LLithium Niobate Metasurfaces
Bofeng Gao, Mengxin Ren, a) Wei Wu, Wei Cai, Hui Hu, and Jingjun Xu b) The Key Laboratory of Weak-Light Nonlinear Photonics, Ministry of Education,School of Physics and TEDA Applied Physics Institute, Nankai University, Tianjin 300071,P.R. China School of Physics and Microelectronics, Shandong University, Jinan, Shandong 250100,P.R. China
Lithium niobate is a multi-functional material, which has been regarded as one of the most promising platformfor the multi-purpose optical components and photonic circuits. Targeting at the miniature optical compo-nents and systems, lithium niobate microstructures with feature sizes of several to hundreds of micrometershave been demonstrated, such as waveguides, photonic crystals, micro-cavities, and modulators, et al . Inthis paper, we presented subwavelength nanograting metasurfaces fabricated in a crystalline lithium niobatefilm, which hold the possibilities towards further shrinking the footprint of the photonic devices with newoptical functionalities. Due to the collective lattice interactions between isolated ridge resonances, distincttransmission spectral resonances were observed, which could be tunable by varying the structural parameters.Furthermore, our metasurfaces are capable to show high efficiency transmission structural colors as a result ofstructural resonances and intrinsic high transparency of lithium niobate in visible spectral range. Our resultswould pave the way for the new types of ultracompact photonic devices based on lithium niobate.Lithium niobate (LiNbO , LN) has been regarded as akey material in photonics community because of its com-mercial availability and multi-functional properties, including excellent visible and infrared optical trans-parency, large electro-optic, piezoelectric and second or-der harmonic coefficients, as well as photorefactive ca-pability et al . Their electrical and optical propertiescould be effectively adjusted by ion doping, for exam-ple increasing optical damage resistance by magnesiumor zirconium dopant.
Just like silicon as the back-bone of electronic industry, LN acts as a multi-purposematerial platform for photonics, and opto-electronicsystems. During the past decades, most of the attentionhas been drawn to realize high-density integrated pho-tonic/ optical circuits.
And various techniques havebeen developed to make photonic microstructures usingLN, for example thermal diffusion or proton exchange, laser writing, focussed ion milling, ion etching, and so on. Until now, the waveguides, pho-tonic crystals, micro-cavities, optical frequencyconverters, superbroad-bandwidth modulators with the feature sizes of several to hundreds of microm-eters have been fabricated. However, how to further re-duce the sizes of the LN based photonic elements andmeanwhile extend their functionalities are still in pursuitfor novel photonic systems with ultra-compact fashion.The idea of metasurfaces may help. The metasur-faces allow to funnel light from the far-field to a deepsubwavelength scale, and manipulate light behavior toachieve optical functionalities unprecedented in the nat-ural materials. Originally, metasurfaces have beenmade of plasmonic metals, in which the electromag-netic energy is confined and light-matter interactionsare boosted within nanometer scale, leading to giant a) Electronic mail: ren [email protected] b) Electronic mail: [email protected] chirality, enhanced nonlinearities, and high sen-sitive sensing. However, the intrinsic Ohmic losses hin-der their applications. Recent emerging all-dielectricmetasurfaces is a promising alternative to metallic struc-tures, which are believed to alleviate losses, and aremore accessible to both electric and magnetic resonancesthan plasmonic counterparts, . Until now, various noveloptical elements with nanoscale dimensions, such asmetalens, waveplates, Huygens surfaces, have (a) oooo O864 回衾卢 x-pol.Simu.d=62.5%D (客)L
20 x-pol.Exp. 60 40 20 y-pol.Exp.400 550 700 850 1000 Wavelength (nm) 400 550 700 850 1000 Wavelength (nm)
FIG. 1.
The SEM images and spectral properties ofLN nanograting metasurfaces. a , The typical SEM im-ages of fabricated array with D =500 nm and d =312.5 nm.The cross-section is shown on the right, in which Pt is usedas a protection layer for cross-section cutting. The profile ofLN is round shaped rather than the designed rectangle. Thescale bars are 1000 nm and 500 nm, respectively. b,c Thesimulated and experimental transmission spectra for arrayswith D of 400 nm, 500 nm, and 600 nm for orthogonal polar-izations. The geometric parameters used in simulations hereare reproduced from the cross-section. a r X i v : . [ phy s i c s . op ti c s ] O c t FIG. 2.
The electric (and magnetic) dipole moments induced inside LN and the simulated field maps. a , Thespectra of the electric ( p , red curves) and the magnetic ( m , blue curves) dipole moments’ magnitude induced inside an isolatesingle LN ridge (shown in the inset) for x - and y -polarized incident planar waves, respectively. The m is divided by light speed c considering that the emission of electric dipole is c times larger than the magnetic counterpart in vacuum. b , The resultsfor the periodic arrays with D =500 nm. c , The field maps at wavelengths marked in (b) on the cross-section for one structureperiod. The area enclosed by solid black lines are LN, whereas outside area is air. The light propagates along + z axis. For x -polarization, the crosses represents the j d along - x direction, and dots means along + x direction. The sizes of the symbolsshow the magnitudes. For y -polarization case, the instantaneous distributions of displacement current density j d are presentedby black arrows (length in logarithmic scale). been demonstrated.Here we presented LN as an alternative material torealize metasurfaces in visible and infrared frequencyranges. We fabricated nanograting metasurfaces in thethin crystalline LN film. The nanograting structures areeasy to manufacture, which were shown to support supe-rior resonances and new functionalities, such as magneticmirrors, structural color surface, . As a result of ahigh refractive index contrast between LN ( n >
2) andambient media, both electric- and magnetic- dipole mo-ments are well formed inside LN nanostructures. Fanoshaped resonances are observed in the transmission spec-tra as a result of the collective lattice interactions be-tween isolated LN ridges. The spectral positions of theresonances can be engineered by varying the geometri-cal parameters. Such metasurfaces are proved to exhibitvivid structural colors, which cover the range from pinkto purple dependent on different structural dimensions.The representative scanning-electron micrographs(SEM) of nanograting metasurface are shown in Fig. 1(a).A LNOI wafer (from NANOLN Corporation) with a x -cut 220 nm thick LN membrane on top of a silica insula- tor was adopted. We fabricated the nanograting metasur-faces with different periods ( D ) using focussed ion-beam(FIB, Ga + ). The duty cycles (percentage of LN ridgewidth over one period) of each structures remain 62.5%unchanged. The entire array footprints are 40 × µ m .Due to the angle divergence and scattering of the Ga + beams during fabrications, the practical cross-section ofthe final ridges become round shaped on the top, whichdeviates ideal rectangle design, as shown by the cross-section image. Figure 1(b) and (c) show the simulatedand experimental transmission of metasurfaces for nor-mal incidence with polarization parallel ( x -polarization)and perpendicular ( y -polarization) to the ridges for struc-tures with D =400, 500 and 600 nm ( d =250, 312.5,375 nm), respectively. Electromagnetic simulations wereperformed using the finite element method (Comsol Mul-tiphysics). The material parameters of LN were takenfrom ellipsometric measurements. The light was inci-dent from the far field beneath the sample and outputfrom the air side, which guarantees that the light withwavelength larger than the D would not diffract in thetransmitted direction. The round shape of the ridges FIG. 3.
Evolution of spectra for metasurfaces withdifferent ridge separations. a , The transmission spectrafor metasurfaces with d =312.5 nm unchanged, while D in-creases from 400 to 600 nm. This is equivalent to increasethe separations between the ridges. The real geometric pro-file of the LN ridge was considered. b , The similar results for y -polarization incidence. was considered in the simulation modeling. Both thespectral resonance positions and the lineshape exhibit ahigh sensitivity to both the geometric parameters of themetasurfaces and the light polarizations. The spectrashift to longer wavelength for larger D . For x -polarizedincidence, the spectra exhibit stepwise asymmetric lineprofiles, and the transmission becomes smaller for larger D . In contrast, y -polarized excitation gives distinct nar-row valleys with asymmetric Fano shape centered at 650,760 and 907 nm, respectively. Furthermore, the trans-mission curves become relative flat and nearly 100% ina wide spectral range beyond the right slopes of the res-onance valleys, which mean the structures come closerto satisfying the impedance matching condition for freespace, leading to zero light reflectance. Such high trans-mission benefit from both the high transparency of LNand the structural designs. And it could be used to real-ize the broadband antireflective films with few hundrednanometers thick, which are much more compact thanthe traditional multilayer coating films. The measuredspectra (by a commercial microspectrophotometer fromIdeaOptics Technologies, Fig. 1(c)) match the simulationresults well in the resonant wavelength positions (as in-dicated by the dashed lines), except for the lower trans-mission levels and the reduced quality factors of the res-onances. For example, for the x -polarized incidence, thestep slopes of the measured spectra are more inclinedthan the simulated ones. And the sharp resonance valleyat the blue edge of the spectral step shown in simulationfor 400 nm structure becomes broader in experiment. Forthe y -polarization, the resonance valleys are shallow withreduced contrasts. Such differences may result from theLN structure roughness, the Ga + contamination by FIB,all of which would cause extra optical loss and deteri- orated optical performance, but were not considered inthe simulations. The spectral resonances could be explained by the col-lective interactions between the local responses of eachLN ridges. Due to the relatively higher refractive indexof LN compared to the surrounding air and silica, eachLN ridge with subwavelength width supports Mie reso-nances, which induce both the electric and the magneticresponses. We numerically evaluate the electric and themagnetic dipole moments excited in a single ridge with d =312.5 nm and infinity length along x -direction. Usingthe multipole decomposition approach, the electricand magnetic moments are defined by p = (cid:82) V χ ( r ) E ( r ) dv and m = (cid:82) V r × J ( r ) dv , respectively. χ ( r ), E ( r ), and J ( r ) are the electric susceptibility, local electric field, andthe displacement currents induced within the ridge at po-sition r (defined in a Cartesian basis with coordinatescenter coinciding with the ridge’s geometric center). Thevolume integration is performed within the ridge. Con-sidering that the emission of electric dipole is c timeslarger than their magnetic counterpart with the samemagnitude in vacuum, the m is divided by c for di-rect comparison of their contributions to the far field ra-diation. As shown by Fig. 2(a), both the electric andmagnetic dipole moments are excited in an isolate singleridge (shown by inset). The p shows small resonance dipsat 466 nm and 677 nm (indicated by the dashed lines)for x -polarized light, and a very broad p valley appearsaround 788 nm for the y -polarized excitation. On theother hand, m shows broad resonances centered around678 nm for the x -polarization, while 600 nm and 752 nmfor y -polarization. However, when the LN ridges are ar-ranged in an ordered array (shown in Fig. 2(b) inset), theelectromagnetic fields of any single ridge may influencethe response of neighboring ones. In this way, the individ-ual ridges are coupled together forming collective latticemodes, which facilitates the narrowing and enhancementof the p and m resonances similar to that reported in theplasmonic arrays. As an example, the results of the pe-riodic array with D =500 nm are shown in Fig. 2(b). For x -polarized incidence, both p and m are enhanced dra-matically compared to the isolated single ridge response.The unconspicuous resonance dips for the isolated ridgebecome remarkable distinct and narrow valleys around488 nm and 656 nm (indicated by the dashed lines con-necting Fig. reffig2(a) and Fig. reffig2(b)). The stepwiseline shape of p spectrum resembles that of the transmis-sion spectrum shown in Fig. 1. In the mean time, the m resonance is also narrowed and enhanced as a resultof the lattice periodicity. For y -polarized incidence, thesimilar enhancement and narrowing effects also happen.In this case, the response is dominated by the magneticcomponent peaked around 768 nm, where the broad p valley of isolated ridge narrows to presents a obvious dipwith minimum near zero. The strong m acts a secondarywave source, leading to a significant backward reflectionand a predominant transmission valley in the Fig. 1.The resonant responses are further characterized bythe hot-spots and displacement currents formed withinthe grating layer. Figure 2(c) gives the representativesituation of metasurface with D =500 nm, and the lightis incident along + z -direction. For x -polarization inci-dence, the electric field distributions give similar featuresfor three wavelength shown in the figure, in which elec-tric hot-spots mainly localize at the top and bottom ofthe LN ridge, meanwhile in the air gap between ridges.The magnetic field H y and induced displacement cur-rent j d are shown on the right column. The ridges areassumed to have infinite length in the simulations, thusthe induced displacement current only has the compo-nent parallel to x -direction as a result of translationalsymmetry restriction. At around 737 nm, the j d nearlyshow no circulating features, however, due to the differentmagnitude of j d between the top and bottom of LN ridge,closed line Integral of current is non-zero, hence the effec-tive magnetic response can still be excited following theAmpere’s circuital law. At other wavelengths, anti par-rallel j d s are formed, generating magnetic dipole moment.Under y -polarization incidence, the magnetic fields showstronger magnitudes than the case of x -polarized lightat the wavelengths around resonances. This is consis-tent with the appearance of the circulating displacementcurrents shown by the arrows in most right column inFig. 2(c), which forms strong magnetic dipoles orientedalong x at the resonance around 768 nm. In contrast, atother wavelengths, the displacement currents with dif-ferent circulating directions cancel the formation of m ,leading to much smaller m values at those wavelengths.Such lattice collective resonance is highly sensitive tothe separations between the LN ridges. Figure 3 gives theevolution of spectra for metasurfaces with d =312.5 nmunchanged, while D increases from 400 to 600 nm, imply-ing the ridge separation enlarges. The spectra suffer dra-matical redshift because longer wavelength are needed tomatch the lattice resonance conditions. In the meantime,the coupling between the neighboring ridges, hence thelattice resonance becomes weaker for larger separation.This cause broadening of the resonances, and reduce theirquality factors. Specifically, for x -polarization, the trans-mission level decreases as D increases, and the sharp res-onances valley at the blue edge of the spectrum step dis-appears for larger separations. On the other hand, theresonance valley for y -polarization has much lower con-trast on the left slope for the case of D =600 nm comparedto 400 nm, and the minimum of the valley significant de-viate from the zero level.Such distinct structural resonances of the LN metasur-faces provide an access to a artificial structural colors. AsLN has high transparency in the visible spectral range,it is an ideal platform to realize the high efficiency struc-tural colors in transmission mode, which is in remark-able contrast to the reflection colors demonstrated by theplasmonic and Si based metasurfaces. We fabricateda group of metasurfaces with period varied from 300 to800 nm in 100 nm increment, and illuminated them bywhite light. The bright field transmission optical micro-
FIG. 4.
The structural colors from the LN metasur-faces. a , Photographs of LN metasurfaces, illustrating thevibrant colors and the continuous color change with increas-ing lattice periods from 300 to 800 nm. b , The experimentalspectra for the LN metasurfaces under orthogonal polariza-tions. scope images are shown in Fig. 4(a), whose backgroundgives the color of the unstructured LN film perceivedby a CMOS camera. Vivid colors that obviously dif-fer from the background are achieved in the metasurfacepositions. The photograph Their spectra are shown inFig. 4(b). The spectra, and the perceived colors are tun-able by both the structural geometries and polarizations.Colors ranging from pink, green, blue to purple could beachieved. Such results could be promisingly applied inthe next generation displaying color filters. Furthermore,as a matter of fact, based on various structure designs,the available chromaticity space is unlimited using LN.In conclusion, we presented the first demonstrationof all-dielectric LN metasurfaces. We showed thatnanogratings are flexible structures to engineer Mie op-tical resonances and tune the spectral responses throughsimple varying the geometric parameters, such as the pe-riodic and separation of the ridges. This also provide aversatile toolset to create structural color palettes overthe visible frequency range. Different from the most ofthe previous metasurfaces fabricated using amorphousmaterials, our LN metasurfaces were prepared in crys-talline film. Thus many distinguishable properties rootedin the lattice symmetry could be maintained in the meta-surfaces, such as second order optical nonlinearity, andelectro-optical effects, and so forth. This will unambigu-ously profit the future researches of the switchable andnonlinear dielectric metasurfaces. 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