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Correlating Nanoscopic Energy Transfer and Far-Field Emission to. Unravel Lasing Dynamics in ... Published: January 25, 2018. Letter pubs.acs.org/NanoLett.
Letter pubs.acs.org/NanoLett

Cite This: Nano Lett. XXXX, XXX, XXX−XXX

Correlating Nanoscopic Energy Transfer and Far-Field Emission to Unravel Lasing Dynamics in Plasmonic Nanocavity Arrays Claire Deeb,†,∥ Zhi Guo,§ Ankun Yang,‡,⊥ Libai Huang,§ and Teri W. Odom*,†,‡ †

Department of Chemistry and ‡Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States § Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, United States S Supporting Information *

ABSTRACT: Excited-state interactions between nanoscale cavities and photoactive molecules are critical in plasmonic nanolasing, although the underlying details are less-resolved. This paper reports direct visualization of the energy-transfer dynamics between twodimensional arrays of plasmonic gold bowtie nanocavities and dye molecules. Transient absorption microscopy measurements of single bowties within the array surrounded by gain molecules showed fast excited-state quenching (2.6 ± 1 ps) characteristic of individual nanocavities. Upon optical pumping at powers above threshold, lasing action emerged depending on the spacing of the array. By correlating ultrafast microscopy and far-field light emission characteristics, we found that bowtie nanoparticles acted as isolated cavities when the diffractive modes of the array did not couple to the plasmonic gap mode. These results demonstrate how ultrafast microscopy can provide insight into energy relaxation pathways and, specifically, how nanocavities in arrays can show single-unit nanolaser properties. KEYWORDS: Localized surface plasmons, metal nanoparticle arrays, plasmon lasing, transient absorption microscopy, lattice plasmons

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in detail. Moreover, most work has focused on unit shapes in arrays that were cylindrical, in which dipolar lattice modes produced hot spots at the particle edges.20−23,29 Units composed of small gaps between two nanoparticles, however, are more advantageous for squeezing light into deepsubwavelength volumes.30−32 Lasing action has been observed from gold bowtie arrays,33 but the nanoscopic energy-transfer mechanism at the single-bowtie level remains unknown. Here, we report direct observation of energy-transfer dynamics between organic gain molecules and plasmonic bowtie nanocavities. The three-dimensional (3D) bowties were organized into arrays to examine how neighboring units affected nanoscopic kinetics and macroscopic lasing emission. We found that bowties exhibited single-unit behavior when the diffraction modes of the array did not couple to the LSP gap mode. Transient absorption microscopy measurements showed that local excitation decayed at the site of the pumped bowtie, and lasing action was observed with emission normal to the surface and at off-angles. When the bowties were very weakly diffractively coupled, however, energy was transferred from the pumped bowtie to in-plane neighbors, and only amplified spontaneous emission was observed with characteristics that followed the dispersive propagating lattice plasmons.

nteractions between localized surface plasmons (LSPs) and molecular excitons at the single-particle level underlie plasmon-mediated fluorescence and strong coupling processes.1−5 Coupling strengths depend on both spectral and spatial overlap of dye molecules with the electromagnetic hot spots of the metal nanostructures. For example, fluorescence enhancement depends on spectral overlap between the dye emission and plasmon mode of the nanoparticle as well as the distance between the dye and the particle.6 LSPs can also strongly alter the intensity and spectral profile of emission from nearby fluorescent molecules.7,8 The extent of near-field effects can be visualized by embedding fluorophores into a photopolymer and imaging with scanning probe techniques.9−11 Compared to single nanoparticles, metal nanostructures organized into periodic arrays can produce greater field enhancements on a per-particle basis and surface plasmon resonances with higher quality factors.12−16 When nanoparticles are arranged into arrays of appropriate periodicity, coupling between LSPs of the single unit and diffraction modes of the array can result in collective modes.17−19 These lattice plasmons (or surface lattice resonances) can be used as open nanocavities for lasing action at optical frequencies in the presence of molecular gain.20−24 Although semiquantum models can calculate spatially the subwavelength hot spots that contribute to population inversion and lasing,25,26 and ultrafast dynamics above and below lasing threshold have been measured,21,27,28 the correlation between energy transfer at the nanoscale and emission in the far-field has not been examined © XXXX American Chemical Society

Received: December 12, 2017 Revised: January 15, 2018 Published: January 25, 2018 A

DOI: 10.1021/acs.nanolett.7b05223 Nano Lett. XXXX, XXX, XXX−XXX

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Nano Letters

Figure 1. 3D plasmonic bowtie nanocavities with tunable periodicities and spectrally engineered gap mode. (a) (Left) Scheme of plasmonic bowtie arrays and femtosecond transient absorption microscopy performed on a single nanocavity. A total of three different array periodicities (a0 = 346, 600, and 1200 nm) were tested. The IR-140-BA gain medium is shown in pink. (Right) Jablonski energy diagram showing IR-140 molecules photoexcited to the upper vibrational states of S1. (b) IR-140 PL in BA (red) and absorption of a0 = 1200 nm bowties (black). Inset: SEM image of gold bowtie arrays with enlarged view of a single unit. (c) Kinetics obtained on and away from single bowties (a0 = 1200 nm). The solid lines are fits to the data. Inset: zoomed-in view of early time delays.

Figure 1a represents a cartoon of 3D gold bowtie arrays with periodicity a0 surrounded by liquid gain. Gold bowtie nanocavities were fabricated by PEEL12−16,19,30,34 (a process combining photolithography, etching, electron-beam deposition, and lift-off, see the Methods section in the Supporting Information) and designed to support a LSP gap mode resonance λLSP at ca. 885 nm in a refractive index (n) = 1.52 to ensure spectral overlap with the photoluminescence (PL) of IR140 dye in benzyl alcohol (BA)35 centered at ca. 875 nm (Figure 1b). Different periodicities (a0 = 600 and 1200 nm square array; a0 = 346 nm hexagonal array) were selected so that the diffractive modes36 would or would not couple to the LSP gap mode. The hexagonal array structure had an effective 346 nm square lattice spacing (a0 = 400 × sin 60° = 346 nm).37 Only a0 = 600 nm arrays showed a poor-quality lattice plasmon; the other periodicities did not produce diffractively coupled lattice plasmon modes (Figure S1). We characterized the energy transfer between dyes and bowties by transient absorption (TA) spectroscopy and microscopy.38−43 Single bowties surrounded by IR-140-BA were selectively pumped, and then the decay of the local excitation was spatially resolved as a function of probe time. The time resolution of the energy migration pathways was ∼250 fs. Energy-transfer kinetics were first performed on and away from single 3D bowties in the a0 = 1200 nm array (Figure 1c). Dye molecules were excited at 800 nm with a diffraction-limited pump spot size of ca. 320 nm, and excited-state absorption (ESA) was monitored at 580 nm (see the Methods section in the Supporting Information). Photoexcited states from IR-140 had lifetimes of ∼1 ns (Figures 1c and S2). When the dye surrounded gold bowties, the TA kinetics yielded spectral features similar to the IR-140 dye only when the pump beam was polarized perpendicular to the bowtie axis (Figure 1c).

However, when the pump was polarized parallel to the dimer axis, fast-decay components were present, and TA kinetics could be fit to a sum of two exponentials. This polarization dependence confirms that the fast-decay component is due to interactions between the orientation of transition dipoles of the dye and the localized plasmon fields. The fast-decay component (2.8 ± 0.1 ps) was the dominant relaxation pathway for dye molecules near the bowties. This shorter lifetime reflects the response of a single bowtie compared to 15 ps reported from an ensemble,33 whose longer lifetime may be attributed to small changes in gap sizes among bowties from slight fabrication differences. We characterized 25 individual bowties, and similar kinetics with average fast-decay component of 2.6 ± 1 ps were measured for each. The slow-decay component was 2 ns, representing the dye population that did not interact with the plasmonic particles and was longer than the 1 ns lifetime from IR-140-BA, which suggests that dye lifetimes depend on the surrounding dielectric environment.33 Similar kinetics were also observed from individual bowties in a0 = 346 nm arrays, suggesting that the dye transferred its energy to the LSP gap mode (Figure S3). In the a0 = 600 nm array, however, a slightly shorter fast-decay component (1.9 ps, Figure S3) was observed. To visualize whether bowtie cavities coupled to neighboring units or acted as isolated dimers, we performed transient absorption microscopy (TAM) measurements on 3D gold bowtie arrays with the same unit structure (same LSP) but with different a0 (different diffractive modes). These measurements allowed us to differentiate between plasmonic contributions associated with local energy decay and photonic effects from diffractively coupled bowties. Figure 2a illustrates a TAM image collected at 1 ps pump−probe delay (close to time zero) on an array with a0 = 1200 nm for which the pump and probe beams overlapped in space. A 1 ps delay was chosen to be close but B

DOI: 10.1021/acs.nanolett.7b05223 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 2. Spatial mapping of energy-transfer dynamics in plasmonic nanocavity arrays. (a) TAM image collected at early pump−probe delay on an array with a0 = 1200 nm. Pump and probe beams overlapped spatially, and a piezo electric stage was used to scan the sample to construct TAM images. (b) Excited-state population distribution maps collected on a0 = 1200 nm. In the diffusion maps, the pump beam was fixed, while the probe beam was scanned by a pair of galvanometer mirrors. TAM image and excited-state population distribution maps with (c,d) a0 = 346 nm and (e,f) a0 = 600 nm. The dotted circles in panels a, c, and e show the position of the bowties. The arrows in panels b, d, and f indicate the pump beam position. Data were collected for a pump beam aligned with the dimer axis and are expressed in microvolts on the color scale.

the dye decayed at the same position as the bowtie; no energy transfer was observed between array units for a0 = 1200 nm bowties. Based on the similar kinetics of individual bowties within the array (Figure S3), we expected that a0 = 346 nm bowties would show similar excited-state population distribution maps (Figure 2c). Indeed, single bowties at 1 and 4 ps time delays decayed at the same position as excitation and with no energy transfer to neighboring bowties (Figure 2d). The additional red spots around the pumped bowtie suggest that the first ring of the Airy disk weakly pumped the surrounding cavities. In-plane electromagnetic interactions can exist between individual bowties of the array when diffractive modes couple with the LSP of the units.14,44,45 To determine how lattice spacing affected energy transfer, we selected a0 = 600 nm because the bowties could diffractively couple; note that because the LSP gap mode was not fully optimized for this array spacing, only a poor-quality lattice plasmon mode resulted (Figure S1c). The excited-state population distribution maps were collected at pump−probe delays of 1, 3, and 9 ps and showed clear energy transfer from the pumped bowtie to its neighbors (Figures 2e,f, S4, and S5). Near-field interactions can influence far-field responses, including fluorescent enhancement,5 strong coupling,46 and lasing.21,47 To correlate nanoscale energy transfer recorded by

shorter than the fast-decay component determined from the kinetics in Figure 1c (2.6 ± 1 ps). The TAM images display pump-induced change in the probe transmission (ΔT) with high signal levels (red spots) corresponding to ground-state bleaching (GSB) of dye molecules because of their interaction with gold bowties and low signal levels (blue) indicating photoinduced ESA of the dye. The different signals from bowties can be attributed to some structural inhomogeneity during the process of fabricating 3D bowtie nanoparticles. Only bowties of high quality showed a fast-decay component in the kinetics (red spots). To map the excited-state population dynamics, we fixed the pump beam (spot size ca. 320 nm) on a single bowtie in the TAM image and raster-scanned the probe beam at different pump−probe delays (see the Methods section in the Supporting Information). Hence, the population distribution of dye molecules at a given pump−probe time delay could be observed. Figure 2b depicts a spatial map of the ultrafast dynamics of a0 = 1200 nm bowties at pump−probe delays of 1 and 3 ps, relative times that were chosen because they were also close to but shorter than the measured fast-decay component. At 1 ps, the TAM image represents the initial photogenerated population created by the pump pulse (red spot, indicated by black arrow); at later delay times, the image map reflects energy migration away from the initial excitation. Note that the ESA of C

DOI: 10.1021/acs.nanolett.7b05223 Nano Lett. XXXX, XXX, XXX−XXX

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Figure 3. Lasing action of bowtie arrays with a0 = 1200 nm. (a) Output signals collected at different emission angles α. IR-140-BA surrounding bowtie arrays was pumped with an 800 nm femtosecond-pulsed laser, and light was collected from normal emission (α = 0°) to 50°. (b) FDTD simulated angle-resolved transmission data on bowtie arrays embedded in IR-140-BA gain medium. Black dots indicate the wavelength of the lasing peak at different emission angles. Light was aligned parallel to the bowtie axis. (c) Input−output light−light curve for different input pump pulse energies collected at α = 0°. (d) Line width and output intensity of lasing signal as a function of the pump energy.

Figure 4. Measured angle-resolved optical transmission spectra and corresponding far-field signals of Au bowtie arrays. (a) Angle-resolved transmission measurements on bowtie arrays with a0 = 346 nm in n = 1.52 showing that the LSP did not overlap with any diffraction modes. Black dots indicate the wavelength of the lasing peak at different emission angles. (b) Output lasing signals collected at different emission angles α on a0 = 346 nm. (c,d) Angle-resolved transmission measurements and ASE signals on bowtie arrays with a0 = 600 nm. Black dots represent peak positions of the ASE emission at different angles that follow the dispersive propagating lattice mode.

the LSP gap mode of the bowties.48 Calculations have shown that population inversion is confined to subwavelength vicinities (≤25 nm) of plasmonic nanoparticles and that only these regions effectively participate in lasing action.33,47 We expected similar behavior in single bowties with their intense optical fields localized within the gaps. Simulated angle-resolved transmission data of the bowtie arrays showed that secondorder diffractive modes [(2, 0) and (0, ± 2)] crossed the LSP gap mode; however, the array units did not appear to be efficiently coupled, and the output response suggested that each cavity acted independently (Figure 3b). This conclusion was

TAM with far-field signals of the bowties surrounded by gain, we used the lasing response as a diagnostic signal. IR-140 dye in BA around the bowtie arrays was optically pumped by 800 nm fs pulses with light polarized along the dimer axis; output signals were collected at emission angles ranging from normal emission (α = 0°) to 50° in steps of 5° (see the Methods section in the Supporting Information). Above a threshold condition (0.08 mJ × cm−2), a0 = 1200 nm arrays showed lasing action at all emission angles α collected (Figure 3a; the full data set is given in Figure S6). Lasing was supported by resonant energy transfer from the excited states of the dye to D

DOI: 10.1021/acs.nanolett.7b05223 Nano Lett. XXXX, XXX, XXX−XXX

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also supported by TAM, where the local excitation decayed at the site of the bowtie (Figure 2b). Lasing spectra of bowties with a0 = 1200 nm showed a well-defined threshold with a dramatic change in slope in the input output light−light curve (Figures 3c,d). A clear onset of stimulated emission occurred at 0.08 mJ × cm−2, and the output intensity was increased by 4 orders of magnitude over the spontaneous emission. Angle-resolved transmission measurements from a0 = 346 nm arrays showed that the LSP did not overlap with any diffraction modes (Figure 4a, dark region) and that no diffractively coupled lattice plasmon modes formed (Figure S1). Lasing action from these bowtie arrays was observed at all measured output angles α (Figure 4b) but with decreasing amplitude as α increased (Figure S7), which again supported that each bowtie functioned as a single nanocavity for lasing. TAM measurements also confirmed this conclusion, in which the energy decayed at the site of the pumped bowtie (Figure 2f). The spatial coherence of the lasing signal was determined by mapping the far-field patterns of emitted light at different distances from the bowtie arrays using a charge-coupled device (CCD) beam profiler (see the Methods section in the Supporting Information).21,22 Above the lasing threshold, directional beam emission normal to the surface with a small divergence angle (θ < 1.5°) was observed (Figure S8). The a0 = 600 nm arrays have first-order diffraction modes at the same locations as the second-order diffraction modes of a0 = 1200 nm arrays. Diffraction modes (0, ±1) and (−1, ±1) overlapped with the LSP band (Figure 4c, dark region), and their coupling produced a poor-quality lattice plasmon mode with ∼20 nm line width (Figure S1). Figure 4d illustrates the output emission signal collected from bowtie arrays with a0 = 600 nm surrounded by IR-140 in BA. Emission signals were representative of amplified spontaneous emission (ASE), a process in which spontaneously emitted photons are amplified by stimulated emission through propagating lattice plasmon mode.49 Because the band-edge mode was not optimized, the plasmon channel that mainly dominated the emission was the propagating mode. Figure S9 shows the complete set of ASE data and highlights how ASE signals followed the [(0, ±1) and (−1, ±1)] modes. TAM measurements were again consistent, such that the local excitation was transferred from the pumped bowtie to its neighbors (Figure 2d). Furthermore, far-field beam-profile measurements showed a diverging ASE signal (Figure S10). Conclusions. In summary, we directly imaged the energytransfer dynamics between dye molecules and plasmonic nanocavities and demonstrated how array periodicity could alter nanoscopic kinetics at the nanoscale and lasing emission in the far-field. Without diffractive-mediated coupling, the bowties operated as isolated units, the local excitation decayed at the site of the pumped bowtie, and lasing was observed in the farfield at a range of different angles. When bowties were diffractively coupled, however, energy transfer from the pumped bowtie to its neighbors was observed, and amplified spontaneous emission following the array modes was detected. Our results reveal the importance of understanding the fundamentals of energy-transfer pathways between plasmonic nanocavity arrays and emitters for structuring individual nanolaser cavities.

Letter

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b05223. Additional details on experimental methods. Figures showing optical spectra, transient decay kinetics, local excitation transfer and development, emission signals, lasing and ASE from Au bowtie arrays, and far-field patterns of ASE and lasing (PDF)



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Claire Deeb: 0000-0002-1323-0660 Ankun Yang: 0000-0002-0274-4025 Libai Huang: 0000-0001-9975-3624 Teri W. Odom: 0000-0002-8490-292X Present Addresses ∥

C.D.: MiNaO, Centre de Nanosciences et de Nanotechnologies (C2N), CNRS, Université Paris-Sud, Université Paris-Saclay, 91460 Marcoussis, France. ⊥ A.Y.: Department of Materials Science and Engineering, Stanford University, Stanford, California 94305, United States. Notes

The authors declare no competing financial interest.



ACKNOWLEDGMENTS This work was supported by the National Science Foundation (NSF) under DMR-1608258 (T.W.O.) and DMR-1306514 (C.D. and T.W.O). This work made use of the NUANCE Center facilities, which are supported by NSF-MRSEC, NSFNSC, and the Keck Foundation. L.H. and Z.G. acknowledge the support from the U.S. Department of Energy Office of Basic Energy Sciences through award no. DE-SC0016356. We thank Yi Hua and Alexander J. Hryn for helping with FDTD simulations and providing photoresist post samples.



REFERENCES

(1) Chen, Y.; Munechika, K.; Ginger, D. S. Nano Lett. 2007, 7 (3), 690−696. (2) Carminati, R.; Greffet, J. J.; Henkel, C.; Vigoureux, J. M. Opt. Commun. 2006, 261 (2), 368−375. (3) Holzmeister, P.; Pibiri, E.; Schmied, J. U. R. J.; Sen, T.; Acuna, G. P.; Tinnefeld, P. Nat. Commun. 2014, 5, 5356. (4) Kinkhabwala, A.; Yu, Z.; Fan, S.; Avlasevich, Y.; Müllen, K.; Moerner, W. E. Nat. Photonics 2009, 3 (11), 654−657. (5) Anger, P.; Bharadwaj, P.; Novotny, L. Phys. Rev. Lett. 2006, 96 (11), 113002. (6) Thomas, M.; Greffet, J. J.; Carminati, R.; Arias-Gonzalez, J. R. Appl. Phys. Lett. 2004, 85 (17), 3863−3865. (7) Ringler, M.; Schwemer, A.; Wunderlich, M.; Nichtl, A.; Kürzinger, K.; Klar, T. A.; Feldmann, J. Phys. Rev. Lett. 2008, 100 (20), 681. (8) Zhao, L.; Ming, T.; Chen, H.; Liang, Y.; Wang, J. Nanoscale 2011, 3 (9), 3849. (9) Deeb, C.; Bachelot, R.; Plain, J.; Baudrion, A.-L.; Jradi, S.; Bouhelier, A.; Soppera, O.; Jain, P. K.; Huang, L.; Ecoffet, C.; Balan, L.; Royer, P. ACS Nano 2010, 4 (8), 4579−4586. (10) Deeb, C.; Zhou, X.; Gérard, D.; Bouhelier, A.; Jain, P. K.; Plain, J.; Soppera, O.; Royer, P.; Bachelot, R. J. Phys. Chem. Lett. 2011, 2 (1), 7−11. E

DOI: 10.1021/acs.nanolett.7b05223 Nano Lett. XXXX, XXX, XXX−XXX

Letter

Nano Letters

(44) Zhou, W.; Hua, Y.; Huntington, M. D.; Odom, T. W. J. Phys. Chem. Lett. 2012, 3 (10), 1381−1385. (45) Zhou, W.; Odom, T. W. Nat. Nanotechnol. 2011, 6 (7), 423− 427. (46) Vakeväinen, A. I.; Moerland, R. J.; Rekola, H. T.; Eskelinen, A. P.; Martikainen, J. P.; Kim, D. H.; Törmä, P. Nano Lett. 2014, 14 (4), 1721−1727. (47) Zhou, W.; Dridi, M.; Suh, J. Y.; Kim, C. H.; Co, D. T.; Wasielewski, M. R.; Schatz, G. C.; Odom, T. W. Nat. Nanotechnol. 2013, 8 (7), 506−511. (48) Stockman, M. I. Nat. Photonics 2008, 2 (6), 327−329. (49) Samuel, I. D. W.; Namdas, E. B.; Turnbull, G. A. Nat. Photonics 2009, 3 (10), 546−549.

(11) Zhou, X.; Soppera, O.; Plain, J.; Jradi, S.; Sun, X. W.; Demir, H. V.; Yang, X.; Deeb, C.; Gray, S. K.; Wiederrecht, G. P.; Bachelot, R. J. Opt. 2014, 16, 114002. (12) Auguié, B.; Barnes, W. L. Phys. Rev. Lett. 2008, 101 (14), 143902. (13) Zou, S.; Janel, N.; Schatz, G. C. J. Chem. Phys. 2004, 120 (23), 10871−10875. (14) Kravets, V. G.; Schedin, F.; Grigorenko, A. N. Phys. Rev. Lett. 2008, 101, 087403. (15) Zou, S.; Schatz, G. C. Chem. Phys. Lett. 2005, 403 (1−3), 62− 67. (16) Zou, S.; Schatz, G. C. J. Chem. Phys. 2004, 121 (24), 12606. (17) Chu, Y.; Schonbrun, E.; Yang, T.; Crozier, K. B. Appl. Phys. Lett. 2008, 93 (18), 181108. (18) Vecchi, G.; Giannini, V.; Gómez Rivas, J. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 80 (20), 201401. (19) Hicks, E. M.; Zou, S.; Schatz, G. C.; Spears, K. G.; Van Duyne, R. P.; Gunnarsson, L.; Rindzevicius, T.; Kasemo, B.; Käll, M. Nano Lett. 2005, 5 (6), 1065−1070. (20) Zhou, W.; Dridi, M.; Suh, J. Y.; Kim, C. H.; Co, D. T.; Wasielewski, M. R.; Schatz, G. C.; Odom, T. W. Nat. Nanotechnol. 2013, 8 (7), 506−511. (21) Yang, A.; Hoang, T. B.; Dridi, M.; Deeb, C.; Mikkelsen, M. H.; Schatz, G. C.; Odom, T. W. Nat. Commun. 2015, 6, 1−7. (22) Wang, D.; Yang, A.; Wang, W.; Hua, Y.; Schaller, R. D.; Schatz, G. C.; Odom, T. W. Nat. Nanotechnol. 2017, 12 (9), 889−894. (23) Hakala, T. K.; Rekola, H. T.; Vakeväinen, A. I.; Martikainen, J.P.; Necada, M.; Moilanen, A. J.; Torma, P. Nat. Commun. 2017, 8, 1− 7. (24) Deeb, C.; Pelouard, J.-L. Phys. Chem. Chem. Phys. 2017, 19, 29731−29741. (25) Dridi, M.; Schatz, G. C. J. Opt. Soc. Am. B 2013, 30 (11), 2791− 2797. (26) Dridi, M.; Schatz, G. C. J. Opt. Soc. Am. B 2015, 32 (5), 818. (27) Zhou, X.; Deeb, C.; Kostcheev, S.; Wiederrecht, G. P.; Adam, P. M.; Béal, J.; Plain, J.; Gosztola, D. J.; Grand, J.; Félidj, N.; Wang, H.; Vial, A.; Bachelot, R. ACS Photonics 2015, 2 (1), 121−129. (28) Röder, R.; Sidiropoulos, T. P. H.; Tessarek, C.; Christiansen, S.; Oulton, R. F.; Ronning, C. Nano Lett. 2015, 15 (7), 4637−4643. (29) Schokker, A. H.; Koenderink, A. F. Phys. Rev. B: Condens. Matter Mater. Phys. 2014, 90 (15), 155452. (30) Suh, J. Y.; Huntington, M. D.; Kim, C. H.; Zhou, W.; Wasielewski, M. R.; Odom, T. W. Nano Lett. 2012, 12 (1), 269−274. (31) Hao, E.; Schatz, G. C. J. Chem. Phys. 2004, 120 (1), 357. (32) Fischer, H.; Martin, O. J. F. Opt. Express 2008, 16, 9144−9154. (33) Suh, J. Y.; Kim, C. H.; Zhou, W.; Huntington, M. D.; Co, D. T.; Wasielewski, M. R.; Odom, T. W. Nano Lett. 2012, 12 (11), 5769− 5774. (34) Henzie, J.; Lee, M. H.; Odom, T. W. Nat. Nanotechnol. 2007, 2 (9), 549−554. (35) IR-140. https://www.photonicsolutions.co.uk/upfiles/IR-140. pdf. (36) Gao, H.; McMahon, J. M.; Lee, M. H.; Henzie, J.; Gray, S. K.; Schatz, G. C.; Odom, T. W. Opt. Express 2009, 17, 2334−2340. (37) Yang, A.; Hryn, A. J.; Bourgeois, M. R.; Lee, W.-K.; Hu, J.; Schatz, G. C.; Odom, T. W. Proc. Natl. Acad. Sci. U. S. A. 2016, 113 (50), 14201−14206. (38) Huang, L.; Cheng, J.-X. Annu. Rev. Mater. Res. 2013, 43, 213− 236. (39) Terada, Y.; Yoshida, S.; Takeuchi, O.; Shigekawa, H. Nat. Photonics 2010, 4 (12), 869−874. (40) Gao, B.; Hartland, G. V.; Huang, L. ACS Nano 2012, 6 (6), 5083−5090. (41) Lo, S. S.; Shi, H. Y.; Huang, L.; Hartland, G. V. Opt. Lett. 2013, 38 (8), 1265−1267. (42) Van Dijk, M. A.; Lippitz, M.; Orrit, M. Phys. Rev. Lett. 2005, 95 (26), 267406. (43) Muskens, O. L.; Del Fatti, N.; Vallée, F. Nano Lett. 2006, 6 (3), 552−556. F

DOI: 10.1021/acs.nanolett.7b05223 Nano Lett. XXXX, XXX, XXX−XXX