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Chemically etched fiber tips for near-field optical microscopy: a process for smoother tips Patrick Lambelet, Abdeljalil Sayah, Michael Pfeffer, Claude Philipona, and Fabienne Marquis-Weible An improved method for producing fiber tips for scanning near-field optical microscopy is presented. The improvement consists of chemically etching quartz optical fibers through their acrylate jacket. This new method is compared with the previous one in which bare fibers were etched. With the new process the meniscus formed by the acid along the fiber does not move during etching, leading to a much smoother surface of the tip cone. Subsequent metallization is thus improved, resulting in better coverage of the tip with an aluminum opaque layer. Our results show that leakage can be avoided along the cone, and light transmission through the tip is spatially limited to an optical aperture of a 100-nm dimension. © 1998 Optical Society of America OCIS codes: 050.1220, 060.2370, 180.5810, 240.6700, 350.5730. In scanning near-field optical microscopy ~SNOM! a sharp probe tip of submicrometer dimension picks up optical information in the near field by sending, collecting, or diffracting light at the surface of a sample.1 The near field that is the source of subwavelength optical information is influenced as much by the shape and optical properties of the sample surface as it is by the tip itself.2 Reliable results thus critically depend on the ability to work with well-defined tips whose geometrical and optical properties are characterized and controlled. Most of the SNOM probes used today are based on tapered optical fibers. Tips are obtained either by heating and pulling3,4 or by chemical etching5,6,7 in aqueous solution of fluorhydric acid ~HF!, producing a conical tip. To yield a subwavelength optical probe, the conical tip is coated with an aluminum layer, leaving a nanometer-size aperture at the tip apex. The ideal tip is characterized by high optical transmission through a single hole with dimension of a few tens of nanometers. The optical resolution is of the When this research was performed, the authors were with the Institut d’Optique Appliquée, Ecole Polytechnique Fédérale, 1015 Lausanne, Switzerland. A. Sayah is now with the Institut de Microsystèmes, Ecole Polytechnique Fédérale, 1015 Lausanne, Switzerland. The e-mail address for P. Lambelet is patrick. [email protected]. Received 20 April 1998; revised manuscript received 3 August 1998. 0003-6935y98y317289-04$15.00y0 © 1998 Optical Society of America order of the size of the aperture.8 Furthermore, sharp tips characterized by a small radius of curvature are more appropriate, since they allow the tip to come quite close to the sample surface when this surface is not perfectly flat. The heating and pulling process produces long tips ~1 mm!, with smooth surfaces, allowing good and uniform metallization with aluminum and leading to well-defined apertures. Compromises between optical transmission and aperture diameter lead, for that type of tip, to typical transmissions of 1028–1024 for aperture sizes between 30 and 100 nm.4 The rather low transmission is due to the strong attenuation of the light along the taper as well as to the very small cone angle of the tip ~typically a few degrees!. Chemical etching, on the other hand, allows one to produce fiber tips with much shorter cones ~;200 mm! and thus much larger cone angles. This leads to higher transmission, with the light being guided in the intact core of the fiber down to a few micrometers from the tip apex. Typical transmissions of 1023 have been reported.9 The main drawback of chemical etching is the roughness of the surface obtained after etching with HF, which decreases the quality of the aluminum coating.10 Such tips often show leakage of light along the conical taper. In this paper we propose a new etching technique that allows one to produce much smoother surfaces and thus tips of higher quality. Applying this new technique, we have been able to obtain tips with a smooth metal coating and no leakage along the taper. The high transmission ratio of chemically etched tips can then 1 November 1998 y Vol. 37, No. 31 y APPLIED OPTICS 7289 Fig. 1. An optical fiber is dipped, with its acrylate jacket, in aqueous hydrofluorydric acid covered by oil as a protective layer. The acid diffuses through the acrylate and etches the quartz fiber. After 35 min a sharp tip is formed. be combined with some of the advantages of pulled tips, such as smooth cone surfaces and opaque metal coatings. The new mechanism for fabrication of the tips is based on the chemical etching of a glass fiber through its acrylate jacket, as opposed to the standard chemical etching that is usually performed on bare fibers after removal of the jacket. The etching setup is shown in Fig. 1. An optical fiber, monomode at 633 nm ~FS-SN-3221 from 3M!, is dipped with its acrylate jacket into an aqueous 40% HF solution. The HF solution is covered by oil to protect the fiber against acid vapor and is heated to 60 °C to accelerate the procedure. The acid does not dissolve the acrylate jacket but rather diffuses through it to etch the quartz fiber. After 35 min the fiber is removed and rinsed successively with water, trichlorethylene, and acetone. At this point a tip has been formed inside the acrylate jacket. To remove the jacket, an incision is made a few millimeters above the tip, and the jacket, softened by acetone, can then be pulled by being seized in front of the tip. The jacket is thus removed without damage to the fiber tip after the tip is formed. This method is similar to a previous technique used to make fiber tips by chemical etching.7 However, although the mechanism of chemical etching of the glass is the same, etching through the jacket leads to a different tip formation process. When etching is performed on a bare fiber, the meniscus formed by the acid along the fiber is determined by the acid– oil– quartz interface. As the fiber is etched, its diameter decreases and following the laws of superficial tension; the meniscus height decreases until the tip is fully formed. In the present method acid does not etch the jacket but rather diffuses through it to etch the glass. The meniscus height is determined by the acid– oil–acrylate interface and remains constant during tip formation. The formation of the tip is thus essentially due to a diffusion process. As the ions of acid react with the quartz, new ions have to diffuse from the liquid to the surface of the quartz. In the upper part of the meniscus the layer of liquid is very thin, and thus the ions saturate more rapidly, 7290 APPLIED OPTICS y Vol. 37, No. 31 y 1 November 1998 Fig. 2. SNOM tips obtained by chemical etching of ~a! a fiber with its acrylate jacket, ~b! a bare fiber; ~c! and ~d!, corresponding axial views after metallization. slowing down the etching rate. In the deeper liquid the concentration of ions close to the fiber remains higher owing to more efficient diffusion from the large reservoir. This explains why a tip is formed, although the height of the meniscus does not change. Note that the diffusion through the acrylate is very efficient. With the same acid concentration and temperature, forming a tip across the acrylate jacket takes only 5 min longer than on bare fibers. In Fig. 2 scanning electron microscopy with a field emission electron microscope ~JEOL, F 6300! is used to give an image of a tip obtained by this new technique @Fig. 2~a!# and to compare it with a tip produced under the same conditions but by etching a bare fiber @Fig. 2~b!#. These figures show that the surface of the tip etched through the jacket is very smooth ~comparable to pulled fibers! and does not suffer from the surface irregularities clearly visible in Fig. 2~b!, which are usually encountered in chemically etched tips. Here the radius of curvature of the tip apex is 15 nm in Fig. 2~a! and 30 nm in Fig. 2~b!. This improvement of the quality of the surface shows that the movement of the meniscus along the tip when a bare fiber is etched is the main effect responsible for the irregularities observed on the surface. Keeping the jacket on the fiber stabilizes the meniscus and produces a smoother surface, with etching being governed essentially by diffusion. Although the global shape of the tip, viewed on a larger scale, is symmetric, asymmetries can occur in the last few micrometers of the tip in the form of elongated depressions along the axial direction. This effect, as can be seen in Figs. 2~a! and 2~b!, however, is less pronounced in fibers etched through the jacket. The fiber tips are metallized with a 100-nm-thick layer of aluminum ~deposited at 10 nmys, at a pressure P , 5 3 1026 mbar!. Axial views of the tip apex after metallization are displayed in Figs. 2~c! and 2~d!. A smoother surface is observed on the tip etched through the jacket, although aluminum grains Fig. 3. Angular light distribution from a tip chemically etched ~a! with the acrylate jacket, ~b! without the acrylate jacket. The scale is in numerical apertures, NA. are still present. The aperture is not clearly defined for either type of tip. We attribute the difficulty of obtaining clear circular apertures in the metal layer to the very small radius of curvature ~typically 15 nm! characterizing the quartz tip of etched fibers. In contrast, pulled tips are characterized by a flat end, for which much better defined apertures have been observed. We measured a transmission of log~T! 5 23.3 6 0.8 ~26 samples! for the tips etched through the jacket and log~T! 5 22.8 6 0.8 ~13 samples! for the tips made from bare fibers. This means that the transmission varies between 8 3 1025 , T , 3 3 1022 for tips made with the new etching process and 2 3 1024 , T , 1022 for the other kind of tip. The higher transmission of the latter tips is attributed to leakage of light along the cone resulting from the greater roughness of the surface. To check the effect of roughness, we completely closed the aperture at the tip apex by successively evaporating the aluminum from the side and from the front of the tip. The transmission of the tips etched with the jacket decreased to our detection limit of Tclosed 5 1027, but the transmission of the other tips decreased only by a factor of 10 and was still Tclosed 5 7 3 1024. This result shows the importance of the quality of the surface of the quartz tip for guaranteeing efficient metallization without leakage of light along the cone of the tip. Note that quantitative information on the transmission of the tip is not sufficient to determine its quality; information on leakage along the cone and aperture size are important parameters as well. To further analyze light leakage along the cone, we measured the angular emission of both types of tip in the far field. Figure 3 displays this far-field profile for tips with a 100-nm-thick coating of aluminum, obtained with a water-immersion microscope objective ~numerical aperture NA 5 1.2! by placing the tip apex at the object focal plane of the objective and imaging its back pupil on a CCD camera. Figure 3~a! corresponds to the fiber etched with the jacket and shows a uniform and wide light distribution, which is the fingerprint of a single, small ~,300 nm! aperture. The low-contrast features appearing on this image are due to leaking light along the cone but with a total power much lower than the light emitted by the small aperture. On the other hand, Fig. 3~b!, Fig. 4. Near-field line scan of a 1:1 chromium grating of period 372 nm. Solid curve, topography; dotted curve, near-field optical transmission. Optical resolution, Dx 5 100 6 10 nm. which corresponds to etching a bare fiber, shows a structure containing many speckles, which is due essentially to the light leaking along the cone of the tip. Note that this far-field pattern is highly sensitive to leaking light because the interference contrast of the leaking light with the light coming from the tip depends on their relative total power, not on their actual intensity at the tip surface. Fifty percent of the fibers etched with the acrylate jacket show a far-field distribution similar to Fig. 3~a!, but none of the fibers etched without the jacket can produce such a uniform profile. These results show that the problem of light leakage encountered until now with most etched fibers can be solved by etching through the jacket. The resulting smoother surface of the tip allows one to obtain better aluminum coverage without leakage. This observation also shows that the difficulty encountered until now in obtaining a good coating on chemically etched tips in comparison to that on pulled tips is due not to a different chemical state of the surface but to a higher surface roughness. The problem of light leakage along the cone of the tip is not necessarily critical for imaging in the nearfield, since this leaking light has a poor lateral confinement and just adds a background to the near-field image. But for applications based on photochemical reactions11 this leaking light must absolutely be avoided, since it induces a reaction far from the targeted positions. For such applications it is critical to have only one optical aperture on the SNOM tip. To test the performance of the tip in a near-field optical measurement, a transmission SNOM image of a 1:1 chromium grating ~period 372 nm, chromium thickness 21 nm! deposited on glass is taken with a tip etched through the acrylate jacket. Figure 4 shows a cut through such an image. The profile of the light transmission shows a contrast of h 5 0.28. Measured on the same grating with four tips of each type, the contrast in the optical profile is improved by a factor of 2 ~hjacket 5 0.17 6 0.1 versus hbare 5 0.08 6 1 November 1998 y Vol. 37, No. 31 y APPLIED OPTICS 7291 0.07! for tips etched through the jacket. This difference can be explained by the more prominent background signal resulting from light leakage along imperfectly coated tips obtained by etching bare fibers. From the slope of the sides of the grating, in Fig. 4, a resolution of Dx 5 100 6 10 nm is measured ~10%–90% of the intensity!. In conclusion, we have presented a modified etching technique for producing SNOM tips. The fiber is dipped with its acrylate jacket in an HF solution covered by a protective oil layer. In this way the formation of the tip is governed only by diffusion and results in smoother tips compared with those on chemically etched bare fibers. This smoother surface, comparable with the surface of a pulled fiber, allows one to obtain a high-quality aluminum coating on the tip, leading to minimum light leakage along the cone. The authors are grateful to B. Senior ~Centre Interdépartmental de Microscopie Electronique, Ecole Polytechnique Fédéral Lausanne! for taking the electron micrographs and to M. Gale ~Centre Suisse d’Electronique et de Microtechnologie SA, Zürich! for fabricating the test grating. Financial support from the Swiss National Fund for Scientific Research is acknowledged. References 1. M. A. Paesler and P. J. Moyer, eds. Near Field Optics, ~J. Wiley, New York, 1996!. 7292 APPLIED OPTICS y Vol. 37, No. 31 y 1 November 1998 2. B. Hecht, H. Bielefeldt, Y. Inouye, D. W. Pohl, and L. Novotny, “Facts and artifacts in near-field optical microscopy,” J. Appl. 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