Letter pubs.acs.org/NanoLett
Template-Stripped Asymmetric Metallic Pyramids for Tunable Plasmonic Nanofocusing Sudhir Cherukulappurath,†,§ Timothy W. Johnson,†,§ Nathan C. Lindquist,†,‡ and Sang-Hyun Oh*,† †
Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, Minnesota 55455, United States Physics Department, Bethel University, St. Paul, Minnesota 55112, United States
‡
ABSTRACT: We demonstrate a novel scheme for plasmonic nanofocusing with internally illuminated asymmetric metallic pyramidal tips using linearly polarized light. A wafer-scale array of sharp metallic pyramids is fabricated via template stripping with films of different thicknesses on opposing pyramid facets. This structural asymmetry is achieved through a one-step angled metal deposition that does not require any additional lithography processing and when internally illuminated enables the generation of plasmons using a Kretschmann-like coupling method on only one side of the pyramids. Plasmons traveling toward the tip on one side will converge at the apex, forming a nanoscale “hotspot.” The asymmetry is necessary for these focusing effects since symmetric pyramids display destructive plasmon interference at the tip. Computer simulations confirm that internal illumination with linearly polarized light at normal incidence on these asymmetric pyramids will focus optical energy into nanoscale volumes. Far-field optical experiments demonstrate large field enhancements as well as angle-dependent spectral tuning of the reradiated light. Because of the low background light levels, wafer-scale fabrication, and a straightforward excitation scheme, these asymmetric pyramidal tips will find applications in nearfield optical microscopy and array-based optical trapping. KEYWORDS: Surface plasmons, symmetry breaking, Kretschmann coupling, plasmonic nanofocusing, template stripping, tip-enhanced Raman scattering
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microscopy (NSOM), focusing a radially polarized beam to the apex of the tip demands nanometric alignment tolerance. Likewise, side illumination has been successfully used for tipenhanced Raman spectroscopy (TERS),32 but in this case the alignment is also nontrivial. Alternatively, it is possible to use simpler illumination, for example, normal incidence and linear polarization, by breaking the symmetry of the tip geometry. Symmetry breaking in plasmonic nanostructures has been studied by various groups.33−35 For high-throughput production, it is desirable to develop a fabrication scheme that breaks the symmetry of the tip without using any advanced lithography or tip-by-tip fabrication. For NSOM and TERS applications, it is also desirable to use an illumination method that can reduce the background light levels to increase the near-field contrast. In this paper we present a practical solution based on internal illumination of asymmetric metallic pyramidal shells (i.e., excitation of surface plasmons from “within” the pyramid) that can be fabricated on the wafer-scale. By making one facet of the pyramid optically thin and the other facet optically thick, surface plasmons are launched asymmetrically up the outside of the pyramid using simple plane-wave illumination from within the pyramid. This internal illumination scheme also reduces
ubwavelength confinement of optical energy accompanied by high local field enhancements can benefit many applications, such as super-resolution optical microscopy,1 surface-enhanced spectroscopy,2,3 optical trapping,4−7 nonlinear optics,8−11 and heat-assisted magnetic recording.12,13 While nanofocusing of light is impossible with only conventional far-field optics, metallic nanostructures that convert light into surface plasmons can circumvent the diffraction limit and squeeze optical energy into nanometric volumes.14−16 Nanofocusing of plasmons has been theoretically investigated17,18 and experimentally demonstrated with several geometries such as sharp metallic tips,16,19,20 wedges,15 and nanogaps.21−24 Of these structures, sharp metallic tips are particularly useful since they can be readily integrated into scanning probe systems for nanoscale imaging and spectroscopy. 1,13,25 Plasmonic nanofocusing has been achieved with various metallic tips made via electrochemical etching,8 coating of optical fibers,26 attaching metallic nanoparticles to optical fibers,27,28 and milling with focused ion beams (FIB).19,29,30 The structural symmetry of these tips determines the possible optical illumination schemes for photoexcitation of surface plasmons. To achieve maximum field enhancement at the tip,4,31 tips with a symmetric geometry must be illuminated with either radially polarized light at normal incidence or linearly polarized light at large illumination angles. While symmetric tip geometries are typically easier to fabricate and have been successfully used for near-field scanning optical © 2013 American Chemical Society
Received: September 4, 2013 Revised: October 16, 2013 Published: October 21, 2013 5635
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asymmetric pyramidal shell structures can be mass-fabricated using template stripping (Figure 1b−e) that yields pyramids with ultrasmooth metallic faces and sharp apexes.36−38 To fabricate these structures, pyramidal pits were first made in a Si wafer using crystal-orientation-dependent anisotropic etching of Si in a 30% KOH and 10% isopropanol solution.37 This process exposes slowly etching {111} planes of the Si wafer and yields square pyramidal pits with an opening angle of 70.52°.38 To obtain different metal thicknesses on the pyramidal facets, single-step angled evaporation of silver onto the template was performed. Angled metal deposition on pyramidal pits was used previously for the fabrication of bowtie nanoantennas.39 In our work, we use angled deposition to create an asymmetry in the thickness of metal on opposing facets of pyramidal tips (Figure 1c). Samples were placed at an angle of 15° for a nominal silver deposition thickness of 120 nm to yield 35 nm on one face and 110 nm on the opposite face of the pyramid measured normal to the face. The silver film was then peeled from the Si mold using a UV-curable optical adhesive (Norland Products NOA 61, refractive index: 1.56) and a glass microscope slide as a transparent backing layer, yielding smooth upright pyramids. The internal asymmetry of the pyramid enables Kretschmannlike coupling of plasmons and nanofocusing at the tip, while the ultrasmooth outer surface enables low-loss plasmon propagation36 and well-controlled creation of a nanoscale hotspot only at the pyramidal tip.37 Previously, Garoli et al. reported a nanofocusing method with 2D wedge structures by introducing a phase shifting layer at the base.40 This method of nanofocusing, however, is difficult to implement on NSOM tips as the fabrication process require precise lithography and alignment. In our scheme, wafer-scale fabrication of asymmetric pyramidal tips is straightforward using angled evaporation and template stripping. A photograph of a full wafer containing such asymmetric pyramids is shown in Figure 2a. Figure 2b shows a scanning electron micrograph (SEM) of an array of templatestripped silver pyramids with base widths of 20 μm. The crosssectional SEM of the pyramid, shown in Figure 2c, confirms the different thicknesses on the two opposite faces. To calculate the local intensity enhancement factor, FDTD simulations (FullWAVE software package, RSoft Inc.) were performed for the asymmetric silver pyramids. The dispersion of the dielectric function of silver was taken into account using a Drude-Lorentz model fit to experimentally measured values. The tip of the pyramid was rounded to a radius of 6 nm, which can be achieved experimentally using the template-stripping method. To limit the use of computer memory, a small pyramid with a 1.5 μm base width was used for simulations. The pyramid was internally illuminated using a square waveguide mode (as shown in Figure 1). Figure 3 shows electric field maps at an excitation wavelength of 785 nm for two pyramidal tip geometries. For the symmetric pyramid, the linearly polarized incident beam excites plasmons on the two opposing facets of the pyramid, while for the asymmetric pyramid plasmons are excited only on the thin facet and travel toward the apex. In the asymmetric case, a strong local field at the tip is obtained, as shown in Figure 3a. Longitudinal (along the z- or tip axis) and horizontal (along the x-axis) electric fields are plotted in Figure 3a,b, respectively. Figure 3c,d shows their corresponding electric field plots obtained very close to the tip. In the case of symmetric pyramids, SPPs traveling on the facets toward the apex of the pyramid meet at the tip and interfere destructively due to their opposite phases as depicted in Figure 3 panel e (zcomponent) and panel f (x-component). The exact coupling
background light levels since the samples are not directly illuminated by the light source. Three-dimensional (3D) finitedifference time-domain (FDTD) simulations show that incident light at a wavelength of 785 nm can be squeezed to a nanoscale spot with a volume of λ3/1 000 000 at the apex of a pyramid. Experimentally, we investigate this nanofocusing by observing scattered light using far-field optical measurements and show angle-dependent spectral tuning of reradiated light. Scanning Raman scattering spectra and second harmonic generation (SHG) images confirm large field enhancements. Finally, we show that our devices can be readily extended to a large parallel array of illuminated tips. A schematic of the internal illumination method is depicted in Figure 1a. In this scheme, the light incident through a highindex support medium can excite plasmons on the outer facets of a metallic pyramid. To enable nanofocusing with linearly polarized light through normal incidence internal illumination, we break the symmetry in the thickness of opposing facets and launch surface plasmons on only one side of the pyramids. The
Figure 1. Symmetry breaking for nanofocusing. (a) Schematic of nanofocusing of plasmons using internal illumination of an asymmetrically thick pyramid. The lower dielectric material is optical adhesive with a refractive index of 1.56 and the upper medium is air. One side of the pyramid is 35 nm thick and the opposite side has a metal thickness of 110 nm. Plasmons are excited only on the air−silver interface of the thinner side. Schematic of the template-stripping process for making asymmetric pyramids: (b) inverted pyramidal pattern is formed on a Si wafer using anisotropic KOH etching, (c) metal is deposited at an angle for asymmetric thicknesses, and (d) the inverted pyramid is then filled up with an optical adhesive and UVcured (e) followed by template stripping. 5636
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Figure 2. Wafer-scale template stripping of asymmetric metal pyramids (a) Photograph of a wafer with asymmetric pyramids. (b) SEM of a set of 20 μm sized template-stripped silver pyramids. A large array containing up to 1 million such identical pyramids can be fabricated on a single wafer using this method. (c) A cross-sectional SEM view of an asymmetrically thick pyramid. Asymmetry of the thickness in the pyramids is produced by angled metal evaporation. The porous structure of the epoxy seen here is an artifact generated during FIB cross-sectioning.
Figure 3. Three-dimensional FDTD simulations. Pyramids are illuminated from below using a lineally polarized light at a wavelength of 785 nm. Electric field maps are plotted. (a) Asymmetry in the thickness allows for plasmons to travel on one side thereby reaching the apex to give a strong plasmonic field at the tip. Here the longitudinal field (z-axis) is plotted (b) electric field in the horizontal direction (x-axis) for similar conditions as in (a). (c,d) Enlarged electric field plots near the tip corresponding to (a,b), respectively. The field enhancement for the longitudinal direction close to the tip is around 8. (e) In the case of symmetric thickness, no field confinement is observed at the tip. Here, surface plasmons propagate on both sides toward the apex and interfere destructively due to their phase difference. Electric fields along the vertical axis (z-direction) are plotted in (e) and components in the horizontal direction (x-axis) in (f). The images are saturated for clarity.
mechanism of light to SPP for the asymmetric pyramid is more complicated than prism coupling in planar films, because of the involvement of waveguide modes and reflections from four
internal facets of the pyramid, as well as the presence of edge modes. This geometry is similar to that used by Bouhelier et al.26 for coated fibers and Tanaka et al.41 for tetrahedral tips. 5637
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SPPs can be excited from direct coupling of the dielectric waveguide mode when their phases match, similar to Kretschmann coupling. Another possible coupling route is through edge-plasmon modes that can travel to the tip as proposed by Tanaka et al.41 However our FDTD simulations show that the primary method of SPP excitation is through a waveguide coupling rather than edge mode coupling. The calculated field intensity enhancement near the tip in the case of the asymmetric pyramids is more than 60× when normalized to the incident field intensity, which can yield thousand-fold enhancements in Raman and SHG signals. The field is confined to a spot size of 12 nm (1/e of the maximum intensity) in the longitudinal direction for a tip radius of 6 nm, corresponding to a volume of λ3/1 000 000. In our experiments, the template-stripped pyramids were first illuminated internally with a white light source using a 50× objective (NA = 0.55). This NA means that the excitation beam hits the dielectric-silver interface with a range of angles, some of which can better match the SPP excitation angle of the silver− air interface and allow for more efficient coupling. The far-field scattering from the tip of the pyramid was collected using a 100× (0.9 NA) objective and imaged onto a color CCD (Thorlabs DCU224C). Figure 4a shows a CCD image of light scattered from the tip with the incident light polarized in the direction normal to the asymmetric faces of the pyramid. Because of our fabrication scheme, only one pair of opposing faces has unequal thicknesses and the other pair has equal thicknesses. Rotating the polarization normal to the other two faces reduces the scattering at the tip significantly (Figure 4b) implying that it is indeed the asymmetrical thickness that is responsible for the light at the tip. Any weakly scattered light in the symmetrically thick case could be due to slight asymmetry in the excitation angles arising from a tilt in the sample or beam. We then illuminated the pyramid with a 785 nm laser using the 50× objective. As before, a 100× objective was used to image the tip of the pyramid. CCD images of the pyramid when the objective is focused on the base and on the tip of the pyramid are shown in Figure 4c,d, respectively. When focused on the tip region of the pyramid, a bright spot arising from the scattered laser light can be observed. These images indicate that SPPs are excited by the internal illumination and travel along the facet of the pyramid and get reradiated into far field via scattering from the tip, as expected from the simulations. The output power measured near the tip was approximately 100 μW for an incident power of 10 mW at the objective. Thus the total efficiency of coupling to and scattering from the tip is estimated to be around 1% for most of the tips. The efficiency of our internal illumination scheme is higher than the transmission efficiency of conventional aperture probes and can be compared with recent approaches such as aperture probes utilizing extraordinary optical transmission.42 We believe that owing to lower plasmonic propagation losses on template-stripped metals and reduced background, this scheme can find applications in near-field studies of single-molecule fluorescence and Raman scattering. Furthermore, because our nanofocusing scheme uses large-scale fabrication methods, it is possible to obtain multiple hotspots by illuminating an array of asymmetric pyramids internally. This effect is shown in Figure 4f where hotspots from an array of 1 μm sized pyramids (SEM image shown in Figure 4e) are obtained by illuminating them all with white light simultaneously. The generation of multiple electromagnetic hotspots will be useful for array-based
Figure 4. Optical measurements for light scattered at the tip. CCD images of an asymmetric pyramid that is internally illuminated with a white light source. The position of the pyramids is represented by dotted white lines along with the labels indicating the thin and thick facets. The polarization is perpendicular to the asymmetry in (a) and parallel in (b). A higher intensity at the tip when polarized perpendicular to the thick-thin face of the pyramid clearly indicates asymmetry plays an important role in nanofocusing. (c,d) CCD images of a pyramid illuminated internally with a 785 nm diode laser. In (c), the base of the pyramid was focused while in (d) the tip was in focus. Here the laser light couples to surface plasmons and travel toward the tip where it is scattered to far field. Scale bars in (a−d) are 5 μm. (e) SEM image of template-stripped 1 μm pyramids. (f) Intensity hotspots at the tip of the pyramids with white light internal illumination. Parallel hot spots can be achieved using this scheme.
NSOM, surface-enhanced Raman spectroscopy (SERS), and optical trapping. One interesting consequence of using this Kretschmann-like internal coupling scheme for nanofocusing is that under white light illumination, the peak wavelength of light reradiated by the tip can be tuned by changing the angle of light incident at the metal−dielectric interface. To demonstrate this, the pyramid was illuminated with white light from the backside using a condenser with adjustable NA mounted onto an inverted microscope (Nikon Eclipse). The light from the tip was collected using a 100× objective as before. The field and condenser diaphragms of the microscope are adjusted so that a thin collimated beam illuminated the pyramid. Adjusting the position of the condenser diaphragm allows for small changes in the angle of light incident on the backside of the pyramid. With this method, we could observe varying colors scattered from the tip. Figure 5a−c shows CCD snapshots of green, orange, and red colors, respectively, scattered from the tip for 5638
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Figure 5. Angle-dependent spectral tuning of light at the pyramidal tip. CCD images showing colors of the scattered light [(a) green, (b) orange, and (c) red] on a pyramidal tip by slightly changing the incidence angle under internal illumination scheme. A color CCD camera was used for capturing the images. (d−f) The corresponding spectra obtained from a spectrometer. (g) Electric field intensity at the tip dependence on wavelength and incident angle computed using the FEM method for a two-dimensional wedge with asymmetric thickness. A good match between this and theoretical Kretschmann calculations (white dashed line) suggests that the coupling mechanism in asymmetric wedges is primarily Kretschmann-like. (h) Field intensity plot of an asymmetric wedge illuminated at normal incidence for a wavelength of 785 nm computed using COMSOL. The parameters used for the simulations are similar to those used in Figure 3. At normal incidence, the enhancement is relatively weak while in (i) strong SPP generation on the thinner facet of the wedge and local enhancement at the tip is achieved when the incident angle matches Kretschmann coupling angle (10° as measured from the z-axis).
different angles of incidence. Spectral measurements of the light reradiated from the tip for different angles of incidence, shown in Figure 5d−f, were done using a deep-cooled low-noise CCD camera (Princeton Pixis 400) connected to an imaging spectrometer (Newport MS-257i). Measuring the exact incidence angle is difficult in our current setup. However, a clear shift in the peak position of the spectra is observed, indicating that the incidence angle can be used to spectrally tune the nanoconfined light at the tip. To understand this angular dependence of SPP coupling, we performed 2D finite element method (FEM) simulations on an asymmetric wedge using COMSOL Multiphysics software. Here we used FEM instead of FDTD as it better performs simulations where the angle of incidence needs to be changed to determine the Kretschmann coupling condition without any “staircase” grid artifacts as in the FDTD simulations. Furthermore, our FEMbased simulations were more suited for single-wavelength computations. In Figure 5g, the dependence of intensity at the tip on the wavelength and incidence angle of illumination is
shown. The white dashed line represents data from analytical Kretschmann coupling calculations. A good match between the two indicates that the coupling mechanism is primarily Kretschmann-like. Figure 5h shows the intensity plot for an asymmetric wedge structure illuminated internally at normal incidence for a wavelength of 785 nm while in Figure 5i the incidence angle is tilted by 10°. At the surface plasmon resonance angle, SPPs can be excited on that facet, travel toward and focus at the apex, and scatter into the far-field. The intense local field generated at the metallic tip can enhance spectroscopic processes such as SERS and SHG. To validate the usefulness of our tips for SERS, we coated the silver pyramids with a monolayer of benzenethiol and collected Raman spectra scattered from the tip. The pyramid was illuminated using a 785 nm diode laser loosely focused using the 50× objective. Raman signals were collected using a 100× objective coupled to a fiber-optic Raman spectrometer (Ocean Optics QE 65000) through a multimode fiber. Scanning confocal Raman measurement under external illumination 5639
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launch SPPs, the local field enhancement is much weaker when the incident light is linearly polarized, while in our internally illuminated asymmetric pyramid enhancement of the electric field along the tip axis can be easily achieved without any further lithographic patterning. To demonstrate second harmonic generation (SHG) with our devices, a Nikon multiphoton microscope (A1 RMP+) was used. Template-stripped Ag pyramids were illuminated with femtosecond laser light pulses at a center wavelength of 800 nm using a 40× objective (NA 0.5) and the emitted light was collected from the tip side (in transmission mode) using the condenser of the microscope. SHG signals were collected using a photomultiplier tube after filtering out the excitation laser. The quadratic power dependence confirmed that the signal collected is of second order (data not shown). The excitation power was kept low enough to prevent tip damage while keeping the signal intensity strong enough for detection. The polarization of the incident laser light was fixed in the direction perpendicular to the asymmetric faces of the pyramid so as to achieve maximum intensity at the tip. Figure 6e shows a SHG scan image of four pyramids on the sample. The intense bright spot at the tip is representative of strong local intensity confinement. A zoomed-in scan of the SHG signal from a single pyramid is shown in Figure 6f. In conclusion, these simulations and experimental results demonstrate plasmonic nanofocusing with large field enhancements at the tips of asymmetric pyramids illuminated internally. By breaking the symmetry in the thickness of opposing metallic facets, SPPs are launched on only one side of the pyramids and enable a nanofocusing effect using linearly polarized light. This method of nanofocusing can be achieved from samples fabricated over an entire wafer with a one-step angled metal evaporation and template-stripping process. The Kretschmannlike coupling geometry enables angle-dependent spectral tuning. Additionally, the template-stripping process produces ultrasmooth metal surfaces, enhancing the plasmon propagation lengths. Calculations have previously shown that the large opening angle (70.52°) of the pyramid formed by Si {111} crystal planes is also favorable for scattering light from the nearfield to the far-field compared to adiabatic tapering angles.43 These structures should be well suited for TERS and near-field fluorescence imaging using the internal illumination scheme, and also for fiber-coupled illumination schemes. Our internal illumination method will be of particular use in applications such as plasmonic data storage12,44 and near-field optical spectroscopy1 where the reduction of background noise is critical. Furthermore, because arrays of intense hotspots can be generated by illuminating arrays of pyramids, our approach can facilitate array-based optical trapping, plasmonic sensing, as well as high-throughput imaging applications.
independently confirmed the presence of benzenethiol monolayer on the pyramids. To compare the efficiency of our internal illumination with an external illumination method, we performed experiments in both cases under similar conditions, as illustrated in Figure 6a,b. Raman scattering images obtained
Figure 6. Raman and scanning confocal SHG imaging. (a) Internal illumination scheme. (b) Schematic of external illumination of the pyramid. (c) Raman spectra collected from the tip of a pyramid functionalized with benzenethiol (BZT) and internally illuminated with a 785 nm diode laser. The scattered light from the tip was collected with a 100× objective and the spectra measured with a fiberoptic spectrometer. The signature Raman peaks of BZT are distinguishable. Raman spectra collected from the flat silver region of the same sample yielded very low signal. (d) Comparison of Raman spectra in the internal illumination mode (black) and under external (normal) illumination (red). Raman signals are much stronger when the pyramid is illuminated internally. (e) SHG image from the tip of four different asymmetric pyramids on the sample. Scale bar is 10 μm. (f) A zoomed-in image of one of the pyramids shows confined SHG signal from the tip of the pyramid. Scale bar is 5 μm.
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from the tip of a pyramid and from a flat silver surface are shown in Figure 6c. Strong Raman signature peaks of benzenethiol at 1000, 1024, and 1075 cm−1 can be observed from the tip while only a weak signal was measured from the flat Ag region, as expected. We observed more than a 100-fold increase in the Raman scattering signal at the Ag tip compared to flat Ag surfaces. This result is in agreement with our FDTD simulations. The Raman enhancement in the case of internal illumination is stronger compared to external illumination as shown in Figure 6d. This is expected because for smooth template-stripped pyramids without any grating or defects to
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Author Contributions §
S.C. and T.W.J. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by grants from the Office of Naval Research (ONR) Young Investigator Award (N00014-11-15640
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0645), the National Science Foundation (NSF CAREER Award; DBI 1054191), the Minnesota Partnership Award for Biotechnology and Medical Genomics, and Seagate Technology. T.W.J. acknowledges support from the University of Minnesota Doctoral Dissertation Fellowship. Device fabrication was performed at the University of Minnesota Nanofabrication Center, which receives support from the NSF through the National Nanotechnology Infrastructure Network.
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Template-Stripped Asymmetric Metallic Pyramids for Tunable Plasmonic Nanofocusing Sudhir Cherukulappurath,†,§ Timothy W. Johnson,†,§ Nathan C. Lindquist,†,‡ and Sang-Hyun Oh*,† †
Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, Minnesota 55455, United States Physics Department, Bethel University, St. Paul, Minnesota 55112, United States
‡
ABSTRACT: We demonstrate a novel scheme for plasmonic nanofocusing with internally illuminated asymmetric metallic pyramidal tips using linearly polarized light. A wafer-scale array of sharp metallic pyramids is fabricated via template stripping with films of different thicknesses on opposing pyramid facets. This structural asymmetry is achieved through a one-step angled metal deposition that does not require any additional lithography processing and when internally illuminated enables the generation of plasmons using a Kretschmann-like coupling method on only one side of the pyramids. Plasmons traveling toward the tip on one side will converge at the apex, forming a nanoscale “hotspot.” The asymmetry is necessary for these focusing effects since symmetric pyramids display destructive plasmon interference at the tip. Computer simulations confirm that internal illumination with linearly polarized light at normal incidence on these asymmetric pyramids will focus optical energy into nanoscale volumes. Far-field optical experiments demonstrate large field enhancements as well as angle-dependent spectral tuning of the reradiated light. Because of the low background light levels, wafer-scale fabrication, and a straightforward excitation scheme, these asymmetric pyramidal tips will find applications in nearfield optical microscopy and array-based optical trapping. KEYWORDS: Surface plasmons, symmetry breaking, Kretschmann coupling, plasmonic nanofocusing, template stripping, tip-enhanced Raman scattering
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microscopy (NSOM), focusing a radially polarized beam to the apex of the tip demands nanometric alignment tolerance. Likewise, side illumination has been successfully used for tipenhanced Raman spectroscopy (TERS),32 but in this case the alignment is also nontrivial. Alternatively, it is possible to use simpler illumination, for example, normal incidence and linear polarization, by breaking the symmetry of the tip geometry. Symmetry breaking in plasmonic nanostructures has been studied by various groups.33−35 For high-throughput production, it is desirable to develop a fabrication scheme that breaks the symmetry of the tip without using any advanced lithography or tip-by-tip fabrication. For NSOM and TERS applications, it is also desirable to use an illumination method that can reduce the background light levels to increase the near-field contrast. In this paper we present a practical solution based on internal illumination of asymmetric metallic pyramidal shells (i.e., excitation of surface plasmons from “within” the pyramid) that can be fabricated on the wafer-scale. By making one facet of the pyramid optically thin and the other facet optically thick, surface plasmons are launched asymmetrically up the outside of the pyramid using simple plane-wave illumination from within the pyramid. This internal illumination scheme also reduces
ubwavelength confinement of optical energy accompanied by high local field enhancements can benefit many applications, such as super-resolution optical microscopy,1 surface-enhanced spectroscopy,2,3 optical trapping,4−7 nonlinear optics,8−11 and heat-assisted magnetic recording.12,13 While nanofocusing of light is impossible with only conventional far-field optics, metallic nanostructures that convert light into surface plasmons can circumvent the diffraction limit and squeeze optical energy into nanometric volumes.14−16 Nanofocusing of plasmons has been theoretically investigated17,18 and experimentally demonstrated with several geometries such as sharp metallic tips,16,19,20 wedges,15 and nanogaps.21−24 Of these structures, sharp metallic tips are particularly useful since they can be readily integrated into scanning probe systems for nanoscale imaging and spectroscopy. 1,13,25 Plasmonic nanofocusing has been achieved with various metallic tips made via electrochemical etching,8 coating of optical fibers,26 attaching metallic nanoparticles to optical fibers,27,28 and milling with focused ion beams (FIB).19,29,30 The structural symmetry of these tips determines the possible optical illumination schemes for photoexcitation of surface plasmons. To achieve maximum field enhancement at the tip,4,31 tips with a symmetric geometry must be illuminated with either radially polarized light at normal incidence or linearly polarized light at large illumination angles. While symmetric tip geometries are typically easier to fabricate and have been successfully used for near-field scanning optical © 2013 American Chemical Society
Received: September 4, 2013 Revised: October 16, 2013 Published: October 21, 2013 5635
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asymmetric pyramidal shell structures can be mass-fabricated using template stripping (Figure 1b−e) that yields pyramids with ultrasmooth metallic faces and sharp apexes.36−38 To fabricate these structures, pyramidal pits were first made in a Si wafer using crystal-orientation-dependent anisotropic etching of Si in a 30% KOH and 10% isopropanol solution.37 This process exposes slowly etching {111} planes of the Si wafer and yields square pyramidal pits with an opening angle of 70.52°.38 To obtain different metal thicknesses on the pyramidal facets, single-step angled evaporation of silver onto the template was performed. Angled metal deposition on pyramidal pits was used previously for the fabrication of bowtie nanoantennas.39 In our work, we use angled deposition to create an asymmetry in the thickness of metal on opposing facets of pyramidal tips (Figure 1c). Samples were placed at an angle of 15° for a nominal silver deposition thickness of 120 nm to yield 35 nm on one face and 110 nm on the opposite face of the pyramid measured normal to the face. The silver film was then peeled from the Si mold using a UV-curable optical adhesive (Norland Products NOA 61, refractive index: 1.56) and a glass microscope slide as a transparent backing layer, yielding smooth upright pyramids. The internal asymmetry of the pyramid enables Kretschmannlike coupling of plasmons and nanofocusing at the tip, while the ultrasmooth outer surface enables low-loss plasmon propagation36 and well-controlled creation of a nanoscale hotspot only at the pyramidal tip.37 Previously, Garoli et al. reported a nanofocusing method with 2D wedge structures by introducing a phase shifting layer at the base.40 This method of nanofocusing, however, is difficult to implement on NSOM tips as the fabrication process require precise lithography and alignment. In our scheme, wafer-scale fabrication of asymmetric pyramidal tips is straightforward using angled evaporation and template stripping. A photograph of a full wafer containing such asymmetric pyramids is shown in Figure 2a. Figure 2b shows a scanning electron micrograph (SEM) of an array of templatestripped silver pyramids with base widths of 20 μm. The crosssectional SEM of the pyramid, shown in Figure 2c, confirms the different thicknesses on the two opposite faces. To calculate the local intensity enhancement factor, FDTD simulations (FullWAVE software package, RSoft Inc.) were performed for the asymmetric silver pyramids. The dispersion of the dielectric function of silver was taken into account using a Drude-Lorentz model fit to experimentally measured values. The tip of the pyramid was rounded to a radius of 6 nm, which can be achieved experimentally using the template-stripping method. To limit the use of computer memory, a small pyramid with a 1.5 μm base width was used for simulations. The pyramid was internally illuminated using a square waveguide mode (as shown in Figure 1). Figure 3 shows electric field maps at an excitation wavelength of 785 nm for two pyramidal tip geometries. For the symmetric pyramid, the linearly polarized incident beam excites plasmons on the two opposing facets of the pyramid, while for the asymmetric pyramid plasmons are excited only on the thin facet and travel toward the apex. In the asymmetric case, a strong local field at the tip is obtained, as shown in Figure 3a. Longitudinal (along the z- or tip axis) and horizontal (along the x-axis) electric fields are plotted in Figure 3a,b, respectively. Figure 3c,d shows their corresponding electric field plots obtained very close to the tip. In the case of symmetric pyramids, SPPs traveling on the facets toward the apex of the pyramid meet at the tip and interfere destructively due to their opposite phases as depicted in Figure 3 panel e (zcomponent) and panel f (x-component). The exact coupling
background light levels since the samples are not directly illuminated by the light source. Three-dimensional (3D) finitedifference time-domain (FDTD) simulations show that incident light at a wavelength of 785 nm can be squeezed to a nanoscale spot with a volume of λ3/1 000 000 at the apex of a pyramid. Experimentally, we investigate this nanofocusing by observing scattered light using far-field optical measurements and show angle-dependent spectral tuning of reradiated light. Scanning Raman scattering spectra and second harmonic generation (SHG) images confirm large field enhancements. Finally, we show that our devices can be readily extended to a large parallel array of illuminated tips. A schematic of the internal illumination method is depicted in Figure 1a. In this scheme, the light incident through a highindex support medium can excite plasmons on the outer facets of a metallic pyramid. To enable nanofocusing with linearly polarized light through normal incidence internal illumination, we break the symmetry in the thickness of opposing facets and launch surface plasmons on only one side of the pyramids. The
Figure 1. Symmetry breaking for nanofocusing. (a) Schematic of nanofocusing of plasmons using internal illumination of an asymmetrically thick pyramid. The lower dielectric material is optical adhesive with a refractive index of 1.56 and the upper medium is air. One side of the pyramid is 35 nm thick and the opposite side has a metal thickness of 110 nm. Plasmons are excited only on the air−silver interface of the thinner side. Schematic of the template-stripping process for making asymmetric pyramids: (b) inverted pyramidal pattern is formed on a Si wafer using anisotropic KOH etching, (c) metal is deposited at an angle for asymmetric thicknesses, and (d) the inverted pyramid is then filled up with an optical adhesive and UVcured (e) followed by template stripping. 5636
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Figure 2. Wafer-scale template stripping of asymmetric metal pyramids (a) Photograph of a wafer with asymmetric pyramids. (b) SEM of a set of 20 μm sized template-stripped silver pyramids. A large array containing up to 1 million such identical pyramids can be fabricated on a single wafer using this method. (c) A cross-sectional SEM view of an asymmetrically thick pyramid. Asymmetry of the thickness in the pyramids is produced by angled metal evaporation. The porous structure of the epoxy seen here is an artifact generated during FIB cross-sectioning.
Figure 3. Three-dimensional FDTD simulations. Pyramids are illuminated from below using a lineally polarized light at a wavelength of 785 nm. Electric field maps are plotted. (a) Asymmetry in the thickness allows for plasmons to travel on one side thereby reaching the apex to give a strong plasmonic field at the tip. Here the longitudinal field (z-axis) is plotted (b) electric field in the horizontal direction (x-axis) for similar conditions as in (a). (c,d) Enlarged electric field plots near the tip corresponding to (a,b), respectively. The field enhancement for the longitudinal direction close to the tip is around 8. (e) In the case of symmetric thickness, no field confinement is observed at the tip. Here, surface plasmons propagate on both sides toward the apex and interfere destructively due to their phase difference. Electric fields along the vertical axis (z-direction) are plotted in (e) and components in the horizontal direction (x-axis) in (f). The images are saturated for clarity.
mechanism of light to SPP for the asymmetric pyramid is more complicated than prism coupling in planar films, because of the involvement of waveguide modes and reflections from four
internal facets of the pyramid, as well as the presence of edge modes. This geometry is similar to that used by Bouhelier et al.26 for coated fibers and Tanaka et al.41 for tetrahedral tips. 5637
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SPPs can be excited from direct coupling of the dielectric waveguide mode when their phases match, similar to Kretschmann coupling. Another possible coupling route is through edge-plasmon modes that can travel to the tip as proposed by Tanaka et al.41 However our FDTD simulations show that the primary method of SPP excitation is through a waveguide coupling rather than edge mode coupling. The calculated field intensity enhancement near the tip in the case of the asymmetric pyramids is more than 60× when normalized to the incident field intensity, which can yield thousand-fold enhancements in Raman and SHG signals. The field is confined to a spot size of 12 nm (1/e of the maximum intensity) in the longitudinal direction for a tip radius of 6 nm, corresponding to a volume of λ3/1 000 000. In our experiments, the template-stripped pyramids were first illuminated internally with a white light source using a 50× objective (NA = 0.55). This NA means that the excitation beam hits the dielectric-silver interface with a range of angles, some of which can better match the SPP excitation angle of the silver− air interface and allow for more efficient coupling. The far-field scattering from the tip of the pyramid was collected using a 100× (0.9 NA) objective and imaged onto a color CCD (Thorlabs DCU224C). Figure 4a shows a CCD image of light scattered from the tip with the incident light polarized in the direction normal to the asymmetric faces of the pyramid. Because of our fabrication scheme, only one pair of opposing faces has unequal thicknesses and the other pair has equal thicknesses. Rotating the polarization normal to the other two faces reduces the scattering at the tip significantly (Figure 4b) implying that it is indeed the asymmetrical thickness that is responsible for the light at the tip. Any weakly scattered light in the symmetrically thick case could be due to slight asymmetry in the excitation angles arising from a tilt in the sample or beam. We then illuminated the pyramid with a 785 nm laser using the 50× objective. As before, a 100× objective was used to image the tip of the pyramid. CCD images of the pyramid when the objective is focused on the base and on the tip of the pyramid are shown in Figure 4c,d, respectively. When focused on the tip region of the pyramid, a bright spot arising from the scattered laser light can be observed. These images indicate that SPPs are excited by the internal illumination and travel along the facet of the pyramid and get reradiated into far field via scattering from the tip, as expected from the simulations. The output power measured near the tip was approximately 100 μW for an incident power of 10 mW at the objective. Thus the total efficiency of coupling to and scattering from the tip is estimated to be around 1% for most of the tips. The efficiency of our internal illumination scheme is higher than the transmission efficiency of conventional aperture probes and can be compared with recent approaches such as aperture probes utilizing extraordinary optical transmission.42 We believe that owing to lower plasmonic propagation losses on template-stripped metals and reduced background, this scheme can find applications in near-field studies of single-molecule fluorescence and Raman scattering. Furthermore, because our nanofocusing scheme uses large-scale fabrication methods, it is possible to obtain multiple hotspots by illuminating an array of asymmetric pyramids internally. This effect is shown in Figure 4f where hotspots from an array of 1 μm sized pyramids (SEM image shown in Figure 4e) are obtained by illuminating them all with white light simultaneously. The generation of multiple electromagnetic hotspots will be useful for array-based
Figure 4. Optical measurements for light scattered at the tip. CCD images of an asymmetric pyramid that is internally illuminated with a white light source. The position of the pyramids is represented by dotted white lines along with the labels indicating the thin and thick facets. The polarization is perpendicular to the asymmetry in (a) and parallel in (b). A higher intensity at the tip when polarized perpendicular to the thick-thin face of the pyramid clearly indicates asymmetry plays an important role in nanofocusing. (c,d) CCD images of a pyramid illuminated internally with a 785 nm diode laser. In (c), the base of the pyramid was focused while in (d) the tip was in focus. Here the laser light couples to surface plasmons and travel toward the tip where it is scattered to far field. Scale bars in (a−d) are 5 μm. (e) SEM image of template-stripped 1 μm pyramids. (f) Intensity hotspots at the tip of the pyramids with white light internal illumination. Parallel hot spots can be achieved using this scheme.
NSOM, surface-enhanced Raman spectroscopy (SERS), and optical trapping. One interesting consequence of using this Kretschmann-like internal coupling scheme for nanofocusing is that under white light illumination, the peak wavelength of light reradiated by the tip can be tuned by changing the angle of light incident at the metal−dielectric interface. To demonstrate this, the pyramid was illuminated with white light from the backside using a condenser with adjustable NA mounted onto an inverted microscope (Nikon Eclipse). The light from the tip was collected using a 100× objective as before. The field and condenser diaphragms of the microscope are adjusted so that a thin collimated beam illuminated the pyramid. Adjusting the position of the condenser diaphragm allows for small changes in the angle of light incident on the backside of the pyramid. With this method, we could observe varying colors scattered from the tip. Figure 5a−c shows CCD snapshots of green, orange, and red colors, respectively, scattered from the tip for 5638
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Figure 5. Angle-dependent spectral tuning of light at the pyramidal tip. CCD images showing colors of the scattered light [(a) green, (b) orange, and (c) red] on a pyramidal tip by slightly changing the incidence angle under internal illumination scheme. A color CCD camera was used for capturing the images. (d−f) The corresponding spectra obtained from a spectrometer. (g) Electric field intensity at the tip dependence on wavelength and incident angle computed using the FEM method for a two-dimensional wedge with asymmetric thickness. A good match between this and theoretical Kretschmann calculations (white dashed line) suggests that the coupling mechanism in asymmetric wedges is primarily Kretschmann-like. (h) Field intensity plot of an asymmetric wedge illuminated at normal incidence for a wavelength of 785 nm computed using COMSOL. The parameters used for the simulations are similar to those used in Figure 3. At normal incidence, the enhancement is relatively weak while in (i) strong SPP generation on the thinner facet of the wedge and local enhancement at the tip is achieved when the incident angle matches Kretschmann coupling angle (10° as measured from the z-axis).
different angles of incidence. Spectral measurements of the light reradiated from the tip for different angles of incidence, shown in Figure 5d−f, were done using a deep-cooled low-noise CCD camera (Princeton Pixis 400) connected to an imaging spectrometer (Newport MS-257i). Measuring the exact incidence angle is difficult in our current setup. However, a clear shift in the peak position of the spectra is observed, indicating that the incidence angle can be used to spectrally tune the nanoconfined light at the tip. To understand this angular dependence of SPP coupling, we performed 2D finite element method (FEM) simulations on an asymmetric wedge using COMSOL Multiphysics software. Here we used FEM instead of FDTD as it better performs simulations where the angle of incidence needs to be changed to determine the Kretschmann coupling condition without any “staircase” grid artifacts as in the FDTD simulations. Furthermore, our FEMbased simulations were more suited for single-wavelength computations. In Figure 5g, the dependence of intensity at the tip on the wavelength and incidence angle of illumination is
shown. The white dashed line represents data from analytical Kretschmann coupling calculations. A good match between the two indicates that the coupling mechanism is primarily Kretschmann-like. Figure 5h shows the intensity plot for an asymmetric wedge structure illuminated internally at normal incidence for a wavelength of 785 nm while in Figure 5i the incidence angle is tilted by 10°. At the surface plasmon resonance angle, SPPs can be excited on that facet, travel toward and focus at the apex, and scatter into the far-field. The intense local field generated at the metallic tip can enhance spectroscopic processes such as SERS and SHG. To validate the usefulness of our tips for SERS, we coated the silver pyramids with a monolayer of benzenethiol and collected Raman spectra scattered from the tip. The pyramid was illuminated using a 785 nm diode laser loosely focused using the 50× objective. Raman signals were collected using a 100× objective coupled to a fiber-optic Raman spectrometer (Ocean Optics QE 65000) through a multimode fiber. Scanning confocal Raman measurement under external illumination 5639
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launch SPPs, the local field enhancement is much weaker when the incident light is linearly polarized, while in our internally illuminated asymmetric pyramid enhancement of the electric field along the tip axis can be easily achieved without any further lithographic patterning. To demonstrate second harmonic generation (SHG) with our devices, a Nikon multiphoton microscope (A1 RMP+) was used. Template-stripped Ag pyramids were illuminated with femtosecond laser light pulses at a center wavelength of 800 nm using a 40× objective (NA 0.5) and the emitted light was collected from the tip side (in transmission mode) using the condenser of the microscope. SHG signals were collected using a photomultiplier tube after filtering out the excitation laser. The quadratic power dependence confirmed that the signal collected is of second order (data not shown). The excitation power was kept low enough to prevent tip damage while keeping the signal intensity strong enough for detection. The polarization of the incident laser light was fixed in the direction perpendicular to the asymmetric faces of the pyramid so as to achieve maximum intensity at the tip. Figure 6e shows a SHG scan image of four pyramids on the sample. The intense bright spot at the tip is representative of strong local intensity confinement. A zoomed-in scan of the SHG signal from a single pyramid is shown in Figure 6f. In conclusion, these simulations and experimental results demonstrate plasmonic nanofocusing with large field enhancements at the tips of asymmetric pyramids illuminated internally. By breaking the symmetry in the thickness of opposing metallic facets, SPPs are launched on only one side of the pyramids and enable a nanofocusing effect using linearly polarized light. This method of nanofocusing can be achieved from samples fabricated over an entire wafer with a one-step angled metal evaporation and template-stripping process. The Kretschmannlike coupling geometry enables angle-dependent spectral tuning. Additionally, the template-stripping process produces ultrasmooth metal surfaces, enhancing the plasmon propagation lengths. Calculations have previously shown that the large opening angle (70.52°) of the pyramid formed by Si {111} crystal planes is also favorable for scattering light from the nearfield to the far-field compared to adiabatic tapering angles.43 These structures should be well suited for TERS and near-field fluorescence imaging using the internal illumination scheme, and also for fiber-coupled illumination schemes. Our internal illumination method will be of particular use in applications such as plasmonic data storage12,44 and near-field optical spectroscopy1 where the reduction of background noise is critical. Furthermore, because arrays of intense hotspots can be generated by illuminating arrays of pyramids, our approach can facilitate array-based optical trapping, plasmonic sensing, as well as high-throughput imaging applications.
independently confirmed the presence of benzenethiol monolayer on the pyramids. To compare the efficiency of our internal illumination with an external illumination method, we performed experiments in both cases under similar conditions, as illustrated in Figure 6a,b. Raman scattering images obtained
Figure 6. Raman and scanning confocal SHG imaging. (a) Internal illumination scheme. (b) Schematic of external illumination of the pyramid. (c) Raman spectra collected from the tip of a pyramid functionalized with benzenethiol (BZT) and internally illuminated with a 785 nm diode laser. The scattered light from the tip was collected with a 100× objective and the spectra measured with a fiberoptic spectrometer. The signature Raman peaks of BZT are distinguishable. Raman spectra collected from the flat silver region of the same sample yielded very low signal. (d) Comparison of Raman spectra in the internal illumination mode (black) and under external (normal) illumination (red). Raman signals are much stronger when the pyramid is illuminated internally. (e) SHG image from the tip of four different asymmetric pyramids on the sample. Scale bar is 10 μm. (f) A zoomed-in image of one of the pyramids shows confined SHG signal from the tip of the pyramid. Scale bar is 5 μm.
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from the tip of a pyramid and from a flat silver surface are shown in Figure 6c. Strong Raman signature peaks of benzenethiol at 1000, 1024, and 1075 cm−1 can be observed from the tip while only a weak signal was measured from the flat Ag region, as expected. We observed more than a 100-fold increase in the Raman scattering signal at the Ag tip compared to flat Ag surfaces. This result is in agreement with our FDTD simulations. The Raman enhancement in the case of internal illumination is stronger compared to external illumination as shown in Figure 6d. This is expected because for smooth template-stripped pyramids without any grating or defects to
AUTHOR INFORMATION
Corresponding Author
*E-mail: [email protected]. Author Contributions §
S.C. and T.W.J. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by grants from the Office of Naval Research (ONR) Young Investigator Award (N00014-11-15640
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0645), the National Science Foundation (NSF CAREER Award; DBI 1054191), the Minnesota Partnership Award for Biotechnology and Medical Genomics, and Seagate Technology. T.W.J. acknowledges support from the University of Minnesota Doctoral Dissertation Fellowship. Device fabrication was performed at the University of Minnesota Nanofabrication Center, which receives support from the NSF through the National Nanotechnology Infrastructure Network.
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