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Directional Fluorescence Emission Mediated by Chemically-Prepared Plasmonic Nanowire Junctions Arindam Dasgupta, Danveer Singh, Ravi P N Tripathi, and G V Pavan Kumar J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b04718 • Publication Date (Web): 25 Jul 2016 Downloaded from http://pubs.acs.org on July 26, 2016

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The Journal of Physical Chemistry

Directional Fluorescence Emission Mediated by Chemically-Prepared Plasmonic Nanowire Junctions Arindam Dasgupta, Danveer Singh, Ravi P. N. Tripathi, and G.V. Pavan Kumar∗ Photonics and Optical Nanoscopy Laboratory, h cross, Indian Institute of Science Education and Research, Pune-411008, INDIA E-mail: [email protected],Telephone:+91-020-2590-8026

∗

To whom correspondence should be addressed

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Abstract The localized interaction between metallic nanostructures and surrounding fluorescent molecules can influence the emission characteristics of the molecule. With this hindsight, herein, by employing a Fourier optical fluorescence microscope, we experimentally show how fluorescence emission from molecules in the vicinity of a chemicallyprepared silver nanowire-dimer-junction can be directed in one or two channels. Measured forward-to-backward ratio of the fluorescence emission in a single channel was as high as 4.3dB, and the observed polar and azimuthal angular spread was as narrow as 150 and 600 , respectively. Interestingly, the angle between the two emission channels mimicked the angle between the nanowires, thus highlighting the prospect of geometrical control of the emitted light. These observations were further corroborated by 3-dimensional finite-difference time-domain simulations. The presented results will have implications in momentum-space engineering of molecular fluorescence emission, and can be extrapolated to single-emitter studies.


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Introduction Effective detection of molecular fluorescence is vital to a variety of imaging technologies used in biology, chemistry and allied research areas. In recent times, there has been enormous amount of research to detect and probe single molecules, at and beyond the diffraction limit of light. 1–4 Concomitantly, there is a significant interest in controlling the directionality of spontaneous emission from molecules. 1 Controlling directionality of emitters has also turned out to be a vital task for optical antenna applications. 5–7 One of the effective ways to influence directionality of spontaneous emission is to couple the emitters to metallic nanostructures that facilitate surface plasmon polaritons (SPPs). 1,8 Such molecule-SPP mode coupling is at the heart of techniques such as surface plasmon coupled emission, 9–13 which can significantly boost the detection sensitivity of fluorescence. A majority of the studies to influence fluorescence directivity utilize plasmonic thin films 9–14 or plasmonic nanoparticles. 5–7,15–17 In such studies the location of fluorescence excitation and detection are same. An interesting prospect would be to have the fluorescence excitation and detection location to be spatially off-set from each other. Such a scenario can be useful to perform remote excitation of fluorescent molecules, and further control its emission directionality. To achieve such capabilities there is an imperative to design metallic nanostructures based on propagating plasmons that can be employed in remote excitation and directionality control of fluorescence at sub-wavelength scale. Motivated by this requirement, herein we show how a fused junction of a serially coupled silver nanowire (Ag NW) dimer can be used as a location where fluorescence can be remotely excited and directed in two separate angular channels. Importantly, these angular emission channels can be designed by controlling the geometrical angle between the Ag NW dimer. When one of the ends of such a nanowire is illuminated, guided SPP modes 18,19 are generated which results in uni-directional emission from the distal end. 20,21 SPPs in chemicallysynthesized nanowires have the unique ability to propagate broadband radiation over a long distances (tens of microns) at sub-wavelength scales. They have also been studied in con3 ACS Paragon Plus Environment


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text of directed fluorescence emission for building optical devices. 22 An advantage of serially coupled silver (Ag) nanowires (NW) is that it facilitates SPPs and LSP on the same platform. 23,24 Highly localized field can be produced at the fused-junction while illuminating either of the free ends. Such junctions can further serve as hotspot for remote excitation of molecular spectroscopy. 23,25 We further use this advantage to direct fluorescence emission.

Experimental Methods Fourier microscope

Figure 1: Schematic of the spatially filtered Fourier optical microscope, built for BFP imaging. M1 , M2 , M3 , M4 are mirrors; L1 , L2 , L3 , L4 are lenses; PBS and BS are Polarization beam splitter and beam splitter respectively. To study the directionality of the emitted photons, we used a specially designed Fourier microscope capable of dual excitation and spatially filtered imaging and fluorescence spectroscopy. In this microscopy method, the back focal plane (BFP) of the microscope objective lens is imaged. Image formed in this plane is used to resolve the directionality of the emitted photons from the spatially filtered sample plane. 26,27 A schematic of the specialized micro4 ACS Paragon Plus Environment

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scope is shown in figure 1. We constructed our set-up on an commercial inverted microscope (olympus i71X). We used a serially coupled Ag NW dimer immobilized on a glass slide as the sample. A laser of 633nm wavelength was split into two and focused on the two ends of the wire through glass substrate using a 1.3NA 100X objective lens. The same lens was used to collect the back-reflected light. The polarization states were controlled using combination of half wave plate and polarization beam splitter. A combination of relay lenses (L1 ,L2 and L3 ), adjusted at the exit port of the microscope provided a set of imaging planes conjugated either with the aperture plane (BFP) or with the field plane (object) of the microscope. A charge coupled device (CCD) camera ( Andor) was used to record the intensity distributions in the respective planes. The spatial filtering of the junction was done using a circular aperture (100µm pinhole) at a conjugate intermediate image plane (see figure 1). BFP was projected on the CCD chip once L3 was taken out of the path. For collection of fluorescence, the light was routed towards spectrometer using a flip mirror after L2 .

Sample preparation for optical studies Ag NW dimers were synthesized by following a well established method based on polyol reduction of Silver Nitrate (AgN O3 ). 23 For detail procedure, see supporting information S1. For current study, chemically synthesized Ag NW dimers were drop casted on a glass micro slide and coated with a 30 nm thick nile blue-A (NBA) doped poly-vinyl alcohol (PVA) layer. A schematic of the prepared sample is presented in figure 2a. NBA has an absorption maximum at 637 nm and the excitation laser used was of 633nm wavelength. The molecules coated on the Ag NW dimer were excited by focusing lasers at free ends and the directionality of the fluorescent light emerging from the vicinity of the junction was further examined in detail.



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Figure 2: Directing fluorescence from molecules in close proximity to Ag NW dimer junction. a) Schematic of sample where a nile blue-doped PVA film was casted over an Ag NW dimer. b) Illustration of image formation in back focal plane of a microscope objective and coordinate system used for directivity measurements. c) Fluorescence Optical image of end 1 illumination of the Ag NW dimer. Inset shows a SEM image of the wire d) BFP image of the emission from the junction. e) Corresponding I vs φ graph. f),g), h) and i), j), k) are the corresponding data for end 2 excitation and end 1 + end 2 excitations, respectively. White dotted circle in the optical images denote the area projected on to the Fourier plane.


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Image formation in BFP The BFP of the microscope objective directly monitors the angular distribution of the farfield light intensity I = I(θ, φ), where θ and φ are polar and azimuthal angles defined in figure 2b.The radial coordinate of the BFP image scales as the numerical aperture (NA) of the objective lens and the relationship is given by,

N A = n.sinθ

(1)

where n=1.52 is the refractive index (RI) of the glass substrate. For our study, we used a 1.3 NA, 100X objective lens. Therefore maximum collection angle was restricted to 590 . The tangential coordinate of BFP on the other hand represents the azimuthal angle φ(0 < φ < 2π). In the schematic (figure 2b), x1 , x2 represent the axes of two wires and k1 , k2 represent the same directions in Fourier plane.

Results and discussions Directional fluorescence emission from Ag NW dimer junction First, the directionality of the fluorescence of the molecule at the junction was monitored by illuminating only one of the free ends at a time. For this experiment, we have used a Ag NW dimer (7µm + 8µm in length, 200nm diameter) coupled at an angle 1070 . Figure 2c shows a fluorescent optical image of the dimer when end 1 of it was excited with a focused laser illumination. Inset in figure 2c is the scanning electron microscope (SEM) image of the same wire. It is evident that the generated propagating SP modes interacted with the molecules coated on the wire which resulted in fluorescence emission through out the wire. The white dotted circle in figure 2c indicates the region of interest (Ag NW dimer junction) that was spatially filtered and projected in the Fourier plane. The BFP image of emission from the junction in figure 2d shows unidirectional fluorescence emission along k1 , same as 7 ACS Paragon Plus Environment


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the direction of orientation of the excited wire. It is to be noted that the emission has a maximum around θmax = 410 , close to glass-air critical angle. The impression of the inner circle in the BFP image signifies glass-air critical angle (θ = θc ). The full width at half maxima (FWHM) of this emission was 150 . I vs φ plot in figure 2e shows the variation of far-field intensity with respect to φ for a constant θ = 410 . From this plot, it is clear that the direction of maximum emission k1 (φ = 470 ) coincides with the orientation of the excited nanowire. The spread of the emission in φ was approximately 550 . Note that there was a comparatively weak emission observed at φ=φmax + 1800 . The directionality of the emission was quantified by forward-to-backward (F/B) ratio in dB given by, f /b = 10log(If /Ib ). In this case, it was measured to be 2.63 dB (for details see supplementary information S2). Next, we illuminated end 2 of the Ag NW dimer and collected the fluorescence emission from the junction. Figure 2f is the optical image of the end 2 excitation and figure 2g is the BFP image of the emission from the junction. Similar to the previous case, we observed unidirectional emission but along the k2 direction. The emission peak was at θ = 410 and FWHM of the emission was 160 . I vs φ plot in figure 2h reveals that the direction of maximum emission now has been flipped to the direction of orientation of the other wire i.e. φ = 3300 and shows a spread of 700 in φ. The calculated F/B ratio in this case was as high as 4.32dB (for details see supplementary information S2). Important inferences that can be drawn from the single end excitation experiments are a) The same Ag NW dimer junction supported unidirectional fluorescence emission while being excited at either of the free ends and b) The direction of emission could be flipped from one to other by changing the excitation point from one end to the other. When both ends of the Ag NW dimer were illuminated simultaneously, excited SPPs in both wires propagated towards the junction. This was evident from the fluorescence optical image shown in figure 2i where fluorescence emission from molecules deposited close to wire surfaces was visible.The BFP image of the emission form the junction in figure 2j shows that fluorescence of the molecules at the junction was being directed to two directions similar to


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the axes of the oriented wires. Both emissions had maxima around θ = 410 and was along the directions φ = 470 (k1 ) and φ = 3300 (k2 ). The angular distribution of the emitted intensity is shown in the I vs φ plot for constant θ = 410 in figure 2k. An important observation in this case was that the angular separation (1070 ) between the two direction of emission mimicked the angle (1070 ) between the two wires in the dimer. This experiment proves that a) fluorescence emission from the junction of the fluorescent molecule coated Ag NW dimer can be influenced to emit in two directions at a same time by illuminating both the free ends simultaneously and b) the angular separation between the emission directions can be controlled by the angle between the wires in the dimer.

Mechanism behind directional fluorescence

Figure 3: a) Schematic illustration of the process happening in the system under study. b) Energy level scheme of the Ag NW-dye molecule system. The arrows signify the transitions. c) Schematic representation of the process II in case of dual end excitation.

Coupled emission of fluorescent molecules and the propagating SP modes in the Ag NW dimers is the main reason behind the observed directional fluorescence emission. This is governed by a number of processes happening in the system under study which has been



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illustrated schematically in figure 3a. The whole phenomenon can be divided into two processes: (i) the excitation of fluorophores deposited on the wires and (ii) their radiative decay. This has been studied in context of radiative decay of quantum emitters in close vicinity of metallic nanowires by Xu et al . 28,29 When the laser is focused at the ends of the nanowire, it launches propagating SPPs in the excited wire which couple out as photons at the junction. We characterized the guided SPs along the nanowires in absence of nile blue molecules for single and dual end excitations by capturing the SPP leakage radiations into the glass substrate. These experimental results are shown in supporting information S3. The tightly confined optical field associated with these guided SPs along the nanowire enables excitation of the molecules coated on it. This has been labelled as process I in figure 3a. The second process (labelled II in figure 3a) involves the radiative decay of the excited molecules near the metallic nanowire via three possible channels: Channel I: radiative emission in free space. Channel II: decay of the excited molecules near the Ag NW generates propagating SPs in the nanowire which propagates towards the junction and the free end. The presence of high refractive index substrate (glass) results in radiative leakage radiation of the Ag NW SP modes into it (related experimental results are presented in supporting information S3).

Channel III: The propagating SP modes generated due

to the decay of molecules in close vicinity of the surface of Ag NW dimer also couple out as free photon at the junction. Therefore, channel I mainly represents molecular emission which is not coupled to the SP modes in the nanowire and channel II/III represents the plasmon coupled fluorescence emission from molecules deposited on the nanowire system. We provide a simplified energy level scheme 28 (see figure 3b) which can be employed to depict a general picture of the interactions happening between the propagating SPs in the Ag NW dimers and the fluorescent molecules. Apart from these three channels, the Ag NW also provides non-radiative decay channels to the molecules which are in direct contact with the metallic surface. All the three radiative decay channels are mainly related to our experimental observations. Out of which, angular distribution of emission from channel III


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i.e, the junction of the Ag NW dimer is main focus of this work. This shows that the junction acts as an optical antenna by modifying the angular distribution of the fluorescence emission. Therefore, single end excitation leads to excitation of molecules deposited along the length of the excited nanowire and their plasmon coupled emission through the junction of the Ag NW dimer results in unidirectional light directed along the direction of orientation of the excited nanowire. In case of dual end excitation, molecules along both the wires are excited (see figure 3c) and their plasmon coupled emission through the junction is the main reason behind the observed dual directional emission.

Numerical simulations We corroborated our experimental observations with numerical analysis of the far-field scattering of fluorescence photons from the junction of a serially coupled Ag NW dimer. Our calculations are based on combined use of 3-dimensional (3D) finite difference time domain (FDTD) method and Green’s function method. The coupled nanowire system was modelled by joining two cylinders, each of 200nm diameter at a specific angle 1070 to match the experimental configuration. Due to computational constraints, the lengths of the wires was shortened to 5µm each. The silver nanowire system was placed on a dielectric slab. The refractive index of silver was taken from experimental values provided by Jhonson and Christy. 30 For the dielectric substrate we used a refractive index value 1.52. The fluorescent molecules deposited on the Ag NW dimer in the experiment were modelled as dipole sources placed on the top of the nanowire. The emission spectrum of the dipole sources were matched to the fluorescence spectrum of NBA which shows a emission peak at 658nm. To calculate the collective emission of the molecules placed along the wire from the Ag NW dimer junction, we used the following procedure. First we calculated the far-field emission pattern from the junction for 5 individual dipole sources by placing one at a time at every 1µm along the excited wire and then added the resulting far-field emission intensities. The random orientation of every dipole source was taken into consideration by adding the 11 ACS Paragon Plus Environment


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Figure 4: Numerical simulations for Uni- and bi-directional fluorescence emission from junction of Ag NW dimer. a) and b) are calculated far-field radiation patterns for single end excitations (end 1 and end 2 were excited separately). The emission patterns are calculated by adding the far-field radiation patterns of five individual dipoles placed at different positions (one at every 1µm) every time on the corresponding nanowire. c) Simulated far-field emission for dual end excitation which was calculated by adding the far-field emission intensities from the junction for individual dipoles, placed at different positions every time along both nanowires. calculated Fourier plane intensities for two orthogonal in-plane orientations and one perpendicular out-of-plane orientation of dipoles in each case. For single end excitations, we added the far-field emissions through the junctions for molecules placed along the corresponding nanowire (Figure 4a and b) and for dual end excitations the far-field patterns for all the molecules placed along the both wires were added (figure 4c). Again, the far-field emission pattern in each case was calculated using the following steps. In first step, the total electric field inside and around the nanostructure was calculated using a commercial FDTD solver (FDTD solutions, Lumericals). Then, the far-field radiation pattern was obtained using the Green’s function method for stratified media. 31,32 Note that, in the experiment, the molecules got excited due to the interaction with the near-field of the propagating SP modes in the Ag NW system. Therefore the intensity of the molecular emission is dependent on the plasmon propagation length (lspp ) of the SP modes in the nanowires. As we move away from the laser excitation point i.e., the ends of the excited nanowire, emission intensity of molecules get decreased by the factor exp(−x/lspp ) where x


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is the distance of the dipole source from the excitation point (nanowire end). This effect was included in the simulation by setting the oscillation amplitude of the dipole placed at the ends to 1 and for the ith dipole, it was set to (exp(−xi /2lspp )). We used lspp = 15µm for our purpose (see supporting information S5). We simulated the aperture used to select the emission from the junction by only including electric field distribution of 500nm of both the wires around the junction to calculate the far-field radiation patterns. Figure 4a and b are the far-field emission patterns from the junction when end 1 and end 2 was excited. This shows a similar unidirectional emission along the direction k1 and k2 respectively which is the orientation of the illuminated nanowire. The emission peak exactly occurred at θ = θc = 41.20 which further supports the experimental observations. Figure 4c is the calculated far-field emission pattern for dual end excitation of the Ag NW dimer. This shows dual directional emission coming out of the junction. Both emissions peaked at θ = θc = 41.20 . The calculated result again supports the experimental observation of angular separation between two emission directions mimicking the coupling angle (1070 ) of the two wires in the dimer. Therefore,from the experiment (figure2) and the numerical simulation (figure4), it is clear that the SPP modes in the nanowire geometry serve two purposes: firstly they excite the molecule, and secondly they direct the emitted fluorescent signal. The emission direction of the molecules is mainly governed by the geometry of the nanowire dimer, and the property of directional emission is governed by the number of excitations. To see how the junction emission (Channel III ) differs from emission of molecules coated along the wire (ChannelII ), we have captured BFP image of fluorescent light from molecules, uniformly coated on the wire. This experimental result is presented in supplementary information S4 and is further proof of plasmon coupled emission of molecules coated on the serially coupled Ag NW dimer.



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Figure 5: Remote excitation fluorescence spectra of nile blue A from the Ag NW dimer junction for single and dual channel excitation. Note that the total power for all the excitations was constant at 50µW . Table 1: Comparison of relative fluorescence enhancement at Ag NW junction for single and dual channel excitations. Mode of excitation

End 1 power (µW) 50

End 2 Power (µW) 0

Relative enhancement

0

50

0.39

25

25

1

0.37

Single channel

Dual channel

Dual excitation leads to enhanced fluorescence from the nanowire junction Apart from directing light, another important aspect of Ag NW dimer junction is that they can facilitate plasmonic hot-spot due to localized density of optical states. 23,25 In our geometry, by changing the excitation point from one to two, and keeping the total excitation power constant, we can enhance the optical interaction at the junction due to hybridization of propagating SPP modes in each arm of the dimer at the junction. Compared to single excitation configuration, the dual-excitation based hybridization of SPP modes can further


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lead to enhanced light-molecule interaction. 25 We recently reported such type of field localization at Ag NW dimer junctions in context of surface enhanced Raman scattering. 25 In the present scenario, we were interested in knowing whether fluorescence signals can be enhanced by dual-excitation compared to single excitation source, with total power kept constant. To probe this, we performed single- and dual-channel remote excitation fluorescence spectroscopy with a 633nm laser source of total power equal to 50µW . That is, for single channel illumination, a 50µW excitation was focused at individual ends at a time, whereas for dual excitation, a 25µW excitation was used at each end of the Ag NW dimer simultaneously. Next, we captured the fluorescence spectra for the three configuration of illumination (end1, end2 and both ends) as shown in figure 5, and compared the 658 nm emission intensity for each configuration (see Table 1). From table 1, we inferred that for the same illumination powers, dual excitation resulted in greater relative enhancement (value of 1) compared to single channel excitation (value of 0.37 and 0.39). To understand the process in a better way, we performed 3D-FDTD simulation of the geometry with same configuration. As in the experiment, we kept length of one wire to be 8µm and the other wire to be 7µm. The angle between two wires was kept 1070 (see figure 6a and b). First we calculated the electric field at the junction for end 1 excitation of the wire (see figure 6a). The excitation electric field amplitude was kept 1V /m. The maximum electric field at the junction in this case was Ej = 2.8V /m. To get a better insight of the process, we calculated the junction electric field Ej as a function of excitation wavelength (λex ). Figure 6b represents the variation in Ej as function of excitation wavelength for single end excitation. It shows that the value of Ej remained approximately same for λex > 550nm. The ripples in the spectrum represents the Fabry-perot modes present in the wire geometry. We also calculated the charge density distribution (figure 6c) around the junction of the Ag NW dimer at a plane that intersects the dimer at the middle(area enclosed by white dotted square in figure 6a) for end 1 excitation with 633nm laser. This shows SPPs generated in the illuminated wire partially tunnelled into the other wire through the junction. While



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Figure 6: a) Electric field distribution around the Ag NW dimer for end 1 excitation with 633nm laser. b) Variation of junction electric field (Ej ) as a function of excitation wavelength for end 1 excitation. c) Charge density distribution around the junction of the Ag NW geometry (area enclosed by white dotted square in the image representing electric field distribution) for 633nm excitation. d), e) and f) are corresponding data for end 1 + end 2 excitation.


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this charge transfer took place, a electromagnetic hotspot was created at the defect at the junction of the Ag NW geometry. Next, we calculated the electric field distribution for dual excitation (see figure 6d). The total excitation power was kept same as in single end excitation by making the electric √ field amplitude of each source 1/ 2V /m. For this calculation, the mutual phase difference between the two excitation beams at the input points (nanowire ends) was kept 0. The maximum electric field value at junction was increased to Ej = 4.1V /m. The variation of Ej with respect to λex in case of dual excitation is shown in figure 6e. From this spectrum, it is clear that in-phase coupling of SPP modes in two wires at the junction gave rise to new resonances. 33 A weak peak near 485nm and broad strong peak around 600nm are now visible in the spectrum. The peak around 600nm represents bonding interactions between SPP modes at the junction. The charge density distribution (figure 6f) around the junction for dual excitation with 633nm laser (close to 600nm) further proves that the in-phase bonding of SPP modes generated in each arm of the Ag NW dimer at the junction leads to the generation of an optical hotspot with enhanced optical fields. To find out the influence of the mutual phase difference at the excitation points on the SPP interferences at the junction, we performed a set of simulations where we measured the variation of junction electric field as a function of excitation wavelength for excitation beams with various mutual phase differences. These results are shown in supporting information S6. These data imply that Ag NW junction can be harnessed to enhance molecular emission in addition to dual-directional fluorescence emission. Such capability of a single geometry to enhance and direct fluorescence emission can essentially be utilized for optical antenna applications.



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Conclusions In conclusion, we have shown that a serially coupled plasmonic nanowire junctions can be used to obtain directional fluorescence emission. The fluorescence of molecules at the junction can be remotely directed to one or two directions depending upon single or dual excitation of the wire ends. The angular separation between the emission directions can be controlled by varying the angle between the two wires. The directional emission is obtained due to coupling between guided plasmon modes and the molecular emission. The Ag NW dimer not only provided dual emission channels, but also facilitated greater field enhancement compared to single channel emission, thus highlighting the prospect of improving the efficiency of detecting molecular fluorescence. An interesting direction for future work would be to optically probe single emitters precisely placed at the junction of the NW dimer. Such conflagrations have recently found enormous interest in nanoscale quantum information processing as single photon sources 34–37 and our Ag NW dimer can be used as an excellent test bed for such applications. As we have emphasized, the NW junction can be accessed via two spatially separated channels, and hence, one can also utilize our geometry for remotely-excited pumpprobe experiments for ultra-fast molecular nanoplasmonics in real and Fourier space.

Supporting Information Available Protocol for sample preparation, details of computational methods, I vs θ graph for forward to backward emission calculation and BFP image of fluorescene emission of molecules coated along the wire. This material is available free of charge via the Internet at http://pubs.acs.org. This material is available free of charge via the Internet at http://pubs.acs.org/.

Acknowledgement This research work was partially funded by DST-SERB Grant (SR/NM/NS-1141/2012(G))and DST Nanoscience Unit Grant (SR/NM/NS-42/2009), India. G.V.P.K. would like to thank


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DST,India, for Ramanujan Fellowship.

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Directional Fluorescence Emission Mediated by ... - ACS Publications

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