Competence Center for Micro- and Nanotechnologies

Silica-based, metal-coated fiber tips (tapers) are used in a variety of spectroscopic and micro-/ nanoscopic applications (e.g. AFM, SNOM, TERS). The optical resolution of such measurement systems is dependent on the diameter of the tapered fiber end, which allows only a few 10 nm. In addition to taper drawing processes the tip shaping of the multimode standard-fibers is done primarily by wet-chemical etching usually in hydrofluoric acid. In the present study we investigated the possibility of the preparation of geometrically predefined, nanoscaled fiber tips by taking advantage of the step- and gradient-shaped concentration profiles of highly doped silica fibers. For this purpose, a gas phase etching process using hydrofluoric acid vapor was applied. The advantage of this approach is that all reaction products are obtained in a gaseous form and therefore surface condensation which often leads to etching inhomogeneities can be avoided. The shaping of the fiber tips base on very different etching rates depending on the etching treatment and doping characteristics of the optical fibers. The influence of the doping characteristics was investigated in phosphorus-, germanium-, fluorine- and boron-doped glass fibers. The etching behavior correlates with the doping-dependent refractive index profile of the fibers. Strong etching gradients take effect in the region of the central index-dip, since the dopant concentration is reduced to zero due to the manufacturing conditions and therefore the etching rate is minimized. Technological studies on the influence of the etching gas atmosphere (setting of the gas orifice) on the time course of the tip shaping and the final geometry were performed on undoped glass fibers. Narrow fiber tips in various shapes and tip radiuses down to less than 15 nm were achieved and geometrically and topologically characterized. For investigations on coating of planar surface plasmon resonance structures the fiber tips were coated with nanometer-sized silver layers by means of vapour deposition and finally subjected to an annealing treatment.