Turning the Corner: Efficient Energy Transfer in Bent Plasmonic

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Letter pubs.acs.org/NanoLett

Turning the Corner: Efficient Energy Transfer in Bent Plasmonic Nanoparticle Chain Waveguides David Solis, Jr.,† Aniruddha Paul,‡ Jana Olson,‡ Liane S. Slaughter,‡ Pattanawit Swanglap,‡ Wei-Shun Chang,‡ and Stephan Link*,†,‡ ‡

Department of Chemistry and †Department of Electrical and Computer Engineering, Laboratory for Nanophotonics, Rice University, Houston, Texas 77005, United States S Supporting Information *

ABSTRACT: For integrating and multiplexing of subwavelength plasmonic waveguides with other optical and electric components, complex architectures such as junctions with sharp turns are necessary. However, in addition to intrinsic losses, bending losses severely limit plasmon propagation. In the current work, we demonstrate that propagation of surface plasmon polaritons around 90° turns in silver nanoparticle chains occurs without bending losses. Using a far-field fluorescence method, bleach-imaged plasmon propagation (BlIPP), which creates a permanent map of the plasmonic near-field through bleaching of a fluorophore coated on top of a plasmonic waveguide, we measured propagation lengths at 633 nm for straight and bent silver nanoparticle chains of 8.0 ± 0.5 and 7.8 ± 0.4 μm, respectively. These propagation lengths were independent of the input polarization. We furthermore show that subradiant plasmon modes yield a longer propagation length compared to energy transport via excitation of super-radiant modes. KEYWORDS: Surface plasmon resonance, surface plasmon polariton, plasmon waveguiding, super- and subradiant modes, BlIPP, silver nanoparticle

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realize plasmonic circuits.33 While analogues of optical elements such as mirrors, beam splitters, and interferometers have been accomplished with a variety of plasmonic waveguide geometries,18,27,29,34,35 losses due to bending SPPs around corners are a severe problem in addition to the already large intrinsic damping in metals. For dielectric stripe waveguides, maximum transmission around a 90° turn, which is the simplest geometry required in plasmonic circuits, has been found theoretically21 and experimentally22 to be a trade-off between decreasing radiative losses due to better SPP momentum matching in the two arms and increasing intrinsic absorptive losses due to a longer overall waveguide as the curvature of the turn is increased. Optimum radii of curvature for these waveguides were reported to be R = ∼ 5−10 μm,21,22 which are quite large considering the goal of shrinking opto-electronic components. Consistent with these results, bending losses in chemically prepared nanowires have been reported to exceed all other propagation losses if the radius of curvature is smaller than R = 10 μm.36 Better performance for SPP propagation around 90° turns can be achieved with channel plasmons supported by V-shaped grooves in thin metal films.25−27,37 At wavelengths above 1600 nm, lossless bending of channel

growing need to transfer information at high speeds using structures smaller than the optical diffraction limit has fueled the development of plasmonic waveguides, which couple light to coherent waves of electrons, known as surface plasmons polaritons (SPPs).1−6 Many different waveguide geometries have been realized including metallic nanowires,7−16 metal stripes,17−20 dielectric stripes on metal films,21−24 and thin metal films patterned with grooves.25−27 The advantage of high spatial confinement is, however, offset by larger losses compared to dielectric waveguides. To characterize SPP propagation, both near-field11,19,20,27 and far-field12−14,23,28 imaging methods have been applied. Far-field methods include the use of fluorophores that either emit light due to coupling to the SPP modes13,29 or are photobleached for longer exposure times.30 Despite a lower resolution compared to near-field optical methods, fluorescence microscopy is easy to implement and recent super-resolution imaging of fluorophore-labeled nanowires31 indicates great promise to even better resolve SPP propagation by fluorescence-based microscopy. We have taken advantage of the SPP-induced photobleaching to develop bleach-imaged plasmon propagation (BlIPP), which we have used to measure SPP propagation in gold nanowires12,28 and chains of gold nanoparticles.32 In addition to propagation in straight waveguides, it is necessary to guide SPPs in more complex structures such as bends and splits and to connect different elements to fully © XXXX American Chemical Society

Received: June 27, 2013 Revised: August 30, 2013

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plasmons in double bend S-shaped waveguides with a radius of curvature of 0.83 μm has been demonstrated.26 However, bending losses increase significantly for the shorter wavelengths desirable for maximum SPP confinement. Larger transmission, especially around sharp 90° turns with R = 0 μm, has theoretically been predicted to be possible when a SPP resonator in form of a void is placed next to the bend.38 Instead metal nanoparticles could also be used as SPP resonators given that they have been shown to be efficient input and output couplers for plasmonic nanowires.9,39 In fact, simulations for chains of nanoparticles have shown that 100% transmission around 90° turns in L-shaped nanoparticle arrangements is possible.40,41 However, while experiments have confirmed SPP propagation in straight nanoparticle chains,32,42−44 lossless bending of SPPs around sharp 90° turns in plasmonic nanoparticle chain waveguides has not been explored in detail.45 In this work, we report on the SPP propagation in chains of spherical silver nanoparticles with a 90° turn, in comparison to straight chains having the same width and overall length. BlIPP experiments at 633 nm yielded the same SPP propagation length of ∼8.0 μm for bent and straight chains. Through the use of extinction spectroscopy and comparison to BlIPP measurements carried out with 785 nm excitation, we further demonstrate that such efficient energy transport at 633 nm is possible due to coupling of incident light to subradiant plasmon modes of the nanoparticle chains. Straight and bent silver nanoparticle chains, as shown in Figure 1A,F, respectively, were produced by a combination of top-down lithography and bottom-up assembly.32,46,47 Electron beam lithography was used to write an array of 15 μm × 200 nm linear and bent trenches into a thin film of poly(methyl methacrylate) (PMMA) photoresist on an optically transparent indium tin oxide (ITO) coated glass slide. The bent structures contained a 90° turn at the half way point making each segment 7.5 μm long. After developing the trenches, they were filled by drop casting a solution of citrate capped 54 ± 5 nm spherical silver nanoparticles onto the center of the trench array and letting the water evaporate. As necessary, this nanoparticle deposition step was repeated several times to obtain completely filled trenches, which were inspected by a combination of darkfield and bright-field microscopy. In the final preparation step, the PMMA polymer surrounding the silver nanoparticle chains was lifted off by using acetone (for more details see the Supporting Information). Scanning electron microscopy (SEM) images of the entire chains (main part) and higher-magnification images for the end and the middle of each chain (insets) in Figure 1A,F verify the formation of close-packed linear and bent assemblies with interparticle distances of only a few nanometers, important for strong near-field coupling between the nanoparticles, the formation of subradiant modes, and hence efficient SPP propagation.32 The chains were typically ∼4 nanoparticles in width and ∼3 in nanoparticles height. It is important to note that reducing the width of the chains down to just 1 nanoparticle while at the same time ensuring close interparticle separations over many micrometers is difficult if not impossible with the current assembly method. The SEM images in Figure 1 were taken after the optical measurements described next in order to avoid the possibility of melting and fusing the nanoparticles together due to prolonged exposure to the electron beam during high-magnification SEM imaging. Lowmagnification SEM imaging at 1000× magnification was,

Figure 1. SPP propagation in straight (left column) and bent (right column) silver nanoparticle chains. (A) SEM image (7500×) of a 15 μm × 200 nm straight nanoparticle chain taken after the BlIPP experiment. Insets show higher magnification images (90 000×) of the chain end that was excited during the BlIPP experiment (bottom left) and an area near the middle of the chain (upper right). (B) Fluorescence image of the dye coated chain shown in A. (C) Fluorescence image of the same chain recorded after 20 min of exposure to 633 nm light focused at the left end of the chain. The laser was circularly polarized and the power was increased to 12 μW to induce dye photobleaching. Fluorescence images were taken with a low excitation power of 30−40 nW. (D) Difference image created by subtracting image C from image B. (E) Width-averaged line section (red data points) along the nanoparticle chain as extracted from image D. The fitting of the line section data is shown by the green line yielding a SPP propagation length of L0 = 7.5 μm. The contribution due to the photobleaching from direct laser excitation is given by the blue line. (F−J) Same as A−E for a bent silver nanoparticle chain. The upper right inset in F shows a higher-magnification SEM image of the 90° turn. The fitting of the photobleach intensity line section in J yielded a SPP propagation length of L0 = 8.0 μm, indicating no change in SPP propagation for the bent silver nanoparticle chain.

however, used to screen for the most promising areas of continuous close-packed chains to be studied optically. SPP propagation at 633 nm in straight and bent silver nanoparticle chains was investigated using BlIPP, which creates a permanent map of the plasmonic near-field through photobleaching of a dye film coated on top of the waveguide. For the BlIPP measurements, the chains were first coated with a thin film of LDS821 dye and then imaged using a home-built sample scanning fluorescence microscope (for more details see the Supporting Information). Figure 1B,G shows fluorescence B

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⟨L0⟩straight = 8.0 ± 0.7 μm and ⟨L0⟩bent = 8.0 ± 0.4 μm. The difference image (Figure 1I) and line section (Figure 1J) for the bent chain furthermore illustrate that at the position of the 90° turn, located 7.5 μm away from the input end, there is no significant drop in the photobleach intensity. We therefore conclude that SPP propagation in these bent silver nanoparticle chains did not suffer bending losses, verifying previous theoretical predictions.40,41 The chain width of several nanoparticles likely contributed to these results as well because in the 200 nm × 200 nm region of the 90° turn about 50 nanoparticles assisted in the energy transfer to their neighbors in all directions. SPP propagation in both straight and bent nanoparticle chains does not depend on the polarization of the excitation light (Figure 2). Simulations for T-shaped nanoparticle chains

images of the straight and bent silver nanoparticle chains recorded with 633 nm excitation at powers of 30−40 nW. The signal in these images originated from the fluorescence of the dye film present everywhere, while the contrast was due to the fluorescence enhancement by the underlying plasmonic nanoparticles. For the photobleaching step in BlIPP, the piezo scanning stage was then positioned such that the laser was focused directly at one end of the nanoparticle chain and the power of the circularly polarized laser was increased to 12− 13 μW. After an exposure time of 5 min, the laser power was decreased back to the initial value and another fluorescence image was taken. This procedure of nondestructive low-power imaging and high-power exposure localized at the waveguide input was performed four times, corresponding to a total photobleaching exposure of 20 min. Figure 1C,H shows the images obtained after 20 min of the 633 nm laser focused on the left end of the nanoparticle chain waveguides. To quantify photobleaching along the nanoparticle chains, difference images were created by subtracting the fluorescence images taking after high power laser exposure (Figure 1C,H) from those taken at the beginning of the BlIPP experiments (Figure 1B,G). These difference images are shown in Figure 1D,I for the straight and bent nanoparticle chains, respectively, and represent the degree of dye photobleaching. At the waveguide input end, the dye fluorescence was completely bleached due to direct excitation with the 633 nm laser. In addition to direct laser-induced photobleaching, the same laser light also coupled to SPP modes, which excited the dye molecules indirectly through their near-field intensity and over time led to irreversible photobleaching as well. This indirect photobleaching resulted in the significant loss of fluorescence intensity along the waveguide away from the point of direct laser excitation as can be clearly seen in the difference images shown in Figure 1D,I. In fact, photobleaching occurred all the way at the other end of the nanoparticle chains, indicating substantial SPP propagation in these silver nanoparticle chains. The SPP propagation length was determined from widthaveraged line sections taken from the difference images. The photobleach intensity as a function of distance from the excitation source was fit to a kinetic rate model that was developed to describe the dye photobleaching caused by a Gaussian laser beam and an exponentially decaying SPP nearfield.30 The red data points in Figure 1E show the widthaveraged line section taken from the corresponding difference image (Figure 1D) and were fitted to our model as given by the green line, yielding the contribution from the Gaussian laser beam (blue line) and, more importantly, a SPP propagation length for this chain of L0 = 7.5 μm. The fitting routine was, however, not only applied to the line section in Figure 1E but also to the line sections from all difference images created from the four 5 min photobleaching exposures. This multistep process was used to fit the propagation length with higher confidence as the time dependence of the dye photobleaching is directly incorporated in the BlIPP model. This procedure furthermore minimized any effects due to sample and focus drift during long periods of high power laser exposure. The bent nanoparticle chains suffer no bending losses for SPP propagation around the 90° turn. The analysis of the photobleach intensity line section in Figure 1J for the bent nanoparticle chain yields a propagation length of L0 = 8.0 μm. Repeating these BlIPP experiments for a total of six straight and five bent silver nanoparticle chains gives the same SPP propagation lengths within the experimental error, that is,

Figure 2. Excitation polarization dependence for SPP propagation in straight (left column) and bent (right column) silver nanoparticle chains. (A) BlIPP difference image taken after 20 min of exposure to 633 nm laser light that was polarized parallel to the main chain axis. (B) Fitting of the width-averaged line section (BlIPP data) as extracted from A yielded a SPP propagation length of L0 = 8.0 μm. (C,D) BlIPP experiment with the excitation polarization polarized perpendicular to the main chain axis resulting in a SPP propagation length of L0 = 7.5 μm. (E,F) Same as A,B for a bent silver nanoparticle chain. A SPP propagation length of L0 = 7.5 μm was measured. (G,H) Same as C,D for a bent silver nanoparticle chain yielding a SPP propagation length of L0 = 7.5 μm. The polarization direction in case of the bent chains refers to the chain segment that was excited by the 633 nm laser. The bleaching power was 12 μW while the imaging power was ∼40 nW for all data shown here.

predicted that SPP propagation can be directed from the junction into either of the two arms by controlling the incident light polarization.48 To test the effect of input polarization on energy transfer around a 90° turn in nanoparticle chains we therefore carried out BlIPP experiments at 633 nm using a linearly polarized excitation source. Figure 2 summarizes the results by showing difference images and width-averaged line sections for straight (Figure 2A−D) and bent (Figure 2E−H) silver nanoparticle chains with polarization parallel (top panels) C

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The average SPP propagation length of ⟨L0⟩ = 8.0 ± 0.5 μm measured at 633 nm for the silver nanoparticle chains was longer than the propagation length of ⟨L0⟩ = 3.9 ± 0.6 μm obtained previously at 785 nm for straight gold nanoparticle chains of the same overall dimensions and similar nanoparticle size.32 The increase of propagation length by a factor of ∼2 for silver compared to gold nanoparticle chains is consistent with lower intrinsic damping losses in silver for visible and near-IR wavelengths. However, considering the dispersion of the metal losses alone, does not address that the propagation length in nanoparticle chains also depends strongly on the type of mode excited. Excitation of subradiant modes leads to improved SPP propagation lengths compared to excitation of super-radiant modes because radiative damping is strongly suppressed.54 To investigate the nature of the SPP mode excited at 633 nm for straight silver chains we examined the far-field extinction spectrum of these plasmonic waveguides. The extinction spectrum of a single silver nanoparticle chain reveals that 633 nm excitation is resonant with subradiant plasmon modes (Figure 3A). The main extinction peak with a

and perpendicular (bottom panels) with respect to the main chain axis at the input end. From the fit to the photobleach intensity line sections, we extracted under parallel excitation SPP propagation lengths of ⟨L0⟩straight = 8.2 ± 0.3 μm and ⟨L0⟩bent = 7.7 ± 0.3 μm. Under perpendicular excitation with the same laser power and exposure time, we obtained SPP propagation lengths of ⟨L0⟩straight = 7.7 ± 0.3 μm and ⟨L0⟩bent = 7.7 ± 0.6 μm for straight and bent nanoparticle chains, respectively. The errors represent the standard deviation calculated from measurements of at least three different nanoparticle chains for each case investigated here. Within the experimental error, we find no significant differences in SPP propagation length and photobleach intensities as a function of excitation polarization. While we have previously shown that SPP propagation is independent of excitation polarization in straight gold nanoparticle chains,32 it is important to stress the results for the bent nanoparticle chains, for which energy transport around the 90° turn was unaffected by the excitation polarization. We attribute this observation to the multiple-particle width of the chain, unlike the single nanoparticle wide chain studied theoretically,41,48 and isotropic nanoparticle shape that allows for plasmon coupling among tightly packed, but disordered silver nanoparticles in all directions, important especially at the 90° turn for the bent nanoparticle chains. These three-dimensional interactions among the spherical nanoparticles and disorder in nanoparticle arrangements within the chain likely lead to mixing of many SPP modes with different polarizations after coupling of light to the plasmonic waveguide. In fact, based on gold nanowire BlIPP studies,12 a combination of SPP modes including higher order modes were likely imaged in the silver nanoparticle chains studied here. While for nanowires electrodynamics modeling can help reveal the nature of the propagating SPP modes, the large number of nanoparticles comprising these chains makes it prohibitive at the moment for these specific chains to utilize simulations for further characterization of the SPP modes. Future improvements in chain preparation yielding narrower chains with less nanoparticles might be able to address this point in more detail. Direct comparison of SPP propagation lengths between nanoparticle chains vs nanowires is also difficult. For the latter SPP propagation lengths varying between 6−11 μm have been measured49−52 for similar wavelengths, although most of those nanowires had smaller diameters of ∼100 nm. Considering that SPP propagation lengths increase with decreasing confinement,2,13,53 the values measured here for nanoparticle chains are smaller compared to nanowires as expected because of reduced radiative damping in nanowires. The histogram in Supporting Information Figure S2 summarizes all BlIPP measurements carried out with a bleaching wavelength of 633 nm for the straight and bent silver nanoparticles chains and with both circular and linear excitation polarizations. From the distributions in Supporting Information Figure S2, average SPP propagation lengths of ⟨L0⟩ = 8.0 ± 0.5 μm and ⟨L0⟩ = 7.8 ± 0.4 μm were obtained. The error in fitting the photobleach intensity line sections was about 0.5 μm, demonstrating that the standard deviation of the propagation length was not caused by sample heterogeneity in terms of varying nanoparticle arrangements from chain to chain or even within the same chain from segment to segment. Excitation of either end of the nanoparticle chains for an array of chains on the same sample furthermore gave the same results within the experimental error.

Figure 3. Dependence of SPP propagation on excitation wavelength in straight silver nanoparticle chains. (A) Extinction spectrum of a straight silver nanoparticle chain showing a maximum at 765 nm. The excitation wavelengths of 633 and 785 nm are indicated by the dashed green and red lines, respectively. (B) BlIPP difference image taken after 20 min of exposure to 785 nm laser light that was circularly polarized. The bleaching power was 13 μW. (C) Fitting of the widthaveraged line section (BlIPP data shown as red data points) as extracted from B gave a SPP propagation length of L0 = 3.8 μm as indicated by the green line. Direct photobleaching by the Gaussian laser beam was extracted from the fitting model as shown by the blue line.

maximum at 765 nm corresponds to the super-radiant plasmon mode, which couples strongly to incident light. In contrast, subradiant plasmon modes at shorter wavelengths do not couple strongly to the far field. The presence of such modes at 633 nm can nevertheless be inferred from the fact that the extinction maximum is red-shifted from the 633 nm excitation. Different order subradiant plasmon modes in linear chains span a broad spectral range and appear at higher energies compared to the lowest energy super-radiant mode.54 Subradiant plasmon modes furthermore can overlap with the super-radiant mode at the blue edge of the main extinction peak. On the basis of both this assignment and on the fact that energy transport in D

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nanoparticle chains by super-radiant modes is less efficient, excitation directly into the super-radiant mode should decrease the SPP propagation length. We tested this hypothesis by performing BlIPP experiments in straight silver nanoparticle chains using 785 nm as the excitation wavelength, which was resonant with the red edge of the extinction spectrum (Figure 3A). The SPP propagation length for excitation of super-radiant plasmon modes at 785 nm was indeed shorter than for 633 nm excitation (Figure 3B,C). For BlIPP experiments with 785 nm excitation the silver nanoparticle chains were coated with cardiogreen as the photobleachable dye film. Figure 3B,C shows the difference image and the photobleach intensity line section of a silver nanoparticle chain after 20 min of exposure to 13 μW of 785 nm laser light focused on the left end of the nanoparticle chain. Fitting the photobleach intensity to our BlIPP model yielded a SPP propagation length of L0 = 3.8 μm, which was smaller by a factor of ∼2 compared to excitation of subradiant modes at 633 nm. It should be noted that the extinction spectrum in Figure 3A has a long wavelength tail, indicating the presence of conductive contact between some of the constituent nanoparticles. However, based on previous optical studies of large nanoparticle superstructures,55,56 the low intensity at wavelengths longer than the super-radiant mode indicates that, although some interparticle touching likely occurred among the thousands of close-packed nanoparticles, these interactions were not a major factor here. Otherwise the propagation length at 785 nm would increase as the metal’s intrinsic dispersion characteristics would dominate SPP propagation losses. In conclusion, we have used BlIPP to characterize SPPs at 633 nm in straight and bent chains composed of 54 nm spherical silver nanoparticles. We measured SPP propagation lengths of ⟨L0⟩straight = 8.0 ± 0.5 μm and ⟨L0⟩bent = 7.8 ± 0.4 μm, indicating that no bending losses occurred due to the 90° turn. These results show the strength of close-packed nanoparticle arrangements to function as plasmonic elements and in particular short interconnects with arbitrary geometries where intrinsic propagation losses are less critical. Furthermore, extinction spectra together with 785 nm BlIPP experiments verified the subradiant nature of the excited plasmon modes. Excitation of subradiant modes at 633 nm led to longer plasmon propagation than excitation of the super-radiant mode at 785 nm, demonstrating the importance of subradiant plasmon modes for SPP propagation in nanoparticle chain waveguides. It needs to be noted that the wavelength dependence measured here, that is, L0785 < L0633, is opposite compared to continuous plasmonic waveguides as intrinsic losses in silver decrease with increasing wavelength. Furthermore, SPP propagation around the 90° turn was independent of the polarization of the input light, likely because the nanoparticle chains were a few nanoparticles wide. The ability of chains to propagate energy over micrometer distances via subradiant plasmon modes in combination with lossless transfer around 90° turns makes them unique waveguide structures that could be combined with other metallic and dielectric waveguide geometries to bring the concept of plasmonic circuits one step closer to reality.

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ASSOCIATED CONTENT

S Supporting Information *

Nanoparticle chain preparation, optical measurements, and distribution of propagation lengths. 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.

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ACKNOWLEDGMENTS This work was funded by the Robert A. Welch Foundation (C1664), ONR (N00014-10-1-0989), ARO (MURI W911NF-121-0407), and NSF (CHE-0955286). D.S. and J.O. were supported by an NSF Graduate Research Fellowship Grant (0940902) and P.S. acknowledges partial support from the Royal Thai Government. We thank Dr. Britain Willingham for helpful discussions.

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dx.doi.org/10.1021/nl402358h | Nano Lett. XXXX, XXX, XXX−XXX