Bow-tie optical antenna probes for single-emitter scanning near-field optical microscopy

A method for the fabrication of bow-tie optical antennas at the apex of pyramidal Si3N4 atomic force microscopy tips is described. We demonstrate that these novel optical probes are capable of sub-wavelength imaging of single quantum dots at room temperature. The enhanced and confined optical near-field at the antenna feed gap leads to locally enhanced photoluminescence (PL) of single quantum dots. Photoluminescence quenching due to the proximity of metal is found to be insignificant. The method holds promise for single quantum emitter imaging and spectroscopy at spatial resolution limited by the engineered antenna gap width exclusively. High-precision engineering of prototype devices that show function through their design and high complexity is a key target of applied nanoscale science and nanotechnology. Resonant optical antennas in the field of nano-photonics, with their architecture stimulated by their radio frequency counterparts, synergistically combine (i) electromagnetic field confinement and enhancement defined by the size of their feed gap width and (ii) impedance matching of optical waves mediated by the effective length of their antenna arms [1]. The control of sub-wavelength confined and enhanced optical fields pushes the limit in optical characterization [2–4], 4 Present address: Nanotechnology Group, ETH Zurich, Tannenstrasse 3, CH8092 Zurich, Switzerland. 5 Author to whom any correspondence should be addressed. Present address: Lichttechnisches Institut, Universität Karlsruhe (TH), Kaiserstrasse 12, D76131 Karlsruhe, Germany. 6 Present address: ETH Zurich, Elektronenmikroskopie-Zentrum ETH Zürich, HPT C 104, Wolfgang-Pauli-Str. 16, 8093 Zürich, Switzerland. 7 Author to whom any correspondence should be addressed. Present address: Nano-Optics and Bio-Photonics Group, Department of Experimental Physics 5, University of Würzburg, Am Hubland, D-97074 Würzburg, Germany. manipulation [5–7], and optimization of single nanoscale light sources for information processing [8–11] on the nanometre scale. In particular, the impedance matching of optical waves opens an efficient pathway to transfer near-field information into the optical far-field, and vice versa [12]. In this paper, we present a strategy for designing bow-tie optical antennas at the apex of Si3N4 atomic force microscopy (AFM) cantilever tips. We demonstrate that, even in this complex geometry, it is possible to control key antenna parameters such as overall length and width of the feed gap by focused-ion-beam milling. Merging well-established scanning probe technology with the concept of resonant optical antennas [13, 14] leads to a powerful new method of scanning optical microscopy in which an engineered optical hot spot is used as an optical probe [15]. We demonstrate the application of scanning optical antennas to the imaging of single quantum dots at room temperature. We show that the emission of individual quantum dots is enhanced when scanned across the antenna feed gap while concomitantly their excited-state lifetime is reduced. The field confinement, characterized by 0957-4484/07/125506+04$30.00 1 © 2007 IOP Publishing Ltd Printed in the UK Nanotechnology 18 (2007) 125506 J N Farahani et al Figure 1. (a) SEM image of a clean AFM cantilever tip before metal evaporation; (b) FIB image of a fully coated AFM tip apex. The patches 1–4 indicate the area for FIB milling. (1) The first two triangular patches are milled, which defines the bow-tie antenna shape; the corner-to-corner distance of the triangles is ≈30 nm. (2) A single line cut with an exposure time of 300 ms creates the antenna feed gap. (3) FIB milling of rectangular boxes sets the total length of the resonant optical antenna, L . (4) Removal of additional metallic patches. the spot size found in the images, matches the dimension of the antenna feed gap well. Our method therefore qualifies for the imaging of single emitters at sub-wavelength spatial resolution only limited by the size of the antenna feed gap. As an initial step for the preparation of antenna probes, pyramidal Si3N4 AFM cantilevers (figure 1(a)) (DNP, Digital Instruments) were coated with a 40 nm thick aluminium film everywhere, including the tip. To this end, the cantilevers were mounted in an evaporation chamber. A tantalum boat was used as an evaporation source which was driven by resistive heating. The metal deposition rate, monitored by a quartz microbalance, was set to 2 nm s−1. At this rate the pressure during evaporation was 10−6 mbar. Characterization by scanning electron microscopy (SEM) after the metallization showed that more than 90% of all tips exhibited smooth and homogeneous metal coatings at the tip apex, while for the remainder the coating was disrupted by particles sticking to the tip. We found experimentally that, for metal film thicknesses >40 nm, the cantilevers start to bend, probably due to a mismatch between the thermal expansion coefficients of the silicon substrate and the Al thin film, rendering the cantilevers unusable for AFM. Nicely coated tips were further processed by focused ion beam (FIB) milling. FIB milling was performed using a Dualbeam Nova 600 Nanolab or a Strata 235 DB from FEI, in which a vertical focused electron beam and a 52◦ tilted focused Ga ion beam can be used alternately to characterize and modify the area of the substrate, respectively. Figure 1(b) shows a top-view scanning ion beam image of a representative tip. We created a well-defined nanometric metal structure by removing parts of the extended metal film, leaving behind the desired pattern. The best results have been achieved using an ion beam current of 10 pA. Figure 1(b) also depicts the sequence of onscale milling patterns that have been applied to create bowtie antennas at the apex of AFM tips. First, we defined the overall shape of the optical antenna by milling two triangles. The triangles were positioned at a finite distance to each other in order to ensure the presence of a metallic bridge at the apex. In a second step, the metal bridge was dissected carefully by a single ion beam line cut. This single line cut defined the width of the antenna feed gap. The gap is cut immediately to avoid focusing problems due to charging. A third milling step set the total length of the bow-tie optical antenna by removing the aluminium in two rectangular boxes positioned symmetrically with respect to the tip apex. The optical antenna engineering procedure was finalized by milling two more rectangles, as indicated in figure 1(b), in order to remove nearby metal. When considering the overall antenna length it is important to take into account the three-dimensional structure of the tip, which deviates strongly from a plane interface. The dimensions of the rectangles were chosen such that the distance between the antenna at the apex and the remaining metal film down the tip shaft was well below one wavelength. The degree of control that was achievable using the outlined procedure is demonstrated by the series of three tips depicted in figures 2(a)–(f), in which the overall antenna length is varied between 120 and 200 nm. As shown in the side views of the tips (figures 2(b), (d), (f)), variation of the dose by reducing the dwell time leads to different milling depths. The antenna of figure 2(a) actually rests on two Si3N4 posts. Due to the finite depth of focus of the ion beam, this leads to a gap size (≈50 nm) that was larger than the nominal Ga-ion beam waist (≈10 nm). The silicon nitride posts may act as diffusion barriers, stabilizing the metal patches. For the other antennas, figures 2(c) and (e), the dose was optimized by decreasing the milling time to yield smaller feed gap widths (≈25 nm) while still making sure that the antenna arms were isolated from each other. To prepare a suitable sample for microscopy, quantum dots in buffer solution (10−9 molar) were spin-coated onto cleaned standard microscope cover slips. In a second step, to ensure that AFM scanning in contact mode did not result in pick up of quantum dots, we spun cast a dilute solution of poly-methylmetacrylate in toluene on top of the quantum dots. The coating resulted in a homogeneous 10 nm thick polymer film which covered the quantum dots almost completely. In AFM topography images of such films the locations of single quantum dots were hardly discernible. We assume no reorientation of the emission dipole moments of the single quantum dots during the experiment. The setup has been described previously [16]. Briefly, it comprises a sample scanning confocal optical microscope combined with a tip-scanning AFM. The excitation light source was a pico-second, mode-locked 532 nm laser (GE-100, Time-Bandwidth) and pulse picker combination (model 350-160, Conoptics) operating at a repetition rate of 5 MHz. Confocal excitation and photoluminescence collection was performed by a high-numerical-aperture, oilimmersion microscope objective (Zeiss, Plan-Apochromat, 63×, 1.4 NA ∞). After appropriate filtering, the collected light (photoluminescence emission around 585 nm) was detected by a single-photon counting avalanche photodiode (SPAD, SPCM-AQR13, Perkin Elmer) connected to a timecorrelated single-photon-counting computer card (TimeHarp 200, PicoQuant, Berlin). For each single detected photon the arrival time was stored with high precision with respect to the start of the experiment as well as with respect to the previous laser pulse. This allowed us to reconstruct maps of the photoluminescence (PL) intensity as well as corresponding lifetime maps. Both types of information are important for an understanding of the interaction of single quantum dots with the optical antenna. The bow-tie antenna used in the experiments is shown in figures 2(a), (b). Its overall length