Optical properties of metasurfaces infiltrated withliquid crystalsAndrew Lininger1,Alexander Y.Zhu.1,Joon-Suh Park,Giovanna Palermod,Sharmistha Chatterjeed,Department of Physics,Case Western Reserve University,Cleveland,OH 44106;PJohn A.Paulson School of Engineering and Applied Sciences,HarvardUniversity,Cambridge,MA 02138;Nanophotonics Research Center,Korea Institute of Science and Technology,02792 Seoul,Republic of Korea;and CNR-NANOTEC Istituto di Nanotecnologia,Department of Physics,University of Calabria,87036-Rende,ItalyContributed by Federico Capasso,June 22,2020 (sent for review April 7,2020;reviewed by Alexandra Boltasseva and I.C.Khoo)Optical metasurfaces allow the ability to precisely manipulate themolecular reorientation which is responsible for refractive indexwavefront of light,creating many interesting and exotic opticalchanges of the LC (26,27).Recent studies on reconfigurablephenomena.However,they generally lack dynamic control overoptics created with conventional sandwiched LC cells with one oftheir optical properties and are limited to passive optical elements.the two plates coated with a metasurface have been reportedIn this work,we report the nontrivial infiltration of nanostruc-(28-30).Opposed to these implementations involving a bulky LCtured metalenses with three respective nematic liquid crystals ofcell,we propose in this work to harness the wetting properties ofdifferent refractive index and birefringence.The optical propertiesthe metalens to replace the air between the planar nanostructuresof the metalens are evaluated after liquid-crystal infiltration towith optically active birefringent and viscoelastic LCs.Thisquantify its effect on the intended optical design.We observe aimplementation allows for controlling phase and amplitude dis-significant modification of the metalens focus after infiltration fortribution in the metalens plane,thereby limiting the opticaleac liquid crystal.These optical changes result from modificationchanges that are unavoidable with thick LC slabs laying aboveof local refractive index surrounding the metalens structure afterthe metasurface.By harnessing the infiltrated LC optical prop-infiltration.We report qualitative agreement of the optical exper-erties the transmitted field can be significantly modified,andiments with finite-difference time-domain solver (FDTD)simulationfurthemore potentially controlled.results.By harnessing the tunability inherent in the orientation de-pendent refractive index of the infiltrated liquid crystal,the metal-Metalens Infiltrationens system considered here has the potential to enable dynamicPhenomenology.Infiltration of the metalens system has been in-reconfigurability in metasurfaces.vestigated for three different common thermotropic rod-shapednematic LCs:MBBA (Sigma-Aldrich),E7 (Merck),and BL009optical metasurfaces liquid crystal reconfigurable metasurface(Beam Co.).As we expect the lensing ability in our metalenssystem to be mainly affected by the refractive index of the in-filtrate material,these NLCs have been chosen to represent awide range of effective refractive index (n)and birefringence (n).the wavefront of light on an unprecedented scale (1-6).TheseThe presence of long-range order in the LC means that for fixeddevices offer control over the phase,amplitude,and polarizationlight propagation direction the NLC acts as a material with astate of the wavefront traversing the structured plane via thelocal interaction of light with metaatoms arranged at the nano-Significancescale(7-12).With current fabrication techniques it is possible toengineer phase,amplitude,and polarization landscapes,allowinglocalized control of the scattered field and molding the flow ofNanostructured metamaterials have been engineered to gen-light to create optical effects which are unparalleled in naturalerate a wide range of optical phenomena,allowing an un-materials (13,14).This technology has shown promise as aprecedented control over the propagation of light.However,radical change in the form-function relationship compared withthey are generally designed as single-purpose devices withoutconventional refractive optical elements (15-18).a modifiable optical response,which can be a barrier to appli-Most engineered metasurfaces have a prescribed geometrycations.In this work,we report the nontrivial infiltration of awhich has been designed to fulfill a single functionality,and asnanostructured planar silica metalens with nematic liquid crystals.such these devices are necessarily passive optical elements.ThisWe then demonstrate a measurable change in the metalens op-tical response after infiltration.Since the orientation-dependentpresents a barrier to potential application where differing opticaloptical properties of liquid crystals can be controlled with exte rnalresponses may be necessary.The opportunity to enable reconfi-stimuli,this technology could potentially enable dynamic controlgurability in optical materials through the application of externalof the metalens optical responsestimuli has been a longstanding goal of photonics.In recent years,there have been numerous attempts to design reconfigurable sysAuthor contributions:F.C.and G.5.designed research:A.L.A.YZ J.-5.P.,G.P..S.C.andAq o'srud'MMw//:sdnytems,including mechanical (11,19),thermal (20-23),and externalJ.B.performed research:A.L.A.Y.Z,J.5.P.G.P.S.C.,J.B.F.C.,and G.5.analyzed data;voltage-based approaches (24).In this work,we report the infil-and A.L.A.YZ.J.-5.P.F.C.and G.S.wrote the paper.tration of nanopillared planar metasurfaces with various nematicliquid crystals (NLCs)by harnessing the wetting properties of theThe authors dedare no competing interest.metasurface.This infiltration is explained by a combination ofPublished under the PNAS license.competing forces,namely the capillary and the resisting hydrody-A.L.and A.YZ.contributed equally to this worknamic forces (25).Since the LC is a birefringent complex fluid,2F.C.and G.S.contributed equally to this work.wetting of the metasurface induces a modification of the refractiveTo whom correspondence may be addressed.Email:capasso@seas.harvard.edu orindex map with local and global order,in tum modifying the phaseand amplitude of the transmitted electromagnetic field.It is well known that LCs respond to extemal stimuli(e.g.,electricdoi:10.1073/pnas.2006336117//D CSupplementalfield,magnetic field,temperature,strain,etc.,)by undergoing aFirst published August 10 2020.20390-20396|PNAS|August25,2020|vol.117|no.34www.pnas.org/gi/doi/10.1073/pnas.2006336117Table 1.Infiltrated LC optical propertiesWe have characterized the infiltration process in our metalensLCnone△nsystem through measurements of the contact angle and scanningelectron microscopy measurements of the metalens geometricalMBBA1.561.680.12parameters (39).Based on these measurements,our wettingE71.521.740.23model predicts that the metalens structure will favor infiltrationBL0091.591.870.28of the selected LCs.We predict an equilibrium stable film fullyinfiltrating the metalens with the film height at the pillar height(2 um).This analysis assumes a regular array of flat-topped cy-modifiable local birefringence.Refractive index data at roomlindrical pillars (measured sidewall angle at 2.8).In reality thetemperature are given in Table 1 (31).The wetting of nano-pillar spacing and size is highly nonunifom throughout thepatterned surfaces is in general nontrivial and heavily dependentmetalens,as can be seen in Fig.1.This nonuniform geometry canupon the geometrical properties of the patterned array and hy-give rise to complex infiltration behavior which is difficult todrophilic attraction between the liquid and the substrate mate-predict on the scale of the metalens,and which is not containedrial,as well as viscous properties of the wetting liquid (25).Aswithin the simplified model.such,the three NLCs were chosen for their predicted infiltrationproperties,as explained below.Experimental Infiltration and Evaluation.Infiltration of the metal-A droplet of wetting liquid placed on top of a nanopatternedens has been observed with polarized optical microscopy byarray typically remains in at equilibrium,either in the Cassie-Baxtercomparing optical transmission in the infiltrated and unin-state (32).by forming a droplet on the surface without wetting thefiltrated cases.Concurrent observation of the infiltration understructure,or the Wenzel state (33),by displacing the air infiltrationparallel polarization(0 polarizers)and crossed polarization(90and filling the structure below the droplet.However,there exists apolarizers)gives information about both the state of infiltrationthird state in which the liquid exceeds the equilibrium Wenzeland local alignment of the LC.In particular,when the lens isstate,propagating an infiltrating film around the droplet andobserved between parallel polarizers the change in transmittedthroughout the microstructure,leading to a full infiltration of thelight intensity indicates a change in LC film thickness.However,nanopatterned array with the wetting material (34,35).See Fig.2Fwhen viewed between crossed polarizers,bright and dark regionsfor an illustration of this case where the fully infiltrated andshow different levels of molecular alignment in the LC.wetting layers are shown during the infiltration process.This stateThe infiltration process for the metalens system is illustratedhas been previously observed in nanopatterned structures infil-in Fig.2.Three subsequent images during infiltration with MBBA.trated with conventional fluids (36-38).observed under parallel polarizations,are seen in Fig.2A-C.TheThe infiltration state arises as the system responds to the in-fully infiltrated and wetting regions are clearly distinguished fromtroduction of a liquid droplet by attempting to minimize the free-the uninfiltrated region by a decrease in transmission intensityenergy differential in the air-liquid-substrate system.Free en-with increasing film height.A boundary line between these regionsergy is a function of the interfacial energy in the air-liquid,has been added for clearer distinction.Between the secondliquid-substrate,and air-substrate interfaces,which are depen-(Fig.2B)and third (Fig.2C)panels there is a clear circumferentialdent upon the substrate and infiltrating material parameters(25,progression of the wetting front around the metalens,which is34).A more extended theoretical discussion about our currentindicative of the observed infiltration.understandings of the infiltration process has been provided(SIThe infiltration of another optical metasurface composed ofAppendix,Fig.S1).B100μm10 umFig.1.SEM images of the metalens at different magnification;metalenses are composed of an array of nanopillars with controlled diameters.Top view ofthe (A)metalens center and (B)metalens outer edge.(C)Tilted and zoomed view of largest pillars on the edge of the second ring,and (D)a tilted and zoomedview of the same area.Lininger et al.200mUn-infitratedUn-infitratedUn-infitmatedcontact angle.LC infiltratedGUn-infitratedUn-Infitrated5001mFig.2.Characterization of LC infiltration in the metalens system.(A-C)Progression of LC(MBBA)infiltration for the metalens(1 cm diameter,glass on glass)system in parallel polarization:(A)uninfiltrated,and (B)partially infiltrated.(C)Infiltrating film progresses circumferentially throughout the structure.(D andE)LC (MBBA)infiltration for a structured substrate composed of TiOz pillars.The film progresses from D to E at full infiltration outward from the initialcontact point without a visible wetting layer,still resulting in full infiltration.Here the same infiltrate LC leads to different wetting behavior based on thesubstrate properties.(F)lllustration of LC infiltration into a regular array of the metalens'SiO2 pillars.The wetting layer is seen preceding full infiltration.Thecontact angle is measured at the edge of the infiltrated film droplet.(G-Optical analysis of metalens central region in crossed polarization duringthe infiltration process:(G)uninfiltrated,(H)partially infiltrated,(fully infiltrated.for reference.In this case the infiltration appears as an increasebrighter during infiltration due to the disordered alignment of thein intensity under parallel polarization,but the infiltrated regionLC.The final LC orientation is strongly disordered,with self-and propagating front are still clearly visible.Inclusion of the TiOaligned domains extending over several rings.metasurface is intended to show that certain other structures andDuring infiltration,the LC tends to fully infiltrate circumfer-materials can be infiltrated using a similar technique,leading toential rings in the metalens before moving radially inward.Wedifferent optical results after infiltration.This can be incorporatedbelieve this effect is due to the radial nonuniformity present inin a wider range of designed optical response and potentiallythe metalens microstructure,such that the liquid experiencesreconfigurable devices.energetically favorable infiltration in the relatively wide channelsA similar progression can be seen under cross-polarization forwhich becomes more difficult in the densely pillared regions.Thethe metalens infiltration in Fig.2 G-/.For the initial unin-filtrated lens,the densely pillared regions are clearly visible whiledisparity in ease of infiltration for the two regions is reflected inthe open rings appear dark.This image also shows a clear mal-the infiltration dynamics,leading to the observed behavior(42).tese cross resulting from conoscopic interference in the focalThe NLC equilibrium state following full infiltration has beenplane.The conoscopic cross shows the variation,as function of theobserved for each of the sampled LCs.We have characterizedazimuthal angle,of the radiation intensity transmitted through theboth the local LC alignment and the state of filling following anpolarizer-metalens-analyzer.The isochrome circles with the min-imum or maximum intensity of the transmitted light correspond tocross-polarization intensity,as discussed above.A typical equilibriuma definite phase difference between the ordinary and extraordi-infiltration state,observed for MBBA in cross-polarization,is shownnary rays.In particular,when the phase difference is equal tomultiples of 2x,it results in a particular interference patterncase the LC alignment is significantly disordered.This can beknown as maltese or conoscopic cross(26,41).The infiltrationexpected from the complex boundary interaction with the metalensproceeds in Fig.2 H and I,leading to full infiltration of thewithout including an alignment medium for controlling this interac-metalens.In this case the fully infiltrated regions are visiblytion.It is important to control the infiltration orientation state as a20392 www.pnas.org/cgi/doi/10.1073/pnas.2006336117Lininger et al.