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Differential Wavevector Distribution of Surface-Enhanced Raman Scattering and Fluorescence in a Film-Coupled Plasmonic Nanowire Cavity Adarsh Vasista, Harshvardhan Jog, Tal Heilpern, Matthew E. Sykes, Sunny Tiwari, Deepak Kumar Sharma, Shailendra Kumar Chaubey, Gary P. Wiederrecht, Stephen K. Gray, and G V Pavan Kumar Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.7b05080 • Publication Date (Web): 15 Dec 2017 Downloaded from http://pubs.acs.org on December 15, 2017

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Dierential Wavevector Distribution of Surface-Enhanced Raman Scattering and Fluorescence in a Film-Coupled Plasmonic Nanowire Cavity α †

Adarsh B. Vasista ,

†

Sunny Tiwari,

α †

Harshvardhan Jog

,

†

‡

‡

Matthew E. Sykes,

†

Deepak K. Sharma,

Wiederrecht,

‡

Tal Heilpern,

Shailendra K. Chaubey,

‡

Stephen K. Gray,

Gary P.

∗,†,¶

and G. V. Pavan Kumar

†Photonics and Optical Nanoscopy laboratory, Department of Physics, Indian Institute of

Science Education and Research, Pune-411008, India ‡Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439,

USA ¶Center for Energy Science, Indian Institute of Science Education and Research,

Pune-411008, India E-mail: [email protected]

Abstract We report on the experimental observation of dierential wavevector distribution of surface enhanced Raman scattering (SERS) and uorescence from dye molecules conned to a gap between plasmonic silver nanowire and a thin, gold mirror. The uorescence was mainly conned to higher values of in-plane wavevectors, whereas SERS signal was uniformly distributed along all the wavevectors. The optical energymomentum spectra from the distal end of the nanowire revealed strong polarisation 1

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dependence of this dierentiation. All these observations were corroborated by fullwave three dimensional numerical simulations, which further revealed an interesting connection between out-coupled wavevectors and paramaters such as hybridized modes in the gap-plasmon cavity, orientation and location of molecular dipoles in the geometry. Our results reveal a new prospect of discriminating electronic and vibrational transitions in resonant dye molecules using a sub-wavelength gap plasmonic cavity in the continuous-wave excitation limit, and can be further harnessed to engineer molecular radiative relaxation processes in momentum space. The study of molecular emission from regions of conned electromagnetic elds has implications not only in fundamental physics of cavity quantum electrodynamics 1,2 and molecular quantum optics, 37 but also in emerging applications such as single-molecule surface enhanced Raman scattering 811 and optical antennae. 1214 To this end, understanding the spectral features and wavevector distribution of molecular emission 12,1519 from conned environments is an important task. In this Letter, we experimentally show how waveguided surface enhanced Raman scattering (SERS) and uorescence from dye molecules conned between a plasmonic silver nanowire and a gold mirror dierentially distribute in momentum space, thanks to the presence of an extended cavity and waveguiding eects in the geometry. Various cavity-based approaches 1,20,21 have been proposed and realized to study molecular emission in conned electromagnetic environments. Of these, gaps between plasmonic nanostructures 2225 have gained prominence because of the enhancement of electric elds due to hybridization of plasmons in the gap. 23 Importantly, plasmonic gaps provide ultra-small mode volumes 13,26 that can be harnessed to enhance the Purcell eect. 20,27,28 In recent times, molecules conned to a gap between a single plasmonic nanoparticle and a plasmonic thin lm have been studied extensively. 17,2931 A variety of interesting prospects such as single molecule SERS, 32 strong coupling physics at the single molecule level, 17 enhanced spontaneous emission 27 and controlled reectance properties 33 have been demonstrated. 2


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A related geometry which has yet to be studied extensively is a chemically-prepared, single crystalline, plasmonic nanowire coupled to a plasmonic thin-lm, in short: lm-coupled plasmonic nanowire (FPN). In this geometry, one can realize an extended one-dimensional gap between the nanowire and the lm, 34,35 and create conned waves with low group velocity 35 and strong electric eld connement in the gap. 26 A unique feature of the FPN geometry is that it concomitantly facilitates a platform to harness eects of a nano-cavity 34 and a nano-waveguide with low scattering losses. 15,36,37 Furthermore, along the long axis of the nanowire, the geometry provides multiple propagating modes, and can be harnessed for remote excitation of molecules 36,38,39 and enhancement of the Purcell eect. 20,34 In the context of the FPN geometry, secondary emission from molecules, such as uorescence and Raman scattering, is yet to be explored. Given that molecules conned to plasmonic gaps have secondary emission characteristics that are dierent 40,41 from the free space emission, it would be interesting to study them in FPN geometry. Specically, it would be relevant to ask the following questions: what will be the wavevector distribution of the waveguided secondary emission (SERS and uorescence) from molecules conned to the gap? What will be the features of polarisation and angle-resolved emission? Answers to such questions can lead to momentum-space engineering of emission of molecules in conned environments and can be further harnessed to control the electronic and vibrational energy decay processes of molecules in a nano-cavity. Potentially, the developed concepts can be extended to probe the coupling strength between a molecule and its environment. A schematic of the system under study is as shown in Fig.1. The gold lm of 170 nm thickness was thermally evaporated on a glass coverslip, over which Nile Blue (NB) molecules were coated using dropcasting. Ag nanowires (NWs) were dropcast on top of this system. The Ag NWs have a polyvinylpyrrolidone (PVP) coating of thickness ∼4 nm. 42,43 The FPN cavity was excited by focussing a 633 nm laser beam using a high numerical aperture objective lens at one end of the wire. Out-coupled light at the distal end was collected from the same objective lens and transformed into momentum space using an optical Fourier transform 3


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and then analysed (see sections S1 and S2 of the Supporting Information for more details on sample preparation and experimental methods).

Figure 1: Schematic of an Ag nanowire (NW) coupled to a 170 nm thick Au lm. The lm was deposited over a glass coverslip. Nile blue (10−6 M) molecules were sandwiched between Ag NW and Au lm. The FPN is excited at one end using a focused 633 nm He-Ne laser beam. The out-coupled light is collected from the distal end using a spatial ltering approach and then analysed for its energy and momentum states. Fig.2(a) shows the optical image of a lm-coupled Ag NW excited by a focused 633 nm laser light at one of the ends. The arrow mark indicates the polarisation of the excitation beam. Input polarisation was longitudinal and along the axis of the nanowire, which excites a propagating mode in the FPN cavity giving maximum out-coupling at the distal end (see section S3 and S4 of the Supporting Information for Input polarisation dependence and mode analysis). The square region indicated at the distal end is the location from which the out-coupled emission was spatially ltered and resolved in terms of energy and in-plane momentum. Note that the location of excitation and signal collection are spatially separated. In Fig.2(b), the optical spectrum obtained from the spatially ltered region is shown. 4


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There are two characteristic features of the spectrum: rst, a sharp line around 590 cm−1 due to the Stokes Raman mode of the NB molecule. The second feature is a broad background spectrum due to uorescence of NB molecules. We were interested in knowing how these two spectral features distribute themselves in momentum or equivalently wave vector (k) space. To this end, we performed k-resolved optical imaging (Fig.2(c)) by capturing the Fourier space image of the emitted light from the spatially ltered distal end of the geometry. An important feature of Fig.2(c) was that the emission intensity was dominant at high values of ky /k0 . Such dominant emission can be attributed to a unidirectional antenna-like emission process, 44,45 where the Ag NW essentially directs the light in specic angles (Fig.1). Of greater interest to us was to resolve the spectrum along ky /k0 indicated by the rectangular region in Fig.2(c). To this end, we performed energy-momentum (E-k) spectroscopy, the result of which is shown in Fig.2(d). There are two interesting features of the E-k spectrum: 1) The broad uorescence-like emission was mainly conned to high values of ky /k0 . 2) The sharp, 590 cm−1 Raman mode appears equally distributed along all values of ky /k0 . This data indicates that the uorescence background and the SERS emission for the given geometry distribute themselves in dierent ways in the k-space. This is the central result of the letter. The cavity between the NW and lm plays a crucial role in the observed eects. The reason why we obtain an enhanced Raman signal from the molecule is because of the Raman hot-spot created between the Au lm and the Ag nanowire. Our geometry of sandwiching the molecular lm between the Au lm and the Ag nanowire is crucial to observe both SERS and uorescence signals simultaneously. To emphasize this we performed a series of control experiments: molecular emission in the absence of the Ag NW; in the absence of the Au lm; signal from a bare Ag NW on a Au lm (absence of molecules) and the eect of a spacer between the Ag NW and Au lm; spectrum captured from molecules on and o the cavity (see sections S5-S9 of Supporting Information). 5


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Figure 2: Fourier space imaging and E-k spectroscopy of a single Ag nanowire.(a) Bright eld image of an Ag nanowire on an Au lm. The incident light is polarised along the axis of the nanowire as shown. (b) Nile Blue spectrum collected from the distal end showing Raman modes overriding uorescence. (c) Fourier space image of the out-coupled light collected from the distal end of the Ag NW-Au lm system after rejecting the Rayleigh component using a notch lter, which shows emission is biased towards higher ky /k0 values. Here, kx and ky are in-plane momentum vectors of the out-coupled light and k0 is wavenumber of light in vacuum. (d) A small portion of the Fourier image around kx /k0 = 0 was ltered using the slit of the spectrometer (200µm wide) and dispersed to obtain the spectra. The energymomentum spectrum clearly shows the dierence in momentum states of the out-coupled Raman and uorescence photons.

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Out-coupled light from the FPN cavity carries specic polarisation signatures which can be probed to understand the coupling of molecules to the cavity modes. To understand the polarisation dependence of the molecular emission from the FPN cavity, we resolved the outcoupled light in two orthogonal polarisation components, viz. longitudinal (along the axis of the wire) and transverse (perpendicular to the axis of the wire). Fig.3(a) shows the output polarisation resolved spectra of the FPN system collected from the distal end of the Ag NW. To understand the emission process further, we measured output polarisation resolved E-k spectra which are shown in Fig.3(b) and (c).

Figure 3: Output polarisation resolved analysis of out-coupled light from distal end of a single Ag NW. (a) Out-coupled light from the Ag NW was resolved along (longitudinal) and perpendicular (transverse) to the nanowire. (b) and (c) are the full energy-momentum spectra of out-coupled light from the distal end of the nanowire resolved along longitudinal and transverse directions, respectively. Fluorescence limits itself to higher ky values for longitudinal polarisation but is distributed over all ky -values for transverse polarisation. On the other hand SERS distributes uniformly over all ky -values for both transverse and longitudinal polarisations. For the longitudinal output polarisation (Fig.3(b)), both the uorescence and SERS signals directionally scatter to higher ky /k0 values. 7

For transverse output polarisation


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(Fig.3(c)), both the uorescence and SERS signals are distributed uniformly over all ky /k0 . However, because the total SERS signal contains relatively more transverse polarisation compared to the uorescence, 2.5 times more (Fig.3(a) and see section S10 of Supporting Information), it is more distributed in momentum space (Fig.2(d)). To understand the origin of dierential wavevector dependence of SERS and uorescence from the FPN cavity, we performed full wave 3D numerical simulations based on the FiniteDierence Time-Domain (FDTD) method. (See section S11 of Supporting Information for additional details of the simulations.) To account for the emission of molecules positioned throughout the gap between the nanowire and gold lm (see section S12 of Supporting Information for spectrum collected from the middle and distal end of the FPN cavity), we considered broadband classical dipoles at six evenly distributed points within the gap structure with orientations along the x, y and z directions, resulting in eighteen dierent simulations. Here y is the longitudinal direction relative to the nanowire and x and z are in-plane and out-of-plane transverse directions, respectively. Calculations were performed separately for each combination of orientation and dipole position, and the resulting eld intensities were added together in order to mimic the incoherent emission from randomly oriented molecules in the nile blue lm (see also 45 ). The out-coupled light was then projected into the far-eld and resolved in terms of the transverse and longitudinal polarisation components. The calculated images in Figs.4(a) and 4(b) correspond to the far-eld transverse (x) and longitudinal (y) intensities in the Fourier plane, respectively, integrated over the energy range corresponding to 350 cm−1 to 1500 cm−1 Raman shift. It is signicant that the longitudinal case, Fig. 4(b), shows a strong degree of directional emission (much larger ky /k0 > 0 content) along kx /k0 = 0, which is qualitatively consistent with the experimental result of Fig. 3(c). In contrast, the transverse emission pattern appears relatively isotropic along the ky /k0 direction but exhibits lobes at non-zero kx /k0 values. This is a result of the symmetry of the coupled nanowire/gold lm; transverse-polarised dipole emission is suppressed along the y-z symmetry plane. We can also consider just the kx /k0 = 0 region 8


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and plot the corresponding spectra resolved as a function of ky /k0 and emission frequency, i.e., no longer frequency-averaged as in Figs.4(a) and (b).

Figure 4: Numerically simulated far eld image and energy-momentum spectra for two orthogonal output polarisations. (a) and (b) are the simulated Fourier space images for the light outcoupled from distal end of the FPN cavity for transverse and longitudinal output polarisations respectively. Excited dipoles were placed at six dierent positions, with x, y and z orientations, inside the cavity separately and the resulting eld intensities were added together to mimic the incoherent emission from random oriented molecules. The outcoupled light was then analysed for two orthogonal output polarisations. (c) and (d) are the corresponding energy - momentum spectra. Figs.4(c) and (d) present the corresponding E-k spectra for the transverse and longitudinal cases, respectively. We should note that it is not possible to achieve an E-k spectrum such as in Figs.3(c) and (d) with a distinct and narrow Raman emission feature using FDTD, owing to the non-quantum-mechanical nature of the calculations. Rather, these spectra should be construed as what to represent a spatially-averaged out-coupling for dipole emission from 9


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within the gap. This out-coupling should be independent of the nature of the emission process, as it is simply a result of the FPN geometry, and should apply to both SERS and uorescence. We see again see in Figs.4(c) and (d) that the longitudinal polarisation case is more directed in the far-eld,whereas the transverse polarisation is less so, consistent with the results of Fig.3. This indicates the longitudinal-polarized dipole emission couples through propagating plasmon modes within the FPN geometry, similar to what is observed in the Rayleigh scattering from samples without nile blue (see section S13 of Supporting Information).

Figure 5: Variation of emission intensity as a function of ky /k0 for three dipolar orientations at Raman shift of 590 cm−1 for (a) transverse and (b) longitudinal output polarisations. Until this point we have focused on the directionality of the emission as it relates to the far-eld polarization. We now consider the relation between dipole orientation and the fareld emission pattern. Fig.5 shows the emission intensity as a function of ky /k0 for the three dipole orientations taken at a Raman shift of 590 cm−1 . In Fig.5(a) the transverse polarization has a broad and at total emission pattern (green dashed curve). We can decompose this signal into contributions from dipoles with x (blue) , y (red) and z (black) orientations. As might be expected, the longitudinal or y-polarized dipoles do not contribute and both x and z-polarized dipoles do contribute. In the case of the longitudinal polarisation, Fig.5(b), 10


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the signal is dominated by z-oriented dipoles (black curve), with very small contribution from the x and y-orientations. Thus the far-eld, longitudinal (y-polarised) signal arises almost exclusively from z-polarised near-eld dipoles in the gap. Consistent with what has been previously noted and in the experiments, the longitudinal polarisation case in Fig.5(b) is also signicantly more directional than the transverse case. As discussed previously, the longitudinal uorescence emission appears relatively stronger, and hence more directed, than the SERS emission at 590 cm−1 . The ratio of the peak heights at this energy indicates SERS is more enhanced for transverse-polarised emission than uorescence. As the outcoupling is identical for both processes, this would imply that x-oriented dipoles are preferentially enhanced for SERS in the FPN geometry. This may explain why we observe dierential wavevector distributions for uorescence and SERS (Fig.2(d)) and warrants further investigation. The Supporting Information, sec. S11, contains additional FDTD results concerning specic dipole positions and polarisations. To conclude, we have shown how surface enhanced Raman scattering and uorescence emission from molecules conned to a nanowire gap-plasmon cavity dierentially distribute themselves in momentum space. This dierential out-coupling has a strong dependence on emission polarisation, which was revealed by energy-momentum spectroscopy experiments and numerical simulations. Our analysis not only shows the role of hybridized modes in inuencing and enhancing the emission process, but also explicitly reveals an intricate connection between the out-coupled wavevectors and the orientation and location of the molecular dipole in the geometry. All these results highlight the capability of the nanowire gap-plasmon cavity to inuence and discriminate competing molecular emission processes such as Raman scattering and uorescence, without having to utilize any time-gating techniques. It would be interesting to extend our experiments to include oriented molecular layers and two-dimensional materials such as MoS2 , where the dipole orientation is xed, and hence the emission processes can be precisely controlled and varied by engineering the cavity parameters. It would also be interesting to look at the strong coupling eects in 11


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FPN cavity. These eects will be dominant when the nanowire diameter is small enough to produce a large eld localisation, rather than propagation of light. Such studies, if done at the single emitter limit and under controlled experimental conditions, can be potentially harnessed to explore cavity molecular quantum electrodynamics eects.

Acknowledgement This work was partially funded by DST-Nano mission Grant, Govt. of India (SR/NM/NS1141/2012(G)) and Center for Energy Science (SR/NM/TP-13/2016). G.V.P.K., G.P.W. and S.K.G. thank IUSSTF grant, Quantum Plasmonics of Hybrid Nano-Assembles/ JC-32014. A.B.V, H.J. and G.V.P.K. thank Mr. Prashant Kale for technical support. H.J. thanks DST, Govt. of India for KVPY fellowship. A.B.V. and H.J. thank Dr. Danveer Singh for his contributions in sample preparation. The theory and analysis work was performed, in part, at the Center for Nanoscale Materials, a U.S. Department of Energy Oce of Science User Facility, and supported by the U.S. Department of Energy, Oce of Science, under Contract No. DE-AC02-06CH11357. α

authors contributed equally to the work

Supporting Information Available Supplementary material containing following details is available free of cost.

• Sample preparation • Experimental setup • Results of control experiments • Details of numerical simulation

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(45) Dasgupta, A.; Singh, D.; Tripathi, R. P.; Kumar, G. P. 1769217698.

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Nano Letters

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Differential Wavevector Distribution of Surface ... - ACS Publications

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