Correlating Nanoscopic Energy Transfer and Far-Field

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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 Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b05223 • Publication Date (Web): 25 Jan 2018 Downloaded from http://pubs.acs.org on January 28, 2018

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Correlating Nanoscopic Energy Transfer and Far-Field Emission to Unravel Lasing Dynamics in Plasmonic Nanocavity Arrays Claire Deeb1†, Zhi Guo2, Ankun Yang3†, Libai Huang2 and Teri W. Odom1,3* 1

Department of Chemistry, Northwestern University, Evanston, Illinois, 60208, USA.

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Department of Chemistry, Purdue University, West Lafayette, Indiana 47907, USA.

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Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois,

60208, USA.

*Correspondence to: [email protected]

Current address: MiNaO - Centre de Nanosciences et de Nanotechnologies (C2N), CNRS,

Université Paris-Sud, Université Paris-Saclay, 91460 Marcoussis, France (C.D.); Department of Materials Science and Engineering, Stanford University, Stanford, California 94305 (A.Y.).

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Abstract Excited-state interactions between nanoscale cavities and photoactive molecules are critical in plasmonic nano-lasing although the underlying details are less resolved. This paper reports direct visualization of the energy transfer dynamics between two-dimensional 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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Interactions between localized surface plasmons (LSPs) and molecular excitons at the singleparticle 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 with 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.2024

Although semi-quantum models can calculate spatially the sub-wavelength 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 in detail. Moreover, most work has focused on unit shapes in arrays that were cylindrical, where 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 deep-subwavelength volumes.30-32 Lasing action has

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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. 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. 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 (Methods) 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 IR-140 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 poorquality 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-

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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. 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 photo-excited 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: Zoom-in at early time delays.

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 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 (Methods). Photo-excited states from IR-140 had lifetimes of ~1 ns (Figure 1c and Figure S2). When the dye surrounded gold bowties, the TA kinetics yielded spectral features similar to the

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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 was 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

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close but 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 (∆T) 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).

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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. Dotted circles in (a), (c) and (e) show the position of the bowties. Arrows in (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 µV on the color scale. 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 pumpprobe delays (Methods). Hence, the population distribution of dye molecules at a given pumpprobe 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 ps and 3 ps, relative times that were chosen

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because they were also close to but shorter than the measured fast-decay component. At 1 ps, the TAM image represents the initial photo-generated 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 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-ps 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 ps, 3 ps, and 9 ps and showed clear energy transfer from the pumped bowtie to its neighbors (Figures 2e-f; Figures S4-S5).

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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 TAM with

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 800nm 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-140BA 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) Linewidth and output intensity of lasing signal as a function of the pump energy. 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° (Methods). Above a threshold condition (0.08 mJ × cm-2), a0 = 1200 nm arrays showed lasing action at all emission angles α collected (Figure 3a, full data set in Figure S6). Lasing was supported by resonant energy transfer from the excited states of the dye to the LSP gap mode of the bowties.48 Calculations

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have shown that population inversion is confined to sub-wavelength 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 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 lightlight curve (Figures 3c-d). A clear onset of stimulated emission occurred at 0.08 mJ × cm-2, and the output intensity was increased by four 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, where 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 (Supporting Information, Methods).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).

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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)

Figure 4. Measured angle-resolved optical transmission spectra and corresponding farfield 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) and (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. overlapped with the LSP band (Figure 4c, dark region), and their coupling produced a poorquality lattice plasmon mode with ~20-nm linewidth (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

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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, where 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 energy transfer 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 far-field 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.

ASSOCIATED CONTENT Supporting Information Sample fabrication, optical transmission, lasing measurements, and transient absorption microscopy measurements. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] 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, NSF-NSC and the Keck Foundation. L.H. and Z.G. acknowledge the support from US Department of Energy, Office of Basic Energy Sciences through award DE-SC0016356. We thank Yi Hua and Alexander J. Hryn for helping with FDTD simulations and providing photoresist posts samples.

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