Ion beam-treated silicon probes operated in transmission and cross-polarized reflection mode near-infrared scanning near-field optical microscopy (NIR-SNOM)

We present aperture probes based on non-contact silicon atomic force microscopy (AFM) cantilevers for simultaneous AFM and near-infrared scanning near-field optical microscopy (SNOM). For use in high-resolution near-field optical microscopy, conventional AFM cantilevers are modified by covering their tip side with an aluminium layer to obtain an opaque coating. To fabricate an aperture, this metal layer is opened at the very end of the polyhedral probe using focused ion beams, thus allowing apertures of <200 nm in diameter to be created. We describe our concept and investigate the characteristics of these probes by discussing images in transmission and cross-polarized internal reflection mode. Apart from probe characterization, our attention is devoted to the dependence of both signal amplitude and resolution on the tip-to-sample distance and to the polarization effects observed in the course of these investigations. To exclude artefacts induced by distance control, the measurements analysed here are performed in constant-height mode.