Low-Cost Infrared Resonant Structures for Surface-Enhanced

As an alternative fabrication method, nanosphere lithography (NSL) is a technique, which relies on self-assembled spherical particles serving as shado...
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Low-Cost Infrared Resonant Structures for Surface-Enhanced Infrared Absorption Spectroscopy in the Fingerprint Region from 3 to 13 μm Jón Mattis Hoffmann,† Xinghui Yin,‡,§ Jens Richter,†,∥ Andrea Hartung,† Tobias W. W. Maß,† and Thomas Taubner*,†,§ †

Institute of Physics (IA), RWTH Aachen University, 52074 Aachen, Germany Chair for Technology of Optical Systems, RWTH Aachen University, 52074 Aachen, Germany § Fraunhofer Institute for Laser Technology ILT, 52074 Aachen, Germany ‡

ABSTRACT: We use low-cost colloidal lithography with micrometer-sized polystyrene spheres to fabricate arrays of triangular gold microstructures on different infrared-transparent substrates while varying the structures’ lateral size. The refractive index n of the substrate in the infrared spectral range can be varied strongly, e.g., from n = 1.4 (calcium fluoride) to n = 4.0 (germanium) for the used materials. Variation of antenna size and substrate material allows us to tune the spectral resonance position of the fabricated antennas from 3 to 13 μm and therefore to cover the absorption bands of the infrared fingerprint region and functional groups. The easy handling and good tunability is demonstrated with surface enhanced infrared absorption (SEIRA) spectroscopy measurements on poly(methyl methacrylate) (PMMA) covered antennas on two different substrate materials (calcium fluoride and silicon) but equal spectral resonance positions of the antennas to ensure the comparability. Additional near-field measurements show that large antennas on a low-index substrate yield stronger local field enhancement compared to smaller antennas on a higher-index substrate, despite the same far-field response.

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INTRODUCTION Infrared spectroscopy is a powerful characterization method that allows for the investigation of chemical properties of a sample by directly probing molecular vibrations. Characteristic absorption bands are given by the distinct energies needed to excite different stretching and bending vibrations. Some important molecular bonds and the range of their corresponding absorption bands are shown in Figure 1.1−3 These

ments provided by these antennas when excited at the resonance frequency increase the sensitivity4−6 and enable the detection of, e.g., self-assembled monolayers of octadecanethiol (ODT) on a single antenna.7 Further improvement can be achieved by optimizing the coupling of antennas, arranged in an array, by controlled adjustment of the array geometry.8−10 The antenna arrays for these studies were prepared by electron beam lithography. The advantage of this method lies in the large variety of possible geometries and the high reproducibility. However, these advantages come along with high costs and long preparation times. As an alternative fabrication method, nanosphere lithography (NSL) is a technique, which relies on self-assembled spherical particles serving as shadow mask for evaporated metal.11−15 Even though we mainly use micrometer-sized spheres, we keep to the already wellestablished term of nanosphere lithography. NSL allows for fast and easy preparation of arrays of triangular structures on a large amount of substrate materials. The spectral resonance position of the triangular antennas can easily be shifted by changing the structure size, which is achieved by choosing different sphere sizes as a shadow mask.14,16−19 Furthermore, the surrounding medium of the antenna, in general the substrate and the analyte on top of the antennas, has an influence on the spectral behavior. Changing

Figure 1. Regions of characteristic vibrational absorption bands of some typical molecular bonds.1−3 Functional group range (4000− 1500 cm−1) and fingerprint region (1500−500 cm−1).

absorption bands form the so-called fingerprint region from 500 to 1500 cm−1, in which molecules can be identified via complex, unique absorption characteristics, and the region of functional groups from 1500 to 4000 cm−1. The sensitivity of infrared spectroscopy can be increased by using resonant microstructures for surface-enhanced infrared absorption (SEIRA) spectroscopy. The strong field enhance© XXXX American Chemical Society

Received: March 8, 2013 Revised: April 26, 2013

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the refractive index of the environment shifts the resonance position.5,14,17,20 The suitability of NSL-prepared structures for SEIRA has been demonstrated, however, no advantage of NSL arrays compared to disordered metal island films could be found.5 We assume this to be due to averaging the signal over several square millimeters during spectral measurements, including defects and nonresonant larger structures and nonideal matching of the spectral resonance position of the antennas to the absorption band of the analyte. The reduction of the measurement area to a few hundred square micrometers allows us to choose nearly defect-free regions. Furthermore, systematic matching of antenna resonance and absorption band increases the SEIRA enhancement drastically.7 Here, we show the influence of five different substrate materials (calcium fluoride (CaF2), zinc sulfide (ZnS), zinc selenide (ZnSe), silicon (Si), and germanium (Ge)) and the antenna size on the antenna resonance of triangular structures in the infrared. This allows us to create nanostructures with spectral resonances spanning nearly the complete infrared fingerprint and functional bonds regions from 500 to 4000 cm−1. A map of experimentally acquired spectral resonance positions as a function of the experimental parameters, antenna size, and substrate material shows the large versatility as possible substrate for SEIRA spectroscopy. The comparison of two exemplary SEIRA measurements of poly(methyl methacrylate) (PMMA) on tuned antenna-samples with high refractive index (nSi = 3.4) and low refractive index (nCaF2 = 1.4) substrate is presented. Finally, near-field measurements demonstrate higher field enhancements for antennas on CaF2 compared to antennas on Si.

Figure 2. (a) Preparation steps of nanosphere lithography: 1. Polystyrene (PS) sphere deposition, 2. Metal deposition, 3. Sphere detachment, 4. Optical microscope image of actual measurement area of a sample prepared with 4.5 μm diameter spheres resulting in 1.57 μm long antennas on CaF2. (b) FTIR far-field transmission spectra of triangular antennas on ZnS (red, L = 1.58) and CaF2 (black, L = 1.57; blue, L = 0.97).

source can influence the result: Displacement of the sample with respect to the vertical axis between source and sampletable leads to an evaporation angle and therefore different shadowing by the spheres, which results in differently shaped structures. Additional tilting and rotating of the sample and/or multiple evaporation steps define the final shape of the structures.21−24 In this work, we limit the process to simple triangular structures prepared in one evaporation step. Afterward, the spheres are removed with an adhesive tape. In the following, the triangles will be described by their length L (the distance from one tip to the opposite edge), as measured with a scanning electron microscope (SEM). The spectral sample characterization is carried out with a Fourier transform infrared (FTIR) spectrometer attached to a microscope. The microscope (Bruker) uses nontransmitting blade apertures to limit the measurement area. Depending on the size of the aperture and structures, the area contains only several tens of structures, which contribute to the measurements. In this way, it is possible to reduce averaging errors and to avoid array defects and larger structures formed by misalignment of the spheres in the self-assembling process. However, it has to be noted, that the size of the aperture does not change the spectral resonance position or amplitude of the resonance. A larger measurement area and increased number of antennas will, however, improve the signal-to-noise ratio. The FTIR spectrometer can be operated in transmission or reflectance mode in which the extinction of the antennas will result in a dip or a peak in the spectra, respectively. The minimum, respectively, maximum, is called spectral resonance position, given as wavenumber νres or wavelength λres. Figure 2b shows the spectra for three sample systems with two substrate materials and two antenna lengths. The black curve corresponds to a sample with CaF2 substrate and structures with a length of 1.57 μm (42 antennas within the measurement area). By keeping the substrate material and

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RESULTS AND DISCUSSION The samples are prepared by means of NSL with micrometersized polystyrene spheres serving as a shadow mask (Figure 2a). In the first fabrication step, two parameters can be chosen: First, the substrate material, which determines the refractive index n, and second, the sphere diameter d, which influences the size of the structures. The substrates are cleaned with acetone and isopropanol and treated in a plasma etcher for 120 s at 87 W with 0.3 mbar of ambient air (except for ZnSe because of its toxicity). The plasma treatment allows for an additional cleaning and hydrophilization of the substrate for a better spreading of the sphere solution. Afterward, we deposited the solution containing the spheres by drop coating on the substrate. This allows for fast processing and a wide range of possible substrates. During evaporation of the solvent, the spheres form monolayers of closely packed hexagonal arrays but also areas with defects and multilayers. However, the drop coating technique creates homogeneous, defect-free monolayers of at least several hundred square micrometers. Because we focus on measurements of areas with sizes of a few hundred square micrometers, the achieved monolayer area is sufficient. In the next step, a layer of the desired metal (in our case gold) is deposited onto the sample via thermal evaporation. In our experiments, no additional adhesion layer was used. This is not necessary because the spheres are not in contact with the antenna structures, and therefore, no mechanical force is applied to the antennas during the detachment step. This is contrary to, e.g., structures written in a single layer electronbeam resist, where an additional adhesive layer is essential. The orientation of the sample with respect to the evaporation B

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changing the antenna length to 0.97 μm, the resonance position shifts to larger wavenumbers (blue curve, 30 antennas). By keeping the antenna length at 1.58 μm and switching to the higher index material ZnS as substrate the resonance position appears at smaller wavenumbers (red curve, 25 antennas). Hence, the combination of substrate and antenna length allows for a wide tuning range, as shown in the literature for antennas prepared via electron beam lithography.25 The maximum extinction efficiency per antenna, i.e., the ratio of the extinction cross-section to the antenna area, for the cases given in Figure 2b are around 5.5 for both CaF2 samples and slightly higher with roughly 7 for the ZnS sample. This is in the order of the values given in the work of Crozier for triangular antennas,26 but nearly an order smaller compared to arrays of linear antennas, optimized for constructive coupling.8,25 Figure 3a shows the resonance positions for five different substrates (CaF2, n = 1.4; ZnS, n = 2.2; ZnSe, n = 2.4; Si, n =

more than 20 antenna length measurements with the same nominal length. The solid lines are obtained by finite-difference time-domain (FDTD) simulations (Lumerical) using the corresponding refractive indices and triangular shaped antennas. For these simulations, the used length, shape, and arrangement of the antennas in a hexagonal pattern have been taken from SEM images of the samples. For ideal linear antennas, the relation between antenna length and resonance wavelength is given by L = λres/(2n), which is plotted as dashed lines for the different substrates. As can be seen in Figure 3a, this is still a good approximation for triangular micrometersized gold antennas. However, the match between experiment, simulation, and the simple λ/2 model differs strongly depending on the substrate material. Simulation and model match quite well for Ge and Si and differ more for ZnSe, ZnS, and CaF2. The measurements are well-described by the model in the case of Ge and ZnSe and by the simulation in the case of ZnS and ZnSe. The Si data lies below both theoretical methods. For the low index material CaF2, the simple λ/2 model and the simulation differ strongly; however, the experimental data lies between both theories. All measurements show scattering of the data points. We attribute this to the variation in the structure geometry within the measurement area and systematic errors in the length measuring process. The length variation of antennas in an area of around 400 square micrometers still seems to be large enough to introduce deviations. Figure 3b gives an overview of a fraction of the large possible tuning range accessible and of the variation of the achieved structure sizes for certain fixed sphere sizes. The achieved resonances cover the regions presented in Figure 1. As already mentioned, the actual structure size is influenced by the position on the sample holder during the evaporation process or, generally, on the evaporation angle. This leads to a distribution in the triangle length for every sphere diameter rather than a fixed length and thus a variation in the spectral resonance position. However, this variation can be beneficial because the controlled lateral shift of the sample or tilting of the sample holder allows for a finer tuning of the structure size and, therefore, of the resonance position. This feature can be used for exact resonance matching of different samples, as will be shown below. In Figure 4, we present two representative FTIR measurements of antenna enhanced infrared spectroscopy, taken in reflection mode with a gold mirror as the reference area. For this experiment, we use Si (blue line) and CaF2 (black line) substrates. The antennas’ spectral resonance positions of both samples are matched to the absorption band at 1725 cm−1 of a PMMA layer. To ensure the comparability and to avoid different line shapes,7 we used the before mentioned fine-tuning to exactly match the resonance of the triangles on Si substrate to the ones on CaF2. A reference measurement of PMMA on a solid gold film is presented in green. All spectra show the distinct dip at around 1725 cm−1 caused by the absorption of the stretching vibration of the CO functional bond within the PMMA molecule. The features around 2300 cm−1 are due to absorption of CO2, present in the atmosphere. The occurrence of this feature may change drastically between different measurements because the experiments are carried out under ambient conditions and no housing for the spectrometermicroscope is used. Fits to the antenna spectra excluding the range of the PMMA absorption band are shown in red. The reference absorption spectrum and the baseline corrected

Figure 3. (a) Spectral resonance position λres in dependence of the antenna length for different substrate materials. Experimental data (symbols), FDTD simulations (solid lines), and calculations using the equation for the fundamental mode of ideal linear antennas: λres = 2nL (dashed lines). The error bars on the symbols reflect the errors from the length measurement of the antennas. (b) Overview of the spectral resonance positions for each substrate material and the structure−size variation for equal sphere sizes. A rough guide for the material transmittance is highlighted in blue and given as corresponding percentage for 2−3 mm thick substrates. The minimum and maximum antenna lengths are labeled for each substrate.

3.4; Ge, n = 4.0) and several different structure sizes. Experimental data points are given as symbols for the different substrate materials (CaF2, black square; ZnSe, red circle; ZnS, green triangle; Si, blue inverted triangle; Ge, orange diamond). The error bars are determined by the standard deviation of C

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Figure 4. SEIRA spectroscopy using triangular gold antennas on silicon (blue line, with an offset of −0.23 for clarity) and calcium fluoride (black line) as substrate materials and a reference spectrum of PMMA on a solid gold film (green). The red curves represent polynomial fits to the measured data. The inserted curves (bottom) show the reference absorption on a gold film and the enhanced absorption spectra of PMMA on antennas.

Figure 5. S-SNOM images of samples with (a) Si and (b) CaF2 substrates (note the different signal amplitude (S2) scales). The white arrows mark the incidence direction of the light as well as the projection of its electric field on the sample plane, which defines the orientation of the near-field pattern. Line profiles along the black and red dashed lines in panels a and b of samples with (c) Si and (d) CaF2 substrates show the increased near-field for CaF2.

antenna-enhanced absorption spectra, given by the difference between fits and measurements, are shown at the bottom. The 35 nm PMMA layer on a pure gold film leads to an absorption of nearly 0.45%. On the antenna samples, the absorption is larger for the CaF2 substrate with 3.1% compared to 2.4% on Si substrate, resulting in enhancement factors of roughly 5.3 to 6.9. These are the experimentally relevant factors for the used PMMA layer thickness, but they can further be increased by improving the sample layout. The actual enhanced signals originate only from the small volume around the antenna tips, which exhibit the strong local fields.7,27 For the SEIRA enhancement, the analyte outside of the antenna fields is unnecessary. Therefore, it is possible to drastically reduce the amount of sample material to this volume without losing signal in the absolute absorption spectra. Compared to optimized arrays of linear antennas prepared by electron beam lithography, two characteristics of the NSL antennas can lead to smaller enhancement: First, the hexagonal pattern of the array prevents the controlled antenna positioning for constructive interference as possible for structures prepared by electron beam lithography.8 Second, reduced extinction efficiency of triangular antennas compared to linear antennas can result in less effective antenna excitation.26 The difference of 0.7% in the enhanced absorption spectra of both samples is interesting because the PMMA layer thickness of 35 nm on CaF2 is even smaller than the 41 nm on the silicon sample but still shows a stronger absorption feature. Additionally, the number of measured antennas on CaF2 is decreased because the measurement area stays the same, whereas the size of the structures and the array is different. Therefore, in the case of CaF2 substrate, less antennas contribute to the signal, but a larger enhancement is achieved. Much higher absorbance for sample systems with CaF2 substrate compared to Si substrate has also been shown in simulations.10 A possible explanation may be found in the near-field behavior of the antennas. To clarify this, Figure 5 shows two near-field measurements carried out with a scattering-type scanning near-field optical microscope (s-SNOM from Neaspec). An s-SNOM probes the near-fields of a sample with subwavelength resolution28 allowing, e.g., antenna field mapping27,29,30 and near-field spectroscopy.31−34 Figure 5a,b shows optical near-field images of parts of the actual areas used for the measurements in Figure 4 with Si and CaF2 substrate, respectively. As explained in our previous work, triangular

antennas scanned with a metallic tip show a characteristic signal gradient of the near-field when excited in resonance.32 In the visible spectral range, similar patterns around triangular nanostructures have been imaged.35 Here, they could be explained by a position-dependent coupling of the tip dipole with its image dipole. However, this explanation is not suitable for our case: if imaged with infrared light of a wavelength not matching the spectral antenna resonance, no resonance patterns are observed (not shown). In this case, only a homogeneous material contrast is visible. However, in this work, the shape of the resonance pattern is not the main subject. Instead, we merely focus on the comparison of the increased antenna fields of resonant structures on Si and CaF2 substrate. The line profiles along one triangle on each substrate are shown in Figure 5c,d, respectively. One can see that the maximum absolute value obtained on CaF2 is roughly 2.5 times larger than that obtained on Si. To avoid a change in the absolute signal level between both samples, identical laser power is used for both measurements, and the samples have been switched while the laser was running (quantum cascade laser from Daylight Solutions). Additionally, because of the higher reflectance of Si compared to CaF2, the tip is around 1.77 times stronger irradiated by indirect illumination. However, the stronger nearfields of the antennas on CaF2 compensate for this effect. Furthermore, the distortion of the antenna field due to the metallic tip32,36 may be different for the triangles of different size, but because of the same shape and similar field pattern, we do not expect this to be the main influence for the huge difference in the antenna fields. In conclusion, the measurements show a clearly higher near-field enhancement of antennas on CaF2. The differences in these local antenna fields have direct influence on the measured far-field because the intensity of the infrared absorption scales nonlinearly with the square of the local field enhancement,37,38 and the scattering, also part of the extinction, scales with the local field enhancement to the power of four.39 A possible explanation for the higher fields may be found in literature, where it has been shown that focused near-fields of transmission lines on low index materials such as CaF2 exceed those on higher index materials such as Si.40 This is caused by the decreased damping of the surface plasmons due to weaker interaction of the electric field with the lossy metal in the case D

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of CaF2.40,41 Another simplified interpretation for the damping of a geometrically confined, resonant antenna can be found in the method of image charges.42−44 Image charges in the substrate, induced by the resonantly excited antenna, reduce the strength of the antenna dipole. With increasing refractive index of the substrate material, the strength of the induced charges grows, leading to an increased damping of the antenna resonance. The larger field enhancement of the antennas on CaF2 can compensate the larger quantity of the smaller antennas on Si. Beyond that, optimized placement of the analyte material only in the area of the enhanced antenna fields will benefit even more from the stronger fields of antennas placed on low index materials such as CaF2.

(5) Jensen, T.; Van Duyne, R.; Johnson, S.; Maroni, V. SurfaceEnhanced Infrared Spectroscopy: A Comparison of Metal Island Films with Discrete and Nondiscrete Surface Plasmons. Appl. Spectrosc. 2000, 54, 371−377. (6) Aroca, R.; Ross, D.; Domingo, C. Surface-Enhanced Infrared Spectroscopy. Appl. Spectrosc. 2004, 58, 324A−338A. (7) Neubrech, F.; Pucci, A.; Cornelius, T. W.; Karim, S.; GarciaEtxarri, A.; Aizpurua, J. Resonant Plasmonic and Vibrational Coupling in a Tailored Nanoantenna for Infrared Detection. Phys. Rev. Lett. 2008, 101, 157403. (8) Adato, R.; Yanik, A. A.; Amsden, J. J.; Kaplan, D. L.; Omenetto, F. G.; Hong, M. K.; Erramilli, S.; Altug, H. Ultra-Sensitive Vibrational Spectroscopy of Protein Monolayers with Plasmonic Nanoantenna Arrays. Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 19227−19232. (9) Pucci, A.; Neubrech, F.; Weber, D.; Hong, S.; Toury, T.; De La Chapelle, M. L. L. Surface Enhanced Infrared Spectroscopy Using Gold Nanoantennas. Phys. Status Solidi B 2010, 247, 2071−2074. (10) Liberman, V.; Adato, R.; Jeys, T. H.; Saar, B. G.; Erramilli, S.; Altug, H. Rational Design and Optimization of Plasmonic Nanoarrays for Surface Enhanced Infrared Spectroscopy. Opt. Express 2012, 20, 11953−11967. (11) Fischer, U.; Zingsheim, H. H. Submicroscopic Pattern Replication with Visible Light. J. Vac. Sci. Technol. 1981, 19, 881−885. (12) Deckman, H.; Dunsmuir, J. J. Natural Lithography. Appl. Phys. Lett. 1982, 41, 377−379. (13) Hulteen, J.; Treichel, D.; Smith, M.; Duval, M.; Jensen, T.; Van Duyne, R. Nanosphere Lithography: Size-Tunable Silver Nanoparticle and Surface Cluster Arrays. J. Phys. Chem. B 1999, 103, 3854−3863. (14) Haynes, C. L.; Van Duyne, R. P. Nanosphere Lithography: A Versatile Nanofabrication Tool for Studies of Size-Dependent Nanoparticle Optics. J. Phys. Chem. B 2001, 105, 5599−5611. (15) Fischer, U. C.; Heimel, J.; Maas, H. J.; Hartig, M.; Hoeppener, S.; Fuchs, H. Latex Bead Projection Nanopatterns. Surf. Interface Anal. 2002, 33, 75−80. (16) Jensen, T. R.; Malinsky, M. D.; Haynes, C. L.; Van Duyne, R. P. Nanosphere Lithography: Tunable Localized Surface Plasmon Resonance Spectra of Silver Nanoparticles. J. Phys. Chem. B 2000, 104, 10549−10556. (17) Chan, G. H.; Zhao, J.; Hicks, E. M.; Schatz, G. C.; Van Duyne, R. P. Plasmonic Properties of Copper Nanoparticles Fabricated by Nanosphere Lithography. Nano Lett. 2007, 7, 1947−1952. (18) Chan, G. H.; Zhao, J.; Schatz, G. C.; Duyne, R. P. V. Localized Surface Plasmon Resonance Spectroscopy of Triangular Aluminum Nanoparticles. J. Phys. Chem. C 2008, 112, 13958−13963. (19) Patoka, P.; Giersig, M. Self-Assembly of Latex Particles for the Creation of Nanostructures with Tunable Plasmonic Properties. J. Mater. Chem. 2011, 21, 16783. (20) Duval Malinsky, M.; Kelly, K. L.; Schatz, G. C.; Van Duyne, R. P. Nanosphere Lithography: Effect of Substrate on the Localized Surface Plasmon Resonance Spectrum of Silver Nanoparticles. J. Phys. Chem. B 2001, 105, 2343−2350. (21) Kosiorek, A.; Kandulski, W.; Chudzinski, P.; Kempa, K.; Giersig, M. Shadow Nanosphere Lithography: Simulation and Experiment. Nano Lett. 2004, 4, 1359−1363. (22) Gwinner, M. C.; Koroknay, E.; Fu, L.; Patoka, P.; Kandulski, W.; Giersig, M.; Giessen, H. Periodic Large-Area Metallic Split-Ring Resonator Metamaterial Fabrication Based on Shadow Nanosphere Lithography. Small 2009, 5, 400−406. (23) Cataldo, S.; Zhao, J.; Neubrech, F.; Frank, B.; Zhang, C.; Braun, P. V.; Giessen, H. Hole-Mask Colloidal Nano Lithography for LargeArea Low-Cost Metamaterials and Antenna-Assisted Surface-Enhanced Infrared Absorption Substrates. ACS Nano 2012, 6, 979−985. (24) Zhao, J.; Zhang, C.; Braun, P. V.; Giessen, H. Large-Area LowCost Plasmonic Nanostructures in the NIR for Fano Resonant Sensing. Adv. Mater. 2012, 24, OP247−OP252. (25) Weber, D. Nanogaps for Nanoantenna-Assisted Infrared Spectroscopy. Ph.D. thesis, Ruperto-Carola-University of Heidelberg, 2011.

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CONCLUSIONS In summary, we have presented the huge flexibility of nanosphere lithography (NSL) as alternative fabrication method of nanoantennas for SEIRA spectroscopy. Polystyrene spheres as shadow mask for lithography enable a low-cost, fast, and easy fabrication method with the possibility to create structures on various substrate materials. The variety in the refractive index of possible substrate materials exceeds those for the visible spectrum, which allows for a larger tunability. As a result, it is possible to cover the infrared fingerprint and functional bonds regions from 3 to 13 μm. The possibility for fine-tuning the resonance position allowed us to compare two SEIRA measurements of PMMA on perfectly matching antenna-resonances on different substrates, resulting in SEIRA spectra, which are enhanced 5.3 to 6.9 times over that obtained on a pure gold film. Additional near-field scans with a s-SNOM reveal the larger local fields of antennas on calcium fluoride substrate compared to those on silicon substrate. These increased fields can compensate the larger quantity of the smaller antennas in a same sized measurement area on Si substrate, resulting in an even larger enhancement factor.

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AUTHOR INFORMATION

Corresponding Author

*(T.T.) E-mail: [email protected]. Present Address ∥

Integrated Photonics Laboratory, RWTH Aachen University, 52074 Aachen, Germany. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Excellence Initiative of the German federal and state governments and the Ministry of Innovation, Science, Research and Technology of the German State of North Rhine-Westphalia and by the Fraunhofer ATTRACT grant No 692220.

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