Review ArticleADVANCEDPHOTONICSTunable metasurfaces towards versatilemetalenses and metaholograms:a reviewJaekyung Kim,*t Junhwa Seong,"t Younghwan Yang,t Seong-Won Moon,"Trevon Badloe,"andPohang University of Science and Technology,Department of Mechanical Engineering,Pohang,Republic of KoreaPohang University of Science and Technology,Department of Chemical Engineering,Pohang,Republic of KoreaPOSCO-POSTECH-RIST Center for Flat Optics and Metaphotonics,Pohang,Republic of KoreaAbstract.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 throughthe integration with mechanically moving components and electrically tunable elements.Two interestingapplications,in particular,are to vary the focal point of metalenses and to switch between holographicimages.We present the recent progress on tunable metasurfaces focused on metalenses andmetaholograms,including the basic working principles,advantages,and disadvantages of each workingmechanism.We classify the tunable stimuli based on the light source and electrical bias,as well asothers such as themmal and mechanical modulation.We conclude by summarizing the recent progress ofmetalenses and metaholograms,and providing our perspectives for the further development of tunablemetasurfaces.Keywords:tunable metasurface;active metasurface;reconfigurable metasurface;multifunctional metahologram;varifocalmetalens.Received Aug.31,2021;revised manuscript received Dec.10,2021;accepted for publication Dec.28,2021;published onlineMar.7,2022.The Authors.Published by SPIE and CLP under a Creative Commons Attribution 4.0 International License.Distribution orreproduction of this work in whole or in part requires full attribution of the original publication,including its DOl.[D0t10.1117/1.AP.4.2.024001)1 Introductionoptical applications have been implemented,such as beamsplitters,20-2 absorbers,metalenses,metaholograms,40Metasurfaces are composed of two-dimensional (2D)periodicarrays of subwavelength-scale artificial elements,called meta-atoms.They have attracted great attention due to their abilitytural color.47-52The functionality and efficiency of metasurfaces have beento manipulate the properties of electromagnetic waves.-scontinuously increased by improving the methods to designSeveral design methods have been proposed with various shapesand compositions of meta-atoms.The Pancharatnam-Berrymeta-atoms,and the development of their material composition.Achromatic metalenses have been fabricated using complex(PB)phase,also called geometric phase,has been investigatedgeometric-structured meta-atoms that have a wide phase-using rectangular-shaped meta-atoms that impart a phase delaydispersion set,and therefore enable achromatic focusing withproportional to their rotation angle.Propagation phase hassingle-layered metasurfaces.Furthermore,complex-amplitudebeen investigated by exploiting an effective refractive index tometaholograms have been proposed by varying the conversionmanipulate the retardation phase by changing the volume ratio,aspect ratio,and height of meta-atoms.Resonant effects suchefficiency of meta-atoms to enable three-dimensional (3D)as plasmonic resonance,Mie resonance,and Fabry-Perotimages.ss5 In terms of the materials,the use of resin withresonance have also been exploited.By exploiting these re-embedded nanoparticles has been proposed as a methodto achieve mass production of dielectric metasurfaces throughsources,optical elements can be highly miniaturized and varioussingle-step direct nanoimprinting.Low-loss hydrogenatedamorphous silicon has been proposed for low-cost depositionAddress all correspondence to Junsuk Rho,jsrho@postech.ac.krof visibly transparent thinfilms,and the fabrication cost is muchThese authors contributed equally to this work.lower and its modulation efficiency is compatible with that ofAdvanced Photonics024001-1Mar/Apr 2022.Vol.4(2)Kim et al.:Tunable metasurfaces toward versatile metalenses and metahologramstitanium dioxide (TiO)and gallium nitride(GaN)metasur-where r=Vx2+y2 is the radial distance from the center to eachfaces that work at visible frequencies.point,f is the focal length,and A is the wavelength.The distanceTunable metasurfaces with multiple functionalities throughbetween the focal point and a point on a metasurface varies with rthe active control of electromagnetic waves have been activelyso as to correct for the distance difference.and the meta-atomsstudied.3162 Tunable metasurfaces are made up of meta-should satisfy the phase retardation [Eq.(1)to achieve construc-atoms that are controlled by external stimuli such as electricaltive interference at the focal point.To design tunable metalensesbiases or high-intensity light sources.Electrical,themmal,andmeta-atoms must have properties that can change their opticalmechanical stimuli have been used to induce two or more opti-responses so that they satisfy Eq.(1).cal responses in single-or double-layered metasurfaces.Also,To achieve tunable metalenses,some mechanisms such asmanipulation of the polarization state of the incident light thathelicity or spin sensitive geometric phase,liquid crystals(LCs)changes the output wavefront has been used to provide tunableor graphene-integrated lenses,and PCMs or stretching methodsfunctionalities for metasurfaces.Tunable metasurfaces providehave been used.Active materials such as LCs or graphene canmultiple functionalities,however,generally have limitations inbe integrated with metalenses to modulate the phase profilethat the efficiency is generally worse than conventional passivefor achieving focus tuning.Their electrically controllable char-metasurfaces,due to inherent problems such as the properties ofacteristics such as different alignment (LC)and Fermi level andtunable materials and design principles.carrier density (graphene)can be used,depending on the exter-In this review.we define "tunable metasurface"as one thatnal electric field.Moreover,modulating refractive index usingcan induce two or more optical responses due to variations inPCMs can be used in achieving tunable metalenses.Finally,athe incident light,or to changes to the meta-atom configurationmethod of changing the geometrical parameters of the metalensor relative distances between two adjacent metasurfaces.through stretching flexible substrates can be used.Additionally,we present recent advances in tunable metasurfa-ces,in particular,tunable metalenses and metaholograms.Tunable metalenses and metaholograms are important applica-2.2 Tunable Metalenses by Light Sourcetions of tunable metasurfaces.In the case of tunable metalenses,Tunable metalenses can be realized to control properties ofthere is an advantage that it can be applied to an ultrathin zoomlight sources such as the polarization state.Spin-decoupledlens that can replace bulky optical components required formetalenses have been achieved using PB phase to integrate theconventional optical devices.Meanwhile,owing to the highproperties of multiple convex and concave lenses into one meta-capacity of tunable metaholograms,it is expected to be one ofsurface:one phase profile focuses left-circularly polarizedthe fundamental technologies of future metasurface integrated(LCP)incident light,while the other profile focuses right-circu-devices,such as holographic memory devices and ultrahigh-larly polarized (RCP)light.Therefore,the focal point changesdensity display applications.However,once the fabricationwhen the polarization of incident light changes [Fig.1(a)].is undertaken,it is difficult to change the period and size ofAdditionally,the intensity of multiple focal points can be tunedthe meta-atoms.In addition,complete modulation principlesby controlling the ellipticity of incident light [Fig.1(b)].Usingcapable of nanoscale local pixel control have not been estab-only the geometric phase,it has the simplicity of designinglished.a spin decoupled metalens instead of using both the propagationIn this review,we first briefly introduce the fundamentals ofphase and geometric phase because of no need for scanning lotsmetalenses and metaholograms.Most tunable optical responsesof parameters.However,the proposed metalens has a low effi-are obtained by controlling the light source or through an ap-ciency of <50%in theory.plied voltage,so we classify tuning methods as (1)controllingMultiple focal points can be generated by controlling thethe light source,(2)electrical tuning,and (3)non-electricalcircular polarization state of incident light.Helicity-dependenttuning.Non-electrical tuning includes heat-induced phasemultifocal metalenses can create multiple focal points inchange materials (PCMs),mechanical deformation,and changesdifferent directions when the polarization of the incident lightof a relative position of cascaded metasurfaces.Finally,we sum-changes.34 These metalenses are composed of anisotropic rodsmarize the overall contents,and suggest future directions ofhat have different orientations and can be considered as halfresearch on tunable metasurfaces.waveplates with a high efficiency.Its polarization-conversionefficiency is ~97%at 0.64 THz under LCP illumination.2 Tunable MetalensesFurthermore,a spin-selected metalens that has a 0.98 numeri-cal aperture (NA)value (simulated data)can focus incident light2.1 Design Principles of Metalensesat two focal points depending on the spin state of the incidentConventional refractive and diffractive lenses have a tradeoff re-light.It is composed of a unit structure of silicon nanobricks,lationship between miniaturization and optical characteristics.and the desired phase profile is implemented by the PB phase.For example,to achieve achromatic focusing.several diffractiveTwo silicon nanobricks (red and blue)on the metalens act as aor refractive lenses must be used,but it can be achieved usingconvex lens or a concave lens when the spin state of the incidentsingle-layer metasurfaces.s To design metalenses,the desiredlight changes [Fig.1(c)].This spin-selected metalens is usefulphase profile should be physically constructed using meta-atoms.for applying detecting techniques and spin controlled photonics.To focus an incident plane wave at a lens focal point,the targetThe focal point can be adjusted by combining the PB phasephase at a point P(x,y)on a metalens should satisfy the phaseand the propagation phase.5 One spin-multiplexed metalensretardation.uses the PB phase and propagation phase of TiOz nanorods.7It makes a polarzation-independent hyperbolic phase and apolarization-dependent linear phase,depending on the polariza-tion state of the incident light,and therefore has different focalpoints for LCP and RCP light [Fig.1(d)].The diameter and NAAdvanced Photonics024001-2Mar/Apr 2022.Vol.4(2)Terms of Use:https:/www.spiedigitallibrary.org/terms-of-useKim et al.:Tunable metasurfaces toward versatile metalenses and metahologramsRCP(d)Fig.1 Tunable metalenses by light source.(a)Schematic illustration of spin-decoupledmetalenses.50(b)Simulated and experimental electric field intensity distributions at line y=-1.5 mm for multiple polarization states of incident light(LCP,LECP,LP,RECP,RCP from leftto right).(c)Schematic of spin selective metalenses that can focus RCP and LCP incident beamsto different focal points.70(d)Schematic view of metalenses using the combination of the PB phaseand propagation phase."1(e)Schematic of step-zoom metalenses in which the focal length ischanged according to the linear polarization of the incident light.72(f)Schematic of metalensesdoublet that has different functions depending on the polarization of the incident light.73of this lens are 1.8 and 0.05 mm,respectively.Although the NAtwo lenses are used in tandem to implement a three-functionis low,this lens demonstrated diffraction-limited focusing.lens doublet by varying the circular polarization state of theFurthermore,TiO2 is used to obtain high modulation efficien-incident light and the distance between the lenses [Fig.1(f)]cies in the visible band by exploiting its low loss extinctionThis metalens doublet has the advantages of making the imag-coefficient and high refractive index.This metalens has an ad-ing system simple and compact because no additional opticalvantage of high focusing efficiency (maximum of 70%).But it iscomponents are required for the multifunctional system.more complex for designing a metalens;it is a tradeoff relation-Therefore,it has great promising perspectives for applicationsship between efficiency and designing simplicity.in portable imaging systems.A step zoom metalens that has dual focal lengths and0.21 NA value has been demonstrated using double-sidedmetasurfaces.?2 These metasurfaces are composed of an array2.3 Tunable Metalenses by Electrical Biasof silicon nanobricks,and the desired phase is obtained byElectrically tunable metalenses can be realized by applyingchanging the lengths of their long and short axes.Under x po-an extemal voltage bias on active materials,such as LCs7-larized light,the first metasurface operates as a concave lens,and graphene.Transmission-type terahertz metalenses thatand the second one operates as a convex lens.In contrast,undercombine dielectric metasurfaces with photopatterned LCs havey-polarized light,both metasurfaces operate as convex lensesachieved tunable chromatic aberration.s When the voltage bias[Fig.1(e)].Consequently,focal lengths vary depending onis applied to the LCs,their geometric phase modulation van-the linear polarization state of incident light.This double-sidedishes,and only the resonant phase in the metalenses remains,metasurfaces design technique has advantages such as compact-so the function of the device changes from achromatic to dis-ness,simplicity,and flexibility;and it has great potential forpersive focusing [Fig.2(a)].By integrating two functions intoapplications in biomedical sciences,optical communications,one metalens,it has great promising perspectives for applica-and wearable electronics.tions in spectroscopy and imaging systems.Additionally,a metalens doublet that has different functionsA varifocal metalens that switches between NA 0.21 and 0.7depending on the polarization of the incident light and the dis-(simulated data)has been obtained by putting twisted nematictance between two lenses has been reported."3 The first 0.258(TN)LCs under a metalens substrate.Depending on theNA metalens is composed of TiO2 nanocylinders with differentvoltage applied to the electrode,the TN LCs convert thediameters to implement the propagation phase.It is thereforepolarization state of the incident light,and achieve differentpolarization-independent.The second metalens has an NA offocal points for different polarization states of incident light0.66 and is composed of TiO2 rectangular meta-atoms to imple-[Fig.2(b)].Using the combination of a metalens and TN LCs,ment the PB phase,making it a polarization-dependent lens.Theit has advantages of high image quality and fast response timeAdvanced Photonics024001-3Mar/Apr 2022.Vol.4(2)