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Dec 14, 2017 - A major challenge facing plasmon nanophotonics is the poor dynamic tunability. A functional nanophotonic element would feature the real...
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Letter Cite This: Nano Lett. 2018, 18, 302−307

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Magnetic Control of the Chiroptical Plasmonic Surfaces Irina Zubritskaya,*,† Nicolò Maccaferri,§,∥ Xabier Inchausti Ezeiza,§ Paolo Vavassori,*,§,⊥ and Alexandre Dmitriev*,†,‡ †

Department of Physics, University of Gothenburg, Gothenburg 412 96, Sweden Geballe Laboratory for Advanced Materials, Stanford University, Stanford, California 94305-4045, United States § CIC nanoGUNE, Donostia−San Sebastian 20018, Spain ∥ Istituto Italiano di Tecnologia, Genova 16163, Italy ⊥ IKERBASQUE, Basque Foundation for Science, Bilbao 48013, Spain ‡

S Supporting Information *

ABSTRACT: A major challenge facing plasmon nanophotonics is the poor dynamic tunability. A functional nanophotonic element would feature the real-time sizable tunability of transmission, reflection of light’s intensity or polarization over a broad range of wavelengths, and would be robust and easy to integrate. Several approaches have been explored so far including mechanical deformation, thermal, or refractive index effects, and all-optical switching. Here we devise an ultrathin chiroptical surface, built on two-dimensional nanoantennas, where the chiral light transmission is controlled by the externally applied magnetic field. The magnetic field-induced modulation of the far-field chiroptical response with this surface exceeds 100% in the visible and near-infrared spectral ranges, opening the route for nanometer-thin magnetoplasmonic light-modulating surfaces tuned in real time and featuring a broad spectral response. KEYWORDS: Chiroptics, magnetoplasmonics, 2D nanoantennas, active tuning

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self-assembled nanocomposites.14 The magnetic tunability of chiral transmission was recently demonstrated in hybrid magneto-chiral system where 3D chiral gold nanoantennas were combined with the magneto-optically active multilayers.15 Here we propose a plasmonic surface with chiroptical response that is controlled by an external magnetic field over a broad spectral range. For this we design a 2D composite trimer nanoantennas comprising three near-field-coupled nanosized disks of diameters close to 100 nm and identical height of 30 nm, of which one is made of a ferromagnetic material and the other two are made of a noble metal. Individual trimer nanoantennas fill up the macroscale surface, produced with an affordable, highly parallel and cm2-scale bottom-up nanofabrication as pictured on the scanning electron micrograph in Figure 1a.16 We consider the simple 2D symmetric geometry and rely on the interplay between the plasmon phases in the trimer,17 making the nanodisks identical in size but of two different materials, namely gold and nickel. The use of two materials breaks the 2D rotational symmetry, endowing the handedness to the trimer that results in a chiroptical response in otherwise structurally symmetric

he dynamic control of the far-field optical response in nanophotonic devices is an essential requirement. It has been approached recently in the visible and near-infrared spectral ranges by introducing to the plasmon-based devices the thermo-,1,2 and magneto-mechanical1 tuning or other external mechanical forces,3 for example, employing plasmonic metamaterials controlled electromechanically,4 by all-optical switching,5,6 or by adding optically reconfigurable phase-change materials.7,8 However, the remaining issues of the limited extent of the tunability in both amplitude and spectral bandwidth in combination with the required switching time scales, typically down to at least the picosecond range, and the requirement of an easy integration in devices so far prevented the development of a practical adaptive nanoplasmon-based optics. One of the advanced plasmonic functionalities where the urge for a dynamic switching is on the rise is the plasmon nanoantennas-enabled chiroptics. Chiroptical nanoantennas in two- and three-dimension (2D and 3D) draw the intense research interest due to their ability to introduce the additional spin degrees of freedom of light to various optical components,9 including nanophotonics-based optical communication10 and the plasmon-enhanced chiral sensing.11 Similarly to other plasmon nanoantennas, the tunability with chiroptical plasmonics has been addressed by the phase-change materials,12 and, specific to the chiral nanoantennas, by the DNA-mediated structural changes13 and by the mechanically reconfigurable © 2017 American Chemical Society

Received: September 26, 2017 Revised: December 2, 2017 Published: December 14, 2017 302

DOI: 10.1021/acs.nanolett.7b04139 Nano Lett. 2018, 18, 302−307

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Figure 1. Structurally symmetric chiroptical magnetoplasmonic nanoantennas. (a) SEM of the Au−Au−Ni trimer nanoantennas macroscopic surface, produced with the bottom-up colloidal lithography. (b) FDTD-calculated electric near-field of the plasmonic Au trimer nanoantenna, illuminated with the RCP and LCP light at 705 nm (nanodisks in the antenna are 100 nm in diameter, 30 nm height, and the internanodisk gaps are 20 nm). Scale bar: 100 nm. (c) Bimetallic magnetoplasmonic Au−Au−Ni nanoantenna (same size and internanodisk gaps as in the Au trimer) illuminated with the RCP and LCP light at 690 nm. (d) Calculated transmission of the LCP and RCP light by the purely plasmonic (Au−Au−Au trimer, top) and magnetoplasmonic (Au−Au−Ni, bottom, vertically shifted for clarity) nanoantennas. Details of the calculations are provided in Supporting Information.

nanoantenna.18 Such bimetallic antennas then display the differential transmission of the left- (LCP) and right- (RCP) circularly polarized light (see the simulated far-field chiroptical response in Figure 1d; the finite-difference time-domain (FDTD) simulations details are given in the Supporting Information). Ultimately, the chiroptical transmission is traced to a markedly different near-field plasmonic response of the trimer nanoantenna to the left- and right-circularly polarized light (compare purely Au trimer of Figure 1b with Au−Au−Ni trimer of Figure 1c). The near-field responses of the achiral Au nanotrimer to LCP and RCP, naturally, are mirror-symmetric and can be superimposed by the in-plane rotation of the mirror images of each other. Conversely, the two near-field images for RCP and LCP on Au−Au−Ni trimer cannot be superimposed by any in-plane symmetry operation (identity excluded). The chiral transmission of a trimer bimetallic antenna can be explained as follows. Considering the nanodisks “electromagnetic pairs” by which the chiroptical response of a trimer bimetallic antenna could be represented (Figure 2a, highlighted orange and green), the chiroptical effects emerge in a spectral range where the longitudinal plasmonic (optically bright) mode of each pair is excited (note the green and orange arrows of Figure 2a, marking the dipole modes). As these are orthogonal “pairs”, each longitudinal mode is excited in sequence by the rotating E field of the incident circularly polarized light with approximately quarter-period phase delay (Figure 2a). They behave as two weakly coupled oscillators sharing one element and thus are driving each other at resonance with an additional quarter-period phase delay. The net result of these two phase delays is that at the longitudinal resonance, the RCP light excites mainly the bimetallic Au−Ni dimer (Figure 2a right panel) longitudinal plasmon where phase delays of the stimuli, E field and coupling are concurrent and lead to a constructive interference. The incident RCP light excites only weakly the monometallic Au−Au dimer longitudinal resonance, where the opposite phases of stimuli result in destructive interference. The opposite happens when LCP light impinges on the trimer

(Figure 2a left panel; a detailed analysis of these interference effects is presented in the Supporting Information). If the pairs are identical as in monometallic antennas, the LCP and RCP internal responses of the trimer are symmetric and the far-field chiroptical response is absent. In a bimetallic antenna, the resonant mode of one of the pairs is spectrally shifted and otherwise perfect phase-compensation and/or phase-concurrence at the RCP and the LCP transmission are modified; the internal responses lose any symmetry leading to the emergence of the chiroptical effects. Simply put, one can visualize the trimer antenna having two resonant levels, corresponding to the two longitudinal modes of the nanodisks pairs (see Figure 2b, the excitation efficiencies of the modes schematically match the arrow thicknesses, i.e., higher efficiency is shown with thicker arrows). The left panel of Figure 2b, marked “no field”, depicts the emergence of the chiroptical response in the bimetallic trimer with a characteristic shape of the differential transmittance (CDT) from the energy difference of the monometallic (green in the schematics of the trimer) and bimetallic (orange) longitudinal modes and their different excitation efficiencies by the corresponding CP light. We leverage on the presence of the plasmon resonances in metallic nanoferromagnets to add the magnetoplasmonic functionality to the system. In magnetoplasmonics, magneticfield-dependent optical response can be added to the plasmonic elements by the magneto-optically active ferromagnetic materials.19−23 At the same time, plasmonic nanoantennas provide strongly enhanced magneto-optical effects in such hybrid systems.24,25 Here we demonstrate that adding the magnetoplasmonic functionality enables the active magnetic control of the plasmon-based chiroptics. In the schematics of Figure 2b, we apply the magnetic field parallel (right panel, H red) or antiparallel (middle panel, H blue) to the light propagation direction. The magneto-chiral response of the Ni nanodisk element selectively affects the overall coupling efficiency of the RCP (LCP) light to the nanoantenna, schematized in Figure 2b and confirmed by the calculated 303

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Figure 2. The principles of chiroptics in symmetric bimetallic nanoantennas. (a) Chiroptical response of a bimetallic trimer antenna with highlighted electromagnetic pairs and their longitudinal resonances (green and orange), excited with quarter-period phase delay. (b) Chiroptical response (chiral differential transmission, CDT, left panel) and magneto-chiral response of a bimetallic trimer antenna with magnetization direction antiparallel (middle panel) and parallel (right panel) to the light propagation direction. Note the schematics of the nanoantennas resonances to the left. (c) FDTD-calculated magnetically modulated electric near-field of bimetallic Au−Au−Ni trimer antenna at 690 nm for LCP and RCP light. Scale bar: 100 nm. (d) Calculated magnetically modulated CDT of the bimetallic trimer antenna. (e) Chiroptical response (left panel) and magneto-chiral response of a bimetallic dimer antenna with magnetization direction antiparallel (middle panel) and parallel (right panel) to the light propagation direction. (f) FDTD of the electric near-field of bimetallic Au−Ni dimer antenna at 645 nm for LCP and RCP light under applied magnetic field. (g) Calculated magnetic-field dependent CDT of bimetallic dimer antenna. Scale bar in (c.f) is 100 nm.

the Ni−Ni nanodimer and Ni−Ni−Ni nanotrimer antennas, Figure S12). In more detail, the FDTD near-field maps (Figure 2c) allow assessing separately the effect of the applied magnetic field of the opposite polarity on the nanoantenna, induced by the LCP (Figure 2c, upper panel) and RCP (Figure 2c, lower panel) light. While the magnetic field-induced changes are chiefly visible around the ferromagnetic element (Ni nanodisk), they spill out to the entire nanoantenna via the near-field coupling of the elements and result in the significant changes in the far-field chiral differential transmission (CDT) (Figure 2d). Figure 2c,f evidences that the presence of the third element (Au nanodisk) not adjacent to the Ni one, is crucial in determining the larger magnetic tuning of the chiro-optical response of the nanotrimer compared to a simple dimer nanoantenna (compare also middle and right schematics of Figure 2b,e). The magnetoplasmonic bimetallic nanoantenna spatially directs the nearfield of the incoming light independently of its handedness in the near-field, in stark contrast with a structurally symmetric case (compare the Au−Au and Au−Ni nanogaps in Figure

FDTD maps of the electric near-field at 690 nm, the Au−Au− Ni resonance (Figure 2c), leading to the modulation of the chiral transmission in the far-field (Figure 2d). Note that the trimer antenna with two different metals, one of which is a ferromagnet, is essential for the emergence of chiroptical response. The latter results from the electromagnetic pairs phase delays interplay and provides the means for achieving further active control of chiral transmission. In contrast, a dimer antenna with two metals, one of which is a ferromagnet, exhibits the modulation of the optical response by the magnetic field due to the purely magneto-optical effect but in the absence of the magnetic field the overall chiroptical response is zero, making this geometry unsuitable for the tunable chiroptics. That is, for an isolated Au−Ni pair (Figure 2e,f) the effect of the applied magnetic field is limited to a weak chiral transmission (Figure 2g), purely originating in the magnetic nature of the Ni nanodisk (see also Supporting Information for the experimental magneto-optical transmission of the array of Ni nanodisk and Ni−Au nanodimer antennas, Figures S6 and S7; and numerically calculated magneto-optical transmission of 304

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Nano Letters 1b,c). Here the bimetallic antenna is a 2D architecture and the strongly enhanced near-field chiral region is presented as the easily accessible internanodisk gap with readily available selectivity of the materials for surface chemical functionalization (i.e., Au−Au and Au−Ni nanogaps, in this case). This might render this type of bimetallic antenna a potentially powerful optically chiral biological and chemical sensor.26 This chiral resonant near-field sorting is further controlled by the small structural changes of the nanoantenna. For example, changing the nanoscale gaps between the nanodisks and/or the disks diameter effectively ramps up the chiral selectivity of the nearfield intensity and consequently both the far-field chiroptical response and its magnetic tunability (see Supporting Information, Figures S9 and S10), as shown below (see also Supporting Information, Supplementary Figure S1). Figure 3 brings forward the exceptional degree of the experimental tunability of this system by the magnetic field. To show this we track the CDT as we apply ±3 kOe perpendicular to the nanoantennas plane (see the schematics of Figure 3a), which is sufficient to fully magnetically saturate the system (magnetic tunability of trimer nanoantennas by external magnetic fields below saturation is shown in Figure S8 of Supporting Information). The essential tuning element is the magneto-optically active Ni nanodisk of the nanoantenna, while its near-field coupling to the rest of the elements provides the desired changes in the overall far-field chiroptical response. The bimetallic antenna accommodates a large structural flexibility and a whole palette of strongly chiral optical surfaces can be produced by slightly modifying the structural parameters of the system with nanofabrication. Specifically, by varying the size of the Au and Ni nanodisks, the interparticle separations between the Au and Ni elements and the nanoantennas number density on the surface, we alter the absolute values of the differential transmission and its spectral range (compare left panels of Figure 3b−e, gray curves). Note that Figure 3d presents the experimental realization of the simulated model nanotrimer of Figure 2c,d with the same nanoantennas geometry (i.e., nanodisk sizes and nanogaps), the number density, and excellently matching far-field chiroptical spectrum. The extent of the magnetic tunability follows the structural modification, as a rule reaching 25% to 150% in the spectral region of the maximum differential transmission in the visible and extending further up to an order of magnitude in the near-IR (Figure 3f). For the latter, while the plasmon-enabled optical chirality is slowly decreasing as the nanoantennas are gradually becoming off-resonant, purely magneto-optical effects take over the farfield response of the system. We plot the CDT modulation at the peak of the differential chiral transmission and 50 nm off the peak for comparison (Figure 3f). The latter allows directly appreciating how the slight adjustments in nanoantenna design (elements sizes and spacing) result in a significant rise of the magnetic tunability of the system. We note here that while the tunability range is indeed considerable, the tuned parameter itself, that is, the differential transmission (CDT) of 0.2−0.3°, is in the same order as the largest achieved in 2D magnetoplasmonic systems27 and plasmonic chiroptics.28 The insensitivity of the CDT spectra to the in-plane rotation of nanoantennas surfaces confirms the chiroptical origin of the effect as opposed to linear dichroism due to nanoantennas inplane anisotropy (see Figure S11, Supporting Information). As mentioned above, one of the critical parameters for the real-time tunable plasmonic surfaces is the speed of operation. So far reported the slow-tuned systems reach significant

Figure 3. Magnetic tunability of the chiroptical transmission. (a) Schematics of magnetically controlled CDT through a surface of magnetoplasmonic trimer nanoantennas. (b−e) Experimental CDT through a surface of trimer nanoantennas with geometrically tuned structural parameters with (blue, red) and without (gray) applied magnetic field of ∼±3 kOe (left panels, insets depict the to-scale schematics of the nanoantennas). SEM overviews of the corresponding plasmon surfaces; scale bar of 1 μm is common for all the micrographs (right panels). (f) Magnetic tunability of the corresponding surfaces presented in (b−e), expressed as (ΔCDT/CDT) at different wavelengths: the peak CDT wavelength (central bar), and the wavelengths +50 nm (right bar) and −50 nm from the peak CDT wavelengths (left bar).

changes of the tuned parameter (c.f., tuning the transmission,1,2,4,6,8 absorption,6 reflection,4,6,8 or CD1,13,14) at the expense of the operation speed and the prospect of integration 305

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confirms the chiro-optical origin of the measured differential transmission. FDTD Simulations of Magnetically Tunable CDT. We calculated the same signal as in Formula (2) of Supporting Information by considering the system magnetized at saturation along the normal to the sample surface. The solid red and blue curves are the CDT calculated with the magnetic field applied parallel and antiparallel to the wave-vector of the incoming light, respectively.

and manufacturing simplicity. The fast-tuned plasmonic systems at megahertz1,4 or the all-optically (i.e., terahertz) tuned systems,6,8 conversely, suffer from the small tunability range. With the present nanoantennas, the operation speed, in practice, is solely limited by the field-induced magnetization switching in a ferromagnetic element. Generally, the magnetization process is described by Landau−Lifshitz−Gilbert dynamic equation29 where the damping factor, being large for ferromagnets, sets the fundamental limit for magnetization switching speed, which is typically in the order of 1 ns. However, the application of specifically shaped magnetic field pulses allows reaching time scales of ∼100 ps30 (see Supporting Information for the experimental details). Thereby with the currently commercially available sources of the fast-modulated magnetic fields31 we foresee that the developed here magnetoplasmonic surfaces would readily reach the ultimate speed for magnetically induced switching up to 10 GHz over the visible and near-IR spectral ranges while maintaining the exceptionally large tunability range of up to 100−150%. Summarizing, we produced a class of highly tunable by the magnetic field macroscale bottom-up plasmonic chiroptical surfaces. The tuned parameter is the chiroptical transmission, enabled by the nanoantenna design that accommodates ferromagnetic plasmonic elements. The already significant chiroptical response of this system is further tuned up to 150% by the external magnetic field. The presented compact 2D design promises the easy integration and potentially fast operation in the broad spectral range, enabling this type of functional plasmonic surfaces entering the realm of practical optical devices. Methods. Nanofabrication. We use a combination of HCL16 and electron beam angular evaporation to fabricate the magneto-chiral trimer nanoantennas surfaces. During the fabrication we can control and tune with a high precision several structural parameters such as nanoantenna surface coverage, nanoantenna size, and the interparticle distance in Au−Au and Ni−Au pairs in the trimer nanoantenna. See Supporting Information for fabrication details. Measurements of Circular Differential Transmission (CDT) and Magnetically Tunable CDT. We use a supercontinuum white laser source (Fianium) in optical wavelength region 450− 1100 nm to perform the spectroscopic measurements of CDT and magnetically tunable CDT measured with external magnetic field. The output from the laser is supplied by multimode optical fiber and collimated in a 5 mm beam that is sent then to Glan-Thompson polarizer set at 45°. The beam polarization is further controlled by a photoelastic modulator (PEM) with a peak retardation set to 0.25 wavelengths, modulating the light polarization between LCP and RCP at PEM frequency 50 kHz. The transmitted intensity is recorded by a photodetector and the signal is fed to a lock-in amplifier locked to the PEM frequency 50 kHz. More detailed description of the measurement setup and a derivation of the CDT signal can be found in Supporting Information. The magnetically tunable CDT spectra are recorded via an externally applied DC magnetic field at magnetic saturation H and −H with the magnitude 5 kOe (∼0.5T). Although our measurement setup has been specifically designed to be insensitive to any birefringence effect, the measured transmission spectra could be affected by linear dichroism. Thereby, we have also measured the CDT spectra by rotating the samples in-plane. The invariance of the recorded CDT spectra

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b04139. Additional information and figures (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: (I.Z.) [email protected]. *E-mail: (P.V.) [email protected]. *E-mail: (A.D.) [email protected]. ORCID

Alexandre Dmitriev: 0000-0002-2231-9333 Author Contributions

I.Z. and N.M. authors contributed equally. I.Z., N.M., P.V., and A.D. devised the concept. I.Z. performed nanofabrication; I.Z. and N.M. performed optical, magneto-optical experimental measurements. N.M., X.I.E., and P.V. performed numerical simulations and analytical calculations. I.Z., N.M., P.V., and A.D. wrote the manuscript. All authors contributed to the discussions. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS I.Z. and A.D. acknowledge the Swedish Foundation for Strategic Research (SSF) Future Research Leader Grant, Knut and Alice Wallenberg Foundation project “Harnessing light and spins through plasmons at the nanoscale” (2015.0060). N.M. and P.V. acknowledge support from Basque Government (Project PI2015_1_19) and Spanish Ministry of Economy, Industry, and Competitiveness [Project FIS201564519-R (MINECO/FEDER) and the Maria de Maeztu Units of Excellence Programme − MDM-2016-0618]. N.M. acknowledges support from the Doctoral Program of the Department of Education, Linguistic Policy, and Culture of the Basque Government (Grant 2015_2_0113) and from the Predoctoral Mobility Program of the Basque Government under the Grant EP2015-1-44.

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ABBREVIATIONS 2D, two-dimensional; CP, circularly polarized; RCP, rightcircularly polarized; LCP, left-circularly polarized; CD, circular dichroism; MCD, magnetic circular dichroism; FDTD, finitedifference time-domain; DNA, deoxyribonucleic acid; HCL, hole-mask colloidal lithography

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REFERENCES

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