbital angular momentum, the focal length can be switched from 5 mm to 35 mm with large DOFs.

Designing Highly Directional Luminescent Phased-Array Metasurfaces with Reciprocity-Based Simulations

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.

Ultracompact Nanophotonics: Light Emission and Manipulation with Metasurfaces

Internet of Things (IoT) technology is prosperous for the betterment of human well-being. With the expeditious needs of miniature functional devices and systems for adaptive optics and light manipulation at will, relevant sensing techniques are thus in the urgent stage of development. Extensive developments in ultrathin artificial structures, namely metasurfaces, are paving the way for the next-generation devices. A bunch of tunable and reconfigurable metasurfaces with diversified catalogs of mechanisms have been developed recently, enabling dynamic light modulation on demand. On the other hand, monolithic integration of metasurfaces and light-emitting sources form ultracompact meta-devices as well as exhibiting desired functionalities. Photon-matter interaction provides revolution in more compact meta-devices, manipulating light directly at the source. This study presents an outlook on this merging paradigm for ultracompact nanophotonics with metasurfaces, also known as metaphotonics. Recent advances in the field hold great promise for the novel photonic devices with light emission and manipulation in simplicity.

Anisotropic Radiation in Heterostructured "Emitter in a Cavity" Nanowire

Tailorable synthesis of axially heterostructured epitaxial nanowires (NWs) with a proper choice of materials allows for the fabrication of novel photonic devices, such as a nanoemitter in the resonant cavity. An example of the structure is a GaP nanowire with ternary GaPAs insertions in the form of nano-sized discs studied in this work. With the use of the micro-photoluminescence technique and numerical calculations, we experimentally and theoretically study photoluminescence emission in individual heterostructured NWs. Due to the high refractive index and near-zero absorption through the emission band, the photoluminescence signal tends to couple into the nanowire cavity acting as a Fabry–Perot resonator, while weak radiation propagating perpendicular to the nanowire axis is registered in the vicinity of each nano-sized disc. Thus, within the heterostructured nanowire, both amplitude and spectrally anisotropic photoluminescent signals can be achieved. Numerical modeling of the nanowire with insertions emitting in infrared demonstrates a decay in the emission directivity and simultaneous rise of the emitters coupling with an increase in the wavelength. The emergence of modulated and non-modulated radiation is discussed, and possible nanophotonic applications are considered.