Letter pubs.acs.org/NanoLett
Ultrabright Bowtie Nanoaperture Antenna Probes Studied by Single Molecule Fluorescence Mathieu Mivelle,† Thomas S. van Zanten,† Lars Neumann,† Niek F. van Hulst,†,‡ and Maria F. Garcia-Parajo*,†,‡ †
ICFO-Institut de Ciencies Fotoniques, Mediterranean Technology Park, 08860 Castelldefels, Barcelona, Spain ICREA-Institució Catalana de Recerca i Estudis Avançats, 08010 Barcelona, Spain
‡
ABSTRACT: We report on a novel design for the fabrication of ultrabright bowtie nanoaperture antenna (BNA) probes to breach the intrinsic trade-off between power transmission and field confinement of circular nanoapertures as in near-field scanning optical microscopy (NSOM) or planar zero mode waveguides. The approach relies on the nanofabrication of BNAs at the apex of tapered optical fibers tuned to diameters close to their cutoff region, resulting in 103× total improvement in throughput over conventional NSOM probes of similar confinement area. By using individual fluorescence molecules as optical nanosensors, we show for the first time nanoimaging of single molecules using BNA probes with an optical confinement of 80 nm, measured the 3D near-field emanating from these nanostructures and determined a ∼6-fold enhancement on the single molecule signal. The broadband field enhancement, nanoscale confinement, and background free illumination provided by these nanostructures offer excellent perspectives as ultrabright optical nanosources for a full range of applications, including cellular nanoimaging, spectroscopy, and biosensing. KEYWORDS: Bowtie nanoaperture antenna, near-field scanning optical microscopy, single molecule fluorescence, nanophotonics, optical antennas, nanoimaging
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such, nanoaperture configurations are ideal for ultrahigh sensitive detection, making these devices highly attractive for applications in cell biology and biosensing.19,20 Indeed, circular nanoapertures fabricated at the apex of tapered fiber tips as in near-field scanning optical microscopy (NSOM) have been extensively used for nanoimaging of lipids and protein receptors on intact cell membranes with a spatial resolution of ∼70 nm.21−23 Similarly, planar nanoapertures have been exploited for detecting individual biomolecules in solution at very high concentrations,24 single molecule real-time DNA sequencing,25 and for measuring molecular stoichiometry and diffusion on living cells.26−28 Unfortunately, the power throughput of subwavelength circular apertures decays as the fourth power of the aperture size29 limiting in practice the effective dimensions of these nanoapertures to ∼100 nm. Different nanoaperture concepts have been proposed in the past few years to increase both the power transmission and the effective field confinement toward useful applications. The designs include ridge,30,31 C-shape,32,33 tip-on-aperture,34,35 and bowtie nanoapertures36 among others. In particular, bowtie nanoaperture antennas (BNA) consisting of two triangle openings faced tip-to-tip and separated by a small opening gap provide a superconfined spot with an intense local field and broadband response in the visible regime.37,38 Moreover, the
he improvement of nanoscale fabrication technologies in recent years has allowed the investigation of optical phenomena on metallic nanoparticles, leading to the concept of nanoantennas,1 such as dipoles,2,3 bowtie configurations,4,5 diabolo,6,7 and dimers,8,9 among others. Nanoantennas are being increasingly exploited to optically interconnect free-space propagating optical waves into localized fields for applications in nanolithography,10,11 optical tweezing,12 nonlinear optics,13,14 and enhancement and control of the fluorescence emission from single emitters.15−17 In particular, planar gold bowtie nanoantennas allow for broadband nanoscale light confinement in regions as small as 20 nm2 and have been shown to enhance the fluorescence of single molecules by factors up to 1300.17 Yet, driving of the antennas requires farfield diffraction-limited illumination with a spot ≥70 000 nm2, resulting in a significant excitation of many fluorescent molecules by the far-field contribution and superimposed to the local signal obtained by the excitation of one or just a few molecules located in close proximity to the antenna gap. Different approaches have been explored with the aim of reducing the unwanted effects of the far-field excitation. An elegant concept includes the use of adiabatic compression through a metallic tapered waveguide, such that excitation of the nanostructure occurs micrometers away from the actual tip end where the field is highly localized.18 Simpler designs include the use of circular subwavelength apertures surrounded by an opaque film, providing truly background-free near field sources together with a high degree of field confinement. As © 2012 American Chemical Society
Received: September 14, 2012 Revised: October 20, 2012 Published: October 25, 2012 5972
dx.doi.org/10.1021/nl303440w | Nano Lett. 2012, 12, 5972−5978
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Letter
effective confinement region of BNAs can be readily tuned by controlling the excitation polarization.37,38 BNAs have been successfully used as nanometer-sized light sources for nanolithography10 and as high throughput near-field probes.39,40 Recently, isolated BNAs milled in aluminum films have been used for enhanced molecular fluorescence in aqueous conditions.41 Although this method has a higher throughput since molecules diffuse rapidly through the small illumination volume allowing for an increased statistics, it suffers from the uncontrolled positioning of randomly oriented fluorescence molecules within the BNA gap. In this paper, we report on the direct coupling between the enhanced near-field excitation provided by BNAs and individual fluorescent molecules by fabricating BNAs at the apex of tapered fiber probes. Our design combines the best properties of bowtie antennas, i.e., broadband field enhancement and nanoscale confinement, together with background-free illumination provided by the nanoaperture. Importantly, by scanning the BNA probe in close proximity to individual molecules embedded in a thin polymer layer, the relative position and orientation of the antenna with respect to the molecules can be fully controlled. Using this approach we show single molecule nanoimaging using a BNA scanning probe and reveal the full 3D vectorial components of the optical near-field of BNAs using individual molecules as nanoscale optical sensors. Furthermore, direct comparison of the response upon confocal and BNA probe excitation for each individual molecule allowed determination of the field enhancement provided by BNA probes. Our results show a ∼6-fold enhancement on the fluorescence emission of individual molecules when the BNA is properly excited and aligned to the dipole emitter. In addition, fabrication of BNA probes on tapered optical fibers near the cutoff region, as shown in here, provides about 3 orders of magnitude higher throughput than circular aperture probes of similar dimensions, making these bright nanostructures ideal candidates for a large number of highly sensitive applications, including biosensing, spectroscopy, and nanoimaging of biological samples. The BNAs were fabricated at the end face of aluminumcoated tapered optical fibers, following a similar procedure as described by Neumann et al.42 The taper was created by heatpulling a single mode (633 nm wavelength) optical fiber and subsequently depositing a 150 nm thick aluminum coating around the fiber to prevent light leakage from the tapered region. Aluminum was chosen because of its small skin depth at optical wavelengths. The coated probes were then taken to the focus ion beam (FIB) to create a well-defined glass opening with diameters close to the cutoff region (500−700 nm) such as to sustain the lowest order mode (TM01). This milled end face was subsequently coated with a 120 nm thick aluminum layer and the BNA was directly carved into the metal using the FIB. Figures 1a−d display scanning electron microscopy (SEM) images during the different stages of the nanofabrication process of BNAs. Our fabrication approach allowed for extreme reproducibility of BNA probes with the gap between the metallic arms being as small as 50 nm (Figure 1d−f). We used finite difference time domain (FDTD) simulations to calculate the electric field intensity distributions at the BNA end. Figure 2a shows the schematics of a BNA probe together with the parameters taken into account for the simulations. The simulations consider a volume spanning ±2.6 μm in x and y around the BNA end face. The refraction index and taper angle of the dielectric body of the probe were chosen to be 1.448 and
Figure 1. SEM images showing the different fabrication steps of a BNA probe. (a) A standard heat-pulled fiber is coated with 5 nm of Ti and 150 nm of aluminum. (b) The coated fiber is milled by FIB to obtain a 500−700 nm opening diameter. (c) The end facet of the tapered fiber is coated again with a high quality aluminum layer of 120 nm. (d) The final step in the fabrication process consists on a second FIB milling where the bowtie nanoaperture is directly fabricated faceon at the end-facet of the aluminum layer. (e and f) Examples of two other BNA probes showing gap regions of 50 nm and BNA length ∼300 nm. The scale bars are 200 nm.
32°, respectively, and the aluminum dielectric constant is given by the Drude model at λ = 633 nm. The BNA is located at x = y = z = 0. In the z direction, the simulation extends to 1 μm in air and terminates at 7 μm into the body of the probe. All six boundaries of the computation volume are terminated with convolutional-periodic matching layers43 to avoid parasitic unphysical reflections around the probe. The nonuniform grid resolution varies from 25 nm for portions at the periphery of the simulation to 5 nm for the region in the immediate vicinity of the BNA (±200 nm in x and y and −200 to 100 nm in z). Figure 2b−i shows the results of the simulations in the (x, y) plane at 30 nm beyond the BNA when a linearly polarized Gaussian beam (λ = 633 nm) is launched in the probe at z = −6 μm and propagating in z. Figure 2b−e corresponds to the near-field intensities for the different components of the electric field when the BNA is excited using a polarization parallel to its metallic arms, and Figure 2f−i shows the corresponding simulations for perpendicular excitation polarization. Figure 2b,f are the total intensities at the end of the BNA (Etot2) normalized by the intensity calculated at the tapered aperture, i.e., without the aluminum layer or the BNA (E02). Thus Etot2/ E02 directly estimates the degree of field intensity enhancement of the BNAs for the two polarization conditions. In the case of parallel polarization, the BNA highly confines the optical signal in the gap zone with a full-width-at-half-maximum (fwhm) of about 80 × 90 nm2, while providing a 35-fold intensity enhancement (Figure 2b−e). In the case of perpendicular polarization, the intensity distribution is not confined to the gap but localized in the two BNA triangles, with still an enhancement for each lobe of up to 3.5 (Figure 2f−i). In addition, the three components of the electric field are delocalized around the BNA for both polarization conditions, having different intensity values that vary up to 2 orders of magnitude. From these results it is clear that parallel polarization excitation of the BNA allows a localized and strong intensity field at the BNA end, consistent with recent simulations performed by Lu et al.41 To experimentally measure the three-dimensional optical near-field distribution emanating from BNA probes for different 5973
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Letter
Figure 3. Schematic representation of the experiment. The sample containing TDI molecules are imaged by confocal or by the BNA probe using an inverted optical microscope. In the confocal excitation mode, the polarized beam from a He:Ne laser is passed through a λ/4 plate to obtain circularly polarized light. The polarization of the light injected in to the BNA probe is controlled by means of a polarizer and a λ/2 plate. The sample is scanned with respect to the confocal illumination or the BNA probe, to generate an image. The fluorescence emission from individual fluorophores is collected in a confocal configuration. The detection path contains a polarizing beam splitter cube to separate the fluorescence into two polarization components, which are subsequently detected by the two APDs.
Chichester, to determine the full 3D orientation of individual molecules using conventional subwavelength NSOM probes.44 Individual molecules were excited at λ = 633 nm either in a confocal fashion (circularly polarized light) or directly by the BNA probe using appropriate polarization conditions. In the latter case, a shear-force feedback loop was also used to keep the BNA in close proximity to the sample (
