High Magneto‐Optical Activity and Low Optical Losses in Metal‐Dielectric Au/Co/Au–SiO2 Magnetoplasmonic Nanodisks

• DOI: 10.1002/adma.201103634
• Publication Date: 2011-12-24T21:38:43Z
• Authors: Juan Carlos Banthí; David Meneses‐Rod...; Fernando García; María Ujué González; Antonio García‐Ma...; Alfonso Cebollada; Gaspar Armelles
• Keywords: Electromagnetic Fields; Optical Phenomena; Magnetic Phenomena; Electric Impedance; Cobalt; Gold; 02 engineering and technology; Silicon Dioxide; 0210 nano-technology; Nanostructures

Metal-dielectric Au-Co-SiO2 magnetoplasmonic nanodisks are found to exhibit large magneto-optical activity and low optical losses. The internal architecture of the is such that, in resonant conditions, electromagnetic field undertakes a particular spatial distribution. This makes it possible maximize at magneto-optically active layers minimize other, optically lossy ones. Combining materials with ferromagnetic plasmonic properties has recently become an area research.1-12 generation so-called allows one study wide variety physical phenomena which both characters (magnetic plasmonic) intertwined, i.e., control by acting on other vice versa.13 For example, characteristics different kinds structures can be tuned action external magnetic field, basically its capability off diagonal elements dielectric tensor system, which, for material case, sizable.5, 10, 12, 14, 15 On hand, also enhance (MO) continuous films nanostructured systems upon excitation their characteristic surface plasmon resonances.1, 2, 5-7, 9, 11, 16, 17 In this second since MO proportional (EM) material,2, 17-19 mechanism lies light harvesting effectively concentrates EM region. From applied point view, increased relevant context telecommunications, where they could used construct ultrafast switches integrated isolators/circulators elements; offer potential development high-speed modulators high density holographic data storage 3D displays serve sensing as well.20 Obtaining these therefore pertinent several application niches, constitute very promising choice. Different have been proposed so far, being transition metal (Au, Ag, etc.) component either (Fe, Co, Ni)5, 6, 9-11, 13-16, 21, 22 or oxide (Fe Co oxides garnets).7, 23 enhancement comes along increase absorption, limits use transmission related devices. current situation still further improvement, additionally increasing reduction absorption. Here we present our approach reach objective performing strong redistribution nanoscale metal-dielectric materials, takes place inserting (SiO2) into Au/Co/Au adequate positions. layer insertion gives rise hybrid modes new distribution field.24-26 We will show how selecting position inside structure effective way components (Co), while reducing non-MO active, elements, providing system enhanced moderate Three structures, Au/Co/SiO2/Au, Au/SiO2/Co/Au, nanodisks, sketched Figure 1a, were fabricated means colloidal lithography evaporation (see Experimental Section). thickness each was nm Au, 10 20 SiO2. An identical amount Au order compare modification induced response SiO2 layer. 1b shows representative atomic force microscopy (AFM) image systems. ensemble randomly distributed over clearly observable, average around 7.5 × 108 disks cm−2 interdisk distance 350 nm. warrants lack interaction. actual shape that truncated cones25, 26 approximate values upper lower nanodisk diameters 70 110 respectively, slightly larger fully metallic nanodisks. a) Sketch composition b) AFM selected (15 Au/10 Co/20 SiO2/15 Au). 2a extinction spectrum normal incidence Au/Co/SiO2/Au (solid line). clear peak 750 nm, from now denote HWP (high wavelength peak), shoulder 600 (LWP, consistent results obtained Au/SiO2/Au nanodisks,24 some small differences due presence damps blueshifts resonances.13 activity, whose spectral dependence same range shown 2c. depict, line, measured intensity IΦ structure, defined modulus () complex Kerr rotation Φ (Φ = θ + iϕ), ϕ ellipticity. As observed, two features present: region another LWP comparison, sample plotted Figures 2a,c dashed lines. only located between structure.27 localized resonance (LSPR) induces activity.6 effect split resonances, originating plasmons top (Au/Co) bottom (Au) separated sees correspondence activity: higher extinction, stronger previous studies kind systems.6, 7, 11 Au): solid line. experimental Co/15 Au) comparison: c) spectra line) (dashed Calculated (b) (d) based scattering matrix formalism if placed layer, drastic change observed respect case. seen 3a, appears reversed, exhibiting HWP. (Figure 3c) is, however, similar preceding sample, Remarkably, implies specific obtain configuration corresponding region, contradicting naïve rule thumb absorption provides activity. Au/SiO2/Co/Au Au/20 SiO2/10 d), spectra, formalism, These indicate first, within modifications properties, second, maximized minimized. Regarding samples, than equivalent and, moreover, does not necessarily insight underlying effects, performed numerical simulations details depicted 2b,d, 3b,d, showing excellent agreement experiments, confirming validity modeling describe structures. Since ruled magnitude absorbing respectively,2, 4 (namely square electric field) Y–Z plane (left) (right) regions. reference, ref. 17. see regions disk. Additionally, comparing regions, explains structure. near Comparing one. Again, concentration case understand total (related activity) samples.28 findings whole governs value main responsible magnitudes decoupled us design (square |E |2) through finite difference time domain (FDTD) for: spectral, d) sketches left side figure represent cones taken account cylinders decreasing diameter. illumination conditions middle scheme. Notice here incoming beam polarization X–Z plane, plane. LSP resonances excited dipolar character, evident plots but outside all interior (the terms layer) smeared out. applications, convenient simultaneously defining merit, ratio MO/Ext. hand pure 5 ratio, data, metal- without spacer. peaks (in spite wavelength. mainly single peak, more intense those simultaneous influence merit exhibits featureless behavior. weak maximum simply no present. MO/Ext, (thick lines) (thin comparison calculated Faraday divided transmission. Bi-substitute YFe2O5 garnet structure29 made Fe nanoparticles embedded matrix.30, 31 opens door Moreover, controlled modifying structural parameters (diameter, thickness), tune reduced maximized. conclusion, losses inserted positions nanostructure. effects particularly important wavelengths excited. excitation, (Co) non-MO-active system. conversion versus novel spectrally selection Nanodisks Fabrication: samples lithography32 evaporation. procedure hole-mask templates following. First, polystyrene particles (8% w/v, 100 sulfate latex, INVITROGEN) deposited electrostatic self-assembly onto glass substrate precoated ≈360 poly(methyl methacrylate) (PMMA, wt% diluted anisole, MW 950 000, Microchem Corp.). PMMA treated oxygen plasma (50W, 450 mTorr) make hydrophilic, then precursor film adsorbed positively charged using 0.02% (by weight) poly(diallylmethylammonium chloride) (PDDA, 400 000–500 Sigma Aldrich), rinsed water 60 s, finally dried nitrogen gas. Negatively substrates solution interactions. Particle concentrations used. Adsorption 1 min allow adsorption saturation better uniform experiments. Excess under running blown dry Milli-Q (Millipore) times. spheres disk 12%. After this, thin nm) thermal Polystyrene retired tape striping, underwent treatment (50 sccm O2, 50 W, °C, 10−5 Torr, 90 s), attacked uncovered drilled holes. Subsequently, introduced ultrahigh vacuum (UHV) deposition where, after 2 Ti improve adhesion substrates, grown. Ti, SiO2, electron (0.1 Å s−1, 0.5 0.25 s−1 rates, pressure 10−8 10−9 mbar during deposition) (0.25 rate). acetone lift-off, oriented obtained. Optical Characterization: Extinction acquired spectrometer (Andor, Shamrock SR-303i) coupled fiber Olympus microscope bright-field configuration. Illumination condenser 0.1 aperture (N.A.) collected N.A. (×2, 0.06). signal circle 300 μm Magneto-Optical characterization carried out (reflection) spectroscopy polar electromagnet perpendicular surface, illuminated monochromatic Xe lamp followed monochromator. By modulating photoelastic modulator recovering reflected photomultiplier lock-in configuration, ellipticity determined.33 measurements insured Numerical Calculations: complementary methodologies, developed authors method (SMM) adapted able consider elements,34 (proprietary software Lumerical Solutions). cases calculations wave linearly polarized x-direction, cone stacking diameter sketch 4). SMM base 1521 k-points FDTD real space grid 40 points per direction constants employed ellipsometric grown techniques here.21 Periodic boundary cases, ensuring periodic because (350 enough interaction introduce diffraction effects. acknowledge funding support EU (NMP3-SL-2008-214107-Nanomagma), Spanish MICINN ("FUNCOAT" CONSOLIDER INGENIO 2010 CSD2008-00023, MAGPLAS MAT2008-06765-C02-01/NAN PLASMAR MAT2010-10123-E), Comunidad de Madrid ("NANOBIOMAGNET", S2009/MAT-1726 "MICROSERES-CM", S2009/TIC-1476, JAE Doc fellowship D.M.-R.).