Nanoapertures without Nanolithography - ACS Photonics (ACS

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Nanoapertures without nanolithography Alessandro Tuniz, Henrik Schneidewind, Jan Dellith, Stefan Weidlich, and Markus A. Schmidt ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b01265 • Publication Date (Web): 19 Dec 2018 Downloaded from http://pubs.acs.org on December 20, 2018

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Nanoapertures without nanolithography Alessandro Tuniz1,2,3*, Henrik Schneidewind1, Jan Dellith1, Stefan Weidlich1,4 and Markus A. Schmidt1,5,6** 1. Leibniz Institute of Photonic Technology (IPHT Jena), Albert-Einstein-Str. 9, 07745 Jena, Germany 2. Institute of Photonics and Optical Science (IPOS), School of Physics, University of Sydney, Sydney, NSW 2006, Australia 3. The University of Sydney Nano Institute, University of Sydney, Sydney, NSW 2006, Australia 4. Heraeus Quarzglas GmbH & Co. KG, Quarzstr. 8, 63450 Hanau, Germany 5. Abbe School of Photonics and Faculty of Physics, Max-Wien-Platz 1, 07743 Jena, Germany 6. Otto Schott Institute of Materials Research, Fraunhoferstr. 6, 07743 Jena, Germany KEYWORDS:

nanoapertures, lab-on-a-fiber, hybrid fibers, scanning near-field optical

microscopy (SNOM), nanofabrication

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ABSTRACT: We propose and experimentally demonstrate the implementation of lithographyfree nano-apertures on optical fibers. By sputtering metallic nanofilms onto the end face of step index fibers that contain central nano-channels, fiber-integrated nano-apertures are instantaneously implemented without the use of any kind of lithographic step. In accordance with simulations, the experiments show diffraction-limited nano-spots in the far-field at the location of the nano-aperture for sufficiently thick films. We reproducibly implement a series of devices by sputtering Al and Pt nano-films, reaching aperture diameters as small as 40 nm and showing spectrally broadband operation. Due to its simplicity, scalability and potential for large-scale production the nano-aperture enhanced fiber concept will be highly relevant for lab-on-a-fiber applications and for the development of future fiber-based nano-probes with high spatial resolutions.


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The ability to detect and deliver light to nanometer spatial dimensions – particularly on a flexible and convenient fiber platform - enables the observation and control of physical processes down to the molecular level, with applications such as nano-detection and spectroscopy1, particle trapping2 and manipulation3, catalysis4, remote sensing5, and lab-on-a-fiber technology6. Of particular importance for nano-imaging is scanning near-field optical microscopy7 (SNOM) relying on measuring spatial light distributions by laterally scanning nanotips that include subwavelength features. To achieve deep sub-diffraction spatial resolutions, appropriately designed fiber-integrated metallic nano-structures that allow for nano-concentration of electromagnetic radiation are currently being employed8, including metallic nanotips9,10, nanoparticles11, nano-apertures12, and nano-antennas13. One of most straightforward and widely employed methods to achieve nanoscale light localization at visible wavelengths relies on deep-subwavelength nano-apertures milled into metallic nanofilms12, transmitting and collecting light with the respective spatial resolutions defined by the diameter of the aperture used. Nano-apertures are particularly attractive for SNOM when integrated onto the end face of optical fibers, offering a powerful and flexible device for nano-optics that is available commercially14. One key challenge in the field of nano-aperture based probes is the implementation of the nanoscale aperture on the fiber end-face, which typically requires either planar lithographic methods that are hard to adapt to the fiber geometry and demand many processing steps15–18 or laborious metal-deposition techniques (e.g., angular evaporation onto a tilted and constantly rotated fiber) that are difficult to control at rather poor reproducibility levels. As a result, current fabrication schemes do not allow for fiber nanoprobe implementation at large scales, leading to high fabrication costs at comparably low success rates. The most widely used type of fiber-integrated nano-probe is an optical fiber with a metal coated tapered end section that includes a nano-hole at its apex19. At a particular distance from the apex (i.e., local inner radius of the fiber) the propagating fundamental mode cuts off and the electromagnetic field in the remaining section is solely evanescent with only a fraction of the field reaching the aperture20,21. As a consequence, these types of nano-aperture probes suffer from low throughputs, particularly when aperture opening and taper angle are small – for example, typical commercially available probes have half cone angles of ~12°, leading to a transmitted power fraction of 10-11 for aperture diameters of 20 nm20. Since the diameter of the aperture ultimately determines the spatial resolution, various planar-based fabrication approaches are being followed to achieve nano-scale feature sizes on a fiber end-face15, including ion beam milling22, electron beam lithography6 and deposition mask lithography11. It is possible to improve the nano-aperture performance e.g., by modifying the aperture shape into a bowtie3,23, combining it with adjacent nano-antennas13, nanotips24, and plasmonic resonances25, or harnessing the plasmonic enhancement of metal-dielectric-metal waveguides1. To the best of our knowledge, all nano-aperture enhanced fiber probes reported so far require at least one nanolithographic fabrication step15, which is time consuming, requires significant expertise to run, and can only address one sample at a time. Here, we propose and experimentally demonstrate the implementation of lithography-free nanoapertures via depositing metallic nano-films on nanobore optical fibers (NBFs). This only


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requires one single thin-film deposition step, avoiding any kind of lithographic bottleneck via a straightforward fabrication approach. By sputtering Al and Pt nanofilms onto the end faces of NBFs, nano-apertures integrated onto the fiber facet are instantly created, showing deep subwavelength light localization and diffraction-limited nano-spots for films exceeding a defined thickness. Various types of fiber devices have been implemented, experimentally characterized and simulated, revealing all relevant experimental details required for operation, including the film thicknesses, and the improvement in power transmission compared to a homogenous unstructured film. The nano-aperture enhanced NBF and its principle of operation are schematically shown in Fig. 1. The underlying waveguide is a cylindrical silica single-mode step-index fiber (GeO2doped core, diameter 3 μm, core-cladding refractive index contrast 4·10-3) incorporating a central nanochannel (or nanobore) of diameter dbore26,27. The fiber end is covered by a metallic nanofilm that has a thickness such that light transmission through the film is almost completely suppressed (i.e., the metal film thickness t (here between 50 nm and 200 nm) is several times the skin depth). At the location of the nano-hole the metal film is perforated, creating a fiber-integrated nanoaperture that resembles the shape of the hole of the NBF (e.g., Fig. 1e shows an example of a 40nm diameter on-fiber Aluminium (Al) nano-aperture, obtained by coating the nanobore fiber in Fig. 1d). The fiber itself supports a single linearly polarized mode at visible wavelengths with a doughnut-shaped profile with a significant fraction of the total power located inside the central nano-channel (Fig. 1b)27,28. This part of the fiber mode is effectively transmitted through the nano-aperture (Fig. 1c), thereby producing a nano-spot at the coated fiber output (Fig. 1e) that is below the diffraction limit, corresponding to a diffraction-limited spot in the far field, as experimentally confirmed (Fig. 1f).


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Figure 1. (a) Schematic of the concept of lithography-free nano-aperture implementation via metallic nano-film deposition on the end face of nanobore optical fibers (core diameter: 3 µm; bore diameter: ~100 nm). (b) Simulated spatial distribution of the intensity of the fundamental fiber mode along one arbitrary transverse line of the empty nanobore fiber cross section. (c) Distribution of the near-field when the nanobore fiber is coated by a sufficiently thick metal film. (d) Example uncoated nanobore fiber (hole diameter: 100 nm). (e) The smallest nano-aperture implemented so far (aperture diameter 40 nm, window size: 400 nm x 400 nm, using the fiber shown in (d), metal: aluminium). (f) Corresponding measured far-field distribution showing a diffraction-limited nanospot (window size: 5 µm × 5 µm; color scales linearly with intensity; wavelength: 633 nm). Several kilometers of NBFs that are single mode at visible wavelengths have been realized by drawing a single hole preform (fabrication details reported in26), with the drawing conditions adjusted to achieve nominally constant bore diameters over hundreds of meters (here 100nm < dbore < 200 nm, core/cladding refractive index contrast: Δn = 4×10-3). Two types of NBFs (NBF1 (Fig. 2a): dbore = 110nm; NBF2 (Fig. 2d): dbore = 170nm) of nominal length of 100 m were fabricated. Pieces of about 30 cm length were prepared by cleaving and placed one end into a custom-built sputtering machine. The deposition of the high purity metal films was carried out in


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a high vacuum apparatus to minimize contaminations and to ensure a good film adhesion to the fiber end face. The deposition of metal into the nano-channel of the NBF is minimal since the mean free path of atoms in the argon sputtering atmosphere is in the order of 1 cm and thus orders of magnitude larger than the nanobore diameter. The thickness of the metal films, which is controlled by a previously determined deposition rate, is subject to two limitations: (1) the metal must be thick enough two avoid unwanted transmission through the film; (2) the metal must be thin enough to avoid covering the entire hole. Aluminium and platinum were chosen due to the small skin depth (~6nm at optical frequencies20,29), which uniquely enables using relatively thin films (200 nm produce closed apertures, which excludes gold from been used here due to the large skin depth (a comparison of the transmission through different bulk metal films is shown in Fig. S1.) Scanning electron micrograph (SEM) images of the resulting nano-apertures at different deposition times, i.e., different film thickness, are shown in the different panels of Fig. 2 (Al: Figs. 2 c and d (blue background); Pt: Figs. 2 e and f (orange background)). A nano-aperture is produced at the same locations as the nano-hole with almost circular shapes. We observe that the nano-apertures shut for film thicknesses above ~120 nm for Al, and ~200 nm for Pt, allowing for the implementation of nano-apertures with diameters daper smaller than the original nano-hole of the NBF simply by adjusting the film thickness appropriately. The aperture diameters as function of metal film thickness obtained for 45 different samples are shown Fig. 2(g), clearly revealing that thicker films impose smaller nano-apertures. These plots reveal that for both metals an increasing film thickness reduce the diameter of the nano-aperture, with the final aperture diameter depending on the initial diameter of the bore in the fiber. The Pt nano-apertures show a less pronounced error bar, meaning that the fabricated nano-apertures are more circular for the same type of underlying fiber, which might be related to a different film growth process, i.e., growth of grains, which is more pronounced for Al. Reported values for the grain size of Al films are as large as 100-1000 nm20 which is much larger than for Pt (2-30nm grain size)30. A direct comparison of example images of plane and on-fiber Al and Pt metal films are shown in the Figs. S2 and S3. Fig. 2(h) shows the back-scattered electron SEM image of the cross section of a Pt-coated NBF2 (200nm thickness) obtained after a focused ion beam cut at the location of the nano-hole, illustrating the hole-closing effect. The inset of Fig. 2(h) reveals that that the metal partially coats the interior of the nano-hole down to a depth of ~1.5µm with a strongly decreasing thickness towards increasing distances from the fiber end face. By also placing commercially available (solid-core) single mode fibers (Thorlabs 630HP) into the deposition chamber, aperture-free reference samples that include films of identical thicknesses are produced. Note that, unless otherwise stated, for the remainder of this work we will consider only the NBF with dbore = 110nm (NBF1), possessing the smaller nano-channel, and thus yielding smaller nano-apertures.


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Figure 2. SEM micrographs of various nano-apertures created at the end faces of the NBFs using sputtering. Two types of NBFs with different bore diameters are considered: Top row (a)-(c): dbore = 110nm (NBF1), bottom row (d)-(f): dbore = 170nm (NBF2). The labels on the top of the images refer to the respective nanofilm thickness. The images in the blue (orange) box refer to Aluminium (Platinum). The most left-handed images show the end face of uncoated NBFs. Each window size is 400 nm × 400 nm. The slight halos around some of the aperture are likely to result from the conical funnel-type shape of the nano-hole, i.e., from the increasing film thickness towards larger radial distances from the nano-aperture centre as visible in (h), which leads to different contrasts in the SEM imaging. The lower two plots show the results of a quantitative analysis of the experimentally implemented nano-apertures (nano-aperture diameter as function of film thickness for Al (blue) and Pt (orange); (g): NBF2, (inset): NBF1). Each point represents the average of the major- and minor- axis of one nano-aperture, with the error bar being the absolute difference between the average diameter and the major/minor axis. The total number of investigated samples is 45. (h) Back-scattered electron SEM image of the cross section of a Ptcoated NBF2 (200nm thickness) obtained by focused ion beam milling. Inset: zoomed-out image, revealing that the metal partially coats the interior of the NBF down to a depth of ~1.5µm.


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The experimental characterization relies on coupling light from a He:Ne laser (emission wavelength 633 nm, Thorlabs HNLS008L-EC) into the uncoated part of the NBF (20× objective) that was manually cleaved. Efficient in-coupling is achieved by imaging the input fiber end-face while launching light into the imaged core. This procedure is simplified by the fact that the metal coating on the other end of the fiber effectively acts as a mirror, enabling immediate core identification on the uncoated side. Once light is launched, the output is collected by a 40× objective and focused on a charge-coupled device (CCD) camera or an optical spectrum analyzer. The camera image is used to fine-tune the input coupling, until a maximum in transmitted power is achieved. Tests of uncoated fiber samples reproducibly yield coupling efficiency of about ~50%. Due to order-of-magnitude differences in expected output transmission (Fig. 3a), we calibrate and utilize neutral density (ND) filters to prevent the CCD camera from saturating. The mode images captured by the CCD camera are then used to obtain the total power transmitted by each sample by integrating the measured intensity distribution across a fixed camera chip area, corresponding to the center of the fiber (~6 µm × 6 µm). To demonstrate the feasibility of a lithography-free nano-aperture implementation via NBFs, we simulate the light transmission properties of the nano-aperture enhanced NBF geometry (Fig. 1) using finite element modelling (COMSOL). Specifically we consider the fundamental NBF mode (HE11-mode, wavelength 633nm, illustrated in Fig. 1a) propagating towards an Al-based nano-aperture (εAl = -32.8397 +10.8603i, measured by in-house ellipsometry) and calculate the fraction of power that is transmitted into the far-field (i.e., the transmission) by integrating the Poynting vector S across the device cross section 𝜂𝑡 = ∫𝐴|𝑺|(𝑥,𝑦,𝑧 = 𝑧𝑜𝑢𝑡)𝑑𝑥𝑑𝑦 ∫ |𝑺|(𝑥,𝑦,𝑧 = 0)𝑑𝑥𝑑𝑦, where z = 0 refers to the mode profile just 𝐴

before the beginning of the metal film and zout to the longitudinal far-field location which is considered here to be zout= t + λ/2. Here we calculate 𝜂𝑡 as function of film thicknesses for three aperture diameters of Al-coated NBF (Fig. 3a, blue: 100nm, red: 200nm, orange: 300nm)) and compare the results to the transmitted power fraction of a corresponding homogenous film 𝜂𝑡,𝑓 (grey dashed line), assuming that the central nano-aperture diameter equals that of the corresponding nano-hole of the NBF (dbore = daper). The simulations reveal two regions of interest with regard to the transmitted power fraction (Fig. 3a), which are separated by the fraction of ∫ 𝑆𝑧𝑑𝐴 modal power inside the bore of the NBF used 𝜂𝑏 = 𝐴𝑏𝑜𝑟𝑒 ∫𝐴 𝑆𝑧𝑑𝐴 (ηb=5·10-3 for ∞

dbore=200 nm, horizontal dashed green line in Fig. 3a). For t < 60 nm (Fig. 3a(i)), the transmitted power fractions of film and nano-aperture enhanced NBF are close and both larger than ηb, which is a result of the nanofilm being too thin to efficiently block the core mode. In the second region (Fig. 3a(ii)), 𝜂𝑏 > 𝜂𝑡 > 𝜂𝑡,𝑓 so that a significant fraction of power is transmitted through the nano-aperture, clearly showing an increase in transmitted power compared to the homogenous film case. Analyzing 𝜂𝑏 as a function of dbore (inset in Fig. 3a) reveals that the fraction of power inside the empty bore, i.e., that can princially be maximally transmitted is between 0.01% and 0.1%. For large bore diameters, 𝜂𝑏 drops as a result of the strong evanescent decay at the air-core interface, whereas for small values of dbore the decrease in ηb is associated with the overall small fraction of cross section of the nanobore on the entire fiber core area. Note that a local intensity enhancement occurs in the air-gap for sub-100 nm nanobore diameters, which is a result of the boundary conditions28.


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While larger apertures lead to an increase in the transmitted power (Fig. 3a, yellow line), very small apertures (Fig. 3a, blue line) impose the transmitted power to be insignificant and close to that of the aperture-less case, which overall is a result of the high loss of the aperture mode, leading to small throughputs. This behaviour is confirmed through calculations of the device transmission as a function of bore diameter for different film thicknesses (Fig. 3b): For thin films (Fig. 3b, red, 40nm), the contribution of the light from the core is always larger than that of the light going through the aperture, and the aperture contribution is in fact negligible. Thicker films (Fig. 3b, green, 120nm) yield the transmission through the aperture to dominate over that of the core mode, while the overall transmission is very low for very thick films (Fig. 3b, purple, 200nm), which is comparable to the aperture-less film particular at small aperture diameters (Fig. 3b, dotted line). A direct comparison of experimentally measured far-field patterns for the uncoated/coated fibers (40 nm and 80 nm) with corresponding 3D finite element simulations confirms the experimentally observed behaviour (Figs. 3b-d) and in particular shows a strong light locatization (Fig. 3d) for the NBF coated with 80 nm Al film (with respect to the incident mode (Fig. 3b)), i.e., a diffraction limited nano-spot that solely arises due to the presence of the nano-aperture. The transition from a core-dominated to an aperture-dominated transmission regime is visible by inspecting the transverse spatial Poynting vector distribution at the output (Figs. 3f and 3g show the Poynting vector along one transverse line crossing the origin in the near- and far- fields, respectively).


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Figure 3. Three-dimensional finite element simulations and experiments elucidating the concept presented here, using Al as example metal (wavelength: 633 nm). (a) Calculated fraction of transmitted power as a function of nano-film thickness for different aperture diameters (blue: 100nm, red: 200nm, orange: 300nm) compared to the same fiber that contains a homogenous unstructured film (aperture-less situation, dashed grey line). Inset: fraction of power located in the bore as a function of bore diameter. The dots refer to the three bore diameter considered in that figure. (b) Transmission vs. nano-aperture diameter for different film thicknesses (red: 40nm, green: 120nm, purple: 200nm). The dotted lines refer to the transmission through an unstructured film. Note that the simulations assume the diameters of bore and nano-aperture to be identical. (c-e) Measured far-field intensities (left column) and corresponding simulated mode patterns, showing both far-field (taken at a distance λ/4 from the end-face, central column) as well as near-field (taken at a distance λ/60 from the end-face, right column) distributions: (b) Uncoated NBF; (c) t = 40 nm; (d) t = 80 nm film. Each window size is 5 µm × 5 µm. Dashed white circles in (b)-(d) indicate the fiber core/cladding interface. The FWHM of the measured nano-spot is 646 nm, corresponding to the resolving power of the microscope objective (diffraction limit). Near-field (f) and far-field (g) Poynting vector distributions along one transverse line crossing the origin of the simulation area as function of film thickness for a nano-aperture with a diameter of 200nm. The magnitude of the Poynting vector ranges linearly from minimum (black) to maximum (white).


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To experimentally verify the nano-aperture enhanced NBF concept, we fabricated a series of Al and Pt coated NBFs and step index fibers, and measured the respective transmitted power fractions for various film thicknesses (Fig. 4). Metal-coated NBFs reveal increasing values of 𝜂𝑡 compared to the coated step-index fibers (grey circles), as a result of light transmitted through the nano-aperture. The lateral reduction of the field diameter that initially corresponds to that of the incoming core mode to a diffraction-limited nano-spot for thicker films is particularly striking. Measured mode images are shown in the right-handed columns of Figs. 4a,b, clearly revealing nanoscale light localization provided by the nano-aperture. The observed behavior qualitatively agrees with simulations, and is a direct consequence of the optical properties of the metals used. For metal film thicknesses above a certain threshold (60nm for Al and 120nm for Pt), the films efficiently block the fraction of mode that is present within core and cladding, allowing to transmit only the fraction of mode that is localized within the nano-bore domain. This particular fraction of power inside the bore 𝜂𝑏 is the reason for the higher values of transmitted power fraction for the nano-aperture enhanced NBFs, compared to the coated stepindex fibers (𝜂𝑡 > 𝜂𝑡,𝑓). Note that due to its dielectric properties (i.e., longer skin depth), the Pt coated NBFs demand significantly thicker films compared to Al. Al-coated samples require film thickness above 80 nm for the nano-aperture effect to be visible, whereas the Pt-coated samples demand a film thickness of at least 120 nm.


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Figure 4. Experimental verification of the concept presented here. (a) Measured transmitted power fraction as function of nano-film thickness for NBF1 coated by Al (blue circles; the blue dotted line is a guide-to-the-eye). A related curve for a coated step index fiber that contains a homogenous unstructured film is represented by the grey circles (grey dotted line: fit). Corresponding measured farfield distributions are shown in the images of the right-handed column ((i) 40 nm, (ii) 60 nm, (iii) 80 nm and (iv) 120 nm). (b) Transmitted power fraction vs. film thickness when Pt is used as metal (orange (grey) dots: NBF (step-index fiber) sample). The orange dotted line is a guide-to-the-eye. Measured output intensity distributions for the various Pt film thicknesses are shown on the right ((i) 40 nm, (ii) 60 nm, (iii) 120 nm, (iv) 200 nm). Each window size is 5 µm × 5 µm. Dashed white circles show estimated fiber core/cladding interface. To reveal the spectral behavior of the concept experimentally, we repeat the above presented measurements using a supercontinuum light source (NKT SuperK COMPACT) and analyze the output spectra with either an optical spectrum analyzer (OSA) or a CCD camera. The spectra of three Al-sputtered samples, normalized to the transmission of an uncoated NBF, are shown in Fig. 5a. The transmission reduction towards thicker films is again observed and results from the reduced transmission through the metal films. While the retrieved output mode image of the


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60 nm Al-film fiber correspond the spatial profile of fundamental mode of the NBF (Fig. 5b), the sample with the 80 nm Al film reveals a colored diffraction-limited spot at the fiber center as a result of the light solely transmitting through the nano-aperture (Fig. 5c). The color range of the nano-spot spans from green to deep red, which is confirmed by the spectral characterization (red curve in Fig. 5a), clearly revealing that the nano-aperture enhanced fiber supports broadband operation. For the thinnest Al film, the transmission is spectrally flat (Fig. 5, magenta), indicating that core-mode transmission dominates. For thicker films, the experimentally measured curves reveal a slope in the spectral distribution of the transmission, which comes from the fact that at a certain wavelength the transmission level of the bulk film drops below that of the nano-aperture, with the later showing a distinct spectral dependence as suggested by the theory introduced by Bethe31. This is true for all wavelengths for the 80nm films (Fig. 5, red), while for the 60nm film we have a combination of bulk film transmission at short wavelengths (flat transmission 650nm). Note that the dip at 770 nm for the 80nm case is close to the noise floor and is thus a measurement artefact. A qualitatively equivalent behavior is found for the Pt coated samples as shown in Figs. 5d,e.

Figure 5. Spectral properties of the nano-aperture functionalized NBFs. (a) Spectral distribution of the transmitted power fraction of NBF1 coated by Alfilms for three different thicknesses (purple: t = 40 nm, blue: t = 60 nm, red: t = 80 nm). Also shown are measured spatial intensity profiles of two samples that include Al-films of different thickness ((b) t = 60 nm, λ = 650 nm (blue dot in (a); (c) t = 80 nm, broadband excitation (horizontal arrow in (a))). The two images on the right show corresponding images for Pt coated NBFs ((d) t = 40nm, λ = 650 nm. (c) t = 200nm, broadband excitation). Dashed white circles show the estimated core/cladding interfaces. The simulations and experimental characterizations presented highlight the feasibility, simplicity, and effectiveness of the concept of lithography-free nano-aperture implementation via NBFs, which enables light localization to nanoscale dimension using aperture sizes as small as


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40 nm. In contrast to commonly employed near-field fiber probes, the approach presented here is compatible with large-scale fabrication, since hundreds or even thousands of fibers can be coated within one deposition run simultaneously, thus defining a clear advantage compared to individual device fabrication. Beside a clean and flat fiber end face, which is straightforwardly and instantaneously achieved by cleaving, the fabrication does not demand any further steps associated with nano-fabrication such as lithography, milling, or etching. Our approach does not require any kind of tapering, resulting in mechanically robust and highly reproducible devices. Finally, since the light is solely guided inside the core at any location inside the fiber (in contrast to commonly used fiber taper-based probes, which show domains of single- and multimodeness), our concept can handle any type of cladding shape (and is therefore independent of any potential taper angle) provided that the guided mode is not perturbed. Thereby our NBF approach allows for the implementation of new und unconventional types of fiber probe geometries, particularly in the vicinity of the apex. The measured transmitted power fractions for sufficiently thick films (~80 nm for Al, ~120 nm for Pt) are between 10-5 and 10-6, which is of the same order as state-of-the-art nano-aperture probes. By further engineering the NBF properties, we believe it should be possible to improve 𝜂𝑡 by several orders of magnitude. One potential idea is to integrate a recessed micrometer-long high refractive index strand into the nano-channel just before the metal film. If designed appropriately, light from the HE11 core mode would couple into the high index region, increasing 𝜂𝑡 and thus 𝜂𝑏. Another approach, which is currently being investigated, relies on integrating a planar dielectric film-based cavity between film and fiber, which resonantly enhances the light transmission through the nano-aperture at specific wavelengths. Moreover, due to the increasing localization of light inside the bore, we expect the device performance to improve for an etched cladding/core (resulting from a decrease in the mode field diameter32), which is the opposite of what occurs in current near field aperture probes. Note in particular that the latter option naturally suggests using suspended core fibers with micron-scale cores33 and central nano-holes. Improving detection and delivery of light to- and from- nanoscale dimensions using near field microscopy continues to be an active and attractive research area in nanophotonics, due to its multidisciplinary character in fields such as bioanalytics, disease diagnostics, semiconductor inspection, quantum metrology, and nanoscience. This introduces a new fabrication paradigm for making metal nanostructures on a fiber without using lithography, where a macroscopic fiber preform defines the nanostructure. In particular, the simplicity and high sample yield of this technique allows to address a multitude of applications, of which SNOM is only one example, and naturally defines a new technology within the context of “lab-on-a-fiber”34 which typically requires patterning nanostructures on the fiber end face via additional processing steps, e.g., ebeam lithography 6 or self-assembly35, before coating with metal. Here, we avoid additional postprocessing steps by essentially using the fiber preform as a template, which is suited for largescale manufacturing. The advantages of this concept, especially in contrast to approaches that rely on lithography, are manyfold; these include, for instance, interfacing nano-apertures directly with fiber circuitry, reducing production costs for nanostructuring metal films, and opening the door to many applications including, but not limited to, sub-wavelength aperture based focusing36, enhanced transmission through sub-wavelength holes37, single nano-object trapping38, or sub-wavelength bow-tie nano-antennae with order-of-magnitude enhancements23, e.g. for particle trapping and manipulation3. All mentioned areas demand


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ACS Photonics

cost-efficient access to fiber probes with deep sub-wavelength spatial resolution that can straightforwardly be used within near field microscopy and that are available in large quantities. Note that while Si-cantilever based micro-machined aperture SNOM probes exist and can be mass-produced, they still require subsequent processing steps for interfacing them with optical fibers 39,40 Here we have proposed and experimentally demonstrated the implementation of lithography-free nano-apertures via depositing metallic nano-films on nanobore optical fibers. This scheme relies on only one single film deposition step, avoiding any kind of lithography and thus providing a practical pathway for the implementation of deep subwavelength fiber-based nanoprobes at large scales. The device consists of a step index fiber including a longitudinal nanochannel in its core that has one of its ends coated with a metallic nanofilm. The deposition of the metal film onto the nanobore fiber end face results in high-quality nano-apertures that transmit light through apertures with diameters as small as 40 nm. Various types of fiber probes have been implemented via Al and Pt nano-films solely using sputtering of films with thickness between 40 nm and 200 nm. In accordance with simulations, experiments confirm that the fiber mode reaching the aperture is partially transmitted through the nano-aperture, leading to a diffractionlimited nano-spot measured in the far-field. This experimental study provides a valuable benchmark for future aperture- and lab-on-a-fiber designs, and presents a novel and straightforward concept for future fiber-based nanoprobes that allow for unconventional core/cladding geometries. The use of a single and reproducible thin film deposition step naturally lends itself to large sample yields of extremely high quality. ASSOCIATED CONTENT Additional figures (metal nanofilm transmission calculations, SEM images of nano-apertures and metal grains) are available free of charge via the internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *[email protected] **[email protected] Notes The authors declare no competing financial interest. Author Contributions A. T. performed the experiments and simulations. S. W. fabricated the empty nanobore fiber. H. S. and J.D. performed the metal sputtering and focused ion beam milling. M.A.S. and AT developed the idea and supervised the research. The manuscript was written by A.T. and M.A.S. through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources


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The authors also acknowledge funding by the Thuringian State (Projects 2015FGI0011 and 2015-0021) partly supported by the European Social Funds (ESF) and the European Regional Development Fund (ERDF). A.T. acknowledges funding from the University of Sydney Postdoctoral Fellowship scheme. REFERENCES (1)

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Tuniz, A.; Chemnitz, M.; Dellith, J.; Weidlich, S.; Schmidt, M. A. Hybrid-Mode-Assisted Long-Distance Excitation of Short-Range Surface Plasmons in a Nanotip-Enhanced StepIndex Fiber. Nano Lett. 2017, 17, 631–637.


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nanobore optical fiber (NBF)

(a)

|E|2 [a.u.]

|E|2 [a.u.]

1 2 HE11 mode 3 4 5 6 7 8 9 10 11 12 (b) 13 14 15 hollow 16 nanobore core 17 d bore 18 19 20nanobore cladding 21 (b) core 22 23 24 HE11 25 mode 26 (c) 27 nanometal 28 aperture coated 29 near-field core 30 31 2 32 0 -2 x [μm] 33

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metal nanofilm

nanoaperture (c) t

nanospot

(d)

uncoated NBF

(e)

coated NBF

core

100 nm hole ACS Paragon Plus Environment

40 nm aperture

(f)

experiment (coated NBF)

aperture diameter [nm]

aperture diameter [nm]

1 110 nm 2 NBF1 3 (d) (e) 4 5 6 7 170 nm 8 NBF2 9 (g) 10 200 NBF2 11 12 13 14 150 15 16 17 120 18 100 19 80 20 21 40 0 22 50 23 0 24

40nm

40nm

120nm (c) 80nm ACS Photonics

60nm

Al-coated NBF1 60nm 80nm

120nm (f)

80nm

120nm

160nm

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80nm

Pt-coated NBF1 120nm 160nm

200nm

Al-coated NBF2

Pt-coated NBF2 (h) aluminum platinum

nano-aperture Pt

NBF 1

SiO2 100 150 50 film thickness [nm]

200

200nm ACS Paragon Plus Environment

50 100 film thickness [nm]

150

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1.5μm

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10 10 10 10

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x 10

(i) core light transm.

d=300

nm

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d=2

4

2

0

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100

300

00n d) d m ot te d lin d= 10 e: un 0n m ifo rm film

(ii) bore light transm.

10 -2

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λ = 633 nm

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t = 40nm 10

-4

10

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t = 120nm 10 10

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t = 200nm

dotted line: uniform film

bore diameter [nm]

10 -7 20

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60 80 100 Al film thickness t [nm]

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0

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max

40 nm Al film

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80 nm Al film

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simulation (far field)

simulation (near field)

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10

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-2

(i)

(i)

Aluminium

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(ii)

(iii) (iv)

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Al coated SMF (no hole) Al coated Fiber 1

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Aluminium

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60nm film

60nm film

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