Optical metasurfaces towards multifunctionality and tunability

Broadband and polarization-independent complex amplitude modulation using a single layer dielectric metasurface

Precise control over amplitude and phase across the entire space is crucial for generating user-defined wavefronts and has significant value for designing flexible optical systems. Metasurfaces have emerged as compact and effective platforms for such control, offering high spatial resolution and continuity. However, traditional methods only work at specific wavelengths or polarization states, and the demonstration of full space complex amplitude control for broadband and unpolarized light remains limited. In this study, we leverage the principle of dual meta-atom interference to simultaneously modulate amplitude and phase using a single layer metasurface. Using a randomly polarized light source, nanoprinting and Fourier holography displays of complex patterns are achieved within the wavelength range of 480-640 nm, and the results are consistent with simulations. This approach presents several key advantages: continuous, precise and robust modulation of complex amplitude as well as polarization-independent and broadband response, which significantly reduce constraints on the light source's property and fabrication and make it well-suited for a variety of practical applications, including holographic displays, high-capacity communications, computational imaging, and laser beam processing.

A Single Sub-Millimetric Metasurface-Based Optical Element for Lattice Bessel Beam Excitation Enabling Brain Activity Recordings In Vivo

Bessel beams (BBs) are propagation-invariant optical fields that retain a narrow central intensity profile over longer propagation lengths than Gaussian beams (GBs). Due to this property, they have been adopted in fluorescence-based light sheet microscopy (LSM) to obtain 2D longitudinally-extended light-sheets. Yet, current approaches for generating BB lattices in LSM focus on regular excitation patterns and involve complex and bulky optics, limiting integration capability and versatility. Here, a flexible method is presented to obtain BB-arrays with arbitrary geometries by encoding on a single sub-millimetric surface all the optical transformations required. This method is applied using a single metasurface to encode the generation of a linear array of BBs, avoiding the use of conjugation and focusing optics. With respect to the current strategies, this approach, allowing for the independent design of each beamlet of the array, increases the degrees of freedom while making optimal use of the available light with no rejection, thus facilitating its integration into optical systems. According to this method, we fabricated a metasurface-based optical element for generating a linear BB-array of excitation in an LSM configuration and recorded neuronal activity at cellular resolution from the zebrafish larva brain. Thus, the proposed approach greatly extends the BB-array versatility and the application scenarios.

Polarization-independent full-color holographic movie with a single metasurface free from crosstalk

Metasurface holograms offer advantages, such as a wide viewing angle, compact size, and high resolution. However, projecting a full-color movie using a single hologram without polarization dependence has remained challenging. Here, we report a full-color dielectric metasurface holographic movie with a resolution of 512 × 512. Eight frames were multiplexed across blue (445 nm), green (532 nm), and red (633 nm) color channels, achieving a maximum reconstruction rate of 5.6 frames per second. The superposition of the three wavelengths was achieved by adjusting the resolution and position of each target image while maintaining a constant pitch of the meta-atoms. Additionally, we identified the positions of crosstalk images generated that occur due to fabrication errors and proposed and demonstrated conditions and corrections to ensure they do not overlap with the intended images. The superimposition of phase distributions for each wavelength was achieved using the least squares error method, based on a library of over 20,000 types of meta-atoms. These results are anticipated to advance the future development of three-dimensional metasurface holographic movies.

Meta-coupler empowered dynamic wavefront control with on-chip polarization reconfiguration

Metasurfaces consisting of subwavelength structures have shown unparalleled capability in light field manipulation. However, their functionalities are typically static after fabrication, limiting their practical applications. Though persistent efforts have led to dynamic wavefront control with various materials and mechanisms, most of them work in free space and require specialized materials or bulky configurations for external control. This deviates from the original intention of metasurface to realize compact and integrated devices. Here, we leverage the on-chip geometric metasurface associated with polarization reconfiguration of the guided wave, enabling three functions simultaneously: guided wave radiation, polarization multiplexing, and dynamic wavefront manipulation. We demonstrate proof-of-principle functionalities, including intensity-continuously tunable multifocal metalens, and dynamic zoom metalens as well as dynamic holography, based on a metasurface dressed lithium-niobate-on-insulator waveguide. Such an integrated platform for dynamic wavefront shaping implies the prospect of advancements in chip-integrated multifunctional meta-devices.

Integrated optical probing scheme enabled by localized-interference metasurface for chip-scale atomic magnetometer

Emerging miniaturized atomic sensors such as optically pumped magnetometers (OPMs) have attracted widespread interest due to their application in high-spatial-resolution biomagnetism imaging. While optical probing systems in conventional OPMs require bulk optical devices including linear polarizers and lenses for polarization conversion and wavefront shaping, which are challenging for chip-scale integration. In this study, an integrated optical probing scheme based on localized-interference metasurface for chip-scale OPM is developed. Our monolithic metasurface allows tailorable linear polarization conversion and wavefront manipulation. Two silicon-based metasurfaces namely meta-polarizer and meta-polarizer-lens are fabricated and characterized, with maximum transmission efficiency and extinction ratio (ER) of 86.29 % and 14.2 dB for the meta-polarizer as well as focusing efficiency and ER of 72.79 % and 6.4 dB for the meta-polarizer-lens, respectively. A miniaturized vapor cell with 4 × 4 × 4 mm3 dimension containing 87Rb and N2 is combined with the meta-polarizer to construct a compact zero-field resonance OPM for proof of concept. The sensitivity of this sensor reaches approximately 9 fT/Hz1/2 with a dynamic range near zero magnetic field of about ±2.3 nT. This study provides a promising solution for chip-scale optical probing, which holds potential for the development of chip-integrated OPMs as well as other advanced atomic devices where the integration of optical probing system is expected.

ITO-Based Electrically Tunable Metasurface for Active Control of Light Transmission

In recent years, the rapid development of dynamically tunable metasurfaces has provided a new avenue for flexible control of optical properties. This paper introduces a transmission-type electrically tunable metasurface, employing a series of subwavelength-scale silicon (Si) nanoring structures with an intermediate layer of Al2O3-ITO-Al2O3. This design allows the metasurface to induce strong Mie resonance when transverse electric (TE) waves are normally incident. When a bias voltage is applied, the interaction between light and matter is enhanced due to the formation of an electron accumulation layer at the ITO-Al2O3 interface, thereby altering the resonance characteristics of the metasurface. This design not only avoids the absorption loss of metal nanostructures and has a large modulation depth, but also shows compatibility with complementary metal oxide semiconductor (CMOS) technology.

Tunable third harmonic generation based on high-Q polarization-controlled hybrid phase-change metasurface

Dielectric metasurfaces have made significant advancements in the past decade for enhancing light-matter interaction at the nanoscale. Particularly, bound states in the continuum (BICs) based on dielectric metasurfaces have been employed to enhance nonlinear harmonic generation. However, conventional nonlinear metasurfaces are typically fixed in their operating wavelength after fabrication. In this work, we numerically demonstrate tunable third harmonic generation (THG) by integrating a dielectric metasurface with the phase-change material Ge2Sb2Te5 (GST). The hybrid phase-change metasurface can support two BICs with different electromagnetic origins, which are transformed into two high-Q quasi-BICs through the introduction of structural asymmetry. The two quasi-BICs are selectively excited by controlling the polarization of incident light, and their wavelengths are tunable due to the phase transition of GST. Notably, the efficiency of THG is significantly enhanced at the fundamental wavelengths corresponding to the two quasi-BICs, and the operating wavelength for THG enhancement can be dynamically tuned through the GST phase transition. Furthermore, the wavelength of THG enhancement can be further tuned by manipulating the polarization of pump light. Additionally, a high-Q analog of electromagnetically induced transparency is numerically achieved through the interaction between a low-Q Mie resonance and a quasi-BIC mode, which also improves the THG efficiency. The high-Q polarization-controlled hybrid phase-change metasurface holds promise for applications in dynamically tunable nonlinear optical devices.

Minimalist design of multifunctional metasurfaces for helicity multiplexed holography and nanoprinting

Recently, polarization multiplexing has become a common strategy to enhance the information capacity of metasurfaces. Nevertheless, the intricate design of anisotropic nanostructures forming a polarization multiplexed metasurface poses a significant challenge, increasing the requirements for manufacturing processes and diminishing overall robustness. Herein, we present a minimalist metasurface comprised of only two kinds of nanostructures to achieve the integration of continuous-amplitude modulated nanoprinting and eight-step phase-only helicity-multiplexed holography. Specifically, the nanoprinting image governed by Malus's law can be observed in the orthogonally polarized light path, while holographic images can be switched by changing the chirality of the incident circularly polarized light. More importantly, the geometric phase and the propagation phase of the metasurface are optimized simultaneously according to the target images. Thus, the metasurface does not require optimizing many kinds of nanostructures to achieve the phase but only needs two kinds of nanostructures, forming a minimalist metasurface that significantly relieves the design and fabrication burden. Moreover, the proposed methodology is universal and applicable not only in polarization multiplexing but also in other multi-channel multiplexing technologies. Consequently, the proposed scheme holds promising applications in image display, information encryption, data storage, anti-counterfeiting, and more.

Achromatic and Coma-Corrected Hybrid Meta-Optics for High-Performance Thermal Imaging

Long-wave infrared (LWIR) imaging, or thermal imaging, is widely applied in night vision and security monitoring. However, the widespread use of LWIR imagers is impeded by their bulky size, considerable weight, and high cost. While flat meta-optics present a potential solution to these limitations, existing pure LWIR meta-optics face constraints such as severe chromatic or coma aberrations. Here, we introduce an approach utilizing large-scale hybrid meta-optics to address these challenges and demonstrate the achromatic, coma-corrected, and polarization-insensitive thermal imaging. The hybrid metalens doublet is composed of a metasurface corrector and a refractive lens, featuring a full field-of-view angle surpassing 20° within the 8-12 μm wavelength range. Employing this hybrid metalens doublet, we showcase high-performance thermal imaging capabilities both indoors and outdoors, effectively capturing ambient thermal radiation. The proposed hybrid metalens doublet holds considerable promise for advancing miniaturized, lightweight, and cost-effective LWIR optical imaging systems.

High Q lithium niobate metasurfaces with transparent electrodes for efficient amplitude and phase modulation

Lithium niobate (LN)-based metasurfaces have demonstrated remarkable potential in integrated electro-optically adjustable metadevices with the maturation of thin film LN on insulator (LNOI) technology. Here, we proposed a type of high Q factor tunable metasurface with etchless LN, which is electrically driven in the vertical direction by using transparent conductive film. A transmission amplitude modulation of over 60 dB at a voltage of 20 V is realized through guided mode resonances created at the LN layer with a Q factor of 1320. Meanwhile, phase modulation is also realized with a reflective design by adding a gold layer at the bottom of the metasurface. With a gate voltage of 80 V, about 1.75π phase modulation is achieved while keeping reflection over 92%. Our proposed device achieves effective modulation of optical amplitude and phase in the near-infrared band, which lays a good foundation for the development of high performance LN-based active nanophotonic devices.

Eigendecomposition-free inverse design of meta-optics devices

The inverse design of meta-optics has received much attention in recent years. In this paper, we propose a GPU-friendly inverse design framework based on improved eigendecomposition-free rigorous diffraction interface theory, which offers up to 16.2 × speedup over the traditional inverse design based on rigorous coupled-wave analysis. We further improve the framework's flexibility by introducing a hybrid parameterization combining neural-implicit and traditional shape optimization. We demonstrate the effectiveness of our framework through intricate tasks, including the inverse design of reconfigurable free-form meta-atoms.

Collimated flat-top beam shaper metasurface doublet based on the complex-amplitude constraint Gerchberg-Saxton algorithm

Collimated flat-top beam shapers primarily consisting of freeform lenses have a wide range of applications and pose challenges in terms of processing and integration when the diameter is less than millimeters. Metasurfaces represent a promising solution to planarize optics, can mimic any surface curvature without additional fabrication difficulty, and are suitable for flat-top optics. The conventional metasurface design approach relies on imparting the required phase using meta-atoms and encounters challenges in amplitude modulation due to near-field coupling and varying transmittances among meta-atoms with different phases, making the design of flat-top beam shapers difficult. In this paper, we propose a complex-amplitude constraint Gerchberg-Saxton algorithm for designing a collimated flat-top beam shaper metasurface doublet, which avoids the influence of strong near-field coupling on the amplitude distribution and simultaneously considers the amplitude-phase characteristics of the meta-atoms. A collimated flat-top beam with exceptional homogeneity U p of approximately 0.01, a wavefront error less than 0.1λ, and a transmittance higher than 86 % is experimentally obtained, comparable to commercial products based on freeform lenses. A collimated flat-top beam shaper metasurface doublet for generating flat-top beam is introduced, promoting efficient integration with laser systems.

Spin-Selective Angular Dispersion Control in Dielectric Metasurfaces for Multichannel Meta-Holographic Displays

Angle-dependent next-generation displays have potential applications in 3D stereoscopic and head-mounted displays, image combiners, and encryption for augmented reality (AR) and security. Metasurfaces enable such exceptional functionalities with groundbreaking achievements in efficient displays over the past decades. However, limitations in angular dispersion control make them unfit for numerous nanophotonic applications. Here, we propose a spin-selective angle-dependent all-dielectric metasurface with a unique design strategy to manifest distinct phase information at different incident angles of light. As a proof of concept, the phase masks of two images are encoded into the metasurface and projected at the desired focal plane under different angles of left circularly polarized (LCP) light. Specifically, the proposed multifunctional metasurface generates two distinct holographic images under LCP illumination at angles of +35 and -35°. The presented holographic displays may provide a feasible route toward multifunctional meta-devices for potential AR displays, encrypted imaging, and information storage applications.

Multi-Wavelength Selective and Broadband Near-Infrared Plasmonic Switches in Anisotropic Plasmonic Metasurfaces

Anisotropic plasmonic metasurfaces have attracted broad research interest since they possess novel optical properties superior to natural materials and their tremendous design flexibility. However, the realization of multi-wavelength selective plasmonic metasurfaces that have emerged as promising candidates to uncover multichannel optical devices remains a challenge associated with weak modulation depths and narrow operation bandwidth. Herein, we propose and numerically demonstrate near-infrared multi-wavelength selective passive plasmonic switching (PPS) that encompasses high ON/OFF ratios and strong modulation depths via multiple Fano resonances (FRs) in anisotropic plasmonic metasurfaces. Specifically, the double FRs can be fulfilled and dedicated to establishing tailorable near-infrared dual-wavelength PPS. The multiple FRs mediated by in-plane mirror asymmetries cause the emergence of triple-wavelength PPS, whereas the multiple FRs governed by in-plane rotational asymmetries avail the implementation of the quasi-bound states in the continuum-endowed multi-wavelength PPS with the ability to unfold a tunable broad bandwidth. In addition, the strong polarization effects with in-plane anisotropic properties further validate the existence of the polarization-resolved multi-wavelength PPS. Our results provide an alternative approach to foster the achievement of multifunctional meta-devices in optical communication and information processing.

Dynamic Spectral Modulation on Meta-Waveguides Utilizing Liquid Crystal

The integration of metasurfaces and optical waveguides is gradually attracting the attention of researchers because it allows for more efficient manipulation and guidance of light. However, most of the existing studies focus on passive devices, which lack dynamic modulation. This work utilizes the meta-waveguides with liquid crystal(LC) to modulate the on-chip spectrum, which is the first experimentally verified, to the authors' knowledge. By applying a voltage, the refractive index of the liquid crystal surrounding the meta-waveguides can be tuned, resulting in a blue shift of the spectrum. The simulation shows that the 18.4 dB switching ratio can be achieved at 1550 nm. The meta-waveguides are prepared using electron beam lithography (EBL), and the improved transmittance of the spectrum in the short wavelength is experimentally verified, which is consistent with the simulation trend. At 1551.64 nm wavelength, the device achieves a switching ratio of ≈16 dB with an active area of 8 µm × 0.4 µm. Based on this device, an optoelectronic computing architecture for the Hadamard matrix product and a novel wavelength selection switch are proposed. This work offers a promising avenue for on-chip dynamic modulation in integrated photonics, which has the advantage of a compact active area, fast response time, and low energy consumption compared to conventional thermal-light modulation.

Perovskite single-pixel detector for dual-color metasurface imaging recognition in complex environment

Highly efficient multi-dimensional data storage and extraction are two primary ends for the design and fabrication of emerging optical materials. Although metasurfaces show great potential in information storage due to their modulation for different degrees of freedom of light, a compact and efficient detector for relevant multi-dimensional data retrieval is still a challenge, especially in complex environments. Here, we demonstrate a multi-dimensional image storage and retrieval process by using a dual-color metasurface and a double-layer integrated perovskite single-pixel detector (DIP-SPD). Benefitting from the photoelectric response characteristics of the FAPbBr2.4I0.6 and FAPbI3 films and their stacked structure, our filter-free DIP-SPD can accurately reconstruct different colorful images stored in a metasurface within a single-round measurement, even in complex environments with scattering media or strong background noise. Our work not only provides a compact, filter-free, and noise-robust detector for colorful image extraction in a metasurface, but also paves the way for color imaging application of perovskite-like bandgap tunable materials.

Dual-Channel Switchable Metasurface Filters for Compact Spectral Imaging with Deep Compressive Reconstruction

Spectral imaging technology, which aims to capture images across multiple spectral channels and create a spectral data cube, has been widely utilized in various fields. However, conventional spectral imaging systems face challenges, such as slow acquisition speed and large size. The rapid development of optical metasurfaces, capable of manipulating light fields versatilely and miniaturizing optical components into ultrathin planar devices, offers a promising solution for compact hyperspectral imaging (HSI). This study proposes a compact snapshot compressive spectral imaging (SCSI) system by leveraging the spectral modulations of metasurfaces with dual-channel switchable metasurface filters and employing a deep-learning-based reconstruction algorithm. To achieve compactness, the proposed system integrates dual-channel switchable metasurface filters using twisted nematic liquid crystals (TNLCs) and anisotropic titanium dioxide (TiO2) nanostructures. These thin metasurface filters are closely attached to the image sensor, resulting in a compact system. The TNLCs possess a broadband linear polarization conversion ability, enabling the rapid switching of the incidence polarization state between x-polarization and y-polarization by applying different voltages. This polarization conversion facilitates the generation of two groups of transmittance spectra for wavelength-encoding, providing richer information for spectral data cube reconstruction compared to that of other snapshot compressive spectral imaging techniques. In addition, instead of employing classic iterative compressive sensing (CS) algorithms, an end-to-end residual neural network (ResNet) is utilized to reconstruct the spectral data cube. This neural network leverages the 2-frame snapshot measurements of orthogonal polarization channels. The proposed hyperspectral imaging technology demonstrates superior reconstruction quality and speed compared to those of the traditional compressive hyperspectral image recovery methods. As a result, it is expected that this technology will have substantial implications in various domains, including but not limited to object detection, face recognition, food safety, biomedical imaging, agriculture surveillance, and so on.

Dual-Functional Tunable Metasurface for Meta-Axicon with a Variable Depth of Focus and Continuous-Zoom Metalens

Optical metasurfaces have been widely investigated for their versatile ability to manipulate wavefront and miniaturize traditional optical components into ultrathin planar devices. The integration of metasurfaces with multifunctionality and tunability has fundamentally transformed optics with unprecedented control over light propagation and manipulation. This study introduces a pioneering framework for the development of tunable metasurfaces with multifunctionality, and an example of a tunable metasurface of dual functionalities is proposed and numerically verified as one of the tunable meta-axicon for generating Bessel beams with a variable depth of focus (DOF) and a continuous-zoom metalens. Specifically, this design achieves dual-functional phase modulation by helicity-multiplexing from the combination of the geometric phase as well as the propagation phase and realizes tunability for both functionalities through rotational actuation between double metasurface layers. As a result, dual functionalities with continuous tunability of the proposed TiO2 metasurface are enabled independently for the left and right circularly polarized (LCP and RCP) incidences at 532 nm. Specifically, LCP light triggers the metasurface to function as a tunable axicon, generating non-diffracting Bessel beams with variable numerical apertures (NA) and DOFs. Conversely, the RCP incidence induces it to operate as a continuous-zoom metalens and generates variable spherical wavefront focusing on diverse focal lengths. This study not only initially implements the design of tunable meta-axicon, but also achieves the integration of such a tunable meta-axicon and continuous-zoom metalens within a single metasurface configuration. The proposed device could find potential applications in biological imaging, microscopic measurement, laser fabrication, optical manipulation, multi-plane imaging, depth estimation, optical data storage, etc.

A Review on Micro-LED Display Integrating Metasurface Structures

Micro-LED display technology has been considered a promising candidate for near-eye display applications owing to its superior performance, such as having high brightness, high resolution, and high contrast. However, the realization of polarized and high-efficiency light extraction from Micro-LED arrays is still a significant problem to be addressed. Recently, by exploiting the capability of metasurfaces in wavefront modulation, researchers have achieved many excellent results by integrating metasurface structures with Micro-LEDs, including improving the light extraction efficiency, controlling the emission angle to achieve directional emission, and obtaining polarized Micro-LEDs. In this paper, recent progressions on Micro-LEDs integrated with metasurface structures are reviewed in the above three aspects, and the similar applications of metasurface structures in organic LEDs, quantum dot LEDs, and perovskite LEDs are also summarized.

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.

MEMS Tunable Metasurfaces Based on Gap Plasmon or Fabry-Pérot Resonances

Tunable metasurfaces promise to enable adaptive optical systems with complex functionalities. Among possible realizations, a recent platform combining microelectromechanical systems (MEMS) with gap-surface plasmon (GSP) metasurfaces offers high modulation efficiency, broadband operation, and fast response. We compare tunable metasurfaces operating in GSP and Fabry-Pérot (FP) regions by investigating polarization-independent blazed gratings both numerically and experimentally. Peak efficiency is calculated to be ∼75% in both cases (∼40% in measurements), while the operation bandwidth is found larger when operating in the GSP region. Advantages of operating in the FP region include relaxed assembly requirements and operation tolerances. Additionally, simulation and experimental results show that coupling between neighboring unit cells increases for larger air gaps, resulting in deteriorated efficiency. We believe the presented analysis provides important guidelines for designing tunable metasurfaces for diverse applications in miniaturized adaptive optical systems.

Toward a universal metasurface for optical imaging, communication, and computation

In recent years, active metasurfaces have emerged as a reconfigurable nanophotonic platform for the manipulation of light. Here, application of an external stimulus to resonant subwavelength scatterers enables dynamic control over the wavefront of reflected or transmitted light. In principle, active metasurfaces are capable of controlling key characteristic properties of an electromagnetic wave, such as its amplitude, phase, polarization, spectrum, and momentum. A 'universal' active metasurface should be able to provide independent and continuous control over all characteristic properties of light for deterministic wavefront shaping. In this article, we discuss strategies for the realization of this goal. Specifically, we describe approaches for high performance active metasurfaces, examine pathways for achieving two-dimensional control architectures, and discuss operating configurations for optical imaging, communication, and computation applications based on a universal active metasurface.