Optical Metasurfaces Research Articles

Intracavity spatially modulated metasurfaces for a wavelength-tunable figure-9 vortex fiber laser.

Intracavity optical metasurfaces with compact and flexible light manipulation capabilities, effectively enrich the implementation of miniaturized and user-friendly orbital angular momentum (OAM) laser sources. Here we demonstrate a wavelength-tunable figure-9 Yb-doped vortex fiber laser solely with standard non-polarization-maintaining single-mode fibers, which utilizes a gap-surface plasmon (GSP) metasurface as the intracavity mode regulation component to generate OAM beams, extending the avenues and related applications for cost-effective OAM laser sources. Gained by the broadband operation range of the metasurface, the figure-9 fiber laser could emit OAM light with center wavelength tunable from 1020 nm to 1060 nm and of high mode purity (about 90%). OAM beams with different topological charges such as l = ±1 have been obtained by changing the metasurface design. The proposed fiber laser with the intracavity GSP metasurface provides a reliable and customized output of OAM beams at the laser source, holding great promise for a wide range of applications in optical communications, sensing, and super-resolution imaging.

Electrically tunable topological phase transition in non-Hermitian optical MEMS metasurfaces.

Exceptional points (EPs), unique junctures in non-Hermitian open systems where eigenvalues and eigenstates simultaneously coalesce, have gained notable attention in photonics because of their enthralling physical principles and unique properties. Nonetheless, the experimental observation of EPs, particularly within the optical domain, has proven rather challenging because of the grueling demand for precise and comprehensive control over the parameter space, further compounded by the necessity for dynamic tunability. Here, we demonstrate the occurrence of optical EPs when operating with an electrically tunable non-Hermitian metasurface platform that synergizes chiral metasurfaces with piezoelectric MEMS mirrors. Moreover, we show that, with a carefully constructed metasurface, a voltage-controlled spectral space can be finely tuned to access not only the chiral EP but also the diabolic point characterized by degenerate eigenvalues and orthogonal eigenstates, thereby allowing for dynamic topological phase transition. Our work paves the way for developing cutting-edge optical devices rooted in EP physics and opening uncharted vistas in dynamic topological photonics.

Multiresonant all-dielectric metasurfaces based on high-order multipole coupling in the visible.

In many cases, optical metasurfaces are studied in the single-resonant regime. However, a multiresonant behavior can enable multiband devices with reduced footprint, and is desired for applications such as display pixels, multispectral imaging and sensing. Multiresonances are typically achieved by engineering the array lattice (e.g., to obtain several surface lattice resonances), or by adopting a unit cell hosting one (or more than one) nanostructure with some optimized geometry to support multiple resonances. Here, we present a study on how to achieve multiresonant metasurfaces in the visible spectral range by exploiting high-order multipoles in dielectric (e.g., diamond or titanium dioxide) nanostructures. We show that in a simple metasurface (for a fixed particle and lattice geometry) one can achieve triple resonance occurring nearly at RGB (red, green, and blue) wavelengths. Based on analytical and numerical analysis, we demonstrate that the physical mechanism enabling the multiresonance behavior is the lattice induced coupling (energy exchange) between high-order Mie-type multipoles moments of the metasurface's particles. We discuss the influence on the resonances of the metasurface's finite size, surrounding material, polarization, and lattice shape, and suggest control strategies to enable the optical tunability of these resonances.

Scattering characteristics of silicon nanoprisms: A theoretical investigation across monomeric to hexameric structures

Dielectric nanostructures exhibit intriguing optical properties and outstanding advantages in designing optical nanoantennas and metasurfaces compared to plasmonic nanostructures. This study employs classical electrodynamic methods to comprehensively explore the scattering characteristics of silicon triangular nanoprisms in monomer and oligomer forms. For monomeric nanoprisms, the scattering spectra reveal two distinct and prominent resonance peaks attributed to magnetic dipole (MD) and electric dipole (ED) modes. Reducing interparticle gaps within dimeric structures leads to noticeable blueshifts in MD resonance peaks with stable intensities, in contrast to the nearly constant position and significantly reduced intensities of the ED resonance peaks. A pronounced Fano-like resonance was observed upon transitioning to tetrameric and hexameric configurations, resulting from the coupling between MD and ED modes. A broad resonance peak also emerges in the long-wavelength region due to MD-to-MD coupling. The simulations conducted herein hold significant theoretical implications, advancing our comprehension of the scattering properties of dielectric nanoparticles and contributing valuable insights into fundamental nanophotonics.

Hybridization of electromagnetic multipoles in a nanoscatterer in the presence of another nanoscatterer

Coupling between multipolar modes of different orders has not been investigated in depth, despite its fundamental and practical relevance in the context of optical metamaterials and metasurfaces. Here, we use an electromagnetic multipole expansion of both the scattered fields and the oscillating electric currents to reveal the multipolar excitations in a nanoparticle positioned close to another nanoparticle. The considered single-particle multipoles radically differ from multipoles excited in a pair of nanoparticles. Using the expansion, we reveal the multipole character of the electric currents and the contributions of the multipole moments to the scattering cross section of each particle, including the effect of their interaction. We find that light scattered by the particles plays the role of an inhomogeneous incident field for each of the particles, leading to hybridization of the originally independent orthogonal multipole resonances. For an incident plane wave polarized along the nanoparticle pair, the hybridization of the dipole and quadrupole resonances gives rise to a significant narrowband resonance in the spectrum of the dipole scattering, which can be of interest for various applications, e.g. in surface-enhanced fluorescence and Raman spectroscopy. In general, this work shows that the multipole-multipole interaction between nanoparticles must be treated by taking into account also such hybridized multipole resonances.

Towards rapid colorimetric detection of extracellular vesicles using optofluidics-enhanced color-changing optical metasurface.

Efficient transportation and delivery of analytes to the surface of optical sensors are crucial for overcoming limitations in diffusion-limited transport and analyte sensing. In this study, we propose a novel approach that combines metasurface optics with optofluidics-enabled active transport of extracellular vesicles (EVs). By leveraging this combination, we show that we can rapidly capture EVs and detect their adsorption through a color change generated by a specially designed optical metasurface that produces structural colors. Our results demonstrate that the integration of optofluidics and metasurface optics enables spectrometer-less and label-free colorimetric read-out for EV concentrations as low as 107 EVs/ml, achieved within a short incubation time of two minutes.

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.

Colloidal Self-Assembly of Silver Nanoparticle Clusters for Optical Metasurfaces.

Optical metasurfaces are two-dimensional assemblies of nanoscale optical resonators and could constitute the next generation of ultrathin optical components. The development of methods to manufacture these nanostructures on a large scale is still a challenge, while most performance demonstrations were obtained with lithographically fabricated metasurfaces that are restricted to small scales. Self-assembly fabrication routes are promising alternatives and have been used to produce original nanoresonators. Reports of self-assembled metasurface fabrication, however, are still scarce. Here, we show that an emulsion-based formulation approach can be used both for the fabrication of complex colloidal resonators, presenting a strong interaction with light, in particular due to simultaneous magnetic and electric modes of resonance, and for their deposition in homogeneous films. This fabrication technique involves emulsification of an aqueous suspension of silver nanoparticles in an oil phase, followed by controlled drying of the emulsion, and produces silver colloidal clusters. We show that the drying process can be controlled in a liquid emulsion, producing a metafluid, as well as in a sedimented emulsion, producing a metasurface. The structural control of the synthesized colloidal clusters is demonstrated with electron microscopy and X-ray scattering techniques. Using a polarization-resolved multiangle light scattering setup in the visible wavelength range, we conduct a comprehensive angular and spectroscopic study of the optical resonant scattering of the nanoresonators in a metafluid and show that they present strong optical magnetic resonances and directional forward-scattering patterns, with scattering efficiencies of up to 4. The metasurfaces consist of homogeneous films, of variable surface density, of colloidal clusters that have the same extinction properties on the surface and in the fluid. This experimental approach allows for large-scale production of metasurfaces.

Flexible Metasurfaces for Multifunctional Interfaces.

Optical metasurfaces, capable of manipulating the properties of light with a thickness at the subwavelength scale, have been the subject of extensive investigation in recent decades. This research has been mainly driven by their potential to overcome the limitations of traditional, bulky optical devices. However, most existing optical metasurfaces are confined to planar and rigid designs, functions, and technologies, which greatly impede their evolution toward practical applications that often involve complex surfaces. The disconnect between two-dimensional (2D) planar structures and three-dimensional (3D) curved surfaces is becoming increasingly pronounced. In the past two decades, the emergence of flexible electronics has ushered in an emerging era for metasurfaces. This review delves into this cutting-edge field, with a focus on both flexible and conformal design and fabrication techniques. Initially, we reflect on the milestones and trajectories in modern research of optical metasurfaces, complemented by a brief overview of their theoretical underpinnings and primary classifications. We then showcase four advanced applications of optical metasurfaces, emphasizing their promising prospects and relevance in areas such as imaging, biosensing, cloaking, and multifunctionality. Subsequently, we explore three key trends in optical metasurfaces, including mechanically reconfigurable metasurfaces, digitally controlled metasurfaces, and conformal metasurfaces. Finally, we summarize our insights on the ongoing challenges and opportunities in this field.

Elimination of reflection losses in gradient metasurface optical elements

Optical metasurfaces can achieve arbitrary wavefront control using subwavelength nanostructures, enabling imaging, sensing, and display applications using ultrathin structures. Metasurface optical elements that rely on propagation phase achieve this control using varying fill fractions of high refractive index materials, resulting in surface regions with significantly different effective refractive index. The intrinsic need for index variation makes it a non-trivial task to eliminate reflection losses across a complete optical element. Here, we investigate the optical performance of high-index metasurfaces, modified to suppress surface reflections. We demonstrate that a drastic reduction of surface reflections across an entire gradient metasurface is possible through the application of a single optimized anti-reflective layer with a thickness and refractive index that are substantially different from the traditionally expected values.

Asymmetric Full-Color Vectorial Meta-holograms Empowered by Pairs of Exceptional Points.

Holography holds tremendous promise in applications such as immersive virtual reality and optical communications. With the emergence of optical metasurfaces, planar optical components that have the remarkable ability to precisely manipulate the amplitude, phase, and polarization of light on the subwavelength scale have expanded the potential applications of holography. However, the realization of metasurface-based full-color vectorial holography remains particularly challenging. Here, we report a general approach utilizing a modified Gerchberg-Saxton algorithm to achieve spatially aligned full-color display and incorporating wavelength information with an image compensation strategy. We combine the Pancharatnam-Berry phase and pairs of exceptional points to address the issue of redundant twin images that generally appear for the two orthogonal circular polarizations and to enable full polarization control of the vectorial field. Our results enable the realization of an asymmetric full-color vectorial meta-hologram, paving the way for the development of full-color display, complex beam generation, and secure data storage applications.

High Efficiency Ultra-Thin Normal-Incidence Ge-On-Si Photodetector Based on Optical Metasurface

The much thicker intrinsic absorption layer (IAL) in normal-incidence Ge-on-Si photodetectors (NIPD) usually causes a contradiction between responsivity and bandwidth. In response to this issue, here, we simulate the design of an NIPD with geranium (Ge) layers based on a “fishnet” metasurface, leading to a reduced device thickness as thin as 380[Formula: see text]nm. The optical simulation results show that the light field can be perfectly localized in the 210[Formula: see text]nm IAL, and the absorptivity is as high as 99.45% at 1550[Formula: see text]nm, which is even better than bulk materials. Moreover, the electrical simulation results suggest that the horizontal size of the photosensitive region can be reduced to 11.2 [Formula: see text]m, while the responsivity of the photodetector is close to 1 A/W at −1[Formula: see text]V bias voltage, which is nearly 23 times that of a bulk device with the same thickness, and the 3dB bandwidth is up to 40[Formula: see text]GHz, which can be compared with waveguide photodetectors. Besides, this device also demonstrates a high signal-to-noise ratio with a low dark current of 28.68 nA, making it an excellent PD for opto-electrical communication.

Nanostructure-based orbital angular momentum encryption and multiplexing.

The orthogonality among the OAM modes provides a new degree of freedom for optical multiplexing communications. So far, traditional Dammann gratings and spatial light modulators (SLMs) have been widely used to generate OAM beams by modulating electromagnetic waves at each pixel. However, such architectures suffer from limitations in terms of having a resolution of only a few microns and the bulkiness of the entire optical system. With the rapid development of the electromagnetic theory and advanced nanofabrication methods, artificial nanostructures, especially optical metasurfaces, have been introduced which greatly shrink the size of OAM multiplexing devices while increasing the level of integration. This review focuses on the study of encryption, multiplexing and demultiplexing of OAM beams based on nanostructure platforms. After introducing the focusing characteristics of OAM beams, the interaction mechanism between OAM beams and nanostructures is discussed. The physical phenomena of helical dichroism response and spatial separation of OAM beams achieved through nanostructures, setting the stage for OAM encryption and multiplexing, are reviewed. Afterward, the further advancements and potential applications of nanophotonics-based OAM multiplexing are deliberated. Finally, the challenges of conventional design methods and dynamic tunable techniques for nanostructure-based OAM multiplexing technology are addressed.

Optical transparent metasurface lenses and their wireless communication efficiency enhancement

This paper presents the design of an optically transparent metasurface tailored for the 2.4 GHz Wi-Fi band. It is optically transparent and attaches to both sides of the glass to improve communication efficiency. The shape of focusing region is a rectangle with an area of 5 cm by 5 cm and a length of 10 cm. The metasurface attaches to both sides of the glass and realizes area focusing. To meet the requirements for area focusing, the metasurface possesses a double-layer structure of a Jerusalem cross and a circle, and the conductive thin film is a conductive and optically transparent copper mesh. The spatial distribution of field strength in a microwave unreflected chamber is scanned to verify the regional focusing effect of the metasurface. Compared with ordinary glass, the metalens achieves field enhancement of more than 7.3 dB in the designed aggregation region, with an average download speed increasing 20.2 Mb/s. Subsequently, the download speed and network speed stability in different scenarios are tested. The standard deviation is used to calculate the dispersion of the download speed. The results demonstrate that in the focusing area, comparing with ordinary glass, the average download speed of the signal across is increased by 13.8 Mb/s in the indoor environment, accompanied by a reduction in the standard deviation by 0.5. In the stairwell, the average download speed of the signal across of the metalens is observed to increase 12.1 Mb/s, accompanied by a reduction in the standard deviation by 1.4. In conclusion, the metasurface lens demonstrates the better ability to significantly reduce the standard deviation of download speed data in both indoor and stairwell test environments than in air and ordinary glass. This results in the effective smoothing out of the speed uctuations and the enhancing of signal transmission stability. Therefore, the ability of metalens to effectively reduce the amplitude of download speed fluctuations in various indoor environmental contexts confirms its key role in adapting to complex environments and improving the wireless communication performance. Moreover, the download speed of signals passing through the metalens is increased by more than 12 Mb/s in both test environmentsthan that of ordinary glass. This effectively improves not only the signal strength but also the communication efficiency. Concurrently, the designed optically transparent metasurface lens is straightforward in structure and user-friendly, and at the same time, it is moveable and can be positioned according to the needs of communication enhancement. The optically transparent metasurface lens scheme proposed in this study provides a potential solution to the high penetration loss problem currently encountered in indoor wireless communication.

A Quasi-Bound States in the Continuum Dielectric Metasurface-Based Antenna-Reactor Photocatalyst.

Metasurfaces are a class of two-dimensional artificial resonators, creating new opportunities for strong light-matter interactions. One type of nonradiative optical metasurface that enables substantial light concentration is based on quasi-Bound States in the Continuum (quasi-BIC). Here we report the design and fabrication of a quasi-BIC dielectric metasurface that serves as an optical frequency antenna for photocatalysis. By depositing Ni nanoparticle reactors onto the metasurface, we create an antenna-reactor photocatalyst, where the virtually lossless metasurface funnels light to drive a chemical reaction. This quasi-BIC-Ni antenna-reactor drives H2 dissociation under resonant illumination, showing strong polarization, wavelength, and optical power dependencies. Both E-field-induced electronic and photothermal heating effects drive the reaction, supported by load-dependent reactivity studies and our theoretical model. This study unlocks new opportunities for photocatalysis that employ dielectric metasurfaces for light harvesting in an antenna-reactor format.

Dynamic multifunctional metasurfaces: an inverse design deep learning approach

Optical metasurfaces (OMs) offer unprecedented control over electromagnetic waves, enabling advanced optical multiplexing. The emergence of deep learning has opened new avenues for designing OMs. However, existing deep learning methods for OMs primarily focus on forward design, which limits their design capabilities, lacks global optimization, and relies on prior knowledge. Additionally, most OMs are static, with fixed functionalities once processed. To overcome these limitations, we propose an inverse design deep learning method for dynamic OMs. Our approach comprises a forward prediction network and an inverse retrieval network. The forward prediction network establishes a mapping between meta-unit structure parameters and reflectance spectra. The inverse retrieval network generates a library of meta-unit structure parameters based on target requirements, enabling end-to-end design of OMs. By incorporating the dynamic tunability of the phase change material Sb2Te3 with inverse design deep learning, we achieve the design and verification of dynamic multifunctional OMs. Our results demonstrate OMs with multiple information channels and encryption capabilities that can realize multiple physical field optical modulation functions. When Sb2Te3 is in the amorphous state, near-field nano-printing based on meta-unit amplitude modulation is achieved for X-polarized incident light, while holographic imaging based on meta-unit phase modulation is realized for circularly polarized light. In the crystalline state, the encrypted information remains secure even with the correct polarization input, achieving double encryption. This research points towards ultra-compact, high-capacity, and highly secure information storage approaches.

Diffractive order Mueller matrix ellipsometry for the design and manufacture of polarization beam splitting metasurfaces.

Optical metasurface technology promises an important potential for replacing bulky traditional optical components, in addition to enabling new compact and lightweight metasurface-based devices. Since even subtle imperfections in metasurface design or manufacture strongly affect their performance, there is an urgent need to develop proper and accurate protocols for their characterization, allowing for efficient control of the fabrication. We present non-destructive spectroscopic Mueller matrix ellipsometry in an uncommon off-specular configuration as a powerful tool for the characterization of orthogonal polarization beam-splitters based on a-Si:H nanopillars. Through Mueller matrix analysis, the spectroscopic polarimetric performance of the ±1 diffraction orders is experimentally demonstrated. This reveals a wavelength shift in the maximum efficiency caused by fabrication-induced conical pillars while still maintaining a polarimetric response close to ideal non-depolarizing Mueller matrices. We highlight the advantage of the spectroscopic Mueller matrix approach, which not only allows for monitoring and control of the fabrication process itself, but also verifies the initial design and produces feedback into the computational design.

Forward and backward propagation guided-mode waves of electric dipole resonances in an h-BN metasurface

The generation of a backward propagation (BP) and forward propagation (FP) guided mode in a double ridge h-BN metasurface (DRM) is investigated. h-BN is a hyperbolic metamaterial with two Reststrahlen frequency bands (RBs). The incident wave can excite guided modes in DRM, where the Poynting vector parallel component of the FP mode is identical and the BP mode is opposite to that of the incident wave. Based on the optimized structure, the frequency range of the BP mode for TE waves was found near the type II hyperbolic band (HB II), while for TM waves, it was found in the gap between the range of HB I and HB II in h-BN. To comprehensively understand the physics underlying BP and FP modes, we present the electric and magnetic field intensities in DRM, the electric field profile of beam steering, and the radiated powers of multipole resonances. The electric dipole (ED) moment contributes most significantly to the FP and BP modes, with its power being much greater than that of other multipolar moment modes. A proportion of BP mode power and incident power decreases with increasing incident angle of TE waves. There are two peaks of the proportion with TM waves. The maximum proportion is near 75% ofTE waves and 16% ofTM waves. The DRM has shown promising potential in the field of sensors based on BP mode, with a sensitivity of 3.9675 µm/RIU of TE waves and 5.1479 µm/RIU of TM waves. These findings suggest that DRMs hold significant promise for the development of optical metasurfaces, optical switches, and high-performance sensors.

Cost-Effective and Environmentally Friendly Mass Manufacturing of Optical Metasurfaces Towards Practical Applications and Commercialization

AbstractOptical metasurfaces consisting of two-dimensional nanostructures have rapidly developed over the past two decades thanks to their potential for use as optical components, such as metalenses or metaholograms, with ultra-compact form factors. Despite these rapid developments, major challenges for the commercialization of metasurfaces still remain: namely their mass production and use in real-life devices. A lot of effort has been made to overcome the limitations of electron beam lithography which is commonly used to fabricate metasurfaces. However, a breakthrough in mass production is still required to bring the cost of metasurfaces down into the price range of conventional optics. This review covers deep-ultraviolet lithography, nanoimprint lithography, and self-assembly-based fabrication processes that have the potential for the mass production of both cost-effective and environmentally friendly metasurfaces. We then discuss metalenses and future displays/sensors that are expected to take advantage of these mass-produced metasurfaces. The potential applications of mass-produced optical metasurfaces will open a new realm for their practical applications and commercialization.

Dispersion engineered metasurfaces for broadband, high-NA, high-efficiency, dual-polarization analog image processing

Optical metasurfaces performing analog image processing – such as spatial differentiation and edge detection – hold the potential to reduce processing times and power consumption, while avoiding bulky 4 F lens systems. However, current designs have been suffering from trade-offs between spatial resolution, throughput, polarization asymmetry, operational bandwidth, and isotropy. Here, we show that dispersion engineering provides an elegant way to design metasurfaces where all these critical metrics are simultaneously optimized. We experimentally demonstrate silicon metasurfaces performing isotropic and dual-polarization edge detection, with numerical apertures above 0.35 and spectral bandwidths of 35 nm around 1500 nm. Moreover, we introduce quantitative metrics to assess the efficiency of these devices. Thanks to the low loss nature and dual-polarization response, our metasurfaces feature large throughput efficiencies, approaching the theoretical maximum for a given NA. Our results pave the way for low-loss, high-efficiency and broadband optical computing and image processing with free-space metasurfaces.