Photonic Multitasking Interleaved Si Nanoantenna Phased Array

Dynamic Spatial-Selective Metasurface with Multiple-Beam Interference

The emergence of the metasurface has provided a versatile platform for the manipulation of light at the nanoscale. Recent research in metasurfaces has explored a plethora of dynamic control and switching of multifunctionalities, paving the way for innovative applications in fields such as imaging, sensing, and communication. However, current dynamic multifunctional metasurfaces face challenges in terms of functional scalability and selective activation. In this work, we introduce and experimentally demonstrate a strategy that utilizes multiple plane waves to create arbitrary periodic patterns on the metasurface, thus enabling the dynamic and arbitrary spatial-selective activation of its embedded multiplexed functionalities. Furthermore, our strategy facilitates dynamic light control through mechanical translation, as demonstrated by a high-speed, dynamically switchable beam deflection scenario. Our method effectively overcomes the limitations associated with traditional spatially multiplexing techniques, offering greater flexibility and selectivity for dynamic control in multifunctional metasurfaces.

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.

Photonic spin Hall effect driven broadband multi-focus dielectric metalens

The multi-focus metalens can couple the light into multiple channels in optical interconnections, which is beneficial to the development of planar, miniaturized, and integrated components. We propose broadband photonic spin Hall effect (PSHE) driven multi-focus metalenses, in which each nanobrick plays a positive role for all focal points. Three PSHE driven metalenses with four, six, and eight focal points have been designed and investigated, respectively. Under the incidences of left-/right-handed circularly polarized (LCP/RCP) light, these metalenses can generate regularly distributed two, three, and four RCP/LCP focal points, respectively. The uniformity of the focusing intensity has been investigated in detail by designing an additional four six-focus metalenses with different focus distributions. The uniqueness of these metalenses makes this design philosophy very attractive for applications in spin photonics, compact polarization detection, multi-imaging systems, and information processing systems.

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.

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.

Long-infrared dual-wavelength linear-polarization-multiplexed confocal metalens based on an all-silicon dielectric

The metalens has vast applications in biomedicine and industrial manufacturing due to their ultrathin structure and vital ability to manipulate the properties of light waves for long-infrared systems. However, it is difficult for metalens to achieve the confocal function with high focusing efficiency, wide wavelength bandwidth, and low structural complexity. Here, we propose and experimentally demonstrate an all-silicon dielectric metalens composed of arrays of minimalist meta-atoms with a single rectangular nanopillar arranged on a periodic square lattice substrate, which realizes the confocal function of the orthogonal-linear-polarized light with wavelengths of 10.6 µm and 9.3 µm, with focusing efficiencies of 64.94% and 60.03%, respectively. Also, it reveals nearly the diffraction-limited focusing performance. In addition, the metalens can realize precise long-infrared thermal imaging. Moreover, the proposed metalens is compatible with the standard complementary metal oxide semiconductor processes, which can effectively reduce the manufacturing cost and provide a feasible solution for developing planar integrated multifunctional micro-nanophotonic devices in the long-infrared field.

Achromatic and wide field of view metalens based on the harmonic diffraction and a quadratic phase

Although metalens has made breakthroughs in various imaging applications due to its ultrathin, lightweight, and multi-functionality, simultaneously achieving wide field of view (WFOV) and achromatic imaging remains a challenge. Here, we demonstrate a harmonic metalens with a quadratic phase profile that enables WFOV imaging and achromatic imaging at certain discrete wavelengths. First, we quantitatively explain why the quadratic phase enables WFOV imaging using its Fourier Transform (FT). Second, we derive the complex-amplitude transmittance formula of a harmonic metalens. The derived formula integrating with the Angular Spectrum theory can calculate the transmitted field distributions of a harmonic metalens. Third, we propose an achromatic WFOV metalens based on the harmonic diffraction and quadratic phase at a large numerical aperture (NA=0.76), which enables achromatic imaging at wavelengths λ = 600 nm, 1200 nm with a wide field of view (FOV) of 100°. The proposed scheme will undoubtedly expand the applications of metalens in the imaging area.

Design of a frequency-multiplexed metasurface with asymmetric transmission

Metasurfaces presenting diversified functionalities have broadened the prospect of manipulating the phase, amplitude, and polarization from the optical to microwave fields. Although the frequency-multiplexing strategy is one of the intuitive and effective approaches to expand the number of channels, demonstrations reporting on the combination between directional asymmetric transmission and frequency-multiplexing via an ultrathin flat device are limited. In this study, a novel, to the best of our knowledge, strategy is proposed to generate four independent holographic images under opposite illumination directions at two operating frequencies, utilizing a single metasurface composed of two types of metallic resonators and one grating layer. Specifically, each scattering channel with independent information makes full use of the whole metasurface. Simulation and experimental results show good agreement, highlighting the attractive capabilities of the multi-functional metasurface platform, which provides more freedom for the manipulation of electromagnetic waves.

Multifunctional resonant wavefront-shaping meta-optics based on multilayer and multi-perturbation nonlocal metasurfaces

Photonic devices rarely provide both elaborate spatial control and sharp spectral control over an incoming wavefront. In optical metasurfaces, for example, the localized modes of individual meta-units govern the wavefront shape over a broad bandwidth, while nonlocal lattice modes extended over many unit cells support high quality-factor resonances. Here, we experimentally demonstrate nonlocal dielectric metasurfaces in the near-infrared that offer both spatial and spectral control of light, realizing metalenses focusing light exclusively over a narrowband resonance while leaving off-resonant frequencies unaffected. Our devices attain this functionality by supporting a quasi-bound state in the continuum encoded with a spatially varying geometric phase. We leverage this capability to experimentally realize a versatile platform for multispectral wavefront shaping where a stack of metasurfaces, each supporting multiple independently controlled quasi-bound states in the continuum, molds the optical wavefront distinctively at multiple wavelengths and yet stay transparent over the rest of the spectrum. Such a platform is scalable to the visible for applications in augmented reality and transparent displays.

Inverse design of ultracompact multi-focal optical devices by diffractive neural networks

We propose an efficient inverse design approach for multifunctional optical elements based on adaptive deep diffractive neural networks (a-D²NNs). Specifically, we introduce a-D²NNs and design two-layer diffractive devices that can selectively focus incident radiation over two well-separated spectral bands at desired distances. We investigate focusing efficiencies at two wavelengths and achieve targeted spectral line shapes and spatial point-spread functions (PSFs) with optimal focusing efficiency. In particular, we demonstrate control of the spectral bandwidths at separate focal positions beyond the theoretical limit of single-lens devices with the same aperture size. Finally, we demonstrate devices that produce super-oscillatory focal spots at desired wavelengths. The proposed method is compatible with current diffractive optics and doublet metasurface technology for ultracompact multispectral imaging and lensless microscopy applications.

Optical metalenses: fundamentals, dispersion manipulation, and applications

Metasurfaces, also known as 2D artificial metamaterials, are attracting great attention due to their unprecedented performances and functionalities that are hard to achieve by conventional diffractive or refractive elements. With their sub-wavelength optical scatterers, metasurfaces have been utilized to freely modify different characteristics of incident light such as amplitude, polarization, phase, and frequency. Compared to traditional bulky lenses, metasurface lenses possess the advantages of flatness, light weight, and compatibility with semiconductor manufacture technology. They have been widely applied to a range of scenarios including imaging, solar energy harvesting, optoelectronic detection, etc. In this review, we will first introduce the fundamental design principles for metalens, and then report recent theoretical and experimental progress with emphasis on methods to correct chromatic and monochromatic aberrations. Finally, typical applications of metalenses and corresponding design rules will be presented, followed by a brief outlook on the prospects and challenges of this field.

Multifunctional metasails for self-stabilized beam-riding and optical communication

The photonic propulsion of lightsails can be used to accelerate spacecraft to relativistic velocities, providing a feasible route for the exploration of interstellar space in the human lifetime. Breakthrough Starshot is an initiative aiming to launch lightsail-driven spacecrafts accelerated to a relativistic velocity of 0.2c via radiation pressure of a high-power laser beam in order to probe the habitable zone of Alpha Centauri, located 4.2 light years away from the Earth, and transmit back the scientific data collected in the flyby mission to an Earth-based receiver. The success of such a mission requires the lightsail to provide maximal acceleration while featuring beam-riding stability under the illumination of an intense laser beam during the launch phase. Moreover, the large-area lightsail can be harnessed to improve the margin in the photon-starved downlink channel throughout the communication phase by maximizing the gain of the transmitter despite extending the acceleration period and reducing the stability margin due to the elimination of a portion of the propulsion segments. Owing to the potential of metasurfaces to serve as low-weight versatile multifunctional photonic components, metasurface-based lightsails or metasails are deemed to be ideal candidates to simultaneously address the requirements of photonic propulsion and optical communication in laser-driven deep-space probes. Here, we demonstrate the design of a multifunctional metasail for providing high acceleration and enabling the self-stabilized beam-riding of a spacecraft with a detached payload from the sail while maximizing the transmission gain in the downlink optical communication. The metasail consists of two interleaved sub-arrays of dielectric unit cells operating based on the Pancharatnam-Berry geometric phase, optimized to meet the propulsion and communication requirements, respectively. The beam-riding stability of the sail is analyzed through simulation of the motion trajectory during the acceleration phase, while taking into account the effect of the relativistic Doppler shift, and the downlink communication performance is enabled by providing the required conjugate phase by the metasail elements, resulting in beam collimation. The obtained results verify the multifunctionality of the platform and point toward the promise of metasails for extended mission applications.

Dual-Wavelength Polarization-Dependent Bifocal Metalens for Achromatic Optical Imaging Based on Holographic Principle

Recently, ultrathin metalenses have attracted dramatically growing interest in optical imaging systems due to the flexible control of light at the nanoscale. In this paper, we propose a dual-wavelength achromatic metalens that will generate one or two foci according to the polarization of the incident. Based on geometric phase modulation, two unit cells are attentively selected for efficient operation at distinct wavelengths. By patterning them to two divided sections of the metalens structure plane, the dual-wavelength achromatic focusing effect with the same focal length is realized. In addition, the holographic concept is adopted for polarization-dependent bifocal generation, in which the objective wave is originated from two foci that are respectively formed by two orthogonal polarization states of circularly polarized light, namely Left-handed circularly polarized (LCP) light and Right-handed circularly polarized (RCP) light. The incident light is considered as the reference light. The achromatic focusing and polarization-dependent bifocusing are numerically verified through simulations. The proposed design opens the path for the combination of multi-wavelength imaging and chiral imaging, which may find potential applications, such as achromatic optical devices and polarization-controlled biomedical molecular imaging systems.

Additively Manufactured Multi-Material Ultrathin Metasurfaces for Broadband Circular Polarization Decoupled Beams and Orbital Angular Momentum Generation

Controlling the wavefront and manipulating the polarization of the electromagnetic wave using an ultrathin flat device are highly desirable in many emerging fields. To shape the wavefront between two decoupled orthogonal circular polarization states, that is, the right-hand circular polarization (RCP) and the left-hand circular polarization (LCP), most state-of-the-art metasurfaces (MSs) combine the propagation phase and Pancharatnam-Berry phase into meta-atoms. This article proposes a different strategy to fully decouple the LCP and RCP and control their wavefronts independently. By taking advantage of the conductive and dielectric multi-material-integrated additive manufacturing technique, the proposed transmissive MS has an ultrathin thickness (0.11 free-space wavelength) and controls the LCP and RCP wavefronts independently under linearly polarized incidence illumination. The proposed meta-atom consists of a receiving antenna on the top, a transmitting antenna at the bottom with a strip-line connecting them. The strip-line introduces the same phase shifts for both RCP and LCP waves, while the transmitting antenna with in-plane rotation leads to the opposite phase shifts for RCP and LCP waves. Therefore, the phase delays from the strip-line and the angular rotation of the transmitting antenna provide two degrees of freedom, enabling independent beam shaping of LCP and RCP waves. Two MSs with different functionalities are printed for proof-of-concept, and the performances are experimentally verified.

Nanophotonic Color Routing

Recent advances in low-dimensional materials and nanofabrication technologies have stimulated many breakthroughs in the field of nanophotonics such as metamaterials and plasmonics that provide efficient ways of light manipulation at a subwavelength scale. The representative structure-induced spectral engineering techniques have demonstrated superior design of freedom compared with natural materials such as pigment/dye. In particular, the emerging spectral routing scheme enables extraordinary light manipulation in both frequency-domain and spatial-domain with high-efficiency utilization of the full spectrum, which is critically important for various applications and may open up entirely new operating paradigms. In this review, a comparative introduction on the operating mechanisms of spectral routing and spectral filtering schemes is given and recent progress on various color nanorouters based on metasurfaces, plasmonics, dielectric antennas is reviewed with a focus on the potential application in high-resolution imaging. With a thorough analysis and discussion on the advanced properties and drawbacks of various techniques, this report is expected to provide an overview and vision for the future development and application of nanophotonic color (spectral) routing techniques.

Dielectric Metalens: Properties and Three-Dimensional Imaging Applications

Recently, optical dielectric metasurfaces, ultrathin optical skins with densely arranged dielectric nanoantennas, have arisen as next-generation technologies with merits for miniaturization and functional improvement of conventional optical components. In particular, dielectric metalenses capable of optical focusing and imaging have attracted enormous attention from academic and industrial communities of optics. They can offer cutting-edge lensing functions owing to arbitrary wavefront encoding, polarization tunability, high efficiency, large diffraction angle, strong dispersion, and novel ultracompact integration methods. Based on the properties, dielectric metalenses have been applied to numerous three-dimensional imaging applications including wearable augmented or virtual reality displays with depth information, and optical sensing of three-dimensional position of object and various light properties. In this paper, we introduce the properties of optical dielectric metalenses, and review the working principles and recent advances in three-dimensional imaging applications based on them. The authors envision that the dielectric metalens and metasurface technologies could make breakthroughs for a wide range of compact optical systems for three-dimensional display and sensing.

All-dielectric multifunctional transmittance-tunable metasurfaces based on guided-mode resonance and ENZ effect

Electrically tunable metasurfaces open new doors for manipulating the phase, amplitude and polarization of light in ultrathin layers. Compared with metal assisted metasurfaces, all-dielectric transmission metasurfaces-with outstanding feature of low loss, especially incorporating with new electro-optical materials-show great potential for the next generation flat optics. In this study, by combining the epsilon-near-zero effect in indium tin oxide (ITO) with guided-mode resonance, we propose novel electrically tunable all-dielectric metasurface architectures with versatile functions for widespread potential application. The inserted periodic ITO and hafnium oxide layers sandwiched in silicon act as two metal-oxide-semiconductor capacitors in a single period to disturb the resonance wavelength in the near-infrared spectral range under the voltage applied. For the one-dimensional structure, the transmittances of this metasurface at 1512 and 1510 nm change 20 and -14 dB under 0∼5 V bias voltage, respectively. In addition, the bilayer structure performs well in double-waveband applications, indicating that more layers can support more operation wavebands. Meanwhile, the two-dimensional structure works as a polarization insensitive device when setting the same structural parameters in both orthogonal directions. The proposed architecture, with various merits including ultra-compact size, high-speed and complementary metal-oxide-semiconductor compatibility, provides a multifunctional and multi-degree-of-freedom design, as well as enormous potential applications in more complicated flat optics.

Ultrafast laser-inscribed nanogratings in sapphire for geometric phase elements

We systematically studied femtosecond laser-inscribed self-organized nanogratings and geometric phase elements such as a polarization diffraction focusing lens and Q-plate in sapphire crystal. Besides the void structures observed in the focus, nanogratings with periods of 150~300 nm were observed, depending on a nanoslit that took the role of a seeding effect by localized light field enhancement. The non-polarized refractive index change and birefringence were measured with values around 1∼2×10^-3 and 6×10^-4, respectively. Based on the laser-inscribed form birefringence, a geometric phase lens and Q-plate were successfully demonstrated in sapphire with high imaging and a focusing effect. We expect that our findings may promote the understanding of laser-induced nanogratings in bulk and potential applications in geometric phase elements.

Recent Progress on Ultrathin Metalenses for Flat Optics

As technology advances, electrical devices such as smartphones have become more and more compact, leading to a demand for the continuous miniaturization of optical components. Metalenses, ultrathin flat optical elements composed of metasurfaces consisting of arrays of subwavelength optical antennas, provide a method of meeting those requirements. Moreover, metalenses have many other distinctive advantages including aberration correction, active tunability, and semi-transparency, compared to their conventional refractive and diffractive counterparts. Therefore, over the last decade, great effort has been focused on developing metalenses to investigate and broaden the capabilities of metalenses for integration into future applications. Here, we discuss recent progress on metalenses including their basic design principles and notable characteristics such as aberration correction, tunability, and multifunctionality.

Spin-dependent dual-wavelength multiplexing metalens

The Pancharatnam-Berry (PB) phase is generally utilized to realize a single wavelength spin-dependent function or dual-wavelength functions but operating only in one spin state. A dual-wavelength multifunctional metasurface relying on both spins has been rarely designed due to the rather complicated degrees of freedom to be considered. In this Letter, both dynamic and PB phases are adopted, instead of a pure PB phase, to propose a multiplexing metasurface that can independently and simultaneously manipulate left- and right-handed circularly polarized incidences at dual wavelengths. It is demonstrated experimentally as well as numerically that such spin-dependent dual-wavelength metalenses can make circularly polarized incidences of different wavelengths split into and focus at multi-dimensional positions. Our work demonstrates a new avenue in designing spin-dependent dual-wavelength multifunctional optical devices.

High-Efficiency Fabrication of Geometric Phase Elements by Femtosecond-Laser Direct Writing

The nanoresolution of geometric phase elements for visible wavelengths calls for a flexible technology with high throughout and free from vacuum. In this article, we propose a high-efficiency and simple manufacturing method for the fabrication of geometric phase elements with femtosecond–laser direct writing (FsLDW) and thermal annealing by combining the advantages of high-efficiency processing and thermal smoothing effect. By using a femtosecond laser at a wavelength of 343 nm and a circular polarization, free-form nanogratings with a period of 300 nm and 170-nm-wide grooves were obtained in 50 s by laser direct ablation at a speed of 5 mm/s in a non-vacuum environment. After fine-tuning through a hot-annealing process, the surface morphology of the geometric phase element was clearly improved. With this technology, we fabricated blazed gratings, metasurface lens, vortex Q-plates and “M” holograms and confirmed the design performance by analyzing their phases at the wavelength of 808 nm. The efficiency and capabilities of our proposed method can pave the possible way to fabricate geometric phase elements with essentially low loss, high-temperature resistance, high phase gradients and novel polarization functionality for potentially wide applications.

A Fully Phase-Modulated Metasurface as An Energy-Controllable Circular Polarization Router

Geometric metasurfaces primarily follow the physical mechanism of Pancharatnam-Berry (PB) phases, empowering wavefront control of cross-polarized reflective/transmissive light components. However, inherently accompanying the cross-polarized components, the copolarized output components have not been attempted in parallel in existing works. Here, a general method is proposed to construct phase-modulated metasurfaces for implementing functionalities separately in co- and cross-polarized output fields under circularly polarized (CP) incidence, which is impossible to achieve with solely a geometric phase. By introducing a propagation phase as an additional degree of freedom, the electromagnetic (EM) energy carried by co- and cross-polarized transmitted fields can be fully phase-modulated with independent wavefronts. Under one CP incidence, a metasurface for separate functionalities with controllable energy repartition is verified by simulations and proof-of-principle microwave experiments. A variety of applications can be readily expected in spin-selective optics, spin-Hall metasurfaces, and multitasked metasurfaces operating in both reflective and transmissive modes.

Dual-layer achromatic metalens design with an effective Abbe number

Planar achromatic metalenses with a thickness of the order of wavelength have attracted much attention for their potential applications in ultra-compact optical devices. However, realizing single-layer achromatic metalenses across a wide bandwidth requires that the corresponding meta-atoms have complex cross-sections for correct phase profile and dispersion compensation. Herein, we introduce an effective Abbe number and use lens maker equations to design a dual-layer achromatic metalens in which we compensate the dispersion by using a plano-convex liked metalens combined with a plano-concave liked metalens. The stacked metalens are designed based on simple high refractive index dielectric cylindrical meta-atoms with different radii, which simplify the design and fabrication processes. We demonstrate that a dual-layer achromatic metalens has a small focal length difference across the visible wavelength range and an average focusing efficiency above 50%, which proves that the design method is promising for many potential applications in multi-functional flat optical devices.

Chromatic Dispersion Manipulation Based on Metalenses

Metasurfaces are 2D metamaterials composed of subwavelength nanoantennas according to specific design. They have been utilized to precisely manipulate various parameters of light fields, such as phase, polarization, amplitude, etc., showing promising functionalities. Among all meta-devices, the metalens can be considered as the most basic and important application, given its significant advantage in integration and miniaturization compared with traditional lenses. However, the resonant dispersion of each nanoantenna in a metalens and the intrinsic chromatic dispersion of planar devices and optical materials result in a large chromatic aberration in metalenses that severely reduces the quality of their focusing and imaging. Consequently, how to effectively suppress or manipulate the chromatic aberration of metalenses has attracted worldwide attention in the last few years, leading to variety of excellent achievements promoting the development of this field. Herein, recent progress in chromatic dispersion control based on metalenses is reviewed.

Temporal color mixing and dynamic beam shaping with silicon metasurfaces

Metasurfaces offer the possibility to shape optical wavefronts with an ultracompact, planar form factor. However, most metasurfaces are static, and their optical functions are fixed after the fabrication process. Many modern optical systems require dynamic manipulation of light, and this is now driving the development of electrically reconfigurable metasurfaces. We can realize metasurfaces with fast (>10⁵ hertz), electrically tunable pixels that offer complete (0- to 2π) phase control and large amplitude modulation of scattered waves through the microelectromechanical movement of silicon antenna arrays created in standard silicon-on-insulator technology. Our approach can be used to realize a platform technology that enables low-voltage operation of pixels for temporal color mixing and continuous, dynamic beam steering and light focusing.

Metasurfaces-based imaging and applications: from miniaturized optical components to functional imaging platforms

This review focuses on the imaging applications of metasurfaces. These optical elements provide a unique platform to control light; not only do they have a reduced size and complexity compared to conventional imaging systems but they also enable novel imaging modalities, such as functional-imaging techniques. This review highlights the development of metalenses, from their basic principles, to the achievement of achromatic and tunable lenses, and metasurfaces implemented in functional optical imaging applications.

Broadband achromatic metalens in terahertz regime

Achromatic focusing is essential for broadband operation, which has recently been realised from visible to infrared wavelengths using a metasurface. Similarly, multi-terahertz functional devices can be encoded in a desired metasurface phase profile. However, metalenses suffer from larger chromatic aberrations because of the intrinsic dispersion of each unit element. Here, we propose an achromatic metalens with C-shaped unit elements working from 0.3 to 0.8 THz with a bandwidth of approximately 91% over the centre frequency. The designed metalens possesses a high working efficiency of more than 68% at the peak and a relatively high numerical aperture of 0.385. We further demonstrate the robustness of our C-shaped metalens, considering lateral shape deformations and deviations in the etching depth. Our metalens design opens an avenue for future applications of terahertz meta-devices in spectroscopy, time-of-flight tomography and hyperspectral imaging systems.

Controlling the degrees of freedom in metasurface designs for multi-functional optical devices

This review focuses on the control over the degrees of freedom (DOF) in metasurfaces, which include the input DOF (the polarization, wavelength and incident angle of the input light and some dynamic controls), parameter DOF (the complex geometric design of metasurfaces) and output DOF (the phase, polarization and amplitude of the output light). This framework could clearly show us the development process of metasurfaces, from single-functional to multi-functional ones. Advantages of the multi-functional metasurfaces are discussed in the context of various applications, including 3D holography, broadband achromatic metalenses and multi-dimensional encoded information. By combining all the input and output DOF together, we can realize ideal optical meta-devices with deep subwavelength thickness and striking functions beyond the reach of traditional optical components. Moreover, new research directions may emerge when merging different DOF in metasurfaces with other important concepts, such as parity-time symmetry and topology, so that we can have the complete control of light waves in a prescribed manner.

Metasurface-enhanced optical lever sensitivity for atomic force microscopy

We proposed the integration of metasurfaces on atomic force microscopy (AFM) cantilevers to improve the optical lever sensitivity. The metasurface positioned at the back of an AFM cantilever enables anomalous reflection of the laser beam and provides a nonlinear relationship between the incidence and reflection angles following the generalized Snell's law. Therefore, the displacement of the reflected laser spot at the photodetector can be amplified. The metasurface was composed of 30 nm thick Au nano-discs with different diameters in an array of 1500 nm × 300 nm super cells. Using the fabricated metasurface mounted on a precise angle dial and a macroscale cantilever as prototypes, the concept of a metasurface-enhanced cantilever was experimentally ascertained. Results proved that the optical lever sensitivity can be easily increased. Finite element analysis showed that integration of the thin metasurface does not have a significant impact on the cantilever's mechanical properties including stiffness and eigenfrequencies. The proposed metasurface-enhanced optical lever sensitivity may have potential applications in improving functional performances of AFM instruments and cantilever-based sensors, such as allowing much smaller imaging forces and boosting the signal-to-noise ratio.

Multifunctional metaoptics based on bilayer metasurfaces

Optical metasurfaces have become versatile platforms for manipulating the phase, amplitude, and polarization of light. A platform for achieving independent control over each of these properties, however, remains elusive due to the limited engineering space available when using a single-layer metasurface. For instance, multiwavelength metasurfaces suffer from performance limitations due to space filling constraints, while control over phase and amplitude can be achieved, but only for a single polarization. Here, we explore bilayer dielectric metasurfaces to expand the design space for metaoptics. The ability to independently control the geometry and function of each layer enables the development of multifunctional metaoptics in which two or more optical properties are independently designed. As a proof of concept, we demonstrate multiwavelength holograms, multiwavelength waveplates, and polarization-insensitive 3D holograms based on phase and amplitude masks. The proposed architecture opens a new avenue for designing complex flat optics with a wide variety of functionalities.

High-Efficiency Metalenses with Switchable Functionalities in Microwave Region

Regarding miniaturized and integrated systems, a single flat device that possesses diversified functionalities is highly desirable in optical to microwave regimes. With this perspective, bifunctional metalenses constructed by meta-atoms with integrated response to propagation phase and geometric phase are proposed for independent manipulation of right-handed and left-handed circularly polarized waves. The derived general criterion is verified in the microwave region from three bifunctional metalenses operating in transmission manner. The proof-of-concept measurements show that all these metalenses exhibit two independent functionalities that can be switched by flipping the helicity of the incident illumination. Very high efficiencies of around 80%, with peak value of 91%, are achieved by the ultrathin metasurfaces of thickness 0.15λ₀. The proposed metasurfaces provide a promising route for the realization of reconfigurable lenses and antennas in wireless communication systems.

Energy-Tailorable Spin-Selective Multifunctional Metasurfaces with Full Fourier Components

Compact integrated multifunctional metasurface that can deal with concurrent tasks represent one of the most profound research fields in modern optics. Such integration is expected to have a striking impact on minimized optical systems in applications such as optical communication and computation. However, arbitrary multifunctional spin-selective design with precise energy configuration in each channel is still a challenge, and suffers from intrinsic noise and complex designs. Here, a design principle is proposed to realize energy tailorable multifunctional metasurfaces, in which the functionalities can be arbitrarily designed if the channels have no or weak interference in k-space. A design strategy is demostrated here with high-efficiency dielectric nanopillars that can modulate full Fourier components of the optical field. The spin-selective behavior of the dielectric metasurfaces is also investigated, which originates from the group effect introduced by numerous nanopillar arrays. This approach provides straightforward rules to control the functionality channels in the integrated metasurfaces, and paves the way for efficient concurrent optical communication.

Recent Advances in MEMS Metasurfaces and Their Applications on Tunable Lens

The electromagnetic (EM) properties of metasurfaces depend on both structural design and material properties. microelectromechanical systems (MEMS) technology offers an approach for tuning metasurface EM properties by structural reconfiguration. In the past 10 years, vast applications have been demonstrated based on MEMS metasurfaces, which proved to have merits including, large tunability, fast speed, small size, light weight, capability of dense integration, and compatibility of cost-effective fabrication process. Here, recent advances in MEMS metasurface applications are reviewed and categorized based on the tuning mechanisms, operation band and tuning speed. As an example, the pros and cons of MEMS metasurfaces for tunable lens applications are discussed and compared with traditional tunable lens technologies followed by the summary and outlook.

Metamaterial Lensing Devices

In recent years, the development of metamaterials and metasurfaces has drawn great attention, enabling many important practical applications. Focusing and lensing components are of extreme importance because of their significant potential practical applications in biological imaging, display, and nanolithography fabrication. Metafocusing devices using ultrathin structures (also known as metasurfaces) with superlensing performance are key building blocks for developing integrated optical components with ultrasmall dimensions. In this article, we review the metamaterial superlensing devices working in transmission mode from the perfect lens to two-dimensional metasurfaces and present their working principles. Then we summarize important practical applications of metasurfaces, such as plasmonic lithography, holography, and imaging. Different typical designs and their focusing performance are also discussed in detail.

Spin-Selected Dual-Wavelength Plasmonic Metalenses

Several novel spin-selected dual-wavelength metalenses have been proposed and investigated based on the plasmonic metasurface consisting of two kinds of rotary rectangle gap nanoantennas (RGN), which are designed based on merging two or four polarity-inverse lenses corresponding to different wavelengths (765 and 1300 nm). The spin-selected dual-wavelength metalenses with two similar and two different vertical and lateral focal points have also been proposed respectively, which can focus two wavelengths with inverse spin states to arbitrary special positions. The three-dimensional metalens with four focal points have also been proposed, which can focus four beams with inverse spin states and different wavelengths to preset positions. Moreover, a spin-dependent achromatic metalens has also been proposed, which can focus left circularly polarized (LCP) incidence with different wavelengths to the same position. Our work opens up new avenues toward establishing novel spin-selected and wavelength-selected metadevices, and is significant for the development of spin-controlled photonics and particles manipulation. In addition, it provides a new idea for solving the problem of data transmission from optical fiber communication to visible light communication.

Multifocal metalens with a controllable intensity ratio

A multifocal metalens is proposed based on the optical metasurface consisting of subwavelength gratings etched on silver film. The larger transmission of grating makes the proposed metalens have high focusing efficiency, and the exquisite design of the metasurface enables the metalens to focus the light at multiple spots with the controllable intensity ratio. The intensity ratio of focal spots is controlled by adjusting grating structures. The numerical simulations give the good presentation, and the experiment measurement provides the favorable verification for the performance of the proposed multifocal metalens in light focusing and beam splitting. The advantages of the multifocal metalens, including simple design, compact structure, high efficiency, and the controllable focusing, are a benefit to its applications in optical integration and micromanipulation.

Broadband High-Efficiency Chiral Splitters and Holograms from Dielectric Nanoarc Metasurfaces

Simultaneous broadband and high efficiency merits of designer metasurfaces are currently attracting widespread attention in the field of nanophotonics. However, contemporary metasurfaces rarely achieve both advantages simultaneously. For the category of transmissive metadevices, plasmonic or conventional dielectric metasurfaces are viable for either broadband operation with relatively low efficiency or high efficiency at only a selection of wavelengths. To overcome this limitation, dielectric nanoarcs are proposed as a means to accomplish two advantages. Continuous nanoarcs support different electromagnetic resonant modes at localized areas for generating phase retardation. Meanwhile, the geometric nature of nanoarc curvature endows the nanoarcs with full phase coverage of 0-2π due to the Pancharatnam-Berry phase principle. Experimentally incorporated with the chiral-detour phase principle, a few compelling functionalities are demonstrated, such as chiral beamsplitting, broadband holography, and helicity-selective holography. The continuous nanoarc metasurfaces prevail over plasmonic or dielectric discretized building block strategies and the findings lead to novel designs of spin-controllable metadevices.

A Light-Field Metasurface for High-Resolution Single-Particle Tracking

Three-dimensional (3D) single-particle tracking (SPT) is a key tool for studying dynamic processes in the life sciences. However, conventional optical elements utilizing light fields impose an inherent trade-off between lateral and axial resolution, preventing SPT with high spatiotemporal resolution across an extended volume. We overcome the typical loss in spatial resolution that accompanies light-field-based approaches to obtain 3D information by placing a standard microscope coverslip patterned with a multifunctional, light-field metasurface on a specimen. This approach enables an otherwise unmodified microscope to gather 3D information at an enhanced spatial resolution. We demonstrate simultaneous tracking of multiple fluorescent particles within a large 0.5 × 0.5 × 0.3 mm³ volume using a standard epi-fluorescent microscope with submicron lateral and micron-level axial resolution.

A high-efficiency dual-wavelength achromatic metalens based on Pancharatnam-Berry phase manipulation

The application of metasurfaces requires the reduction or elimination of their chromatic aberration while maintaining a high efficiency. We propose a method for the design of dual-wavelength operating achromatic metalenses, which can focus two different wavelengths at the same position. Phase manipulation was achieved by crossing two crystalline Si nanorods on each pixel carrying phase information for wavelengths of 780 and 660 nm based on the principal of Pancharatnam-Berry (PB) phase. At 660 nm, chromatic aberration was reduced from 1.28 to 0.46 μm in numerical experiments. The focusing efficiency of the circularly polarized light reached 90.2% for 780 nm and 49.7% for 660 nm. This method can be extended to other wavelengths.

Multilayer Noninteracting Dielectric Metasurfaces for Multiwavelength Metaoptics

Metasurfaces provide a versatile platform for manipulating the wavefront of light using planar nanostructured surfaces. Transmissive metasurfaces, with full 2π phase control, are a particularly attractive platform for replacing conventional optical elements due to their small footprint and broad functionality. However, the operational bandwidth of metasurfaces has been a critical limitation and is directly connected to either their resonant response or the diffractive dispersion of their lattice. While multiwavelength and continuous band operation have been demonstrated, the elements suffer from either low efficiency, reduced imaging quality, or limited element size. Here, we propose a platform that provides for multiwavelength operation by employing tightly spaced multilayer dielectric metasurfaces. As a proof of concept, we demonstrate a multiwavelength metalens doublet (NA = 0.42) with focusing efficiencies of 38% and 52% at wavelengths of 1180 and 1680 nm, respectively. We further show how this approach can be extended to three-wavelength metalenses as well as a spectral splitter. This approach could find applications in fluorescent microscopy, digital imaging, and color routing.

Polarization-independent metasurface lens employing the Pancharatnam-Berry phase

Metasurface optical elements, optical phased arrays constructed from a dense arrangement of nanoscale antennas, are promising candidates for the next generation of flat optical components. Metasurfaces that rely on the Pancharatnam-Berry phase facilitate complete and efficient wavefront control. However, their operation typically requires control over the polarization state of the incident light to achieve a desired optical function. Here, we circumvent this inherent sensitivity to the incident polarization by multiplexing two metasurfaces that were designed to achieve the same optical function with incident light of opposite helicity. We analyze the optical performance of different multiplexing approaches, and demonstrate a subwavelength random interleaved polarization-independent metasurface lens operating in the visible spectrum, providing a diffraction-limited spot size for the shared-aperture.

Hybrid optical-electronic convolutional neural networks with optimized diffractive optics for image classification

Convolutional neural networks (CNNs) excel in a wide variety of computer vision applications, but their high performance also comes at a high computational cost. Despite efforts to increase efficiency both algorithmically and with specialized hardware, it remains difficult to deploy CNNs in embedded systems due to tight power budgets. Here we explore a complementary strategy that incorporates a layer of optical computing prior to electronic computing, improving performance on image classification tasks while adding minimal electronic computational cost or processing time. We propose a design for an optical convolutional layer based on an optimized diffractive optical element and test our design in two simulations: a learned optical correlator and an optoelectronic two-layer CNN. We demonstrate in simulation and with an optical prototype that the classification accuracies of our optical systems rival those of the analogous electronic implementations, while providing substantial savings on computational cost.

Two-Photon Microscopy with a Double-Wavelength Metasurface Objective Lens

Two-photon microscopy is a key imaging technique in life sciences due to its superior deep-tissue imaging capabilities. Light-weight and compact two-photon microscopes are of great interest because of their applications for in vivo deep brain imaging. Recently, dielectric metasurfaces have enabled a new category of small and lightweight optical elements, including objective lenses. Here we experimentally demonstrate two-photon microscopy using a double-wavelength metasurface lens. It is specifically designed to focus 820 and 605 nm light, corresponding to the excitation and emission wavelengths of the measured fluorophors, to the same focal distance. The captured two-photon images are qualitatively comparable to the ones taken by a conventional objective lens. Our metasurface lens can enable ultracompact two-photon microscopes with similar performance compared to current systems that are usually based on graded-index-lenses. In addition, further development of tunable metasurface lenses will enable fast axial scanning for volumetric imaging.

"Crypto-Display" in Dual-Mode Metasurfaces by Simultaneous Control of Phase and Spectral Responses

Although conventional metasurfaces have demonstrated many promising functionalities in light control by tailoring either phase or spectral responses of subwavelength structures, simultaneous control of both responses has not been explored yet. Here, we propose a concept of dual-mode metasurfaces that enables simultaneous control of phase and spectral responses for two kinds of operation modes of transmission and reflection, respectively. In the transmission mode, the dual-mode metasurface acts as conventional metasurfaces by tailoring phase distribution of incident light. In the reflection mode, a reflected colored image is produced under white light illumination. We also experimentally demonstrate a crypto-display as one application of the dual-mode metasurface. The crypto-display looks a normal reflective display under white light illumination but generates a hologram that reveals the encrypted phase information under single-wavelength coherent light illumination. Because two operation modes do not affect each other, the crypto-display can have applications in security techniques.

Planar Diffractive Lenses: Fundamentals, Functionalities, and Applications

Traditional objective lenses in modern microscopy, based on the refraction of light, are restricted by the Rayleigh diffraction limit. The existing methods to overcome this limit can be categorized into near-field (e.g., scanning near-field optical microscopy, superlens, microsphere lens) and far-field (e.g., stimulated emission depletion microscopy, photoactivated localization microscopy, stochastic optical reconstruction microscopy) approaches. However, they either operate in the challenging near-field mode or there is the need to label samples in biology. Recently, through manipulation of the diffraction of light with binary masks or gradient metasurfaces, some miniaturized and planar lenses have been reported with intriguing functionalities such as ultrahigh numerical aperture, large depth of focus, and subdiffraction-limit focusing in far-field, which provides a viable solution for the label-free superresolution imaging. Here, the recent advances in planar diffractive lenses (PDLs) are reviewed from a united theoretical account on diffraction-based focusing optics, and the underlying physics of nanofocusing via constructive or destructive interference is revealed. Various approaches of realizing PDLs are introduced in terms of their unique performances and interpreted by using optical aberration theory. Furthermore, a detailed tutorial about applying these planar lenses in nanoimaging is provided, followed by an outlook regarding future development toward practical applications.

High-Efficiency Visible Transmitting Polarizations Devices Based on the GaN Metasurface

Metasurfaces are capable of tailoring the amplitude, phase, and polarization of incident light to design various polarization devices. Here, we propose a metasurface based on the novel dielectric material gallium nitride (GaN) to realize high-efficiency modulation for both of the orthogonal linear polarizations simultaneously in the visible range. Both modulated transmitted phases of the orthogonal linear polarizations can almost span the whole 2π range by tailoring geometric sizes of the GaN nanobricks, while maintaining high values of transmission (almost all over 90%). At the wavelength of 530 nm, we designed and realized the beam splitter and the focusing lenses successfully. To further prove that our proposed method is suitable for arbitrary orthogonal linear polarization, we also designed a three-dimensional (3D) metalens that can simultaneously focus the X-, Y-, 45°, and 135° linear polarizations on spatially symmetric positions, which can be applied to the linear polarization measurement. Our work provides a possible method to achieve high-efficiency multifunctional optical devices in visible light by extending the modulating dimensions.

Multifunctional Metasurfaces Based on the “Merging” Concept and Anisotropic Single-Structure Meta-Atoms

Metasurfaces offer great opportunities to control electromagnetic (EM) waves, attracting intensive attention in science and engineering communities. Recently, many efforts were devoted to multifunctional metasurfaces integrating different functionalities into single flat devices. In this article, we present a concise review on the development of multifunctional metasurfaces, focusing on the design strategies proposed and functional devices realized. We first briefly review the early efforts on designing such systems, which simply combine multiple meta-structures with distinct functionalities to form multifunctional devices. To overcome the low-efficiency and functionality cross-talking issues, a new strategy was proposed, in which the meta-atoms are carefully designed single structures exhibiting polarization-controlled transmission/reflection amplitude/phase responses. Based on this new scheme, various types of multifunctional devices were realized in different frequency domains, which exhibit diversified functionalities (e.g., focusing, deflection, surface wave conversion, multi-beam emissions, etc.), for both pure-reflection and pure-transmission geometries or even in the full EM space. We conclude this review by presenting our perspectives on this fast-developing new sub-field, hoping to stimulate new research outputs that are useful in future applications.

MEMS-tunable dielectric metasurface lens

Varifocal lenses, conventionally implemented by changing the axial distance between multiple optical elements, have a wide range of applications in imaging and optical beam scanning. The use of conventional bulky refractive elements makes these varifocal lenses large, slow, and limits their tunability. Metasurfaces, a new category of lithographically defined diffractive devices, enable thin and lightweight optical elements with precisely engineered phase profiles. Here we demonstrate tunable metasurface doublets, based on microelectromechanical systems (MEMS), with more than 60 diopters (about 4%) change in the optical power upon a 1-μm movement of one metasurface, and a scanning frequency that can potentially reach a few kHz. They can also be integrated with a third metasurface to make compact microscopes (~1 mm thick) with a large corrected field of view (~500 μm or 40 degrees) and fast axial scanning for 3D imaging. This paves the way towards MEMS-integrated metasurfaces as a platform for tunable and reconfigurable optics.

Gradient metasurfaces: a review of fundamentals and applications

In the wake of intense research on metamaterials the two-dimensional analogue, known as metasurfaces, has attracted progressively increasing attention in recent years due to the ease of fabrication and smaller insertion losses, while enabling an unprecedented control over spatial distributions of transmitted and reflected optical fields. Metasurfaces represent optically thin planar arrays of resonant subwavelength elements that can be arranged in a strictly or quasi periodic fashion, or even in an aperiodic manner, depending on targeted optical wavefronts to be molded with their help. This paper reviews a broad subclass of metasurfaces, viz. gradient metasurfaces, which are devised to exhibit spatially varying optical responses resulting in spatially varying amplitudes, phases and polarizations of scattered fields. Starting with introducing the concept of gradient metasurfaces, we present classification of different metasurfaces from the viewpoint of their responses, differentiating electrical-dipole, geometric, reflective and Huygens' metasurfaces. The fundamental building blocks essential for the realization of metasurfaces are then discussed in order to elucidate the underlying physics of various physical realizations of both plasmonic and purely dielectric metasurfaces. We then overview the main applications of gradient metasurfaces, including waveplates, flat lenses, spiral phase plates, broadband absorbers, color printing, holograms, polarimeters and surface wave couplers. The review is terminated with a short section on recently developed nonlinear metasurfaces, followed by the outlook presenting our view on possible future developments and perspectives for future applications.

GaN Metalens for Pixel-Level Full-Color Routing at Visible Light

Metasurface-based components are known to be one of the promising candidates for developing flat optical systems. However, their low working efficiency highly limits the use of such flat components for feasible applications. Although the introduction of the metallic mirror has been demonstrated to successfully enhance the efficiency, it is still somehow limited for imaging and sensing applications because they are only available for devices operating in a reflection fashion. Here, we demonstrate three individual GaN-based metalenses working in a transmission window with extremely high operation efficiency at visible light (87%, 91.6%, and 50.6% for blue, green, and red light, respectively). For the proof of concept, a multiplex color router with dielectric metalens, which is capable of guiding individual primary colors into different spatial positions, is experimentally verified based on the design of out-of-plane focusing metalens. Our approach with low-cost, semiconductor fabrication compatibility and high working efficiency characteristics offers a way for establishing a complete set of flat optical components for a wide range of applications such as compact imaging sensors, optical spectroscopy, and high-resolution lithography, just named a few.