Three-dimensional analysis of light propagation through uncoated near-field fibre probes

The near-field emission from uncoated tapered fibre probes is investigated for different probe geometries. The three-dimensional model calculations ar e based on Maxwell's curl equations and describe the propagation of a 10 fs optical pulse (lambda = 805 nm) through tapers of different lengths and di fferent diameters of the taper exit. The numerical evaluation is done with a finite difference time domain code. Two tapers with cone angles of 50 deg rees, with taper lengths of 1.5 mum and 1.0 mum and exit diameters of 100 n m and 520 nm, respectively are considered. We find that without sample the short taper with large exit diameter optimizes both light transmission and spatial resolution. In the presence of a sample with a high dielectric cons tant, however, the spatial near-field distribution changes drastically for both taper geometries. We find a pronounced increase in spatial resolution, down to about 250 nm inside the medium. This collimation of the near-field distribution arises from interferences between emitted and reflected light from the sample surface and from a collimation effect that the field exper iences in the high-index semiconductor material. The combination of high sp atial resolution and transmission and collection efficiencies makes such pr obes interesting for spectroscopic investigations, as demonstrated by recen t experiments.