FULL PAPEROPTICAMATERIALCheck foBifunctional Metasurfaceswww.advopticalmat.dupdatesA Highly Efficient Bifunctional Dielectric Metasurface EnablingPolarization-Tuned Focusing and Deflection for Visible LightSong Gao,Chul-Soon Park,Sang-Shin Lee,and Duk-Yong Choiexhibiting exotic material properties thatMultifunctional metasurfaces have attracted drastically growing interest,do not exist in nature,offers more flex.serving as a prominent candidate to cope with both device miniaturizationibility in manipulating electromagneticand integration.Conventional transmissive bifunctional metasurfaces workingwaves in an elegant way and providingfor visible light inherently suffer from several demerits owing to the use ofunprecedented phenomena,such as nega.geometric phase and spatial multiplexing scheme.Research endeavors aretive refractive index,▣perfect lensing,.☑and cloaking.!3 Yet,the metamaterialmainly devoted to the implementation of static devices.In this work,a highlyis a composite material,which typicallyefficient bifunctional dielectric metasurface,taking advantage of a unit cellinvolves layered structures,requiring(UC),the building block of which capitalizes on a nanopost in hydrogenatedrelatively complex fabrication processes;amorphous silicon,is proposed and experimentally demonstrated totherefore,in practice,it is not popular forsuccessfully enable polarization-mediated anomalous beam deflection anduse in device miniaturization.A metasur.face,which is regarded as a 2D metamate.focusing in the visible band.Through the tailoring of the UC periodicity,itrial,has recently emerged as a prominentis found to be capable of efficiently tuning the beam deflection angle andalternative to the metamaterial becausefocusing distance.For the fabricated sample,the normal transverse-electricof its ability to cope with the aforemen.incidence within a spectral band from 600 to 715 nm is angle-resolved to ationed issues.An optical metasurfacesingle diffraction order via the beam deflection,while a bright line focus isis particularly supposed to engage sub.wavelength-spaced metallic or dielectricattained at the target focal plane for the normal transverse-magnetic incidencenanostructures (meta-atoms),whose size,at=650 nm.This work will initiate a positive prospect for the developmentshape,and orientation can be adjustedof high-performance tunable multifunctional metasurfaces.to facilitate the manipulation of the lightamplitude,phase,and polarization.Inlight of these salient features,a plethora1.Introductionof ultrathin optical devices,induding beam deflectors,meta.lenses,6.71 waveplates,and metaholograms,were exten.Considering the fact that the light wavefront is mediated bysively studied.the accumulated phase shift over a relatively long propaga.So far,the majority of the invented metasurfaces have exhib.tion distance,conventional optical devices have inevitableited a single,predetermined function.Because of the ever.growing demand for improving the capacity for data storageplex shape,and limited performance.Thus,they are neitherand information management,it is highly desirable to have ain compliance with device miniaturization nor suitable to besingle device with versatile functions.This was recently madedeployed in integrated photonic systems.A 3D metamaterial,possible by the development of a metasurface in which mul.tiple functions can be realized by controlling the polarization,S.Gao,C.-S.Park,Prof.S.-S.Leewavelength,and incidence angle.For instance,by varyingDepartment of Electronic Engineeringthe helicity of incident circuladly polarized (CP)light,two dis.Kwangwoon Universitysimilar holograms can be reconstructed based on a single meta.Seoul 01897,South Koreasurface that contains two sets of merged hologram data.AsE-mail:slee@kw.ac.krDr.D-Y.Choiencoded in the two orthogonal polarizations,such a bifunc.Laser Physics CentreResearch School of Physics and Engineeringtional metasurface may find applications in data storage,Australian National Universityinformation encryption,and anti-counterfeiting.A monolayerCanberra,ACT 2601,Australiaof gap-plasmonic metasurface can deflect and focus normallyDr.D-Y.Choiplanes,respectively,depending on the linear polarization ofJinan Universityincident light.Previous metasurfaces have offered iden.Guangzhou,Guangdong 510632,Chinatical or similar functions,but another category of multifunc.can be found under https://doi.org/10.1002/adom.201801337.tional metasurface,which can perform dissimilar functions,isdesperately awaited for the advantages it will offer in tems ofD0t10.1002/adom.201801337the device integration.R Such metasurfaces could be utilized1801337(1of9)2019 WILEY-VCH Verlag GmbH Co.KGaA,WeinheimDADVANCEDSCIENCE NEWSMATERIALSwww.advancedsciencenews.comwww.advopticalmat.deto substantially reduce the elements'footprint and thus inte.splitting metadevices,whose efficiency can be predicted by agrate multiple imaging systems or optical communicationfew meta-atoms,the beam deflection efficiency of the proposedbifunctional metasurface is critically dictated by the efficiencyvisible bands.24-26]Based on the dielectric/metal-dielec.of every single meta-atom.The phase delays associated withtric-metal configurations,however,they are mostly limitedeach nanopost are further examined and found to exhibit insig.to operating in reflection mode.24-261 Transmissivebifunc.nificant reliance on the periodicity of the UC.The inspectiontional metasurfaces were reported in the microwave,22.231of the tightly confined optical fields inside the nanoposts helpsterahertz,2 midinfrared,s near-infrared,and visiblesupport the comprehension of the underlying mechanism.regimes.In particular,the visible bifunctional plasmonicThe UC periodicity can be used to concurrently and efficientlymetasurfaces count on the Pancharatnam-Berry(P-B)phase soadjust the angle of light deflection and focusing distance.Foras to manipulate the wavefront of CP light.Despite its intrinsica manufactured bifunctional metasurface,the performance haswavelength-independent or dispersion-free characteristics,been meticulously inspected in temms of the beam deflectionleading to the broadband operation,the P-B phase is deemedand focusing,by monitoring the angular distribution of the farto be susceptible to several limitations.For a metasurface capi.field intensity and the light intensity images at different posi.talizing on the P-B phase,the incident CP light is known totions from the sample.The reliability of the proposed approachbe scattered into waves of both the same and opposite polariza.is proven by the excellent consistency between the simulationtions.11 The abrupt phase shift useful for wavefront shapingand measurement resultscan be readily accomplished by simply rotating the orientationof the meta-atoms.However,it is solely effective for the CPlight with the opposite polarization.The useful transmitted2.Results and DiscussionCP light corresponding to the opposite polarization results ina limited polarization conversion efficiency of 25%.321 BecauseTo achieve disparate wavefront modulations,a bifunctionalthe metasurface is formed via a spatial multiplexing of doublemetasurface is required to efficiently impart two distinct spa-metasurfaces delivering distinct functionalities,a functionaltial phase distributions without incuming profound functionalcrosstalk may potentially arise because one function tends tocrosstalk,which means that one function that is on duty is rarelyadd background noise to another.adversely affected by the other function that is off duty.TheIn addition to the progress in the development of a mul.conventional advanced transmissive bifunctional metasurface,tifunctional metasurface,whose functional roles are "static,"operating in the visible spectral band,chiefly relied on a spatiala colossal amount of efforts has been devoted to the creationmultiplexing scheme,and,thus,it was inevitably susceptible toof tunable or reconfigurable metasurfaces for manipulatingfunctional crosstalk and limited efficiency.Our bifunctionalthe electromagnetic wave in a tunable manner.This has beenmetasurface has been conceived based on the fact that an indiachieved by applying external stimuli,such as the electric biasvidual dielectric meta-atom induces flexible phase delays uponvoltage,]heat,and elastic force,to the metasurfacesincident light depending on the polarization.5 It is noted thethat incorporate a transparent conducting oxide,phase-changecrosstalk issue may be mitigated by an off-axis illuminationmaterial,and elastic material,respectively.The former twoapproaches were substantiated in the spectral band covering thesurface is illustrated in Figure 1a.For incident light with themillimeter waves,mid.,and near-infrared.The third scheme iselectric field aligned parallel to the x-axis (transverse-electriccurrently thought to be the most feasible way of producing a(TE)polarization)or yaxis(transverse-magnetic (TM)polariza.tunable metasurface for wavefront shaping in the visible band.tion),the metasurface acts as an anomalous beam deflector andNonetheless,the relevant stretchable metasurface relies on thecylindrical lens,respectively.The UC of the proposed metas.use of the P-B phase,suffering from its disadvantages includingurface consists of a rectangular a-Si:H nanopost atop a siliconthe limited operation for cross-polarized CP light with low effi.dioxide(SiO2)substrate,as shown in the magnified schematicciency,as mentioned above.414 To date,all the proposed tun.in Figure 1a.With regard to the UC,the optical phase shiftimposed by the Si nanopost is expected to fulfill a 2 span,In this paper,we propose and develop a highly efficientusually through the tailoring of its cross-sectional dimensionbifunctional dielectric metasurface that renders polarization.Toward that end,with the assistance of the finite difference timetuned beam deflection and focusing in the visible regime.domain (FDTD)method (FDTD Solutions,Lumerical,Canada).The meta-atom constituting the metasurface unit cell (UC)isthe transmission amplitude and phase distributions are exam-made of a hydrogenated amorphous silicon (a-Si:H)nanopostined as a function of the widths of the nanopost,including din a rectangular shape,imparting polarization-selective phaseand d,The proposed metasurface is designed to work at a vis-shifts for implementing the two functions.For that purpose,ible wavelength of=690 nm.The period of the UC is given bywe first explored the relation between the geometric dimen.P=240 nm (=0.35o),while the a-Si:H nanopost features a sub.sion of the nanopost and the polarization-dependent phasewavelength thickness of t 320 nm.As displayed in Figure 1b.c.shifts.For the constructed bifunctional metasurface,a theoret.it is implied from the calculated transmission amplitude (Ar)ical analysis shows that,upon normal incident light with theand phase (that by selecting a collection of nanoposts withdesignated polarizations,not only anomalous beam deflectionproper dimensions,a full phase tuning of 2n can be obtainedbut also light focusing,exhibiting a high-efficiency and negli.for the TE incident polarization in a highly efficient manner.gible functional crosstalk,can be visualized through the simu.Similarly,the results are explored for the TM incident polariza-lated field profiles.As against conventional polarization beamtion,as detailed in Figure S1 in the Supporting Information.In18013372of9)2019 WILEY-VCH Verlag GmbH Co.KGaA,WeinheimADVANCEDADVANCEDOPTICALSCIENCE NEWSMATERIALSwww.advancedsciencenews.comwww.advopticalmat.deaBeamUnit cellb220Beam Focusing0.9000087650.40.360100140180220d,(nm)22018y1401TE60100140180220d,(nm)Figure 1.a)Schematic of the proposed highly efficient bifunctional metasurface enabling polarization-tuned focusing and deflection for visible light.Shown on the right side is the metasurface UC,incorporating the a-Si:H nanopost.b,c)Calculated amplitude (b)and phase distribution (c)as a functionof the cross-sectional dimension of the nanopost for TE polarization at the designed wavelength of=690 nm.light of the rectangular shape of the nanopost,the profiles ofpolarization-controlled beam deflection and focusing,thusthe transmission amplitude and phase corresponding to the TEdemonstrating that a bifunctional metasurface capitalizingand TM polarizations are in mirror symmetry with respect toon the proposed nanoposts is empowered to lead to two dis.the diagonal line for d=d51 To establish the required phasetinct wavefront modulations.Figure 2a displays a portion ofprofiles relating to the TE and TM cases,a conventional eight.the nanopost-based metasurface.A group of 208 nanoposts inlevel phase modulation,which is appropriate for enabling wave.each column,arranged along the xaxis,may be regarded asfront manipulation,has been chosen.Eventually,64 nanopostsa supercell,delivering a hyperbolic phase profile for TM inci.with varying widths are employed to arbitrarily derive a polari.zation-sensitive wavefront(Figure S1,Supporting Information).to invoke a line focusing for the TM case.With regard to theA bifunctional metasurface involving an array of 208beam deflection,which is achievable for the TE polarization,by 208 meta-atoms has been suggested to accomplishthe supercell may be treated as comprising 26 sub-supercells,abTheorySimulation6SupercellReal(E)51531015-Max-24-16-8081624-24-16-8081624x (um)TheorySimulation625Max20434 umN10500-24-16-8081624-24-16-8081624x (um)x (um)Figure 2.a)Schematic showing the arrangement of the nano posts in the designed bifunctional metasurface.b,c)The theoretical and simulated phaseprofiles for:b)beam deflection under normal TE incidence and c)beam focusing under normal TM incidence.d)The x-component of the transmittedelectric field under normal TE incidence.e)The y-component of the transmitted field intensity under normal TM incidence.18013373of9)2019 WILEY-VCH Verlag GmbH Co.KGaA,Weinheim