Achromatic Metalens over 60 nm Bandwidth in the Visible and Metalens with Reverse Chromatic Dispersion

Metasurface optical trap array for single atoms

Metasurfaces made of subwavelength silicon nanopillars provide unparalleled capacity to manipulate light, and have emerged as one of the leading platforms for developing integrated photonic devices. In this study, we report on a compact, passive approach based on planar metasurface optics to generate large optical trap arrays. The unique configuration is achieved with a meta-hologram to convert a single incident laser beam into an array of individual beams, followed up with a metalens to form multiple laser foci for single rubidium atom trapping. We experimentally demonstrate two-dimensional arrays of 5 × 5 and 25 × 25 at the wavelength of 830 nm, validating the capability and scalability of our metasurface design. Beam waists ∼1.5 µm, spacings (about 15 µm), and low trap depth variations (8%) of relevance to quantum control for an atomic array are achieved in a robust and efficient fashion. The presented work highlights a compact, stable, and scalable trap array platform well-suitable for Rydberg-state mediated quantum gate operations, which will further facilitate advances in neutral atom quantum computing.

Full-Color Imaging System Based on the Joint Integration of a Metalens and Neural Network

Lenses have been a cornerstone of optical systems for centuries; however, they are inherently limited by the laws of physics, particularly in terms of size and weight. Because of their characteristic light weight, small size, and subwavelength modulation, metalenses have the potential to miniaturize and integrate imaging systems. However, metalenses still face the problem that chromatic aberration affects the clarity and accuracy of images. A high-quality image system based on the end-to-end joint optimization of a neural network and an achromatic metalens is demonstrated in this paper. In the multi-scale encoder-decoder network, both the phase characteristics of the metalens and the hyperparameters of the neural network are optimized to obtain high-resolution images. The average peak-signal-to-noise ratio (PSNR) and average structure similarity (SSIM) of the recovered images reach 28.53 and 0.83. This method enables full-color and high-performance imaging in the visible band. Our approach holds promise for a wide range of applications, including medical imaging, remote sensing, and consumer electronics.

The Effect of Height Error on Performance of Propagation Phase-Based Metalens

Metalenses, as a new type of planar optical device with flexible design, play an important role in miniaturized and integrated optical devices. Propagation phase-based metalenses, known for their low loss and extensive design flexibility, are widely utilized in optical imaging and optical communication. However, fabrication errors introduced by thin-film deposition and etching processes inevitably result in variations in the height of the metalens structure, leading to the fabricated devices not performing as expected. Here, we introduce a reflective TiO2 metalens based on the propagation phase. Then, the relationship between the height variation and the performance of the metalens is explored by using the maximum phase error. Our results reveal that the height error of the unit structure affects the phase rather than the amplitude. The focusing efficiency of our metalens exhibits robustness to structural variations, with only a 5% decrease in focusing efficiency when the height varies within ±8% of the range. The contents discussed in this paper provide theoretical guidance for the unit design of the propagation phase-based metalens and the determination of its allowable fabrication error range, which is of great significance for low-cost and high-efficiency manufacturing.

On-chip optical wavefront shaping by transverse-spin-induced Pancharatanam-Berry phase

Pancharatnam-Berry (PB) metasurfaces can be applied to manipulate the phase and polarization of light within subwavelength thickness. The underlying mechanism is attributed to the geometric phase originating from the longitudinal spin of light. Here, we demonstrate, to the best of our knowledge, a new type of PB geometric phase derived from the intrinsic transverse spin of guided light. Using full-wave numerical simulations, we show that the rotation of a metallic nano-bar sitting on a metal substrate can induce a geometric phase covering 2π full range for the surface plasmons carrying an intrinsic transverse spin. Especially, the geometric phase is different for the surface plasmons propagating in opposite directions due to spin-momentum locking. We apply the geometric phase to design metasurfaces to manipulate the wavefront of surface plasmons to achieve steering and focusing. Our work provides a new mechanism for on-chip light manipulations with potential applications in designing ultra-compact optical devices for imaging and sensing.

Unveiling Invariant Optical Properties of Dielectric Meta-Atoms

The phase-shift, transmittance, and polarization properties of meta-atoms are investigated, motivated by their use as building blocks of metasurfaces used in metalenses, holograms, and beam shaping. We studied dielectric nanorod meta-atoms of several geometries, which included cylinders, triangles, squares, hexagons, octagons, and truncated cones. By analyzing light propagation through these meta-atoms for three different wavelengths (632.8, 545, and 50 nm), we show that the phase-shift introduced is independent of their cross-section shape, contrary to the expected behavior. Additionally, we show that the polarization response is independent of the shape and that the transmittance is partially shape-independent. We identify a novel dependence of phase-shift on the effective cross-sectional area of meta-atoms. These meta-atom optical properties are independent of its shape if its geometry has a C3 or larger rotational symmetry. This optical invariance has significant implications for the topological optimization of flat optics.

Design of a refractive-metasurface hybrid annular aperture folded optical system

Folded lenses offer advantages in terms of lightness and thinness, but they have limitations when it comes to correcting aberrations. In this paper, we propose a novel approach to address this issue by incorporating metasurfaces in the design of folded optical systems. Specifically, a folded refractive-metasurface hybrid annular aperture folded lens (AFL) is introduced. The structural characteristics of the AFL imaging system are analyzed to investigate the blocking ratio, thickness, and light collection capability of the ring aperture system. Additionally, a hybrid optical integration design using Zemax software is proposed for the metasurfaces. A quadruple-folded AFL working in the mid-infrared waveband is then designed. The superstructure surface is analyzed, and its processability is discussed. The results demonstrate that the reflective-metasurface hybrid AFL significantly improves the imaging quality of this type of optical system while meeting the required design accuracy.

Design and simulation of an extreme ultraviolet metalens based on the Pancharatnam-Berry phase

Extreme ultraviolet (EUV) radiation plays a key role in the fields of material science, attosecond metrology, and lithography. However, the reflective optical components typically used in EUV systems contribute to their bulky size, weight, and increased costs for fabrication. In this paper, we theoretically investigate transmissive metalens designs capable of focusing the EUV light based on the Pancharatnam-Berry phase. The designed metalens is composed of nanoscale elliptical holes, which can guide and manipulate EUV light due to the higher refractive index of the vacuum holes compared to that of the surrounding material. We designed an EUV metalens with a diameter of 10 µm, which supports a focal length of 24 µm and a numerical aperture of up to 0.2. It can focus 55-nm EUV incident light to a diffraction-limited spot, and the focusing efficiency is calculated to be as high as about 7% over a broad EUV frequency range (50-65 nm). This study reveals the possibility of applying a dielectric metalens in the EUV region without a transmissive optical material.

Broadband Achromatic Imaging of a Metalens with Optoelectronic Computing Fusion

The remarkable ultrathin ability of metalenses gives them potential as a next-generation imaging candidate. However, the inherent chromatic aberration of metalenses restricts their widespread application. We present an achromatic metalens with optoelectronic computing fusion (OCF) to mitigate the impact of chromatic aberration and simultaneously avoid the significant challenges of nanodesign, nanofabrication, and mass production of metalenses, a method different from previous methods. Leveraging the nonlinear fitting, we demonstrate that OCF can effectively learn the chromatic aberration mapping of metalens and thus restore the chromatic aberration. In terms of the peak signal-to-noise ratio index, there is a maximum improvement of 12 dB, and ∼8 ms is needed to correct the chromatic aberration. Furthermore, the edge extraction of images and super-resolution reconstruction that effectively enhances resolution by a factor of 4 are also demonstrated with OCF. These results offer the possibility of applications of metalenses in mobile cameras, virtual reality, etc.

3D-printed multilayer structures for high-numerical aperture achromatic metalenses

Flat optics consisting of nanostructures of high-refractive index materials produce lenses with thin form factors that tend to operate only at specific wavelengths. Recent attempts to achieve achromatic lenses uncover a trade-off between the numerical aperture (NA) and bandwidth, which limits performance. Here, we propose a new approach to design high-NA, broadband, and polarization-insensitive multilayer achromatic metalenses (MAMs). We combine topology optimization and full-wave simulations to inversely design MAMs and fabricate the structures in low-refractive index materials by two-photon polymerization lithography. MAMs measuring 20 μm in diameter operating in the visible range of 400 to 800 nm with 0.5 and 0.7 NA were achieved with efficiencies of up to 42%. We demonstrate broadband imaging performance of the fabricated MAM under white light and RGB narrowband illuminations. These results highlight the potential of the 3D-printed multilayer structures for realizing broadband and multifunctional meta-devices with inverse design.

All-silicon polarization-independent broadband achromatic metalens designed for the mid-wave and long-wave infrared

Metasurfaces demonstrate excellent capabilities in manipulating the phase, amplitude and polarization of light. Metalens, as a typical kind of metasurface devices, shows great prospect in simplifying imaging systems. However, like diffractive optical elements, intrinsic dispersion of metasurfaces is high. Thus, significant chromatic aberration is present in common metalenses, deteriorating imaging quality under broadband illumination condition and limiting their applications. To tackle this problem, broadband achromatic metalenses have been proposed and demonstrated in the visible and near-infrared wavelength regions so far. However, broadband achromatic metalens working in the mid-wave and long-wave infrared is still rare. In this paper, thanks to the ingenious design of meta-units that provide the required local phase and phase dispersion, several all-silicon broadband achromatic metalenses working in the mid-wave infrared (3-5 µm) or long-wave infrared (8-14 µm) wavelengths are proposed. Numerical simulation results demonstrate that the designed broadband achromatic metalenses can provide a near-constant focal length with small deviations and an average focusing efficiency of about 70% over the whole operation bandwidths. In addition, these metalenses hold near diffraction-limited focusing capability and polarization-independent focusing features. The achromatic metalenses proposed here are beneficial for improving imaging quality under broadband illumination and increasing detection efficiency of mid-wave and long-wave infrared detection systems.

Theory and Fundamental Limit of Quasiachromatic Metalens by Phase Delay Extension

The periodic extension of phase difference is commonly applied in device design to obtain phase compensation beyond the system's original phase modulation capabilities. Based on this extension approach, we propose the application of quasiphase delay matching to extend the range of dispersion compensation for meta-atoms with limited height. Our theory expands the limit of frequency bandwidth coverage and relaxes the constraints of aperture, NA, and bandwidth for metalenses. By applying the uncertainty principle, we explain the fundamental limit of this achromatic bandwidth and obtain the achromatic spectrum using perturbation analysis. To demonstrate the effectiveness of this extended limit, we simulate a quasiachromatic metalens with a diameter of 2 mm and a NA of 0.55 in the range of 400-1500 nm. Our findings provide a novel theory for correcting chromatic aberration in large-diameter ultrawide bandwidth devices.

All dielectric reflective metalens based on multilayer meta-atoms

While transmission-mode metalenses have been extensively studied, reflection-mode metalenses remain almost unexplored, presenting advantages in terms of improved efficiency and reduced complexity. In this Letter, we investigate a multilayer dielectric metalens operating in reflection mode at visible wavelengths without a metallic layer. Simulations and analysis demonstrate the performance of the metalens, with an 84% reflectivity the metalens proves its efficacy in reflection mode. At a numerical aperture of 0.15, the metalens achieves a 33% focusing efficiency, which is approximately twice that of similar reflective metalenses, facilitating efficient light manipulation and subwavelength resolution. Additionally, the metalens exhibits a well-defined focal spot with a full width at half maximum of 2.03 µm, approaching the diffraction limit.

Broadband achromatic and wide field-of-view single-layer metalenses in the mid-infrared

Metalenses are considered a promising solution for miniaturizing numerous optical systems due to their light weight, ultrathin thickness and compact size. However, it remains a challenge for metalenses to achieve both wide field-of-view and broadband achromatic imaging. In this work, a single-layer achromatic metalens with a wide field-of-view of 160° in the 3800 nm-4200 nm band is designed and analyzed. The quadratic phase profile of the metalens and the propagation phase of each meta-atom are used to increase the field-of-view and compensate for chromatic aberration, respectively. In addition, the metalens is capable of transverse achromatic imaging. The design can be extended to other optical frequencies, which is promising for applications in unmanned vehicles, infrared detection, etc.

Asymptotic dispersion engineering for ultra-broadband meta-optics

Dispersion decomposes compound light into its monochromatic components, which is detrimental to broadband imaging but advantageous for spectroscopic applications. Metasurfaces provide a unique path to modulate the dispersion by adjusting structural parameters on a two-dimensional plane. However, conventional linear phase compensation does not adequately match the meta-unit's dispersion characteristics with required complex dispersion, hindering at-will dispersion engineering over a very wide bandwidth particularly. Here, we propose an asymptotic phase compensation strategy for ultra-broadband dispersion-controlled metalenses. Metasurfaces with extraordinarily high aspect ratio nanostructures have been fabricated for arbitrary dispersion control in ultra-broad bandwidth, and we experimentally demonstrate the single-layer achromatic metalenses in the visible to infrared spectrum (400 nm~1000 nm, NA = 0.164). Our proposed scheme provides a comprehensive theoretical framework for single-layer meta-optics, allowing for arbitrary dispersion manipulation without bandwidth restrictions. This development is expected to have significant applications in ultra-broadband imaging and chromatography detection, among others.

Deep learning enhanced achromatic imaging with a singlet flat lens

Correction of chromatic aberration is an important issue in color imaging and display. However, realizing broadband achromatic imaging by a singlet lens with high comprehensive performance still remains challenging, though many achromatic flat lenses have been reported recently. Here, we propose a deep-learning-enhanced singlet planar imaging system, implemented by a 3 mm-diameter achromatic flat lens, to achieve relatively high-quality achromatic imaging in the visible. By utilizing a multi-scale convolutional neural network (CNN) imposed to an achromatic multi-level diffractive lens (AMDL), the white light imaging qualities are significantly improved in both indoor and outdoor scenarios. Our experiments are fulfilled via a large paired imaging dataset with respect to a 3 mm-diameter AMDL, which guaranteed with achromatism in a broad wavelength range (400-1100 nm) but a relative low efficiency (∼45%). After our CNN enhancement, the imaging qualities are improved by ∼2 dB, showing competitive achromatic and high-quality imaging with a singlet lens for practical applications.

High NA and polarization-insensitive ultra-broadband achromatic metalens from 500 to 1050 nm for multicolor two-photon endomicroscopy imaging

Multicolor two-photon endomicroscopy has become a highly competitive tool for functional imaging in biomedical researches. However, to make the imaging system miniature and applicable for freely behaving animal brain activity, metalenses have received much attention in compact imaging systems. For high resolution multicolor imaging and maximizing fluorescence collection, there is a challenge metalenses faced to achieve large numerical aperture (NA) and focus the NIR excitation and VIS emission lights of multiple fluorophores to the same distance simultaneously because of the limitation of the group delay range of the meta-units. In this paper, we proposed a high NA and polarization-insensitive ultra-broadband achromatic metalens specifically for achromatically focusing the excitation and emission light of multiple fluorophores commonly used in neuroscience studies. TiO2 and Si meta-unit libraries composed of heights, widths and the corresponding phase and group delay were constructed, and the optimal meta-units were selected by particle swarm optimization algorithm to engineer the dispersion of metalens in the VIS band and NIR band, respectively. Combining dispersion engineering with spatial multiplexing, the proposed metalens achieved the maximal effective NA up to 0.8 and large achromatic bandwidth ranging from 500 nm to 1050 nm, which exhibited the coefficient of variation of focal lengths was only 3.41%. The proposed achromatic metalens could successfully achromatically focus different fluorescence with any polarization, which was suitable for most fluorophores. Our results firmly establish that the proposed metalens can open the door to high resolution and minimally invasive multicolor two-photon functional imaging in intravital deep brain.

High-efficiency broadband achromatic metalenses for visible full-stokes polarization imaging

Polarization-imaging technology has important applications in target detection, communication, biomedicine, and other fields. A polarization imaging system based on metalenses, which provides new possibilities for the realization of highly integrated full-Stokes polarization imaging systems, can solve the problems of traditional polarization imaging systems, such as complex structures, large volumes, and the inability to simultaneously obtain linear and circular polarization states. However, currently designed metalens arrays that can achieve real-time full-Stokes polarization imaging can generally only be used for monochromatic detection, which significantly limits the amount of measured information of the object. Broad-spectrum polarization color imaging allows more image degrees of freedom, enabling more accurate characterization of polarization for multi-target object scenes in complex environments. To achieve broad-spectrum polarization imaging, we propose and design a metalens array that can achieve full-Stokes polarization imaging in the broadband visible range, in which the design process of metalenses for splitting and focusing broadband orthogonal circularly polarized light is emphasized. To design metalenses that can achieve polarization splitting and efficient focusing, we simulate and optimize the height and period of the nano-units and show that smaller periods and larger heights do not always result in higher-performance devices when designing multifunctional metalenses. The designed metalens array can split and diffraction-limited focus the orthogonal polarized incident light to the designated position with average focusing efficiencies of 59.2% under 460-680 nm TM linearly polarized light, 53.1% under TE linearly polarized light, 58.8% under left-handed circularly polarized light, and 52.7% under right-handed circularly polarized light. The designed metalenses can be applied to imaging systems, such as polarization imaging and polarization light-field imaging systems.

Nanophotonic Materials for Twisted-Light Manipulation

Twisted light, an unbounded set of helical spatial modes carrying orbital angular momentum (OAM), offers not only fundamental new insights into structured light-matter interactions, but also a new degree of freedom to boost optical and quantum information capacity. However, current OAM experiments still rely on bulky, expensive, and slow-response diffractive or refractive optical elements, hindering today's OAM systems to be largely deployed. In the last decade, nanophotonics has transformed the photonic design and unveiled a diverse range of compact and multifunctional nanophotonic devices harnessing the generation and detection of OAM modes. Recent metasurface devices developed for OAM generation in both real and momentum space, presenting design principle and exemplary devices, are summarized. Moreover, recent development of whispering-gallery-mode-based passive and tunable microcavities, capable of extracting degenerate OAM modes for on-chip vortex emission and lasing, is summarized. In addition, the design principle of different plasmonic devices and photodetectors recently developed for on-chip OAM detection is discussed. Current challenges faced by the nanophotonic field for twisted-light manipulation and future advances to meet these challenges are further discussed. It is believed that twisted-light manipulation in nanophotonics will continue to make significant impact on future development of ultracompact, ultrahigh-capacity, and ultrahigh-speed OAM systems-on-a-chip.

Dual-wavelength UV-visible metalens for multispectral photoacoustic microscopy: A simulation study

Photoacoustic microscopy is advancing with research on utilizing ultraviolet and visible light. Dual-wavelength approaches are sought for observing DNA/RNA- and vascular-related disorders. However, the availability of high numerical aperture lenses covering both ultraviolet and visible wavelengths is severely limited due to challenges such as chromatic aberration in the optics. Herein, we present a groundbreaking proposal as a pioneering simulation study for incorporating multilayer metalenses into ultraviolet-visible photoacoustic microscopy. The proposed metalens has a thickness of 1.4 µm and high numerical aperture of 0.8. By arranging cylindrical hafnium oxide nanopillars, we design an achromatic transmissive lens for 266 and 532 nm wavelengths. The metalens achieves a diffraction-limited focal spot, surpassing commercially available objective lenses. Through three-dimensional photoacoustic simulation, we demonstrate high-resolution imaging with superior endogenous contrast of targets with ultraviolet and visible optical absorption bands. This metalens will open new possibilities for downsized multispectral photoacoustic microscopy in clinical and preclinical applications.

Geometric-phase-based axicon lens for computational achromatic imaging

Conventional optical imaging systems usually utilize several lenses within a precise assembly to eliminate chromatic aberration, which increases the difficulty of system integration. In recent years, with the rapid development of metasurfaces and liquid crystals (LCs), planar optical elements provide feasible solutions to realize flexible light manipulation and lightweight systems. However, there also exists chromatic aberration, which can be corrected but at the cost of a complex device design. Here, a geometric-phase-based axicon lens is utilized to correct chromatic aberration across a broadband wavelength with the assistance of a post-process algorithm. The axicon lens is fabricated through arranging orientations of liquid-crystal molecules with a standard photoalignment technique, and it produces an approximately invariant point spread function (PSF) at several discrete wavelengths, which is used as the prior information to extract the object in the blurred image. In the experiment, the reconstruction quality is significantly improved after the post-process algorithm. We expect our work to provide further development to reduce the dispersion with both the device design and the computational image technique.

A Quad-Band and Polarization-Insensitive Metamaterial Absorber with a Low Profile Based on Graphene-Assembled Film

A quad-band metamaterial absorber using a periodically arranged surface structure placed on an ultra-thin substrate is demonstrated in this paper. Its surface structure consists of a rectangular patch and four L-shaped structures distributed symmetrically. The surface structure is able to have strong electromagnetic interactions with incident microwaves, thereby generating four absorption peaks at different frequencies. With the aid of the near-field distributions and impedance matching analysis of the four absorption peaks, the physical mechanism of the quad-band absorption is revealed. The usage of graphene-assembled film (GAF) provides further optimization to increase the four absorption peaks and promotes the low-profile characteristic. In addition, the proposed design has good tolerance to the incident angle in vertical polarization. The proposed absorber in this paper has the potential for filtering, detection, imaging, and other communication applications.

Multifunctional Metasurfaces Enabled by Multifold Geometric Phase Interference

Geometric phase is frequently used in artificially designed metasurfaces; it is typically used only once in reported works, leading to conjugate responses of two spins. Supercells containing multiple nanoantennas can break this limitation by introducing more degrees of freedom to generate new modulation capabilities. Here, we provide a method for constructing supercells for geometric phases using triple rotations, each of which achieves a specific modulation function. The physical meaning of each rotation is revealed by stepwise superposition. Based on this idea, spin-selective holography, nanoprinting, and their hybrid displays are demonstrated. As a typical application, we have designed a metalens that enables spin-selective transmission, allowing for high-quality imaging with only one spin state, which can serve as a plug-and-play chiral detection device. Finally, we analyzed how the size of supercells and the phase distribution inside it can affect the higher order diffraction, which may help in designing supercells for different scenarios.

Rapid deep-learning-assisted design method for 2-bit coding metasurfaces

This paper proposes a deep-learning-assisted design method for 2-bit coding metasurfaces. This method uses a skip connection module and the idea of an attention mechanism in squeeze-and-excitation networks based on a fully connected network and a convolutional neural network. The accuracy limit of the basic model is further improved. The convergence ability of the model increased nearly 10 times, and the mean-square error loss function converges to 0.000168. The forward prediction accuracy of the deep-learning-assisted model is 98%, and the accuracy of inverse design results is 97%. This approach offers the advantages of an automatic design process, high efficiency, and low computational cost. It can serve users who lack metasurface design experience.

Polarization-modulated broadband achromatic bifunctional metasurface in the visible light

Achromatic bifunctional metasurface is of great significance in optical path miniaturization among advanced integrated optical systems. However, the reported achromatic metalenses mostly utilize a phase compensate scheme, which uses geometric phase to realize the functionality and uses transmission phase to compensate the chromatic aberration. In the phase compensation scheme, all the modulation freedoms of a nanofin are driven at the same time. This makes most of the broadband achromatic metalenses restricted to realizing single function. Also, the phase compensate scheme is always addressed with circularly polarized (CP) incidence, leading to a limitation in efficiency and optical path miniaturization. Moreover, for a bifunctional or multifunctional achromatic metalens, not all the nanofins will work at the same time. Owing to this, achromatic metalenses using a phase compensate scheme are usually of low focusing efficiencies. To this end, based on the pure transmission phase in the x-/y- axis provided by the birefringent nanofins structure, we proposed an all-dielectric polarization-modulated broadband achromatic bifunctional metalens (BABM) in the visible light. Applying two independent phases on one metalens at the same time, the proposed BABM realizes achromatism in a bifunctional metasurface. Releasing the freedom of nanofin's angular orientation, the proposed BABM breaks the dependence on CP incidence. As an achromatic bifunctional metalens, all the nanofins on the proposed BABM can work at the same time. Simulation results show that the designed BABM is capable of achromatically focusing the incident beam to a single focal spot and an optical vortex (OV) under the illumination of x- and y-polarization, respectively. In the designed waveband 500 nm (green) to 630 nm (red), the focal planes stay unchanged at the sampled wavelengths. Simulation results prove that the proposed metalens not only realized bifunctional achromatically, but also breaks the dependence of CP incidence. The proposed metalens has a numerical aperture of 0.34 and efficiencies of 33.6% and 34.6%. The proposed metalens has advantages of being flexible, single layer, convenient in manufacturing, and optical path miniaturization friendly, and will open a new page in advanced integrated optical systems.

Topologically optimized concentric-nanoring metalens with 1 mm diameter, 0.8 NA and 600 nm imaging resolution in the visible

Metalenses can achieve diffraction-limited focusing via localized phase modification of the incoming light beam. However, the current metalenses face to the restrictions on simultaneously achieving large diameter, large numerical aperture, broad working bandwidth and the structure manufacturability. Herein, we present a kind of metalenses composed of concentric nanorings that can address these restrictions using topology optimization approach. Compared to existing inverse design approaches, the computational cost of our optimization method is greatly reduced for large-size metalenses. With its design flexibility, the achieved metalens can work in the whole visible range with millimeter size and a numerical aperture of 0.8 without involving high-aspect ratio structures and large refractive index materials. Electron-beam resist PMMA with a low refractive index is directly used as the material of the metalens, enabling a much more simplified manufacturing process. Experimental results show that the imaging performance of the fabricated metalens has a resolution better than 600 nm corresponding to the measured FWHM of 745 nm.

Advance of large-area achromatic flat lenses

A new framework of light coherence optimization is proposed to design non-ideal broadband achromatic lenses, enabling large-scale flat lenses' implementation and high performance. The strategy paves the way for practical planar optical devices and full-color imaging systems.

Rapid Cellular-Resolution Skin Imaging with Optical Coherence Tomography Using All-Glass Multifocal Metasurfaces

Cellular-resolution optical coherence tomography (OCT) is a powerful tool offering noninvasive histology-like imaging. However, like other optical microscopy tools, a high numerical aperture (N.A.) lens is required to generate a tight focus, generating a narrow depth of field, which necessitates dynamic focusing and limiting the imaging speed. To overcome this limitation, we developed a metasurface platform that generates multiple axial foci, which multiplies the volumetric OCT imaging speed by offering several focal planes. This platform offers accurate and flexible control over the number, positions, and intensities of axial foci generated. All-glass metasurface optical elements 8 mm in diameter are fabricated from fused-silica wafers and implemented into our scanning OCT system. With a constant lateral resolution of 1.1 μm over all depths, the multifocal OCT triples the volumetric acquisition speed for dermatological imaging, while still clearly revealing features of stratum corneum, epidermal cells, and dermal-epidermal junctions and offering morphological information as diagnostic criteria for basal cell carcinoma. The imaging speed can be further improved in a sparse sample, e.g., 7-fold with a seven-foci beam. In summary, this work demonstrates the concept of metasurface-based multifocal OCT for rapid virtual biopsy, further providing insights for developing rapid volumetric imaging systems with high resolution and compact volume.

All-dielectric terahertz metasurfaces with dual-functional polarization manipulation for orthogonal polarization states

All-dielectric metasurfaces have led to a surge of activities in the field of polarization converters due to their extremely significant potential in the manipulation of terahertz waves. Herein, a versatile all-dielectric metasurface platform that can realize dual-functional polarization manipulation for the orthogonal states of polarization in the terahertz frequency range is proposed. Furthermore, such metasurface platform exhibits the properties of a full-waveplate for one circularly polarized light, and a quarter-waveplate for the orthogonal circularly polarized light. For experimental demonstrations of strategy verification, several representative metasurfaces consisting of subwavelength-scaled all-silicon elliptical cylinders were designed, fabricated, and characterized to demonstrate the capability of dual-functional polarization manipulation, including bifunctional waveplate, near-field imaging, and focusing. The metasurface platform demonstrated here may provide an alternative perspective for the development of compact, versatile polarization terahertz devices, and the design concept can be extended to other frequency ranges as well.

A Single-Celled Metasurface for Multipolarization Generation and Wavefront Manipulation

Due to their unprecedented ability to flexibly manipulate the parameters of light, metasurfaces offer a new approach to integrating multiple functions in a single optical element. In this paper, based on a single-celled metasurface composed of chiral umbrella-shaped metal-insulator-metal (MIM) unit cells, a strategy for simultaneous multiple polarization generation and wavefront shaping is proposed. The unit cells can function as broadband and high-performance polarization-preserving mirrors. In addition, by introducing a chiral-assisted Aharonov-Anandan (AA) geometric phase, the phase profile and phase retardation of two spin-flipped orthogonal circular polarized components can be realized simultaneously and independently with a single-celled metasurface via two irrelevant parameters. Benefiting from this flexible phase manipulation ability, a vectorial hologram generator and metalens array with spatially varying polarizations were demonstrated. This work provides an effective approach to avoid the pixel and efficiency losses caused by the intrinsic symmetry of the PB geometric phase, and it may play an important role in the miniaturization and integration of multipolarization-involved displays, real-time imaging, and spectroscopy systems.

Broadband Achromatic Metalens in the Visible Light Spectrum Based on Fresnel Zone Spatial Multiplexing

Metalenses composed of a large number of subwavelength nanostructures provide the possibility for the miniaturization and integration of the optical system. Broadband polarization-insensitive achromatic metalenses in the visible light spectrum have attracted researchers because of their wide applications in optical integrated imaging. This paper proposes a polarization-insensitive achromatic metalens operating over a continuous bandwidth from 470 nm to 700 nm. The silicon nitride nanopillars of 488 nm and 632.8 nm are interleaved by Fresnel zone spatial multiplexing method, and the particle swarm algorithm is used to optimize the phase compensation. The maximum time-bandwidth product in the phase library is 17.63. The designed focal length can be maintained in the visible light range from 470 nm to 700 nm. The average focusing efficiency reaches 31.71%. The metalens can achieve broadband achromatization using only one shape of nanopillar, which is simple in design and easy to fabricate. The proposed metalens is expected to play an important role in microscopic imaging, cameras, and other fields.

Double-layer polarization-independent achromatic metasurface array for optical fiber bundle coupling in microendoscope

Optical fiber bundle-based microendoscope, which is significant in clinical diagnosis and industrial detection, calls for miniaturization of the probe and high-resolution observation. Here, we propose a double-layer metasurface array borrowing the structures of insect compound eyes to meet both requirements instead of traditional optical components. Each unit in the array aims for an incident field of view, focusing light at the center of the fiber end face with no chromatic aberration at the wavelengths of 470 nm, 530 nm and 630 nm. The metasurface array is composed of a series of isotropic TiO₂ nanopillars which are special selected after considering resonance mode and angular dispersion characteristics, etched on both sides of a silica substrate, with the individual functions of deflecting and focusing. In image space, numerical aperture (NA) is 0.287 and the particular layout of two layers achieve zero telecentricity theoretically, which meet the requirements of optical fiber bundle coupling. A unit for incident angle of 20° is shown to validate our design approach numerically, which obtains a focused spot close to the diffraction limit. The compact and ultrathin metasurface could greatly reduce the size of the probe in optical fiber bundle based microendoscope while ensuring the imaging quality.

Theoretical Design of a Bionic Spatial 3D-Arrayed Multifocal Metalens

With the development of micro/nano-optics, metasurfaces are gaining increasing attention working as novel electromagnetic wave control devices. Among which, metalenses have been developed and applied as a typical application of metasurfaces owing to their unique optical properties. However, most of those previous metalenses can only produce one focal point, which severely limits their applications. Inspired by the fly compound eye, we propose a special kind of spatial multifocal metalens. Our metalenses can reverse the polarization state of the incident circularly polarized light, which is then focused. In addition, a horizontally aligned multifocal metalens can be achieved by designing reasonable phase and region distributions, which is similar to a vertically aligned one. Most significantly, a spatially 3D-arrayed multifocal metalens with low crosstalk is well achieved by combining these two distribution methods. The proposed bionic 3D-arrayed multifocal metalens with amazing focusing effect promises applications in imaging, nanoparticle manipulation, optical communication, and other fields.

Mid-Infrared Broadband Achromatic Metalens with Wide Field of View

Metasurfaces have the ability to flexibly control the light wavefront, and they are expected to fill the gaps of traditional optics. However, various aberrations pose challenges for the application of metasurfaces in the wide angle and wide spectral ranges. The previous multi-aberration simultaneous optimization works had shortcomings such as large computational load, complex structure, and low generality. Here, we propose a metalens design method that corrects both monochromatic and chromatic aberrations simultaneously. The monochromatic aberration-corrected phase distribution is obtained by the optical design, and the chromatic aberration is reduced by using the original search algorithm combined with dispersion engineering. The designed single-layered wide-angle achromatic metalens has a balanced and efficient focusing effect in the mid-infrared band from 3.7 μm to 5 μm and a wide angle of ±30°. The design method proposed has the advantages of low computational load, wide application range, and easy experimental fabrication, which provides new inspiration for the development of generalized software for the design and optimization of metasurfaces.

Broadband behavior of quadratic metalenses with a wide field of view

Metalenses are attracting a large interest for the implementation of complex optical functionalities in planar and compact devices. However, chromatic and off-axis aberrations remain standing challenges. Here, we experimentally investigate the broadband behavior of metalenses based on quadratic phase profiles. We show that these metalenses do not only guarantee an arbitrarily large field of view but are also inherently tolerant to longitudinal and transverse chromatic aberrations. As such, we demonstrate a single-layer, silicon metalens with a field of view of 86° and a bandwidth up to 140 nm operating at both 1300 nm and 1550 nm telecommunication wavelength bands.

Multilayer dielectric metalens

We propose and analyze a metalens whose meta-atoms (nanoscatterers) are integrated by a stack of quarter-wave dielectric layers. Each multilayer meta-atom is a nanopillar, which consists of alternating layers of high- and low-refractive-index materials. We show that the nanopillars of a multilayer metalens may have a smaller aspect ratio than the nanopillars of a standard metalens and have similar optical properties (focusing efficiency and chromaticity).

Sub-wavelength patterned pulse laser lithography for efficient fabrication of large-area metasurfaces

Rigorously designed sub-micrometer structure arrays are widely used in metasurfaces for light modulation. One of the glaring restrictions is the unavailability of easily accessible fabrication methods to efficiently produce large-area and freely designed structure arrays with nanoscale resolution. We develop a patterned pulse laser lithography (PPLL) approach to create structure arrays with sub-wavelength feature resolution and periods from less than 1 μm to over 15 μm on large-area thin films with substrates under ambient conditions. Separated ultrafast laser pulses with patterned wavefront by quasi-binary phase masks rapidly create periodic ablated/modified structures by high-speed scanning. The gradient intensity boundary and circular polarization of the wavefront weaken diffraction and polarization-dependent asymmetricity effects during light propagation for high uniformity. Structural units of metasurfaces are obtained on metal and inorganic photoresist films, such as antennas, catenaries, and nanogratings. We demonstrate a large-area metasurface (10 × 10 mm²) revealing excellent infrared absorption (3–7 μm), which comprises 250,000 concentric rings and takes only 5 minutes to produce.

Fabrication of metasurfaces with nanoscale structures is inefficient for large areas. Here, the authors introduce patterned pulse laser lithography for creating structured arrays with sub-wavelength feature on large-area thin films under ambient conditions.

Inverse design of high degree of freedom meta-atoms based on machine learning and genetic algorithm methods

Since inverse design is an ill-conditioned problem of mapping from low dimensions to high dimensions, inverse design is challenging, especially for design problems with many degrees of freedom (DOFs). Traditional deep learning methods and optimization methods cannot readily calculate the inverse design of meta-atoms with high DOFs. In this paper, a new method combining deep learning and genetic algorithm (GA) methods is proposed to realize the inverse design of meta-atoms with high DOFs. In this method, a predicting neural network (PNN) and a variational autoencoder (VAE) generation model are constructed and trained. The generative model is used to constrain and compress the large design space, so that the GA can jump out of the local optimal solution and find the global optimal solution. The predicting model is used to quickly evaluate the fitness value of each offspring in the GA. With the assistance of these two machine learning models, the GA can find the optimal design of meta-atoms. This approach can realize, on demand, inverse design of meta-atoms, and opens the way for the optimization of procedures in other fields.

Broadband mid-infrared metalens with polarization-controlled at-will chromatic dispersion

Mid-infrared metalenses are promising for potential applications in a range of fields, including molecular detection, imaging, and optical sensing. To fulfil the different requirements of these diverse application scenarios, engineering of the ability to control the chromatic dispersion of these lenses at will is vitally important. Herein, we demonstrate broadband mid-infrared metalenses with polarization-controlled at-will chromatic dispersion capability based on all-silicon metasurfaces. Both polarization-insensitive and polarization-sensitive mid-infrared metalenses with at-will chromatic dispersion capabilities are demonstrated numerically for operation at wavelengths ranging from 3.75 μm to 4.75 μm. The average focusing efficiency of all these metalenses exceeds 40% in the wavelength range of interest, and the highest focusing efficiency ranges up to 67%. The mid-infrared metalenses with at-will chromatic dispersion proposed here may be beneficial for mid-infrared sensing, communications, and imaging applications in the future.

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.

Doublet Metalens with Simultaneous Chromatic and Monochromatic Correction in the Mid-Infrared

Metalenses provide a powerful paradigm for mid-infrared (MIR) imaging and detection while keeping the optical system compact. However, the design of MIR metalenses simultaneously correcting chromatic aberration and off-axis monochromatic aberration remains challenging. Here, we propose an MIR doublet metalens composed of a silicon aperture metalens and a silicon focusing metalens separated by a fused silica substrate. By performing ray-tracing optimization and particle-swarm optimization, we optimized the required phase profiles as well as the sizes and spatial distributions of silicon nanopillars of the doublet metalens. Simulation results showed that the MIR doublet metalens simultaneously achieved chromatic and off-axis monochromatic aberration reduction, realizing a continuous 400 nm bandwidth and 20° field-of-view (FOV). Thanks to its planar configuration, this metalens is suitable for integration with CMOS image sensor to achieve MIR imaging and detection, which has potential application in troubleshooting and intelligent inspection of power grids. This work may facilitate the practical application of metalens-integrated micro/nanosensors in intelligent energy.

Multiwavelength achromatic super-resolution focusing via a metasurface-empowered controlled generation of focused cylindrically polarized vortex beams

Non-invasive imaging beyond the diffraction limit and free from fluorescent labels in the visible is highly desired for microscopy. It remains a challenge to obtain such super-resolution focusing along with multiwavelength achromatic performance in the far field using an integratable and easily designed system. In this work, we demonstrate a straightforward metasurface-based method to realize multiwavelength achromatic generation and focusing of cylindrically polarized vortex beams (CPVBs). Attributed to the extra degrees of freedom of CPVBs and multi-section design, we have realized multiwavelength achromatic super-resolution focusing in the air with focal size tighter than that of normally used schemes like immersion metalenses or focused radially polarized beams. It is expected that this metasurface-empowered ultra-compact design will benefit potential applications which call for high resolution, like optical microscopy, laser processing, etc.

Broad-Band Polarization-Insensitive Metasurface Holography with a Single-Phase Map

The remarkable potential of metasurface holography promises revolutionary advancements for imaging, chip-integrated augmented/virtual reality (AR/VR) technology, and flat optical displays. The choice of constituent element geometry constrains many potential applications purveyed through polarization-independent optical response. The limited capabilities and degree of freedoms in commonly used meta-atoms restrict the design flexibility to break the conventional trade-off between polarization-insensitivity and bandwidth. Here, we propose a geometric phase-enabled novel design strategy to break this conventional trade-off. The proposed strategy ensures the realization of broad-band polarization-insensitivity through a simplified design procedure. An identical output wavefront manipulation is achieved by adjusting the phase delay freedom of geometric phase engineering under different incident polarization conditions. For proof of concept, a metahologram device is fabricated by an optimized complementary metal-oxide-semiconductor (CMOS)-compatible material of hydrogenated amorphous silicon (a-Si:H). This metahologram device reproduces the required hologram with high image fidelity and efficiency under different polarization scenarios of white light incidence. Due to the simple design strategy, low computational cost, and easy fabrication, the proposed technique can be an excellent candidate for realizing polarization-insensitive metahologram devices.

Largest aperture metalens of high numerical aperture and polarization independence for long-wavelength infrared imaging

Because of unique superiorities of planar optical devices based on metalens on manipulating amplitude, phase, polarization, wavelength of incident light, metalenses have great prospects to replace traditional catadioptric optical components, especially in imaging and optoelectronic integration. However, the research of metalens has focused on visible or near-infrared wavelength in the past few years and little attention was paid to the long-wavelength infrared metalens. Here, we demonstrate the largest aperture, high numerical aperture, and polarization-independent metalens operating at long-wavelength infrared. The metalens has a numerical aperture of 0.45 at the center wavelength of10 µm. The aperture of the metalens is 80 mmwhich is much larger than the existing level we know. It has high-resolution imaging ability with focusing incident light down to a spot as small as ∼1.04λ. Ambient light imaging experiments are carried out to show the performance of the metalens. In addition, metalens is flimsy, large-scale and low-cost, which provides an effective solution for the development of ultra-lightweight and compact optical devices for LWIR technology.

Manipulation force analysis of nanoparticles with ultra-high numerical aperture metalens

Metalens optical tweezers technology has several advantages for manipulating micro-nano particles and high integration. Here, we used particle swarm optimization (PSO) to design a novel metalens tweezer, which can get 3-dimensional trapping of particles. The numerical aperture (NA) of the metalens can reach 0.97 and the average focusing efficiency is 44%. Subsequently, we analyzed the optical force characteristics of SiO₂ particles with a radius of 350 nm at the focal point of the achromatic metalens. We found the average maximum force of SiO₂ particles in the x-direction and z-direction to be 0.88 pN and 0.72 pN, respectively. Compared with the dispersive metalens, it is beneficial in maintaining the constant of optical force, the motion state of trapped particles, and the stability of the trapping position.

Focusing Characteristics and Widefield Imaging Performance of the Silicon Metalens in the Visible Range

Conventional optical high numerical aperture lenses are essential for high-resolution imaging, but bulky and expensive. In comparison, metalens-based optical components are the subjects of intensive investigation for their flexible manipulation of light. Methods of detecting and characterizing focal spots and scanning imaging produced by metalenses are well established. However, widefield imaging by metalenses is experimentally challenging. This study demonstrates the design and realization of silicon-based metalenses with numerical apertures of 0.447 and 0.204 in the broadband spectrum of 580–780 nm for microscopic widefield imaging. The optimized aspect ratio of the single nanorod is 5.1:1, which reduces the fabrication difficulty compared to other, more complicated designs and fabrication. Furthermore, we successfully demonstrate widefield imaging by the designed metalens and compare the simulated and the experimentally extracted modulation transfer function curves of the metalens.

Coherently switching the focusing characteristics of all-dielectric metalenses

Flat, gradient index, metasurface optics - in particular all-dielectric metalenses - have emerged and evolved over recent years as compact, lightweight alternative to their conventional bulk glass/crystal counterparts. Here we show that the focal properties of all-dielectric metalenses can be switched via coherent control, which is to say by changing the local electromagnetic field in the metalens plane rather than any physical or geometric property of the nanostructure or surrounding medium. The selective excitation of predominantly electric or magnetic resonant modes in the constituent cells of the metalens provides for switching, by design, of its phase profile enabling binary switching of focal length for a given lens type and, uniquely, switching between different (spherical and axicon) lens types.

Design of Multifunctional Tunable Metasurface Assisted by Elastic Substrate

Metasurfaces with both multifunctionality and tunability hold great application potential in next-generation optical devices. In this paper, we propose a stretchable metasurface composed of arrays of identical dielectric rectangular resonators embedded in the polydimethylsiloxane (PDMS) substrate. It is shown that the metasurface possesses three functions at the operating wavelength of 532 nm. The switching of functions can be implemented by changing the period Px of the metasurface, induced by stretching the PDMS substrate along the x-direction. When the period Px is less than the operating wavelength of 532 nm, the behavior of metasurface can switch between transmissive window and reflective mirror. When the period Px of the metasurface varies from 532 nm to 700 nm, the metasurface act as a dynamic equal-power beam splitter with conversion efficiency higher than 90%, and the corresponding splitting angle can be adjusted from 90° to around 49.5°. Moreover, we achieve the switching of transmissive window/reflective mirror/split-ratio-variable splitter based on the metasurface consisting of arrays of identical L-shaped resonators embedded in the PDMS substrate.

Single-Step Fabricable Flexible Metadisplays for Sensitive Chemical/Biomedical Packaging Security and Beyond

Secure packaging and transportation of light-sensitive chemical and biomedical test tubes are crucial for environmental protection and public health. Benefiting from the compact form factor and high efficiency of optical metasurfaces, we propose a broad-band polarization-insensitive flexible metasurface for the security of sensitive packages in the transport industry. We employ both the propagation and the geometric phase of novel TiO₂ resin-based anisotropic nanoresonators to demonstrate a flexible and broad-band polarization-insensitive metasurface in the visible domain. The ultraviolet nanoimprint lithographic technique (UV-NIL) is used to fabricate high-index TiO₂ nanoparticle-embedded-resin (nano-PER) structures that are patterned on a flexible substrate. This novel approach provides swift single-step fabrication without secondary fabrication steps such as deposition and etching. Moreover, replicating and transforming patterns over flexible substrates make the proposed technique highly suitable for large-throughput commercial manufacturing. As the proposed metahologram manifests high transmission efficiency in the visible domain, such flexible metaholographic platforms could find several exciting applications in bendable/curved displays, wearable devices, and holographic labeling for interactive displays.

Dielectric metalens for miniaturized imaging systems: progress and challenges

Lightweight, miniaturized optical imaging systems are vastly anticipated in these fields of aerospace exploration, industrial vision, consumer electronics, and medical imaging. However, conventional optical techniques are intricate to downscale as refractive lenses mostly rely on phase accumulation. Metalens, composed of subwavelength nanostructures that locally control light waves, offers a disruptive path for small-scale imaging systems. Recent advances in the design and nanofabrication of dielectric metalenses have led to some high-performance practical optical systems. This review outlines the exciting developments in the aforementioned area whilst highlighting the challenges of using dielectric metalenses to replace conventional optics in miniature optical systems. After a brief introduction to the fundamental physics of dielectric metalenses, the progress and challenges in terms of the typical performances are introduced. The supplementary discussion on the common challenges hindering further development is also presented, including the limitations of the conventional design methods, difficulties in scaling up, and device integration. Furthermore, the potential approaches to address the existing challenges are also deliberated.