pubs.acs.org/NanoLett
Three-Dimensional Plasmonic Nanofocusing Nathan C. Lindquist,† Prashant Nagpal,‡ Antoine Lesuffleur,† David J. Norris,‡ and Sang-Hyun Oh*,† †
Department of Electrical and Computer Engineering and ‡ Department of Chemical Engineering & Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 ABSTRACT We demonstrate three-dimensional plasmonic nanofocusing of light with patterned metallic pyramids obtained via template stripping. Gratings on the faces of these pyramids convert linearly polarized light into plasmons that propagate toward and converge at a ∼10 nm apex. Experiments and computer simulations confirm that optical energy is focused into a nanoscale volume (5 × 10-5 wavelength3). Because these structures are easily and reproducibly fabricated, our results could benefit many applications, including imaging, sensing, lithography, and nonlinear spectroscopy. KEYWORDS Surface plasmon, nanofocusing, template stripping
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tip.13 Nanoholes through a gold film can launch plasmons into a laterally tapered region.6 Curved slits18,19 or holes4 can focus plasmons to an in-plane bright spot. In these cases, plasmons were generated and focused on only one side of the device. Full three-dimensional nanofocusing, where plasmons converge to a tip from multiple directions, should also be possible. Radially polarized plasmons could be launched around a circularly symmetric tapered waveguide.12,20,21 Unfortunately, investigations on this approach have so far been limited to theoretical treatments. In addition to fabrication challenges, experimental realization requires nontrivial optical excitation. For example, tapered wires must be illuminated with radially polarized light so that the excited plasmons will interfere constructively as they travel toward the tip.9 This would then create an intense longitudinal field oriented along the tip axis. Alternatively, the edges of a tightly focused Gaussian beam can also excite such a field.10 If linearly polarized light is used instead, the longitudinal tip mode is generated only for large angles of incidence22 or with asymmetric tips.23
he concentration of light into nanoscale “hot spots” requires manipulation of optical energy well below its wavelength. While dielectric structures cannot achieve this due to diffraction, patterned metals that support surface plasmons provide a solution.1-3 Indeed, various metallic films,4 trenches,5 tapers,6,7 gaps,8 and tips9-13 have shown promise for unprecedented control and delivery of optical energy into subwavelength volumes. Surface plasmons are electromagnetic surface waves sustained by density fluctuations of free electrons at a metal interface.14 They are generated when light scatters from a surface pattern such as a grating. Due to their evanescent nature, plasmons are not limited by diffraction. Thus, if they propagate toward and focus at a sharp tip or apex,12 excitation of highly local and extremely intense optical fields is possible. While all nanoscale metallic tips exhibit some local-field enhancement (i.e., due to an optical lightning rod effect), plasmonic nanofocusing schemes use gratings, prisms, or slits to launch plasmons into metallic structures that will then deliver their energy into nanoscale volumes. The spectral behavior is determined both by the excitation geometry and by the shape and local resonances of the tip itself. Previously, metallic tips and tapers have been fabricated via the metallization of fibers,15 electrochemical etching,13 electron-beam induced deposition,16 complex multistep processes,17 or standard metal deposition.6 All of these techniques introduce inherent surface roughness that can degrade performance and lead to sample-to-sample variations. In addition, if the structure is then patterned with focused ion beam (FIB) milling,7,13,15 impurities are implanted in the metal. Despite these limitations, focusing of plasmons has been demonstrated. Gratings milled into the side of an electrochemically etched gold wire can deliver plasmons to the
Due to these implementation issues, the ability to focus photogenerated plasmons from multiple directions toward a specific location on a nonplanar device has proven challenging. In this Letter, we demonstrate such three-dimensional plasmonic nanofocusing of light with patterned gold and silver pyramids obtained via template stripping.24 These structures are easily fabricated, exhibit smooth surfaces without impurities, and require only normal illumination with linearly polarized light. We place periodic gratings on all four faces of a sharp metallic pyramid such that incident light creates plasmons that converge to a nanoscale spot at the tip. Three-dimensional finite-difference time-domain (FDTD) simulations confirm incident light with a wavelength of λ ) 710 nm focusing into a 36 × 36 × 14 nm3 volume at the ∼10 nm tip, producing a 5 × 10-5 λ3 spot. With this welldefined hot spot, we experimentally demonstrate surfaceenhanced Raman scattering (SERS) and second harmonic
* To whom correspondence should be addressed,
[email protected]. Received for review: 12/30/2009 Published on Web: 03/17/2010 © 2010 American Chemical Society
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FIGURE 1. Patterned metallic pyramids for three-dimensional plasmonic nanofocusing. (a) A scanning electron micrograph (SEM) with a schematic overlay demonstrating the use of sharp metallic pyramids with integrated patterning for three-dimensional plasmonic nanofocusing. (b) Top-view SEM of the silicon template used for fabrication. (c) Side-view and (d) top-view SEMs of a silver pyramid patterned with a set of symmetric gratings. (e) Side-view and (f) top-view SEMs of a silver pyramid patterned with a set of asymmetric gratings. The symmetry of the patterning determines the symmetry of the plasmonic field at the pyramid tip. Scale bars: (a) 500 nm; (b) 500 nm; (c) 200 nm; (d) 1000 nm; (e) 200 nm; (f) 1000 nm.
generation (SHG). Because our structures are easily and reproducibly produced, these results could provide benefit for multiple applications, including imaging,25 sensing,26 lithography, and nonlinear spectroscopy.23 For fabrication of metallic pyramids with integrated patterning, we used template-stripping methods.24,27,28 A silicon template with inverted pyramidal pits was first created via anisotropic chemical etching. With FIB we then defined gratings on the smooth faces of these pits. Deposition and removal of a metal film from this template yielded upright, patterned metallic pyramids. The purity of the metal is maintained because only the template is exposed to FIB. In addition, because the template is reusable, multiple copies of the same pyramids with precisely defined nanoscale features were produced. Finally, extremely smooth interfaces are obtained, which eliminates unwanted roughness that can plague plasmonic structures made by conventional methods. Indeed, we show below that even nominal nanoscale roughness introduced through standard metal deposition is enough to degrade the nanofocusing performance of our pyramids. Figure 1 shows scanning electron micrographs (SEMs) of a set of silver structures. Gold pyramids were also fabricated (see Figure S1 in Supporting Information). Light incident from above (Figure 1a) is backscattered into plasmons that travel up the sides and corners of the pyramid, converging at the apex. Using the silicon template (Figure 1b), multiple © 2010 American Chemical Society
samples (>20) of the same pattern were fabricated with different metals. Figure 1c,d shows side-view and top-view SEMs of silver pyramids with gratings running down all four faces. The metal is 200 nm thick; the bumps have a height of 60 nm, have a periodicity of 280 nm, and begin 480 nm from the tip. Since the gratings on opposing faces are placed symmetrically, normally incident linearly polarized light cannot excite longitudinal field components at the tip. If instead, the gratings are placed asymmetrically (Figure 1e,f), plasmons traveling up opposing sides and corners of the pyramid will interfere constructively at the tip, leading to large field confinement and enhancement. To explore the behavior expected of such structures, three-dimensional FDTD calculations were performed for patterned gold pyramids assuming normally incident planewave illumination (Figure 2). A Drude-Lorentz dispersion model was fit to experimental optical constants.29 The tip was also rounded to have a 10 nm radius, consistent with Figure 1. Figure 2a shows the time-averaged electric-field intensity at 710 nm excitation for a pyramid patterned symmetrically with 370 nm periodicity bumps. The field distribution is broadly localized into two large side lobes, with a null at the apex. Figure 2b shows an asymmetrically patterned pyramid with an intense and tightly focused field at the apex. With symmetric patterning, the field is oriented transversally, but with asymmetric patterning, the field is oriented longitudinally at the tip (Figure 2c), with the neces1370
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FIGURE 2. Three-dimensional finite-difference time-domain simulations. Normally incident, linearly polarized light illuminates the pyramids from above with a wavelength of λ ) 710 nm. The electric field intensities are plotted. (a) Symmetric patterning produces a time-averaged plasmonic field with a null at the apex, whereas (b) asymmetric patterning produces a strongly localized field which is oriented (c) longitudinally (z). Note the linear intensity scales in (a) and (b), and the logarithmic intensity scale in (c). (d) Spectral response of the longitudinal field intensity 30 nm above the tip. A single set of bumps is sufficient to excite the localized plasmon at the tip, and adding more bumps collects from a larger area and allows a degree of tunability.
FIGURE 3. Scanning confocal Raman imaging. (a) Top-view and (b) side-view scanning confocal Raman images of a smooth template-stripped asymmetrically patterned silver pyramid. The tip is clearly distinguishable, and the signal from the gratings is very weak. The dashed lines depict the location of the pyramid. (c) A top-view Raman scan of a template-stripped unpatterned silver pyramid showing an enhancement in the Raman signal of (d) 100-fold due to patterning. (e) Top-view and (f) side-view Raman images of a template-stripped sample with an additional 100 nm of thermally evaporated silver. In this case, even nominal nanoscale roughness is sufficient to degrade a well-defined plasmonic nanofocusing effect. Scale bars: (a) 500 nm; (b) 1000 nm; (c) 500 nm; (e) 500 nm; (f) 1000 nm. © 2010 American Chemical Society
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sary symmetry (see Figure S2 in Supporting Information) for nanofocusing.30 By offsetting the opposing gratings by a fraction of the period, such that the plasmons arriving on the apex of the pyramid interfere constructively, it is possible to generate a longitudinally polarized field even though the structure is illuminated with horizontal, linearly polarized light. Figure 2d shows the spectral response of the longitudinal component of the electric field 30 nm above the tip for a single set of bumps and for several grating periodicities. Even a single bump is enough to excite the tip (see video S1 in Supporting Information), while adding more bumps collects from a larger area, as with a bull’s eye pattern,31 and allows a degree of spectral tuning. The periodicity of the gratings on opposing sides of the pyramids should be chosen to maximize the conversion efficiency of incident light into plasmons at a fixed illumination angle. The grating itself was simulated separately with FDTD (not shown) and found to be up to 10% efficient in generating backscattered surface plasmons, depending on the periodicity and incident wavelength. These structures should also generate intense local fields. To test this, we coated gold and silver pyramids with a monolayer of benzenethiol and collected Raman spectra as a function of the position of a diffraction-limited focus of a 752 nm Kr laser. The height of the Raman peak at 1073 cm-1, which characterizes the in-plane C-C-C bending plus C-S stretching mode of the benzenethiol, was used to generate the intensity images shown in Figure 3. For our template-stripped asymmetrically patterned pyramids, a single resolution-limited SERS hot spot was observed, in both the x-y and x-z scans (Figure 3a,b, respectively). Compared to the scan of an unpatterned pyramid on the same substrate (Figure 3c), the signal was enhanced 100-fold (Figure 3d). The combination of a sharp tip with gratings is responsible for significant field enhancement. Analogous results were also obtained for gold pyramids (see Figure S3 in Supporting Information). Due to the well-tailored patterning, the gratings on the faces of the pyramids are not visible in the Raman scans (Figure 3a,b). Typically, any subwavelength bump, groove, particle, or random roughness is an efficient SERS scatterer. Here, the signal generated from the tip overwhelms the signal from the gratings, demonstrating control over the location and intensity of the hot spot. Three-dimensional nanofocusing was only observed with the smooth template-stripped patterned interfaces. When we thermally evaporated another 100 nm of Ag on top of a freshly template-stripped Ag pyramid with asymmetric patterning (see Figure S4 in Supporting Information), Raman scans (Figure 3e,f) revealed that the signal from the tip was now less than the signal from the gratings. Clearly, even nominal nanoscale roughness inherent in standard metal deposition techniques was detrimental to the desired effect. This is demonstrated further by evaporating metal onto unpatterned and symmetrically patterned template-stripped © 2010 American Chemical Society
FIGURE 4. Scanning confocal second harmonic generation (SHG) imaging. (a) SHG images of a template-stripped unpatterned gold pyramid and (b) a template-stripped asymmetrically patterned gold pyramid. The dashed squares outline the approximate extent of the pyramids. (c) The power dependence of the signal from the pyramids shows clear SHG behavior. (d) By using a water immersion lens, the response of the patterned pyramids is better located near the 800-880 nm laser excitation range, leading to a 35-fold SHG enhancement due to patterning. Scale bars: 3 µm.
pyramids. Randomly distributed hot spots appear on their corners and edges (see Figure S5 in Supporting Information). In contrast, the smooth template-stripped patterned pyramids allow high-intensity, reproducible plasmonic nanofocusing. To confirm that the excitation of surface plasmons from incident light occurs via a grating-coupling mechanism, we collected confocal second harmonic generation (SHG) scans on a set of smooth patterned and unpatterned gold pyramids as a function of wavelength. The SHG image of a templatestripped unpatterned pyramid is shown in Figure 4a. A template-stripped pyramid patterned with a set of asymmetric bumps is shown in Figure 4b. The quadratic power dependence confirms SHG behavior (Figure 4c). A 35-fold enhancement in the SHG signal due to patterningsi.e., patterned vs unpatternedswas achieved when the sample was immersed in water to shift the response of the structure toward the 800-880 nm excitation range of our laser (Figure 4d), as confirmed with FDTD calculations (see Figure S6 in Supporting Information). A clear maximum exists for a fundamental excitation wavelength of 870 nm, establishing wavelength-dependent behavior. These results demonstrate three-dimensional nanofocusing with well-defined plasmonic hot spots. The effect relies on having smooth patterned interfaces. Because the hot spot is at the end of a sharp tip, our structure has clear advantages in areas such as scanning-probe microscopy, optical trapping,32 1372
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and high-density data storage.33 While here we illuminate the pyramids from above, these structures consist of metal films that can be sufficiently thin to allow backside excitation. Along with the ability to template multiple copies of a single pyramid or arrays of pyramids on a flexible substrate, our results should benefit a wide variety of applications.
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Acknowledgment. We thank Jerry Sedgewick for assistance with SHG instrumentation. This research was supported by the Minnesota Partnership Award for Biotechnology (N.C.L., A.L., and S.-H.O.) and the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award DE-FG02-06ER46438 (P.N. and D.J.N.). We also utilized resources at the University of Minnesota, including the Nanofabrication Center, which receives partial support from NSF through the National Nanotechnology Infrastructure Network, and the Characterization Facility, which has received capital equipment funding from NSF MRSEC. N.C.L. and P.N. acknowledge support from the University of Minnesota doctoral dissertation fellowship. S.-H. Oh acknowledges support from the 3M Faculty Award and ACS PRF Doctoral New Investigator Award. Supporting Information Available. Experimental methods, Figures S1-S6, and video S1. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES AND NOTES (1) (2) (3) (4) (5) (6) (7) (8) (9)
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