Study of the Chemical Fabrication Process of NSOM Probes and the Modification of the Probe Surface

Abstract

Near-field scanning optical microscopy (NSOM) merges scanning probe technology with the power of high-resolution optical microscopy and provides a natural view into the nanoworld. NSOM requires tapered probes with subwavelength optical apertures and wide cone angles to efficiently channel the illumination light to the tip apex so that it can acquire optical images beyond the diffraction limit. Tapered probes with a range of cone angles can be fabricated through chemical etching of optical fibers using hydrofluoric acid (HF) by varying the etching time. Apart from their use for NSOM imaging, such optical probes can also be transformed into nanosensors by attaching sensing elements to the NSOM probe surface. This work seeks to identify the maximum obtainable cone angle in an NSOM probe fabricated by chemical etching of an optical fiber and to create a nanosensor using this kind of probe. We investigate the progression of cone angles with etching time and propose a model of the etching process. We find that the variation of cone angle as a function of etching time does not follow the expected exponential plateau curve and we compare the experimental result to simulations with multiphysics models of the etching process of an optical fiber. Additionally, functionalization of NSOM probes with different fluorescent molecules is investigated and a fluorescent nanosensor is developed. We observe that the nanosensor is able to detect concentration changes of Cu^(2+) and Fe^(3+) ions in a droplet of sample solution.