RESEARCHRESEARCH ARTICLEaccompanied by polarization conversion to left.handed circularly polarized light (33,34).Thus,each nanofin at(,y)is rotated by an angleAPPLIED OPTICS(2)Metalenses at visible wavelengths:To maximize the polarization conversion effi-ciency,the nanofins should operate as half-Diffraction-limited focusing andwaveplates(11-13,21).This is achieved due tothe birefringence arising from the asymmetriccross section of nanofins with appropriately desubwavelength resolution imagingsigned height,width,and length(Fig.1,Cand D).Simulations using a commercial finite differ-ence time domain(FDTD)solver (Lumerical Inc..Mohammadreza Khorasaninejad,*Wei Ting Chen,Robert C.Devlin,*Jaewon Oh,12Vancouver)in Fig.IF show that conversion effi-Alexander Y.Zhu,Federico Capassociencies as high as 95%are achieved and thatthe metalens can be designed for a desired wave-Subwavelength resolution imaging requires high numerical aperture(NA)lenses,whichlength via tuning of nanofin parameters.Theare bulky and expensive.Metasurfaces allow the miniaturization of conventionalconversion efficiency is calculated as the ratio ofrefractive optics into planar structures.We show that high-aspect-ratio titaniumtransmitted optical power with opposite helicitydioxide metasurfaces can be fabricated and designed as metalenses with NA =0.8.to the total incident power.Diffraction-limited focusing is demonstrated at wavelengths of 405,532,and 660 nmThree distinct metalenses were fabricated withwith corresponding efficiencies of 86,73,and 66%.The metalenses can resolve nanoscalerespective design wavelengths 0)of 660,532,features separated by subwavelength distances and provide magnification as high asand 405 nm.All of these metalenses have the170x,with image qualities comparable to a state-of-the-art commercial objective.same diameter of 240 um and a focal length ofhttpsOur results firmly establish that metalenses can have widespread applications in90 um,yielding a NA =0.8.The fabricationlaser-based microscopy,imaging,and spectroscopy.process uses electron-beam lithography to createthe lens pattern in the resist (ZEP 520A).Thethickness of the resist is the same as the designedetasurfaces are composed of subwave.spots are smaller by a factor of ~15 than thosenanofin height,H,and ALD is subsequently usedlength-spaced phase shifters at an in-from a commercially available,high-NA objectiveto deposit amorphous TiO,onto the developedterface,which allows for unprecedented(100x Nikon CFI 60:NA 0.8).Imaging usingresist.Amorphous TiO is chosen because it hascontrol over the properties of light (Z,2).these metalenses shows that they can yieldlow surface roughness,no absorption at visibleand have advanced optical technologysubwavelength resolution,with image qualitieswavelengths,and a sufficiently high refractiveby enabling versatile functionalities in a planarcomparable to that obtained by the commercialindex (-2.4).Due to the ALD process being con-structure (1-30).Various optical components,formal,a deposition thickness of at least W/2ranging from lenses,holograms and gratings to(where W is the nanofin width)is required topolarization-selective devices,have been demon-Planar lens design and fabricationproduce void-free nanofins (37).However,the de.strated using silicon-based (7-19)and plasmonicTypical high-NA objectives consist of precision-position also leaves a TiO,film of equal thicknessmetasurfaces(3,4,21-27).However,the high in-engineered compound lenses which make themon top of the resist,which is then removed bytrinsic losses of silicon and plasmonic materialsbulky and expensive,limiting their applicationscontrolled blanket reactive ion etching.Finally,in the visible range (400 to 700 nm)have pre-and hindering their integration into compactthe remaining electron beam resist is stripped,andvented the realization of highly efficient meta-and cost-effective systems.Singlet planar lensesonly high-aspect-ratio nanofins remain.Figure 1,Gsurfaces in this region.Although this challengewith high NA in the visible range are in partic.and H,shows optical and scanning electron mi-can be partially overcome by using dielectric ma-ularly high demand due to their potential widecroscope (SEM)images of a fabricated metalens,terials with a transparency window in the visiblespread applications in imaging,microscopy,andrespectively.Additional SEM micrographs of thespectrum (e.g.,GaP,SiN,and TiO2),achievingspectroscopy.Although visible planar lenses canmetalens are shown in fig.S1(35).Because thefull control over the phase of light requires pre-be realized by diffractive components,high NAgeometrical parameters of the nanofins are de-cise,high-aspect-ratio nanostructures,which areand efficiency are not attainable because theirfined by the resist rather than top-down etching,Octoin turn restricted by available nanofabricationconstituent structures are of wavelength scale.high-aspect-ratio nanofins with-90vertical sidemethods.Recently,we have developed an approachwhich precludes an accurate phase profile.walls are obtained.It is important to note thatbased on titanium dioxide (TiO2)(31)preparedFigure 1A shows a schematic of a transmissiveachieving these atomically smooth sidewalls isby atomic layer deposition(ALD)(32),which en-dielectric metalens.The building blocks of thevery challenging with a conventional top-down2022ables fabrication of high-aspect-ratio metasurfacesmetalens are high-aspect-ratio TiO nanofins (Fig.that are lossless in the visible spectrum.Here,we1,B to E).To function like a spherical lens,thebecause inevitable lateral etching results in surfacedemonstrate highly efficient metalenses at visiblephase profile (y)ofthe metalens needs toroughness and tapered/conical nanostructureswavelengths (405.532,and 660 nm)withfollow (25)efficiencies as high as 86%.They have high nu-Characterizing metalens performancemerical apertures (NA)of 0.8 and are capable of2f-v2+y+f2The metalenses'focal spot profiles and efficien-focusing light into diffraction-limited spots.Atcies were measured using the experimental setuptheir respective design wavelengths,these focalwhere is the design wavelength,and y areshown in fig.S2.Figure 2A shows a highly sym-the coordinates ofeach nanofin,and fis the focalmetric focal spot that is obtained for the metal-length.This phase profile is imparted via rotationens at its design wavelength 660 nm.The"Harvard John A.Pauson School of Engineering and Appliedof each nanofin at a given coordinate (y)by anvertical cut of the focal spot is also shown in FigScierces,Harvard University.Cambridge.MA 02138.USA.angle 0n(,y)(Fig.IE).In the case of right-2G with adiffraction-limited (full-width athalf-maximum (FWHM)of 450 nm.Figure 2,Bauthor.Email:capasso@seas.harvard.eduand H,show the focal spot of the metalens11903JUNE2016·V0L.3521ssUE6290sciencemag.org SCIENCERESEARCH RESEARCH AR TICLEvery close to the condition for a perfect spherical100wavefront (37).We also calculated the Strehl ra-tio from the measured beam profiles for the80three metalenses at their design wavelengthsand found that they are close to 0.8 (see mate-용rials and methods and fig.S3).consistent withthe observed diffraction-limited focusing.In addi-tion,due to the use of the geometric phase,thephase profile of the metalens is only dependenton the rotation of the nanofins.This is controlledwith very high precision,as is characteristic ofelectron-beam lithography.Altematively,otherhigh-through put lithography methods such as400450500550600650700deep-ultraviolet (UV)can provide similar fabri-Wavelength (nm)cation accuracy.It is important to note that although themetalenses were designed at specific wavelengths,we still observe wavelength-scale focal spots atwavelengths away from the design.For example,for the metalens designed at =532 nm,wewnloadmeasured focal spot sizes of 720 and 590 nm atwavelengths of=660 and 405 nm,respectively(fig.S4).The broadening of the focal spots withfromrespect to the theoretical diffraction-limited val-ues comes from chromatic aberration becausemetasurfaces are inherently dispersive.Chromaticaberrations in our metalens are more pronouncedthan the lenses based on refractive optics,result-ing in a wavelength-dependent focal length (fig.S5A).This is generally not an issue for laser-Fig.1.Design and fabrication of metalenses.(A)Schematic of the metalens and its building block.therelated imaging,microscopy,and spectroscopyTiO2 nanofin.(B)The metalens consists of TiO2 nanofins on a glass substrate.(C and D)Side and topbecause monochromatic sources with narrowviews of the unit cell showing height H.width W.and length L of the nanofin,with unit cell dimensions S x S.linewidths are used.For example,in Raman(E)The required phase is imparted by rotation of the nanofin by an angle 0 according to the geometricmicroscopes/spectrometers,a 532-nm laser withPancharatnam-Berry phase.(F)Simulated polarization conversion efficiency as a function of wavelengtha linewidth of a few picometers is common.InThis efficiency is defined as the fraction of the incident circularly polarized optical power that is convertedthis case,the linewidth-induced broadening ofto transmitted optical power with opposite helicity.For these simulations.periodic boundary conditions arethe focal spot size and change in focal length isapplied at the x and y boundaries and perfectly matched layers at the z boundaries.For the metalensnegligible.designed atd=660 nm(red curve).nanofins have W=85.L =410.and H=600 nm,with center-to-centerWe also measured the focusing efficiency ofspacing S=430 nm.For the metalens designed at =532 nm(green curve).nanofins have W=95.L=the metalenses.As shown in Fig.3A,themetalens250,and H=600 nm,with center-to-center spacing S=325 nm.For the metalens designed at=405 nmdesigned at 660 nm has a focusing efficiency(blue curve).nanofins have W=40.L=150.and H=600 nm,with center-to-center spacing S=200 nm.of 66%,which remains above 50%in most of the(G)Optical image of the metalens designed at the wavelength of 660 nm.Scale bar,40 um.(H)SEMvisible range.Figure 3A also shows the measuredmicrograph of the fabricated metalens.Scale bar.300 nm.focusing efficiency ofthe metalens designed atAd=532 nm.This metalens has a focusing effi-Technoldesigned at the wavelength of 532 nm and its corthat the metalenses provide smaller (~15 times)ciency of 73%at its design wavelength.In addi-responding vertical cut Moreover,this metalensand more symmetric focal spots.This can be un-tion,we measured the beam intensity profile ofdesign can be extended to the shorter wavelengthderstood because conventional high-NA objecthis metalens in the g cross section within aregion of the visible range,which is of great in-tives are designed to image under broadband40-um span around the focal point (Fig.3B).De-terest in many areas of optics,such as lithographytails of this measurement are discussed in theand photoluminescence spectroscopy.Figure 2Cto be corrected for multiple wavelengths over asupplementary materials (35)(see fig.S2 anddepicts the intensity profile of the focal spot fromrange of angles of incidence to meet industrymaterials and methods).The negligible back-the metalens designed at the wavelengthstandards for the required field of view.This isground signal not only demonstrates excellent2022405 nm with a FWHM of 280 nm (Fig.2D).Al-typically implemented by cascading a series ofphase realization,where the beam converges tothough this wavelength is very close to the bandprecisely aligned compound lenses.Fabricationa diffraction-limited spot,but also shows thegap of TiO=360 nm,the absorption loss isimperfections in each individual optical lens andhigh conversion efficiency ofeach nanofin.Forresidual aberration errors,particularly sphericalthe metalens designed at the wavelength ofTo compare the performance of our metalensesaberration,result in a focal spot size larger405 nm,a measured focusing efficiency of 86%with a commercially available lens we selectedthan theoretical predictions (36).In contrastis achieved.The latter measurement was donea state-of-the-art Nikon objective.This objectiveour metalens is designed to have a phase profileusing a diode laser (Ondax Inc.,Monrovia,CA)has the same NA as our metalenses (0.8)and isfree of spherical aberration for normally incidentbecause the shortest wavelength that ourtunabledesigned for visible light.Focal spot intensitylight,which results in a diffraction-limited spotlaser (SuperK Varia)can provide was -470 nm.profiles of the objective at wavelengths of 660at a specific design wavelength(37).For example532,and 405 nm were measured using the samethe theoretical root mean squares of the waveusing right circularly polarized incident light.setup as in fig.S2 (see Fig.2,D to F).A compar-aberration function (WAFRMs)for the metalensesHowever,the polarization sensitivity of the deison of the corresponding focal spot cross sec.designed for 405,532,and 660 nm are 0.0497sign can be overcome by implementing thephasetions in Fig.2,G to I,and Fig.2.J to L.revealsprofile using circular cross section nanopillars inSCIENCE sciencemagorg3JUNE2016·V0L.3521SsUE62901191RESEARCH RESEARCH ARTICLEAB0.6040.4500nm500nm500nmE0.80.60.60.60.40.40402500nm500nm500nmH0.8080.80.6一450nm280nm0.4Downloaded from https://www.science.org at National University of Defensey (um)K0.80.60.6620nm600nm420nm0.40.2Technology on October 20,101y (um)y (um)2022Fig.2.Diffraction-limited focal spots of three metalenses (NA 0.8)532.and 405 nm have FWHMs 450,375,and 280 nm,respectively.Theand comparison with a commercial state-of-the-art objective.(A to C)symmetric beam profiles and diffraction-limited focal spot sizes are relatedMeasured focal spot intensity profile ofthe metalens designed at (A)d=660.to the quality of the fabricated metalenses and accuracy of the phase(B)d=532.and (C)d=405 nm.(D to F)Measured focal spot intensityrealization.(J to L)Corresponding vertical cuts of the focal spots of theprofiles of the objective (100x Nikon CFI 60.NA =0.8)at wavelengths ofobjective.at wavelengths of (J)660.(K)532.and(L)405 nm.FWHMsofthe(D)660.(E)532.and (F)405 nm.(G to I)Corresponding vertical cuts offocal spots are labeled on the plots.These values are-1.5 times as large asthe metalenses'focal spots.Metalenses designed at wavelengths of 660.those measured for the metalenses.which the phase is controlled via changing theirameter D 2 mm and focal length f=0.725 mmshows the image formed by the metalens Thediameters.giving NA 0.8.First,we characterized the im-light source was a tunable laser(SuperK Varia)aging resolution using the 1951 United States Airset at 530 nm with a bandwidth of 5 nm.BecauseImaging demonstrationForce(USAF)resolution test chart (Thorlabs Inc.,the resulting image was larger than our chargeTo demonstrate the use of our metalens for pracJessup,MD)as the target object.The measure-coupled device(CCD)camera,we projected thetical imaging,we fabricated a metalens with di-ment configuration is shown in fig.S6.Figure 4Aimage onto a translucent screen and took its11923JUNE2016·V0L.352 ISSUE6290sciencemag.org SCIENCE