We propose a method to fabricate a probe with a nanometric protrusion for near-field optical microscopy. The method involves a tapering process based on selective etching of a GeO2-doped fiber in a buffered hydrogen fluoride solution and a metallizing process by vacuum evaporation and chemical polishing. We fabricated a tapered probe which has a protrusion emerging from a metal film. The protrusion has an apex diameter less than 10nm and a foot diameter less than 20nm. Employing the probe, we succeeded in obtaining a highly resolved image of 20nm gold particles.
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Recently, super high resolution optical imaging has been performed beyond the diffraction limit by near-field optical microscopy [1–4] employing a tapered fiber probe with a nanometric apex and a small taper angle. We previously produced a probe with a taper angle of 20°, an apex diameter less than 10nm, and a nanometric protrusion emerging from a metal film  by selective resin coating (SRC) . Employing the probe, we succeeded in obtaining nanometric images [7,8] of biological samples. Such a probe is found to be effective both as a scatterer (and/or a generator) of high spatial frequency evanescent components responsible for high resolution capability and as a suppresser of the low spatial frequency components and the propagating components. However, the SRC method can not be applied to a tapered probe such as a pencil-shaped probe without a flat end portion . In this letter, we propose a method to fabricate a pencil-shaped probe which has a nanometric protrusion emerging from a metal film using a commercial GeO2-doped fiber.
Figure 1 schematically explains the method involving five steps; [A] tapering the cladding, [B] sharpening the core, [C] increasing the taper angle, [D] coating a metal film, and [E] removal of metal covering the apex region. Here, the method is divided into the tapering process of steps [A]–[C] and the metallizing process of steps [D] and [E]. The symbols α, θ, and θ B are defined a s the taper angles of the pencil-shaped fiber. The notations of Oil/HF, BHF, HF are HF acid with a surface layer of an organic solution, a buffered hydrogen fluoride solution with a volume ratio of 40wt.%-NH4F aqueous solution: 50wt.%-HF acid: H2O= X:1:Y, and an aqueous solution containing HF. Throughout this letter, BHF is denoted as X:1:Y. Figures 2(a) and 2(b) show scanning electron micrographs (SEM) of the pencil-shaped fiber obtained with taper angles of θ =10° and α=20° by steps [A]–[C]. The taper angle and the apex diameter are 20° and less than 10nm, respectively. Figures 2(c) and 2(d) are, respectively, SEM images of the magnified top region of a fabricated probe with a protrusion and of the apex region. The dark portion in Fig. 2(d) shows a fabricated probe with a protrusion. The radial thickness of gold film is about 150nm. The foot diameter of silica tip protruding from the metal film is less than 20nm. In the following, we describe the method for fabricating the probe.
In step [A], a dispersion-shifted fiber (Sumitomo Electric Industries, DS1) having the germanosilicate (GeO2) doped core was immersed for 50min in a mixed solution of a volume ratio of 50wt.%-HF acid: 95wt.%-sulfuric acid= 4:1. Here, the fiber has a diameter of 125μm, a core diameter of 4μm, and a GeO2 doping ratio of 9mol%. To suppress the vapor phase evaporation of HF-H2O, the surface of the solution was sealed with a layer of dimethylsilicone oil as was done in meniscus etching of our previous method . Sulfuric acid is added to maintain the symmetry of the conical taper. The fiber has a taper angle of α=20° by this meniscus etching. In step [B], we selectively etched the fiber for 120min in 10:1:1 of BHF. We obtained a pencil-shaped fiber with taper angles of θ B=10° and α=20°. Step [C] was performed by immersing the fiber for 90 seconds in HF-containing solution mixed with a volume ratio of 50wt.%-HF acid: H2O= 1:3. We obtained the pencil-shaped fiber with taper angles of θ=20° and α=20°. In step [D], the fiber was coated with chromium and gold films of thickness 3nm and 200nm, respectively, by a vacuum evaporation unit. Here, the pressure was maintained at 10-6-10-7 Torr. We used electron-beam evaporation for coating with gold. The fiber is tilted with an angle ϕ=50° and rotated. In step [E], the metallized fiber was immersed for 15min in a KI-I2 aqueous solution mixed with a weight ratio of KI:I2:H2O=20:1:80000 at 25°C (±0.5°C).
Next, we discuss the reproducibility of the etching process. In step [B], by denoting the etching rate of the core and cladding as R 1 and R 2, respectively, the taper angle θ B is represented by [10: Eq. (A1)]
The right side is controlled by varying the GeO2 doping ratio and the concentrations of BHF (or X and Y). The pencil-shaped probe with taper angles of θ B=20° and α=20° is fabricated by the etching with a ratio of 1.7:1:1 where R 1/R 2=1. However, it is difficult to produce a probe with an apex diameter less than 10nm with high reproducibility. We consider that this difficulty is attributed to geometric eccentricity of the taper formed by step [A]. The taper has an eccentric radius of around 1μm between the core and the apex of the taper. To improve the reproducibility, we add steps [C]. As a result, we succeeded in obtaining the probe with a taper angle of θ =20° (±3°) and an apex diameter of less than 10nm with 80% reproducibility or more.
Futhermore, we discuss the metallizing process using vacuum evaporation. We performed step [D] with various angle ϕ in a region of 55°–90° and observed the probe by SEM. In the case of a thin coating of 120nm and a tilted angle of ϕ=50°, we evaluated a metal thickness of 30nm covering the fiber apex from the SEM photographs before and after the chemical polishing. For fabricating a probe with a protrusion, step [E] or another removal technique is required At ϕ=50°, we obtained a protrusion-type probe with a foot diameter of 20nm (± 15nm) with about 75% reproducibility or more using 12 fiber samples with apex diameters less than 10nm.
Finally, to demonstrate the imaging capability of the fabricated probe, we observed gold particles of size 20nm fixed on a glass substrate by a near-field optical microscope (NOM). The NOM is operated under illumination mode with the probe illuminating the sample substrate by an argon ion laser with a 488nm wavelength. Details of the experimental system can be found in Ref. 11. The size of the gold particles are well calibrated by TEM to be 20nm (±1nm) and they are just suitable for the high resolution imaging capability of the probe. The obtained image is shown in Fig. 3. Here, the scan area is 200nm×200nm. The dark portions (as marked by the arrow) correspond to single gold particles. Based on power spectral analysis of the image, the estimated size of the single particles is found to be around 22nm. For details of the analysis, refer to Ref. 12. This closely agrees with the actual size of the particle, thus demonstrating the effective functioning is high resolution imaging of the proposed NOM probe.
In conclusion, we proposed a method to fabricate a pencil-shaped fiber with a nanometric tip protruding from metal film. We obtained a probe which has a silica protrusion with an apex diameter less than 10nm and a foot diameter less than 20nm. Employing the probe, we obtained a high-resolution image of 20nm-sized gold particles.
References and links
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