Physics-data-driven intelligent optimization for large-aperture metalenses

Metalenses have gained significant attention and have been widely utilized in optical systems for focusing and imaging,owing to their lightweight,high-integration,and exceptional-flexibility capabilities.Traditional design methods neglect the coupling effect between adjacent meta-atoms,thus harming the practical performance of meta-devices.The existing physical/data-driven optimization algorithms can solve the above problems,but bring significant time costs or require a large number of data-sets.Here,we propose a physics-data-driven method employing an“intelligent optimizer”that enables us to adaptively modify the sizes of the meta-atom according to the sizes of its surrounding ones.The implementation of such a scheme effectively mitigates the undesired impact of local lattice coupling,and the proposed network model works well on thousands of data-sets with a validation loss of 3×10^(−3).Based on the“intelligent optimizer”,a 1-cm-diameter metalens is designed within 3 hours,and the experimental results show that the 1-mm-diameter metalens has a relative focusing efficiency of 93.4%(compared to the ideal focusing efficiency)and a Strehl ratio of 0.94.Compared to previous inverse design method,our method significantly boosts designing efficiency with five orders of magnitude reduction in time.More generally,it may set a new paradigm for devising large-aperture meta-devices.

Temperature-responded tunable metalenses based on phase transition materials

Once the metalenses are fabricated,the functions of most metalenses are invariable.The tunability and reconfigurability are useful and cost-saving for metalenses in realistic applications.We demonstrate this tunability here via a novel hybrid metalens with the strategic placement of an ultra-thin VO;layer.The hybrid metalens is capable of dynamically modulating the focusing intensity of transmitted light at a wavelength of 1550 nm,and demonstrate a 42.28%focusing efficiency of the incident light and 70.01%modulation efficiency.The hybrid metalens’optothermal simulations show an optothermal conversion process of dynamic focusing,and a maximum laser density of 1.76×10^(3)W/cm^(2) can be handled at an ambient temperature lower than 330 K.The hybrid metalens proposed in this work,a light-dose sensitive tunable smart metalens that can protect other instruments/systems or materials from being damaged,has its specific applications such as in anti-satellite blinding,bio-imaging,etc.

Generation of super-resolved optical needle and multifocal array using graphene oxide metalenses

Ultrathin flat metalenses have emerged as promising alternatives to conventional diffractive lenses,offering new possibilities for myriads of miniaturization and interfacial applications.Graphene-based materials can achieve both phase and amplitude modulations simultaneously at a single position due to the modification of the complex refractive index and thickness by laser conversion from graphene oxide into graphene like materials.In this work,we develop graphene oxide metalenses to precisely control phase and amplitude modulations and to achieve a holistic and systematic lens design based on a graphene-based material system.We experimentally validate our strategies via demonstrations of two graphene oxide metalenses:one with an ultra-long(~16λ)optical needle,and the other with axial multifocal spots,at the wavelength of 632.8 nm with a 200 nm thin film.Our proposed graphene oxide metalenses unfold unprecedented opportunities for accurately designing graphene-based ultrathin integratable devices for broad applications.

Near-Field Focusing With Subwavelength Thickness Metalenses via Electromagnetic Susceptibility Models

The design mentality of an optimal metalens model, based on the electromagnetic susceptibility, a synthesis of subwavelength-thick metasurfaces (MSs) is presented in this paper. First, based on the finite difference method of generalized sheet transition conditions, the surface susceptibility function of the MS with spatial discontinuities can be determined. Then, the paper analyzed the remaining corresponding physical field conditions for the scale of metalens. In order to adapt to the physical limitations encountered in the near-field focusing of the metalens, a standard parabolic phase design is proposed in this paper, and its upsides and downsides of the two-phase processing in different aspects are compared. Using COMSOL software with numerical simulation, it can be seen that the standard design can easily obtain high resolution in the near field, while the focusing effect is more stable when the focal length is small by the parabolic phase design.

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.

Ultra-thin, planar, Babinet-inverted plasmonic metalenses

We experimentally demonstrate the focusing of visible light with ultra-thin,planar metasurfaces made of concentrically perforated,30-nm-thick gold films.The perforated nano-voids—Babinet-inverted(complementary)nano-antennas—create discrete phase shifts and form a desired wavefront of cross-polarized,scattered light.The signal-to-noise ratio in our complementary nano-antenna design is at least one order of magnitude higher than in previous metallic nano-antenna designs.We first study our proof-of-concept‘metalens’with extremely strong focusing ability:focusing at a distance of only 2.5 mm is achieved experimentally with a 4-mm-diameter lens for light at a wavelength of 676 nm.We then extend our work with one of these‘metalenses’and achieve a wavelength-controllable focal length.Optical characterization of the lens confirms that switching the incident wavelength from 676 to 476 nm changes the focal length from 7 to 10 mm,which opens up new opportunities for tuning and spatially separating light at different wavelengths within small,micrometer-scale areas.All the proposed designs can be embedded on-chip or at the end of an optical fiber.The designs also all work for two orthogonal,linear polarizations of incident light.

Tunable metasurfaces towards versatilemetalenses and metaholograms:a review

Metasurfaces have attracted great attention due to their ability to manipulate the phase,amplitude,and polarization of light in a compact form.Tunable metasurfaces have been investigated recently through the integration with mechanically moving components and electrically tunable elements.Two interesting applications,in particular,are to vary the focal point of metalenses and to switch between holographic images.We present the recent progress on tunable metasurfaces focused on metalenses and metaholograms,including the basic working principles,advantages,and disadvantages of each working mechanism.We classify the tunable stimuli based on the light source and electrical bias,as well as others such as thermal and mechanical modulation.We conclude by summarizing the recent progress of metalenses and metaholograms,and providing our perspectives for the further development of tunable metasurfaces.

Metalenses: from design principles to functional applications

Lens is a basic optical element that is widely used in daily life,such as in cameras,glasses,and microscopes.Conventional lenses are designed based on the classical refractive optics,which results in inevitable imaging aberrations,such as chromatic aberration,spherical aberration and coma.To solve these problems,conventional imaging systems impose multiple curved lenses with different thicknesses and materials to eliminate these aberrations.As a unique photonic technology,metasurfaces can accurately manipulate the wavefront of light to produce fascinating and peculiar optical phenomena,which has stimulated researchers9 extensive interests in the field of planar optics.Starting from the introduction of phase modulation methods,this review summarizes the design principles and characteristics of metalenses.Although the imaging quality of existing metalenses is not necessarily better than that of conventional lenses,the multi-dimensional and multi-degree-of-freedom control of metasurfaces provides metalenses with novel functions that are extremely challenging or impossible to achieve with conventional lenses.

Multi-foci metalens for spectra and polarization ellipticity recognition and reconstruction

Multispectral and polarized focusing and imaging are key functions that are vitally important for a broad range of optical applications.Conventional techniques generally require multiple shots to unveil desired optical information and are implemented via bulky multi-pass systems or mechanically moving parts that are difficult to integrate into compact and integrated optical systems.Here,a design of ultra-compact transversely dispersive metalens capable of both spectrum and polarization ellipticity recognition and reconstruction in just a single shot is demonstrated with both coherent and incoherent light.Our design is well suited for integrated and high-speed optical information analysis and can significantly reduce the size and weight of conventional devices while simplifying the process of collecting optical information,thereby promising for various applications,including machine vision,minimized spectrometers,material characterization,remote sensing,and other areas which require comprehensive optical analysis.

Ultraviolet metalens and metalens array of focused vortex beams

The solar-blind ultraviolet(UV)wavelength is particularly interesting within the range of 200 nm–300 nm.Here,we propose a focusing metalens,focusing vortex beam(VB)metalens and metalens array that specifically work in the UV band to focus a beam or VB.Firstly,a high numerical aperture(NA)focusing metalens working at a wavelength of 214.2 nm was designed,and the NA reached 0.83.The corresponding conversion efficiency of the unit structure reached as high as 94%,and the full width at half maximum was only 117.2 nm.Metalenses with large NA can act as optical tweezers and can be applied to trap ultracold atoms and molecules.Secondly,a focused VB metalens in the wavelength range of200 nm–300 nm was also designed,which can convert polarized light into a VB and focus the VB simultaneously.Finally,a metalens array was developed to focus VBs with different topological charges on the same focal plane.This series of UV metalenses could be widely used in UV microscopy,photolithography,photonics communication,etc.

Review:Chromatic Dispersion Manipulation Based on Optical Metasurfaces

Metasurfaces are densely arrayed two⁃dimensional(2D)artificial planar metamaterials,which can manipulate the polarization,distribution,and amplitude of light by accurately controlling the phase of the scattering light.The flat metasurface has the potential to substantially reduce the thickness and complexity of the structures and allows ease of fabrication and integration into devices.However,the inherent chromatic aberration of the metasurface originating from the resonant dispersion of the antennas and the intrinsic chromatic dispersion limit their quality.How to effectively suppress or manipulate the chromatic aberration of metalenses has attracted worldwide attention in the last few years,leading to a variety of excellent achievements.Furthermore,utilizing the chromatic dispersion of metasurface to realize special functionalities is also of significant importance.In this review,the most promising recent examples of chromatic dispersion manipulation based on optical metasurface materials are highlighted and put into perspective.

Design of high efficiency achromatic metalens with large operation bandwidth using bilayer architecture

Achromatic metalens composed of arrays of subwavelength nanostructures with spatially varying geometries is attractivefor a number of optical applications. However, the limited degree of freedom in the single layer achromatic metasurfacedesign makes it difficult to simultaneously guarantee the sufficient phase dispersion and high diffraction efficiency,which restricts the achromatic bandwidth and efficiency of metalens. Here we propose and demonstrate a high efficiencyachromatic metalens with diffraction-limited focusing capability at the wavelength ranging from 1000 nm to 1700 nm. Themetalens comprises two stacked nanopillar metasurfaces, by which the required focusing phase and dispersion compensationcan be controlled independently. As a result, in addition to the large achromatic bandwidth, the averaged focusingefficiency of the bilayer metalens is higher than 64% at the near-infrared region. Our design opens up the possibilityto obtain the required phase dispersion and efficiency simultaneously, which is of great significance to design broadbandmetasurface-based optical devices.

Dynamic phase assembled terahertz metalens for reversible conversion between linear polarization and arbitrary circular polarization

If a metalens integrates the circular polarization(CP)conversion function,the focusing lens together with circular-polariz-ing lens(CPL)in traditional cameras may be replaced by a metalens.However,in terahertz(THz)band,the reported metalenses still do not obtain the perfect and strict single-handed CP,because they were constructed via Pancharatnam-Berry phase so that CP conversion contained both left-handed CP(LCP)and right-handed CP(RCP)components.In this paper,a silicon based THz metalens is constructed using dynamic phase to obtain single-handed CP conversion.Also,we can rotate the whole metalens at a certain angle to control the conversion of multi-polarization states,which can simply manipulate the focusing for incident linear polarization(LP)THz wave in three polarization conversion states,in-cluding LP without conversion,LCP and RCP.Moreover,the polarization conversion behavior is reversible,that is,the THz metalens can convert not only the LP into arbitrary single-handed CP,but also the LCP and RCP into two perpen-dicular LP,respectively.The metalens is expected to be used in advanced THz camera,as a great candidate for tradi-tional CPL and focusing lens group,and also shows potential application in polarization imaging with discriminating LCP and RCP.

Off-axis multi-wavelength dispersion controllingmetalens for multi-color imaging

Dispersion control is crucial in optical systems,and chromatic aberration is an important factor affecting imaging quality in imaging systems.Due to the inherent property of materials,dispersion engineering is complex and needs to trade off other aberration in traditional ways.Although metasurface offers an effective method to overcome these limits and results in well-engineered dispersion,off-axis dispersion control is still a challenging topic.In this paper,we design a single-layer metalens which is capable of focusing at three wavelengths(473 nm,532 nm,and 632 nm)with different incident angles(0°,-17°and 17°)into the same point.We also demonstrate that this metalens can provide an alternative for the bulky color synthetic prism in a 3-chips digital micromirror device(DMD)laser projection system.Through this approach,various off-axis dispersion controlling optical devices could be realized.

Design of an all-dielectric long-wave infrared wide-angle metalens

Optical metasurfaces are two-dimensional arrays of nano-scatterers that modify optical wavefronts at subwavelength spatial resolution.They achieve the effect of focusing through phase control under a subwavelength scale,and are called metalenses.They are poised to revolutionize optics by enabling complex low-cost systems.However,there are severe monochromatic aberrations in the metasurfaces.In this paper,the coma of the long-wave infrared optical system is eliminated through a single-layer metasurface.By changing the phase function,this metalens has a numerical aperture of 0.89,a focal length of 150μm and a field of view of 120°(0.4@60 line pairs/mm)that enables diffraction-limited monochromatic imaging along the focal plane at a wavelength of 10.6μm.The designed metasurface maintains a favorable value of the modulation transfer function at different angles.This equipment can be widely used in imaging and industrial processing.

High efficient Al: ZnO based bifocus metalens in visible spectrum

The optical components of the visible light band are widely used in daily life and industrial development. However due to the serious loss of light and the high cost, the application is limited. The broadband gap metasurface will change this situation due to its low absorption and high efficiency. Herein, we simulate a size-adjustable metasurface of the Al doped ZnO (AZO) nanorod arrays based on finite difference time domain method (FDTD) which can realize the conversion of amplitude polarization and phase in the full visible band. The corresponding theoretical polarization conversion efficiency can reach as high as 91.48% (450 nm), 95.27% (530 nm), and 91.01% (65 nm). The modulation of focusing wavelength can be realized by directly adjusting the height of the AZO nanorod. The designed half-wave plate and metalens can be applied in the imaging power modulation halfwave conversion and enriching the spectroscopy.

A review of anomalous refractive and reflective metasurfaces

Abnormal refraction and reflection refers to the phenomenon in which light does not follow its traditional laws of propagation and instead is subject to refraction and reflection at abnormal angles that satisfy a generalization of Snell’s law.Metasurfaces can realize this phenomenon through appropriate selection of materials and structural design,and they have a wide range of potential applications in the military,communications,scientific,and biomedical fields.This paper summarizes the current state of research on abnormal refractive and reflective metasurfaces and their application scenarios.It discusses types of abnormal refractive and reflective metasurfaces based on their tuning modes(active and passive),their applications in different wavelength bands,and their future development.The technical obstacles that arise with existing metasurface technology are summarized,and prospects for future development and applications of abnormal refractive and reflective metasurfaces are discussed.

Terahertz wide-angle metalens with nearly ideal object-image relation

Metalenses are essential components in terahertz imaging systems.However,without careful design,they show limited field of view and their practical applications are hindered.Here,a wide-angle metalens is proposed whose structure is optimized for focusing within the incident angles of±25°.Simulation and experiment results show that the focusing efficiency,spot size,and modulation transfer function of this lens are not sensitive to the incident angle.More importantly,this wide-angle metalens follows the ideal Gaussian formula for the object-image relation,which ensures a wider field of view and better contrast in the imaging experiment.

Metasurface-enabled augmented reality display: a review

Augmented reality(AR)display,which superimposes virtual images on ambient scene,can visually blend the physical world and the digital world and thus opens a new vista for human–machine interaction.AR display is considered as one of the next-generation display technologies and has been drawing huge attention from both academia and industry.Current AR display systems operate based on a combination of various refractive,reflective,and diffractive optical elements,such as lenses,prisms,mirrors,and gratings.Constrained by the underlying physical mechanisms,these conventional elements only provide limited light-field modulation capability and suffer from issues such as bulky volume and considerable dispersion,resulting in large size,severe chromatic aberration,and narrow field of view of the composed AR display system.Recent years have witnessed the emerging of a new type of optical elements—metasurfaces,which are planar arrays of subwavelength electromagnetic structures that feature an ultracompact footprint and flexible light-field modulation capability,and are widely believed to be an enabling tool for overcoming the limitations faced by current AR displays.Here,we aim to provide a comprehensive review on the recent development of metasurface-enabled AR display technology.We first familiarize readers with the fundamentals of AR display,covering its basic working principle,existing conventional-optics-based solutions,as well as the associated pros and cons.We then introduce the concept of optical metasurfaces,emphasizing typical operating mechanisms,and representative phase modulation methods.We elaborate on three kinds of metasurface devices,namely,metalenses,metacouplers,and metaholograms,which have empowered different forms of AR displays.Their physical principles,device designs,and the performance improvement of the associated AR displays are explained in details.In the end,we discuss the existing challenges of metasurface optics for AR display applications and provide our perspective on future research endeavors.

Revolutionary meta-imaging:from superlens to metalens

The refractive-lens technique has been well developed over a long period of evolution,offering powerful imaging functionalities,such as microscopes,telescopes,and spectroscopes.Nevertheless,the ever-growing requirements continue to urge further enhanced imaging capabilities and upgraded devices that are more compact for convenience.Metamaterial as a fascinating concept has inspired unprecedented new explorations in physics,material science,and optics,not only in fundamental researches but also novel applications.Along with the imaging topic,this paper reviews the progress of the flat lens as an important branch of metamaterials,covering the early superlens with super-diffraction capability and current hot topics of metalenses including a paralleled strategy of multilevel diffractive lenses.Numerous efforts and approaches have been dedicated to areas ranging from the new fascinating physics to feasible applications.This review provides a clear picture of the flat-lens evolution from the perspective of metamaterial design,elucidating the relation and comparison between a superlens and metalens,and addressing derivative designs.Finally,application scenarios that favor the ultrathin lens technique are emphasized with respect to possible revolutionary imaging devices,followed by conclusive remarks and prospects.