Gold-coated parabolic tapers for scanning near-field optical microscopy: Fabrication and optimisation

It is well known that the lateral spatial resolution achieved in scanning near field optical microscopy (SNOM) depends critically on the size and geometry of the optical fibre point used as a source of light or as the collecting sensing element. Although several groups have evaluated the importance of raper geometry, there is up to now no indication of how an optimal probe may be produced in practice. In this work we have developed a systematic study for different taper morphologies, including conical and ''parabolic'' shapes. Using the heating and pulling method, we have studied the processing parameters' influence on the aperture size and final geometry of the tips. We have found that both parabolic and conical tips can provide high resolution but, in view of the optical efficiency parabolic probes are superior over conical ones. Parabolic tapers can be fabricated if the glass preform is not allowed to melt to a low viscosity, but softening only enough to be drawn into a taper and then rupturing. Gold coating is used for further light confinement and also to provide good electrical conduction in our combined STM/SNOM system. We discuss different geometries and suggest ways for optimising size and morphology of the tips.