Near-infrared chiral plasmonic metasurface absorbers - OSA Publishing

Vol. 26, No. 24 | 26 Nov 2018 | OPTICS EXPRESS 31484

Near-infrared chiral plasmonic metasurface absorbers LEIXIN OUYANG,1 WEI WANG,1 DANIEL ROSENMANN,2 DAVID A. CZAPLEWSKI,2 JIE GAO,1,3 AND XIAODONG YANG1,4 1

Department of Mechanical and Aerospace Engineering, Missouri University of Science and Technology, Rolla, MO 65409, USA 2 Center for Nanoscale Materials, Argonne National Laboratory, Argonne, IL 60439, USA 3 [email protected] 4 [email protected]

Abstract: Chirality plays an essential role in the fields of biology, medicine and physics. However, natural materials exhibit very weak chiroptical response. In this paper, nearinfrared chiral plasmonic metasurface absorbers are demonstrated to selectively absorb either the left-handed or right-handed circularly polarized light for achieving large circular dichroism (CD) across the wavelength range from 1.3 µm to 1.8 µm. It is shown that the maximum chiral absorption can reach to 0.87 and that the maximum CD in absorption is around 0.70. The current chiral metasurface design is able to achieve strong chiroptical response, which also leads to high thermal CD for the local temperature increase. The highcontrast reflective chiral images are also realized with the designed metasurface absorbers. The demonstrated chiral metasurface absorbers can be applied in many areas, such as optical filters, thermal energy harvesting, optical communication, and chiral imaging. © 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

1. Introduction Chirality represents the asymmetry property of matter, which is important in many fields of science and engineering. Chirality has been introduced in chemistry since the 1870s [1], and the study of chirality advances our knowledge in understanding the mysteries of nature [2– 11]. There are usually a pair of enantiomeric forms for optical chiral materials [12], which are two non-superimposable mirror images with different responses to the left-handed and righthanded circularly polarized (LCP and RCP) light. However, the high-efficiency chiral absorbers are not attainable from natural materials. Recently, metamaterials and metasurfaces known for exhibiting exotic electromagnetic properties [2–11] have been designed to achieve highly-efficient selective chiral absorption with various types of three-dimensional (3D) optical structures, such as double-layered L-shaped antennas [13], double-layered twisted crosses [14,15], single-layered double sectors [5], spirals [16–18], entangled structures [19,20], and letter-shaped structures [21,22]. Moreover, a deep-learning-based model has been utilized to automatically design and optimize 3D chiral metamaterials [23]. Here, we present one kind of highly-efficient chiral plasmonic metasurface absorbers working in the near-infrared wavelength region. The designed chiral metasurface absorber, consisting of a three-layer metal-dielectric-metal structure, can selectively absorb the LCP or RCP normal incident light. The results show 87% maximum chiral absorption and 70% maximum CD in absorption at the resonance wavelength across the wavelength range from 1.3 µm to 1.8 µm. The high CD in absorption is due to the special design of the top-layer metallic patterns. The resonance wavelength with the maximum CD in absorption can be easily tuned by simply changing the geometric parameters of the top-layer metallic patterns. To elucidate the mechanism of optical chiral absorption, electric field distributions and temperature distributions are mapped for the LCP and RCP incidence light. In order to further visualize the potential applications of the chiral metasurface absorbers, the high-contrast near#346712 Journal © 2018

https://doi.org/10.1364/OE.26.031484 Received 25 Sep 2018; revised 10 Nov 2018; accepted 12 Nov 2018; published 14 Nov 2018


infrared reflecctive chiral imaages of a Taich hi logo dependding on differennt incident polaarizations are demonstraated. The resullts can be appliied to many appplications succh as optical fillters [13], thermal absorbers [24,25], optical comm munication deevices [21], annd chiral imaaging and 6–28]. holograms [26

Fig. 1. 1 (a), (b) Schematics of the chiral metasurface m absorrbers in Form A aand Form B. (c-g)) SEM images of the fabrricated chiral abso orbers in Form A w with different geoometric parameterss from Sample 1 to Samp ple 5: P1 = 1050, 1160, 1160, 13600 and 1460 nm; P2 = 400, 380, 380, 380 and a 380 nm; L = 450, 500, 550, 60 00, 650 nm; W1 = 50 nm; W2 = 1100 nm. Scale barr repressents 500 nm.

2. Design an nd characterrization of ch hiral metasurrface absorb bers The designed chiral metasurrface absorberss are composedd of a top 55 nm m-thick gold (A Au) layer patterned with h double-rectaangle resonatorrs, a 130 nm-tthick silica (SiiO2) spacer layyer and a 200 nm-thick gold ground plane p on a glass substrate, witth thicknesses denoted as th, td and tm, T schematics of the design ned unit cells foor two enantiom mer forms are shown in respectively. The Figs. 1(a) and d 1(b). The do ouble-rectanglee pattern consiists of two connnected gold rrectangles with overlapp ped space W1, rectangle widtth W2, and recctangle length L L. The rectanggular unit cell has the periods p of P1 and a P2. The metallic m rectanggle resonators are one simple type of antennas for building metaasurface absorb bers. By comb mbining two reectangle resonaators and otating) the loccations of them m [29], simple ddouble-rectanggle patterns witth broken shifting (or ro mirror symm metries are created in the to op gold layer,, allowing thee designed meetasurface absorber to be b chiral. The multiple reflecctions of lightt inside the chhiral Fabry-Pérrot cavity [22] formed with w the top resonator r layerr and the grouund plane creaates a chiral pplasmonic resonant mod de and thus enh hance the seleective resonantt absorption off the incident ccircularly polarized ligh ht. The thick go old ground plan ne will block th the transmissioon so that the abbsorption is equal to A = 1 – R since T = 0 (where A, T and R deenote the absorrption, transmisssion and he two enantiom mer forms Form rm A and Form m B shown in F Figs. 1(a) reflection, resspectively). Th and 1(b) are mirror m symmetrric with each other, o so that onnly the absorption performannce of the chiral absorb ber in Form A is presentted here. Diffferent from other types oof chiral metamaterialss and metasurffaces that requ uire complicateed and precise fabrication proocedures, the double-reectangle patterrns of the dessigned chiral m metasurface abbsorbers can bbe easily fabricated usiing focused ion n beam (FIB) milling. m The A Au-SiO2-Au muultilayer is depposited on a glass substrrate, with the Au A layer depossited by sputteering and the S SiO2 layer depposited by electron-beam m evaporation. Then the designed double-rrectangle patteern array is m milled into the top gold layer using an FIB F system (FE EI Helios Nannolab 600). Figgures 1(c)-1(g) show the


scanning electron microscop pe (SEM) imag ges of the top vview of the fabbricated chiral aabsorbers in Form A (frrom Sample 1 to Sample 5), where the geo metric parameeters P1, P2 andd L of the unit cells are tuned in orderr to increase th he resonance w wavelength, whhile W1 = 50 nm m and W2 = 100 nm are set as constantts. pectra are meassured with a Foourier transform m infrared specctrometer The opticaal reflection sp (FTIR, Nicoleet 6700) conneected to an inffrared microscoope. A set of a linear polarizzer and a quarter-wave plate is used to specify the circular c polarizzation of the inncident light. N Numerical a also performed to investigate the opticcal responses of the chiral aabsorbers simulations are under both circular polarrizations, wheere the permiittivity of golld is taken ffrom the i part increased by tthree times andd the refractivee index of experimental data with the imaginary measured and ssimulated SiO2 is set as a constant, with a value of 1.45. Figure 22(a) plots the m absorption spectra of the ch hiral metasurfaace absorber inn Form A (Sam mple 3) under LCP and ved that the chiral c plasmonnic resonance iis around 1.6 µm with RCP incidencce. It is observ polarization sensitive s absorrption, where the absorptionn for RCP inccidence is signnificantly stronger than that for LCP incidence so that t the CD inn absorption deefined as CD = |ALCP – ARCP| is high. The slight difference betweeen the simulaated and measuured absorptioon spectra ness arising froom the fabricattion process. T The chiral could be caussed by the defeects and roughn resonance waavelength can be tuned by varying v the geeometric param meters of the top-layer double-rectan ngle patterns. Figure F 2(b) plo ots the measureed absorption sspectra for Sam mple 1 to Sample 5, sho owing the conttinuous increasse of the chirall resonance waavelength from m 1.36 µm to 1.78 µm. The T measured maximum m chiral absorption ccan reach to 0..87 while the m maximum CD in absorpttion is around 0.70. 0

Fig. 2. 2 (a) Simulated and a measured abso orption spectra off the chiral metasuurface absorber inn Form A (Sample 3) un nder LCP and RCP incidence. (b) M Measured absorpttion spectra of thee Sample 5. chiral metasurface absorbers in Form A frrom Sample 1 to S

To reveal the mechanissm responsiblee for the polarrization sensitiive absorptionn, electric mperature disttributions in the t chiral meetasurface abssorber in Form m A are field and tem simulated. Fig gure 3(a) show ws the electric field f distributioons across the pplane 10 nm unnderneath the top surfacce of the spaceer layer for botth the LCP andd RCP incidennt light at the pplasmonic resonance of 1.6 µm (Samp ple 3). It show ws that the chiiral plasmonic resonance is enhanced inside the dielectric spacerr layer under RCP incidennce rather thann LCP incideence. The simulated electric field |E(rr)| and current density J(r) ddistributions att the top surfaace of the g. 3(b), and thee term J·E reprresents the pow wer lost to heaat per unit resonator can be seen in Fig hermore, in ord der to investigate the temperrature distributtion in the absoorber, the volume. Furth heat transfer equation ∇·(-kk∇T) = q is so olved, where T is the temperrature, k is thee thermal a q is the heeat generation density in mettal, q(r) = (ω/22)Im[ε(ω)]ε0|E E(r)|2. The conductivity and incident light power onto on ne unit cell is 10 1 μW, correspponding to aboout 30 µW/μm2. The top oundaries are set s as the fixed room temperaature of 273 K and the side booundaries and bottom bo are periodic, which w can be considered c to be b insulated. Heeat is generatedd mostly inside the toplayer double-rectangle patteern, then cond ducted to the ssurrounding m materials with tthe lower


temperatures, and finally reaches equilibriium. Figure 3( c) plots the sim mulated heat ggeneration butions, q, at the top surfacce of the doubble-rectangle paattern at the pplasmonic density distrib resonance, wiith the maximu um value of 4.9 91x1016 W/m3. The strong abbsorption of thee incident RCP light can n lead to high heat generatio on density withhin the top-layyer metallic patttern, and thus causing a significant local temperrature increasee. Figure 3(d)) shows the ssimulated d att the top surfacce of the doubble-rectangle ppattern. Due too the high temperature distributions thermal cond ductivity of gold, g the top-llayer double-rrectangle patteern achieves a higher temperature th han the surrou undings, while the temperaturre inside the ddouble-rectanglle pattern remains almo ost constant, wh hich can be seen from Figs. 3(f)-(h) with tthe temperaturee profiles along differen nt lines marked d in Fig. 3(e). The thermal ccircular dichroiism (CDT) is ddefined as CDT = |∆TLCPP - ∆TRCP| [30],, where the ∆T T is the local teemperature incrrease inside the doublerectangle patttern due to th he absorption of o incident LC CP or RCP ligght. Under thee specific incident lightt power of 10 μW per unit cell, CDT = 4 K. The relaationship betweeen CDT, incident light power IP and CD can be sim mplified as CD DT = cT ·IP ·CD, where cT is a thermalcoefficient co onstant equal to o 0.56 μW−1K−1 for each unit cell and CD = 0.70 in this caase.

Fig. 3. 3 (a) Simulated electric e field distriibutions across thhe plane 10 nm uunderneath the topp surfacce of the spacer lay yer under LCP and d RCP incidence aat 1.6 µm (Samplee 3). (b) Simulatedd electriic field and curren nt density (arrowss) distributions acrross the top surfaace of the double–– rectan ngle pattern. (c) Simulated S heat generation densityy distributions andd (d) temperaturee distrib butions at the top surface of the dou uble-rectangle patttern. (e) Schematiics of the lines forr tempeerature profiles. (f--h) Temperature prrofiles alone the liines under LCP annd RCP incidence.

Next, the chiral metasu urface absorberrs are used to demonstrate the high-contrrast nearl around thhe chiral plassmonic resonaance. The infrared refleective images of a Taichi logo incident opticcal beam from a laser source at 1.58 µm firrst passes through a combinaation of a linear polarizeer and an achro omatic quarter--wave plate to generate the circularly polarrized light and then the beam b is focuseed normally on nto the Taichi Logo sample using a 20 × objective lens. The refl flected light fro om the samplee is directed aand collected bby an infraredd camera. Figure 4(a) sh hows the schem matic diagram of the Taichi L Logo and the sstructure components of the fabricated d sample, wheere chiral abso orbers in bothh Form A andd Form B are used for different areas. Figure 4(b) is i a SEM imag ge of the fabriccated Taichi Loogo with diameeter of 45 4 displays the t reflected im mages of the Taichi logo uunder differentt incident µm. Figure 4(c) polarizations from circular,, elliptical to linear l polarizaations. Since thhe absorption from the urface patterns in Form A orr Form B depeends on the inncident polarizaation, the chiral metasu areas in Form m A will appeaar brighter (daarker) while thhe areas in Forrm B have the opposite


brightness und der LCP (RCP P) incidence. In n other words, tthe image conttrast of the Taiichi Logo can be tuned d by switching g the incident polarization. H However, wheen the incidennt light is linearly polariized, the Taich hi logo can be barely b observedd as shown in F Fig. 4(c).

Fig. 4. 4 (a) Schematic diagram of the Taichi Logo andd the structure coomponents of thee fabriccated sample with chiral metasurfacce patterns in Form m A or Form B. ((b) SEM image off the fabricated Taichi log go with diameter of o 45 µm. (c) Thee reflected images of the Taichi logoo underr different incidentt polarizations from m circular, ellipticaal to linear polarizzations.

3. Conclusio on The near-inffrared chiral plasmonic metasurface m abbsorbers havee been desiggned and demonstrated to perform circular polarrization-dependdent absorptioon with largee circular he strong chiro optical responsee of the chiral metasurface aabsorber also ggives high dichroism. Th thermal circular dichroism for the local temperature. Thhe chiral plasm monic resonannce of the a can be tuned by sim mply changing the geometricc parameters of the unit metasurface absorber cell. Furtherm more, the high h-contrast neaar-infrared refl flective chiral images depennding on different inciident polarizattions are also realized. Theese results aree promising ffor future applications in i optical filterrs, thermal eneergy harvestinng, optical com mmunication, aand chiral imaging. Funding National Scieence Foundatiion (NSF) (D DMR-1552871,, ECCS-16530032); Office oof Naval Research (ON NR) (N00014 4-16-1-2408); U.S. Departm ment of Energgy, Office off Science (Contract No. DE-AC02-06CH11357). Acknowledg gment The authors acknowledge a the t support fro om the Intelliggent Systems Center and thhe facility support from the Materials Research Cen nter at Missourri S&T. This w work was perfoormed, in part, at the Center for Nano oscale Materiaals, a U.S. Deppartment of Ennergy Office of Science User Facility,, and supporteed by the U.S. Department of Energy, Offfice of Sciencce, under Contract No. DE-AC02-06C CH11357. References 1.

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