Dielectric Slot Tip for Scanning Near-Field Microwave Microscope

While tapered and coated fibers are used as probes for scanning near-field optical microscopy (SNOM), tapered coaxial probes and other structures are used in the microwave regime for broad band measurements. Aperture probes, tapered fibers and tapered waveguides have the inherent disadvantage that the radiation will have to pass a cutoff region. This is not the case for coaxial probes and for appropriately chosen transmission lines based on metallic wires. To enhance the energy transition for a tapered SNOM tip, the cladding can be split by milling longitudinal slits in it. We demonstrate the principle of mode conversion in the microwave region, building a tip for a scanning near-field microwave microscope (SNMM) with a feed similar to a SNOM tip with the slits in the cladding. Transition to a single wire mode is made at the very end of the tip. With this new kind of SNMM tip we scan a test structure and demonstrate a resolution of 1/882 wavelengths for double passage operation.