Zou et al.PhotoniX(2020)1:2htps/dol.org/10.1186/s43074020-00007-9PhotoniXREVIEWOpen AccessImaging based on metalensesXiujuan Zou'2,Gaige Zheng3,Quan Yuan,Wenbo Zang,Run Chen2,Tianyue Li2,Lin Li,Abstractzhusn@njuedu.cnNational Laboratory of Solid StateMetalens,a prominent application of two-dimensional metasurfaces,hasMicrostructures,School of Physics,demonstrated powerful abilities even beyond traditional optical lenses.ByCollege of Engineering and Appliedmanipulating the phase distribution of metalens composed of appropriatelySciences,Nanjing University.Nanjing 210093,Chinaarranged nanoscale building blocks,the wavefront of incident wave can beFull list of author information iscontrolled based on Huygens principle,thus achieving the desired reflected andavailable at the end of the articletransmitted wave for many different purposes.Metalenses will lead a revolution inoptical imaging due to its flat nature and compact size,multispectral acquisition andeven off-axis focusing.Here,we review the recent progress of metalenses presentingexcellent properties,with a focus on the imaging application using these metalenses.We firstly discuss the mechanism for achieving metalenses with high efficiency,largenumerical aperture,controlling the chromatic dispersion or monochromaticaberrations and large area fabrication.Then,we review several important imagingapplications including wide-band focusing imaging,polarization dependent imaging,light field imaging and some other significant imaging systems in different areas.Finally,we make a conclusion with an outlook on the future development andchallenges of this developing research field.Keywords:Metalens,Optical imaging,Dispersion manipulation,AchromaticaberrationIntroductionManipulation of electromagnetic(EM)waves in refraction and reflection through vari-ous structures is of fundamental importance in a great number of applications.Through interaction between light waves and nanostructure it is possible to controlthe light intensity,polarization and propagation direction.Conventionally,light hasbeen manipulated in transmission using refractive lenses,which are devices intendedto focus the light in a single or various spots and then have evolved into numerous op-tical applications operating at all along the EM spectrum.The optical lens has beenplaying a critical role in almost all areas of science and technology since its inventionappeared about 3000years ago.Conventional lenses gradually change the phase andpolarization of light during the propagation.It is difficult to accumulate sufficientphase change once the device size is further reduced to subwavelength scale due to thelimited refractive indices of natural materials.In addition,although these systems canobtain the images with high quality,they are usually bulky,costly,and time-consumingto manufacture with high precision,which are significant limitations,particularly forapplications such as portable and wearable devices.Springer Openindicate if changes were madeZou et al.PhotoniX(2020)1:2Page 2 of 24To meet the growing requirement of device miniaturization and system integration,metasurfaces have emerged as a versatile platform for wavefront shaping [1-7].Meta-surfaces are one kind of such diffractive optical elements with flat subwavelengthimplementations,which consist of quasiperiodic arrays of light resonators (also calledas meta-atoms)[8,9].It can be designed with large numbers of degrees of freedom to con-trol the light wavefront,which enables considerable number of unprecedented functional-ities.As a result,ultra-thin flat optics can be generated by deploying such metasurfaces inthe design.Metasurfaces have been proposed for focusing [10-14],deflecting light [15,16],changing polarization [17-20],generating holograms [17,21-29],producing vortex beams[18,30],manipulating thermalion [31-33],modulating light intensity [34-36],and opticalcloaking [27].Among all these potential optical applications,high-quality imagingof metelenses has received considerably renewed attention in the field of researchin the last decade.Metasurfaces are able to introduce abrupt local phase changes in the EM field ontheir surface by using different sub-wavelength sized resonators [37,38].This makes itpossible to arbitrarily transform the incident phase profile to a new phase profile,as-suming that we have a set of unit cells that are able to introduce any phase shift be-tween 0 and 2n radians.The basic concept of metasurface lenses is that they convertan incident plane wave into a spherical wave by introducing different phase shifts alongtheir lateral axis.Compared with that of state-of-the-art commercial objectives,imagequality captured by optical systems with metalenses is as good and even better [39-41].Metalenses seem to be potentially revolutionary in optical imaging due to its flat natureand compact size,multispectral acquisition and even off-axis focusing.However,Meta-lenses have the same problem of aberrations as ordinary lenses in use,so the elimin-ation of aberrations,including monochromatic aberrations and chromatic aberrationsis quite important for imaging and displaying.Chromatic aberration resulted from ma-terial dispersion,the resonant phase dispersion of unit cells in refractive optical compo-nents and different phase accumulation through light propagation makes the resultingfocal spots wavelength dependent.Therefore,a variety of methods and technologies forimproving image quality have been produced one after another.Here,we will review the progress in the development of metalenses and its applica-tion,especially in imaging.This paper is organized into two major parts.Firstly,thebasic characteristic parameters of existing or reported metalenses will be introducedand analyzed.Next,different types of state-of-the-art imaging technologies of meta-lenses mentioned in the first part used for machine vision,biomedical imaging and op-tical communication will be reviewed,followed by concluding remarks.Review on the applications as imaging lens of metasurfacesMetalenses as a practical application of metasurfaces are two-dimensional metamateri-als [42,43]in which the thickness dimension is ignored.Metamaterials themselves canbe used to guide and control the flow of the EM wave and then the ultra-thin,flexibleessence of matesurfaces to modulate light,which makes the metalens a product thatcombines all the above excellent characteristics and attracts intensive attention of re-searchers in related academic field [44].Metasurfaces can be exploited to control theEM wave's amplitude,phase and polarization with subswavelength resolution and en-gineer the wavefronts at distances less than the wavelength [45].It is discriminatedZou et al.PhotoniX(2020)1:2Page 3 of 24from the conventional optical imaging devices that wavefronts shaping of metalensesare achieved by phase accumulation of wave propagating in the medium [37,46].Ac-cording to the generalized Snell's law deduced by the Fermat principle and the law ofconservation of momentum,the reflected wave and the transmitted wave can be arbi-trarily redirected only by designing the spatial phase response of the optical resonatorsforming the optical interface [47,48].Some metalenses with significant characteristicsare widely required in practical use [49].Thus,in what follows,different efforts to con-trol different-functionalities metalenses are made by numerous groups by utilizing dif-ferent geometries and materials for different incident polarizations of EM wave.Amongthem,we reviewed several of the most crucial factors of the metalenses and their rolein various imaging systems for many different purposes.All these aspects will eventu-ally be reflected in the imaging behavior/quality of the lens,and may lead to color er-rors,image distortion,vignetting,image blur and so on.Key figures of merit for metalensesFocusing efficiency of metalensesLike traditional lenses,when metalenses are exploited in the optical systems,we usuallyconsider their focusing characteristics.When a single wavelength of EM wave is incident,the focus efficiency of metalenses is generally analyzed.The focusing efficiency is definedas the ratio of light intensity from the focal spot at corresponding focal plane to the lightintensity reflected by a metallic mirror(or transmitted through a lens)with the same pixelsizes.Although visible planar lenses can be realized by diffractive components,high effi-ciency are difficult to attain because their constituent structures are of wavelength scale,which precludes an accurate phase profile.Until now,much efforts to improve the focusefficiency of metalenses have been made to improve lens behavior,such as improvingimaging resolution.Several mechanisms of enhancing the focusing efficiency includingemployment of flat mirror,high-contrast index materials,all-dielectric materials or geo-metric phase have been proved to be feasible.Pors et al.designed a focusing plane mirrorcomposed of the metal-insulator-metal configurations in which the top metal layer is aperiodic array of nano-bricks of different sizes.The designed flat mirror that focuses alinearly polarized incident beam with focusing efficiency of 78%theoretically in a broad-band wavelength regime from 700 nm 1000 nm,while the broadband focusing eficiencyreaches approximately 30%experimentally.High NA and long focus length are realizedby considering gradient metasurfaces and changing both lateral dimensions of meta-elements,which can be used in the radiation focusing of optical systems as is shown inFig.1(a)[50].Due to the relative lower focusing efficiency of the reflective metasurfacelens,transmissive lenses are also considered by researchers to enhance the focusing effi-ciency in Fig.1(b).The metalenses,composed of high-contrast silicon nano-posts trans-mitarrays on glass,are fabricated by Faraon's group in one lithographic step and themeasured focusing efficiency is up to 82%in Fig.1(d)[51].The meta-elements are de-signed as the periodic circular amorphous silicon posts placed on the hexagonal lattice,enables the desired phase masks for the wavefront shaping od light at will in Fig.1(c).Themetalens with high NA can be integrated on the optoelectronic chip for imaging applica-tion.Latter,Cappasso and co-workers fabricated and designed high aspect ratio all-dielectric(titanium dioxide meta-atoms and silica substrate)metasurfaces as metalenses,