Letter pubs.acs.org/NanoLett
A Resonant Scanning Dipole-Antenna Probe for Enhanced Nanoscale Imaging Lars Neumann,† Jorick van ’t Oever,†,§ and Niek F. van Hulst*,†,‡ †
ICFO−Institut de Ciencies Fotoniques, Mediterranean Technology Park, 08860 Castelldefels (Barcelona), Spain ICREAInstitució Catalana de Recerca i Estudis Avançats, 08010 Barcelona, Spain
‡
ABSTRACT: We present a scanning antenna probe that provides 35 nm optical hotspots with a 16-fold excitation enhancement. A resonant optical antenna, tuned to operation in the visible, is carved into the aluminum-coated scanning probe. The antenna resonances, field localization, excitation, and polarization response are probed in the near-field by scanning over single fluorescent nanobeads. At the same time, the distance-dependent coupling of the emission to the antenna mode is mapped. Good agreement with theory is obtained. The presented scanning antenna approach is useful for both nanoscale plasmonic mode imaging and (bio)imaging. KEYWORDS: Plasmonics, nanophotonics, optical nanoantenna, dipole antenna, nanoimaging, near-field scanning optical microscopy
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aperture, the imaging process is extremely sensitive to any tilt of the typically wider than 400 nm end-facet. Apertureless near-field scanning optical microscopy (aNSOM), based on scattering tips, addresses these disadvantages by illuminating the apex region from the far-field.20,21 In the apex region, a sharp (atomic force microscopy (AFM)) tip creates the desired confinement and enhancement of the local field. The background that arises from the far-field illumination is compensated by oscillation of the tip22 and detection in cross-polarized schemes.23 An alternative scheme to compensate the far-field illumination includes guiding of the excitation field from a dedicated illuminated area to the apex.24−26 The ultimately achievable resolution relies on the sharpness of the apex and thus on the fabrication process.27 In a second scheme, resonant nanostructures, or optical antennas, are placed at the apex and their respective modes excited by far-field illumination. Resonant optical antennas provide fields localized to an order of magnitude below the diffraction limit11 that at the same time are highly enhanced. Placed on a scanning tip, these highly enhanced and confined fields make optical antennas good candidates for sensitive highresolution microscopy. Examples include spherical28,29 and monopole antennas.11,12 Here, we demonstrate the application of single resonant dipole antennas fabricated on scanning tips for fluorescence microscopy, mapping of antenna modes and emission control. The dipole antennas are scanned in close proximity to single fluorescent beads of 20 nm in diameter, thus permitting
lasmonic nanostructures are attracting considerable interest in science and technology as a tool to tailor optical fields with subwavelength precision. As an interaction between two objects occurs through their respective electromagnetic fields, tailoring optical fields allows to mediate and control this interaction. Here especially resonant optical antennas are in the focus for their ability to efficiently link a nanoscopic object to freely propagating light through coupling in the near-field.1−3 This mediation effect is highly relevant for applications in fields as diverse as photodetection,4,5 photovoltaics,6 nonlinear optics,7−10 and imaging.11,12 As antennas act as efficient mediators between an emitter and the propagating far-field, the use of antennas addresses simultaneously resolution and sensitivity. This is particularly relevant for imaging. A vast number of nanostructures, mainly nonresonant and some resonant, have been designed in the past for imaging. The key criteria in the evaluation of these structures include the field confinement and enhancement provided under diffractionlimited far-field illumination. First, the main nonresonant scheme is represented by near-field scanning optical microscopy (NSOM) that uses tapered optical fibers to compress the light field. At the apex of the fiber, the light field penetrates a suitable nanoaperture. The physical properties of the nanoaperture define the imaging characteristics of a NSOM design and include circular apertures,13,14 bowtie apertures,15−17 triangles,18 and dedicated apertures19 among others. Imaging with a nanoaperture is generally background-free, however the mayor disadvantage is the extremely low transmissivity to the aperture region that imposes a lower bound to the dimensions of the aperture and in consequence to the resolution and sensitivity. Also, as the near-field decays rapidly from the © XXXX American Chemical Society
Received: June 16, 2013 Revised: October 1, 2013
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dx.doi.org/10.1021/nl402178b | Nano Lett. XXXX, XXX, XXX−XXX
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Figure 1. Left: Experiments performed on a standard confocal microscope. Right: A scanning tip carries a resonant optical antenna made of aluminum. Fluorescent emitters are placed a few nanometers beneath and in the focus of a confocal microscope. Far-field illumination excites simultaneously the emitters and the antenna modes that provide an additional evanescent excitation field. The total emitted fluorescence is mapped spatially with photodetectors.
evaporated onto the flat face. Figure 2a shows the scanning tip in this stage.
interaction and coupling in the near-field. A sketch of the proposed scheme is shown in Figure 1. Our approach combines the convenience of apertureless near-field scanning microscopy with the local field enhancement by resonant metallic nanostructures. It avoids the challenge of chemical attachment30 by carving the nanostructure directly into the scanning tip. While this method relies on far-field illumination and is intrinsically not background-free, the resonances of metallic nanostructures provide highly enhanced local fields which help to overcome the background illumination. The confinement of the local field, and thus the imaging resolution, is given by the field distribution of the resonant mode which depends on the size and shape of the antenna. Overall, the proposed scheme requires less critical fabrication steps than engineered aperture probe designs.17 The fiber solely acts as a support, avoiding losses through guidance in a tapered waveguide. Finally, the end-facet only contains the antenna, simplifying the distance control. The versatility of the proposed imaging method relies strongly on the reliable, reproducible, and time-effective fabrication of an antenna structure on a scanning tip. The fusion of focused ion beam (FIB) technology and lithography adapted to ion beams has provided such an efficient tool to fulfill exactly this task. The optical antennas displayed in this letter have been fabricated on a Zeiss Auriga Cross Beam FIB system equipped with a Fibics NPVE lithography system. The lithographic approach guarantees an effective and reproducible fabrication of systematic sets of nanostructures. The dipole antennas in this report were fabricated in aluminum and have lengths between 100 and 580 nm. At an excitation wavelength of 647 nm, these lengths cover the first and second order resonances and touch the third-order resonance.31 Considering both spatial field confinement and enhancement, aluminum is superior to gold as aluminum is a more perfect metal, that features the shortest skin depth. Aluminum antennas can be made thinner and with spatially more confined fields. The spatial confinement compensates for the slightly higher losses compared to gold. Also, in our experiments aluminum adhered better to the scanning tip and showed a considerably higher resistivity to mechanical stress during the scanning process. This is crucial for the durability of the tip and of major importance for practical applications. In the first fabrication step, the scanning tip is prepared by heat-pulling an optical fiber and establishing a flat end face by ion milling. A thin adhesion layer of 2−5 nm of titanium and the structural layer of 60 nm of aluminum are thermally
Figure 2. Fabrication of a scanning dipole antenna probe. (a) The tapered and cut scanning tip is metalized with aluminum. (b) The antenna is frontally milled into the end face and rests on a glass socket. The lithographic fabrication guarantees a constant height of 50 nm and width of 65 nm (inset) across all antennas. All scale bars are 500 nm long.
Next, the dipole antenna is milled frontally into the end face using ion beam lithography. Figure 2b gives the final state of the full probe. The antenna resides on a glass socket which is the result of an exposure to a relatively high ion dose. This socket guarantees that only the antenna and not the supporting fiber interacts with the sample. The lithographic milling ensures that the width of all antennas, shown in the inset, is kept reproducibly at 65 nm. The height of the initial aluminum layer becomes reduced to about 50 nm due to the narrowness of the antenna. To test the imaging properties of the dipole antenna, we embedded fluorescent beads of 20 nm in diameter (Invitrogen FluoSpheres F8783) in a matrix of 60 nm polyvinyl alcohol (PVA). These beads contain a few hundred randomly oriented dye molecules and have a quantum efficiency close to unity. The beads were chosen for their strong signal; however, the method can essentially be extended to any type of emitter of interest. All experiments were performed on a standard confocal inverted microscope extended with an additional scanning system above the focal plane for the scanning dipole antenna tip. Two piezoelectric scanners allow to independently position the fluorescent beads and the scanning tip. A shear-force feedback controls the vertical position of the scanning tip such that antenna and bead interact through the near-field.32 A continuous-wave-laser operating at a wavelength of 647 nm B
dx.doi.org/10.1021/nl402178b | Nano Lett. XXXX, XXX, XXX−XXX
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Figure 3. (a) An antenna of length 170 nm is scanned across a fluorescent emitter fixed in the microscope focus. Its base size is marked by a dashed line. With the polarizations of excitation and detection chosen along the antenna axis, two bright spots of enhanced emission of fluorescence emerge. The smallest feature has a FWHM of only 35 nm. (b) The numerical simulation of the local field intensity near the antenna reflects the measurements well. All scale bars are 100 nm long.
matches the experimentally recorded fluorescence map well. The origin of the increased fluorescence is indeed an enhanced excitation field due to the presence of a dipolar mode in the antenna. Insets in Figure 3a show cross sections of the fluorescence hotspots. In the most narrow dimension, the hotspots have full width at half-maximum (FWHM) of only 35 nm. This recorded FWHM is less than a factor two larger than the size of the probing bead. As this width is a convolution of the size the bead and the spatial extent of the mode, the size of the resolved modal feature is close to the resolution of this configuration. The hotspots are elliptical and wider by roughly 10 nm perpendicular to the axis of the antenna. Our numerical calculations agree well with this observation. As the antennas narrow from the base, the hotspots are expected to be narrower than the antenna base width of 65 nm. This assumption is confirmed by the FWHM 49 and 56 nm of the two spots. To gain an understanding of the resonant properties of the antenna, we scan antennas of different lengths across the fluorescent bead. As the dipolar mode is associated with a particular length of the antenna, we expect the properties of the excitation spots to depend strongly on the parameters of the antenna. Fluorescence maps are in shown in Figure 4. The maps are normalized to the respective emission in the absence of an antenna and are directly comparable. Clearly, only antennas with a very specific length, centered at around 170 nm, support a resonance. The antenna of length 140 nm features a bright right spot, while the left spot is hardly visible. We explain this by different distances of its two extremities to the bead, either through a tilt of the scanning tip with respect to the sample plane or a fabrication-induced inclination of the antenna. The antenna of 180 nm in length already shows only a weak enhancement. This strong length dependence rules out alternative explanations, such as the “lightning rod” effect that is independent of the length.34 It confirms the presence of a resonant dipolar mode on the scanning antenna probe. In Figure 5, we address the quality of the resonance and the resulting enhancement factor. The data points were calculated from recorded fluorescence maps under the influence of antennas ranging in length from 100 to 580 nm, including those shown in Figure 4. We define the enhancement factor of the fluorescence as the ratio between the count rates obtained under illumination polarized along and perpendicular to the
provides the excitation illumination through an NA 1.3 objective. The resulting fluorescence is polarization-resolved on two avalanche photodetectors (APD). In order to characterize the plasmonic mode of the antenna, a single fluorescent bead is positioned exactly in the focus of the microscope. The scanning tip with the antenna is scanned across the bead and the emitted fluorescence spatially resolved. This mode mapping is repeated for a polarized excitation both parallel and perpendicular to the axis of the antenna. Afterward, a such calibrated antenna provides a well-defined excitation source for high-resolution microscopy. As the interaction between bead and antenna occurs through evanescent fields, maintaining a constant distance between both is crucial. Fluctuations could modify excitation enhancement and polarization of the detected fluorescence. We address this issue by using piezoelectric scanners with an intrinsic noise level of
