Chemical Identification of Individual Fine Dust Particles with Resonant

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Chemical Identification of Individual Fine Dust Particles with Resonant Plasmonic Enhancement of Nanoslits in the Infrared Jochen Vogt, Sören Zimmermann, Christian Huck, Michael Tzschoppe, Frank Neubrech, Sergej Fatikow, and Annemarie Pucci ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00812 • Publication Date (Web): 14 Feb 2017 Downloaded from http://pubs.acs.org on February 15, 2017

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Chemical Identification of Individual Fine Dust Particles with Resonant Plasmonic Enhancement of Nanoslits in the Infrared Jochen Vogt,†,ǁ Sören Zimmermann,§ Christian Huck,† Michael Tzschoppe,† Frank Neubrech,†,‡ Sergej Fatikow,§ and Annemarie Pucci†,ǁ,* †

Kirchhoff Institute for Physics, Heidelberg University, Im Neuenheimer Feld 227, 69120 Heidelberg, Germany ǁ

§

InnovationLab GmbH, Speyerer Str. 4, 69115 Heidelberg, Germany

Division Microrobotics and Control Engineering, University of Oldenburg, 26129 Oldenburg, Germany

‡

4th Physics Institute and Research Center SCoPE, University of Stuttgart, Pfaffenwaldring 57, 70569 Stuttgart, Germany

KEYWORDS: infrared, plasmonics, surface-enhanced infrared absorption (SEIRA), dust sensing, inverse nanostructures

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ABSTRACT: We demonstrate the capability of single plasmonically active nanoslits for sensing of small fine dust particles via surface-enhanced infrared absorption (SEIRA) spectroscopy. Investigating phononic excitations of individual spherical silica particles coupled to the plasmonic excitation of single nanoslits, we are able to detect and chemically identify single spheres with diameters of 240 nm by their enhanced phononic signal. The single silica spheres in nanoslits lead to Fano-type phononic signals on the plasmonic background. The enhancement of the phononic silica signal is highest for particles located in the middle of the slit, in accordance with the FDTD-simulated near-field distribution along the slit at resonance. Our results reveal, that resonant plasmonic nanoslits are promising substrates for SEIRA spectroscopy of fine and ultra fine dust particles and guide the way towards SEIRA based dust sensing devices.

In the last decade surface-enhanced infrared (IR) absorption (SEIRA) with plasmonic structures has gained great attention due to its ability to enhance the cross-section of vibrational excitations in the IR spectral region by several orders of magnitude. SEIRA has been successfully applied in IR spectroscopy using various plasmonic systems, for example, linear nanoantennas,1 nanoantenna dimers,2 fan-shaped,3 bowtie like4 or inverse structures.5 The focus of most SEIRA investigations so far was on the spectroscopy of the vibrational modes of small amounts of organic molecules, or proteins.1,6,7 However, SEIRA has also great potential for sensing of species like dust particles, which are of importance in astronomy, environmental physics and also medicine due to their large impact on environmental safety and respiratory as


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well as cardiovascular health.8-10 The possible application of SEIRA to the spectroscopy of such particles has so far been rather rarely investigated,11,12 and not yet with small fine or ultra fine particles with dimensions in the range well below 500 nm. SEIRA spectroscopy can deliver useful information on the chemical composition of such particles with a detection sensitivity down to the single particle level as shown below in this contribution. In addition SEIRA offers the potential for real-time sensing, especially in combination with high intensity broadband IR laser light sources.13,14 With regard to the chemical identification of fine and ultra fine particles, the presented method thus represents a highly sensitive and destruction-free optical alternative to existing particle characterization techniques such as for example mass spectrometry.15,16 The chemical composition of particles can be determined via characteristic vibrational modes that are mainly depending on the dielectric function ( ) of the particle’s material. In general, also the particle size and shape, influence the spectra. For particles large compared to the wavelength, particle size dependent resonances occur, but this is not the case for small particles investigated in the present study. For spherical particles in a surrounding medium with the dielectric constant s with a diameter that is much smaller than the IR wavelengths () the absorption at the specific frequencies of phononic excitations dominates the Rayleigh scattering of the particle. In general the absorption cross-section of such a sphere is given by:17 abs = π

()

Im ()s s

(1)

Formula 1 links the dielectric function of the particle’s material ( ) to distinct particle resonances occurring at frequencies of phononic excitations where the expression is maximized. For strong oscillators with Re!( )" ≤ −2s , the well-known Fröhlich resonance condition Re!( )" = −2s follows. Since the dielectric function (that reflects the material specific absorptions) determines the absorption peak frequencies of the particle, these resonance


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frequencies are likewise a fingerprint of the particle’s chemical composition, as demonstrated in Figure 1c, where simulations of spheres made from different exemplary materials are shown. However, due to the small extinction cross-section of such particles, optical investigations of small amounts are challenging and for well defined individual small fine or ultra fine particles, with diameters well below 1 μm within reasonable time limits out of reach for standard spectroscopy methods, as for example conventional IR reflection absorption spectroscopy or Raman spectroscopy. As in the case of sensing of small amounts of organic molecules, this limitation can be overcome with the application of SEIRA using plasmonic nanostructures. The enhancement of vibrational modes through the high near-fields of plasmonic resonances enables the IR spectroscopic detection of the chemical composition of tiny particles. Although the SEIRA signal strength is sensitive to the particle size, the determination of the particle size from SEIRA measurements is still challenging and requires further investigations, since the SEIRA signal strength depends on several further factors as well, e.g. the exact spatial position in the near-field of the plasmonic structure as discussed below. As recently demonstrated, plasmonic nanoslits are a beneficial substrate for SEIRA sensing.5,18 Inverse nanostructures (e.g. nanoslits) work according to Babinet’s principle and feature the complementary spatial distribution of electric and magnetic near-fields when compared to the respective positive structure (i.e. linear nanorods). This difference has for example recently been studied with electron energy loss spectroscopy,19 and scattering scanning near-field optical microscopy.20 This contribution is the first one that demonstrates resonant plasmonic SEIRA sensing of individual dust nanoparticles. The test particles are spherical silica particles with a nominal diameter of 237 nm. The single spheres are placed in plasmonically resonant nanoslits as


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illustrated in Figure 1a. We analyse the enhancement of the phononic signals of these spheres also with respect to their spatial position in the nanoslit and compare the results to FDTD simulations.

RESULTS AND DISCUSSION For the SEIRA investigations of dust particles we used inverted plasmonic nanoantennas, i.e. nanoslits, working according to Babinet’s principle.5,21-24 Resonant nanoslits feature highly enhanced near-fields, roughly as strong as that of linear nanoantennas, however, with a different spatial distribution.5 In such slits at resonance, the enhanced electric component of the near-field is concentrated in one spot in which the single particle can be placed. A special advantage of nanoslits for dust sensing is the geometrical configuration with the particle trapped in the slit which is very stable against particle loss or displacement. Additionally advantageous for our test studies, the naturally conductive substrate (in our case gold) of inverted nanostructures avoids charging effects under the scanning electron microscope (SEM) and thus enables a high resolution for precise focused ion beam (FIB) treatments and nanomanipulation. For the SEIRA measurements on individual silica particles of this study, we fabricated single nanoslits by means of FIB milling into a thermally evaporated gold film of 100 nm thickness on a CaF2 substrate (see Figure 1a and Methods for details). Similar to nanorods, nanoslits can be tuned over a wide spectral range.5,25 Exemplarily, Figure 1b shows relative reflectance spectra of nanoslits with selected lengths. A slit length of ) = 3.0 μm was chosen for the SEIRA measurements in order to ensure a good spectral match between the plasmonic resonance and the phononic mode of the


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silica spheres (see Figure 1c) leading to maximized enhancement.6 Subsequent to the FIB milling of the nanoslits, individual silica spheres have been partly inserted into the nanoslits at various positions (A − E) using a nanomanipulator, as shown in the atomic force microscopy (AFM) images in Figure 1a and 2a.

Figure 1. a) Exemplary AFM image illustrating the sample layout: Nanoslits have been milled in an evaporated gold film on CaF2 by means of FIB and single silica spheres have been inserted partly into the nanoslits using a nanomanipulator. The enhanced IR signal of the silica sphere depends on the position in the slit. b) Experimental spectra (smoothed) of FIB milled nanoslits in


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a gold film with various length ) (- ≈ 110 nm, 0 ≈ 100 nm). c) Simulated extinction crosssection of spheres consisting of different materials with diameters of 237 nm (see Methods for details). AFM measurements of the fabricated slit-particle arrangements reveal, that the spheres were partly inserted into the nanoslits with remaining heights of 160 − 180 nm above the gold substrate (see Figure 2a and b). Since the diameter of the spheres exceeds the width of the slits, the spheres have been pressed into the slits by a depth of 60 − 80 nm. Considering the much higher hardness of silica (Mohs hardness 7) compared to gold (Mohs hardness 2.5), it can safely be assumed that rather the gold is deformed than the sphere, leading to the final geometry depicted in Figure 2. As extracted from AFM and SEM measurements, the side walls of the nanoslits are not perfectly vertical but rather show an inclination of ≈ 45° at the upper half of the slit as indicated by dashed lines in Figure 2c. Furthermore, not only the gold film but also the CaF2 substrate beneath the nanoslit has been FIB milled by ≈ 50 nm as the line scan in Figure 2b shows.


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Figure 2. a) AFM micrographs of individual FIB milled nanoslits in a gold film on CaF2. Single silica spheres with a nominal diameter of 237 nm have been placed at various positions in the slits. b) AFM line scan through a slit with its silica sphere (position E) parallel (blue, x) and perpendicular (red, y) to the long slit axis. c) Illustration of the geometry of the slit-sphere arrangements. The sketch shows a cut perpendicular to the long slit axis through the center of the sphere (in y-direction). Microscopic IR relative reflectance measurements using synchrotron light have been performed on each of the slit-particle arrangements shown in Figure 2a and 3a (see Methods for details). The resulting spectra are shown in the upper panel of Figure 3b: Excited with light polarized perpendicular to long slit axis, the nanoslits feature an extinction of 3 − 4 % at the fundamental plasmonic resonance at ≈ 1150 cm-1 , which leads to a good spectral match with the resonance of the silica sphere and thus a high plasmonic near-field enhancement.6 Already in the bare relative reflectance spectra in Figure 3b it can be clearly seen, that the enhanced phononic


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signal of the silica spheres rises, as the sphere is placed closer to the middle of the nanoslit. Spectra recorded with light polarized with the electric field parallel to the long axis of the nanoslits (indicated by the symbol ∥ in Figure 3b) show neither a plasmonic nor a phononic signal proving the phononic signal to arise from the enhancement due to the coupling to the plasmonic excitation. For a further analysis of the observed SEIRA signals, the spectra have been accurately baseline corrected using an adapted version of the algorithm proposed by Eilers (see refs 2, 26).

Figure 3. a) SEM micrographs of individual FIB milled nanoslits () = 3.0 μm, - ≈ 110 nm, 0 ≈ 100 nm) in a gold film on CaF2 with single silica spheres (nominal diameter 237 nm) at various positions along the slit. b) Upper panel: Microscopic relative reflectance IR spectra of the individual nanoslits shown in (a). Polarization parallel to the long nanoslit axis is indicated by the symbol ∥. The spectra are vertically shifted for better visibility. Lower panel: Baseline corrected relative reflectance spectra. The enhanced phononic signals (with signal


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strength 8) of the single silica spheres can be clearly identified and get stronger as the sphere is placed closer to the middle of the nanoslit. The baseline corrected relative reflectance spectra are shown in the lower panel of Figure 3b. The enhanced phononic mode of the silica sphere occurring in an asymmetric Fano-type line shape6 can be clearly identified. The maximum signal strength (read out peak-to-peak as indicated in Figure 3b) occurs for the sphere closest to the middle of the slit at position E and amounts to 8SEIRA,E = 0.43 %. The signal strength decreases when the sphere is positioned closer to the tip end of the slit, where it almost vanishes, as observed for position A. In order to estimate the enhancement of the phononic signal, a reference measurement on a large amount of silica spheres has been performed. To this end, a monolayer of silica spheres identical to the ones used for the SEIRA measurements was prepared on a CaF2 substrate. Microscopic IR measurements of this layer yielded an unenhanced reference signal per sphere of 8ref = 0.01 % when measured with the same aperture as used for the SEIRA measurements (see Methods for details). With this reference value the enhancement of the phononic signal due to the plasmonic resonance of the nanoslit is 8SEIRA,E / 8ref ≈ 40 for the strongest signal at position E. In Figure 4, the spatial distribution of the signal enhancement along the nanoslit is compared to the FDTD simulated phononic signal enhancement and the simulated near-field intensity distribution in the nanoslit. The near-field plot in Figure 4a shows the simulated electric field intensity in the nanoslit at 1100 cm= in a plane parallel to the substrate at half of the insertion depth of the spheres. As indicated by white circles, the spheres have been placed at different sites with varying near-field strength.


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Figure 4. a) Simulated electric near-field intensity of a nanoslit when excited with light polarized with the electric field perpendicular to the slit (as indicated). Field intensity recorded at 1100 cm= in a plane parallel to the substrate at half of the insertion depth of the sphere in the slit. The positions where the silica spheres have been placed are indicated. b) Measured (red, mirrored data in gray) and simulated (blue) enhanced phononic signal of single silica spheres (nominal diameter 237 nm) positioned at various sites of plasmonic nanoslits, and simulated average electric near-field intensity (green) at 1100 cm= at the sites, where the silica spheres are located. The simulated enhanced signal was extracted from simulated spectra of the arrangements A − E normalized to the measured maximal plasmonic extinction of the respective structure. The errors for the experimental and simulated values are mainly due to uncertainties in the baseline correction procedure. In line with the electric near-field distribution, Figure 4 shows in (b) the measured enhanced phononic signal 8SEIRA from spheres located at various positions along the nanoslit. Coming from a low level at the tip end of the slit, the measured enhanced signal shows an increase towards its maximum in the center. Please note that the data points plotted in gray are mirrored copies of the


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measured data given in red. The spatial distribution of the measured vibrational signal strength is in good agreement with the one obtained from FDTD simulations of the system (see Methods and the Supporting Information for details) and follows the distribution of the electric near-field intensity in the slit averaged over the volume of the sphere at the respective sites. Please note that the intensity values given in Figure 4a are recorded inside the nanoslit, whereas the ones in (b) are average values in the volume occupied by the respective sphere. Since the main part of each sphere is located outside the nanoslit (as shown in Figure 2), the average near-field intensity is low compared to regions inside the slit. As observed in several former studies, the absolute values of the plasmonic extinction of inverted nanostructures in simulations tend to higher values than typically observed in experiments. This is a result of deviations from the simulated ideal geometry and material properties of the structures as well as the beam alignment. To still enable a comparison between the spatial enhancement profile of the nanoslit structure gained from simulation and experiment, the simulated spectra for the positions A − E (shown in Figure S1 in the Supporting Information) have been normalized to the measured maximal plasmonic extinction of the respective nanoslit structure. In line with the higher extinction observed in simulations, the simulated near-field intensity shown in Figure 4b also exceeds the experimentally observed signal enhancement. Aside from the comparison of absolute values, the spatial profiles of the signal enhancement in experiment and simulation as well as the near-field intensity show an excellent agreement.


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SUMMARY AND CONCLUSION We have demonstrated, that plasmonic nanoslits fabricated by means of FIB milling in a thin gold film, are suitable structures for spectroscopic sensing of individual small fine particles via SEIRA. Single spherical silica particles with a diameter below 240 nm could be chemically identified via the frequency of their material specific phononic mode, which was enhanced by the strong near-field of resonant plasmonic nanoslits. The individual silica spheres located at various positions along similar single plasmonic nanoslits give a SEIRA response that is in accord with the spatial distribution of the near-field intensity along these slits, as we have shown with FDTD simulations. The Fano-type signals of the spheres are present on the plasmonic background for all positions along the slit. The signal is increasing towards its maximum at the slit middle. For the real applications, slit arrays with stronger plasmonic signals5 are recommended. Towards sensing of ultrafine particles with dimensions below 100 nm, narrower nanoslits are beneficial, because they feature even higher field enhancements.5 Based on our study, we expect that sensing of particles also below 200 nm in diameter is feasible with the resonant plasmonic enhancement provided by nanoslits.

METHODS Fabrication of Nanoslits. Nanoslits were fabricated in a gold film by means of focused ion beam milling. First, a 2 nm chromium film was thermally evaporated under a pressure in the 10> mbar range with a rate of 0.7 nm⁄s on a CaF2(100) substrate as an adhesion layer in order to stabilize the gold film against the mechanical impact due to the impression of the silica spheres. Subsequently, a gold film of 100 nm thickness was evaporated with a rate ranging from 0.72 to 1.08 nm⁄s at a pressure below 10> mbar. The evaporation rates were determined


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using a calibrated quartz crystal microbalance. Subsequent to the gold evaporation, nanoslits were written using a focused beam of gallium ions. The average processing time for each slit was 1.9 s applying 30 kV acceleration voltage and 1 pA probe current. The lateral distance between individual slits was set to 50 μm. The sample alignment within the focal plane of the beam was implemented by a slip-stick driven piezo stage with 20 nm closed-loop accuracy. In this way, the ion beam based fabrication of spatially distributed slits was realized without manual refocusing leading to homogeneous slits with similar resonance frequencies. Particle Handling. Silica particles were obtained as an aqueous suspension from Microparticles GmbH (Charge SiO2-FL2067; (0.237 ± 0.01) μm). Water was replaced by purified ethanol through repetitive centrifugation, pipetting and ultrasonic treatment. The ethanolic suspension was applied to a silicon substrate and immediately dried under vacuum. This procedure resulted in individualized silica particles. Pick-up and release of individual silica particles into the slits was achieved by cooperative handling of two tailored end effectors using a dual-probe robotic setup integrated within the vacuum chamber of a scanning electron microscope. Here, electrochemically etched tungsten tips were applied as end effectors. These tips were customized using focused ion beam processing for offering a minimized or maximized contact area to a particle, respectively. In this way, an adhesion guided handling strategy was implemented allowing to insert a silica particle into a slit with sub-50 nm positioning accuracy. The robotic manipulation sequence was controlled using consistent visual feedback provided by the scanning electron microscope. Due to the handling in the SEM, the procedure can lead to a tiny layer of carbon on the sample, which, however, is not disturbing our measurements. Detailed information on the overall setup and the handling strategy can be found in previous work.27


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Microscopic SEIRA Spectroscopy. The microscopic IR reflectance spectra of single nanoslits with silica spheres were recorded using IR synchrotron radiation from the synchrotron ANKA and a Bruker VERTEX 80v Fourier-transform IR spectrometer coupled to a Bruker IRScope II IR microscope, equipped with a mercury cadmium telluride detector cooled with liquid nitrogen. The IR light was polarized with a polarizer inserted in the beam path before the sample. The nonvacuum part of the beam path was constantly purged with nitrogen gas, to avoid IR absorption from atmospheric H2 O and CO2 . The spectra were recorded with a resolution of 4 cm= and several 1000 scans. Using a 36× Schwarzschild objective and a circular aperture, a beam spot of 12.5 μm diameter was realized. The individual nanoslits were localized following the procedure described in ref 2. The measurements of the reference used to calculate the relative reflectance were performed at a spot close to the respective nanoslit on the bare gold substrate. Dielectric Function of Silica Spheres. Silica spheres are not consisting of the perfect glassy SiO2. Their IR dielectric function is a little bit different from the textbook data for SiO2. In order to precisely estimate the unenhanced reference signal of a single silica sphere IR measurements on a well defined large amount of such silica spheres have been performed. To this end, a monolayer of silica spheres from the same batch as used for the nanomanipulation described above has been prepared: An amount of 20 μl of an aqueous suspension of silica spheres (5 g / 100 ml) has been dispersed on an O2-plasma cleaned CaF2 wafer. Spincoating of the suspension at 3000 rpm for 60 s and subsequent spincoating of 20 μl purified ethanol lead to extended areas of homogeneous monolayers of silica spheres on the substrate as the SEM graph in Figure 5a shows. In the lower part of the image the clean substrate can be clearly identified, whereas in the middle and the upper part a monolayer of spheres has formed. The layer height corresponds to the diameter of the spheres as we have checked by means of AFM measurements.


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The IR transmittance of the prepared layer shown in Figure 5b has been measured at the region indicated in Figure 5a (beam spot diameter: 12.5 μm, 100 scans, resolution: 8 cm= ). With an estimated (two-dimensional) filling factor of 83 % of spheres in the layer, a signal per sphere of 0.012 % was estimated from the measurement shown in Figure 5b. Dielectric data of the silica material has been derived by modeling the transmittance with the software SCOUT28 using a Bruggeman effective medium29 for the layer consisting of the sphere material and air. The layer thickness was set to the sphere diameter of 237 nm and the volume filling to 55 % as extracted from the SEM measurements. The material of the silica spheres has been modeled with three Brendel oscillators in the considered spectral region and a high frequency dielectric constant of K = 2.1, analogously to the descriptions of silica in refs 30, 31. The corresponding dielectric data is shown in Figure 5c.


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Figure 5. a) SEM micrograph of a monolayer of silica spheres on a CaF2 substrate. b) IR transmittance in the region indicated in (a) and modeled transmittance. c) Modeled dielectric function of silica. Numerical Simulations. FDTD simulations were performed with the commercial software Lumerical FDTD Solutions (v.8.11.337) on the high performance cluster bwForCluster MLS&WISO (Production). Using the total-field scattered-field approach, perfectly matched layer boundary conditions were chosen in all directions in a distance from the nanoslits of at least one wavelength and the symmetry of the simulated system (anti-symmetric boundary conditions in L-direction) was included in the simulation in order to reduce the required computational power. According to the experiment the simulation layout of the nanoslit-sphere arrangements contained a gold film with a nanoslit and a partly inserted silica sphere on top of a CaF2 substrate. The geometry for each of the slit-sphere arrangements was modeled as sketched in Figure 2c. The dielectric function for gold was described in the Drude model with a plasma frequency p = 60820 cm= , a scattering rate of M = 532 cm= , and a dielectric background of K = 7.9 as derived for a similarly prepared gold film.5 The CaF2 substrate was considered to be dispersionless with a constant refractive index of N = 1.43 and the dielectric function for the silica sphere was modeled on basis of IR measurements on a defined layer of silica spheres as described above. The simulation time was set to at least 1500 fs. The structures were illuminated under normal incidence using a plane wave source with the light coming from the air direction. The bandwidth of the source was set to 950 cm= − 3000 cm= and the light was polarized perpendicular to the long axis of the nanoslit. The simulation mesh was defined by the automatic non-uniform mesh algorithm with an accuracy level of 4 and additional meshes around the nanoslit and the silica sphere with maximum mesh steps of 2 nm in each spatial direction. The


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near-field of the simulated nanoslits was recorded using a 3D field profile monitor around the nanoslit and the sphere in a reference simulation with identical layout except that the silica sphere was replaced by vacuum. The simulations of the bare spheres in vacuum shown in Figure 1c were performed with a simulation time of 5000 fs and broadband plane wave sources. The mesh accuracy was set to level 8 and an additional mesh with maximum mesh steps of 1 nm in each spatial direction was placed around the sphere. The material data for the bare silica sphere was modeled in the same way as for the spheres in the nanoslits, the IR optical material data for silicon nitride and aluminum oxide was taken from ref 30, the data for PMMA was taken from the commercial software WVASE32 from J.A. Woolam Co.. All symmetries provided by the setup were included. Convergence testing of the simulations was performed by decreasing the mesh size and by increasing the simulation time, the number of PML layers and the simulation volume.

Supporting Information. FDTD simulations of silica spheres in nanoslits. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author *Address correspondence to [email protected].

Notes The authors declare no competing financial interest. All authors have given approval to the final version of the manuscript.


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ACKNOWLEDGMENT J.V. acknowledges financial support by the Heidelberg Graduate School of Fundamental Physics. S.Z. and S.F. acknowledge support from German Research Foundation (DFG) under Project GZ: FA 347/39-2. C.H. acknowledges financial support by the Helmholtz Graduate School for Hadron and Ion Research. F.N. thanks the BW-Stiftung (PROTEINSENS) for financial support. We acknowledge the Synchrotron Light Source ANKA for provision of synchrotron light and instruments at their beamlines and we would like to thank Y.L. Mathis, D. Moss, B. Gasharova and M. Süpfle for assistance in using beamline IR2. This research was supported in part by the bwHPC initiative and the bwHPC-C5 project funded by the Ministry of Science, Research and the Arts Baden-Württemberg (MWK) and the German Research Foundation (DFG) provided through associated compute services of the bwForCluster MLS&WISO (Production) at Heidelberg University and the University of Mannheim.

REFERENCES 1.

Neubrech, F.; Pucci, A.; Cornelius, T. W.; Karim, S.; García-Etxarri, A.; Aizpurua, J.

Resonant Plasmonic and Vibrational Coupling in a Tailored Nanoantenna for Infrared Detection. Phys. Rev. Lett. 2008, 101, 157403.


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For Table of Contents Use Only

Chemical Identification of Individual Fine Dust Particles with Resonant Plasmonic Enhancement of Nanoslits in the Infrared Jochen Vogt, Sören Zimmermann, Christian Huck, Michael Tzschoppe, Frank Neubrech, Sergej Fatikow, and Annemarie Pucci


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Chemical Identification of Individual Fine Dust Particles with Resonant

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