Inspection of nano-sized SNOM-tips by optical farfield evaluation

High resolution optical microscopy has many interesting applications in solid state physics, low temperature physics, biology and semiconductor technology. Unfortunately, the lateral resolution of conventional microscopes is limited by the Rayleigh-limit. "Scanning nearfield optical microscopy" (SNOM) seems to be a promising new approach to characterize the properties of materials optically with a high lateral resolution of 50-100nm. The most important part of such a microscope is the scanning probe (a special glass fiber tip). However, the quality of the optical fiber tip is of decisive importance. Since the production process of pulled and coated glass fiber tips is still highly empirical and error-prone, a technique would be useful to determine the tipsʹ quality before they are shipped to the user or mounted in the microscope. The tipsʹ apertures are smaller than lambda/2 and therefore they cannot be measured in a non-destructive way by conventional optical microscopy. This paper discusses an easy and fast method for the optical characterization of common glass fiber SNOM tips. The effective aperture of the tip is measured from the far-field distribution of the emitted intensity recorded by a CCD target. A numerical model is introduced to solve this inverse task and a simple optical setup is presented to detect light emitted by the tip at an angle of up to 90° from the optical axis. Experimental investigation, near/far-field calculations and scanning electron microscope investigations show the working principle of this measurement technique for the analysis and evaluation of a typical nanostructured object