Shielded cantilever with on-chip interferometer circuit for THz scanning probe impedance microscopy

We have realized a microstrip based terahertz (THz) near field cantilever that enables quantitative measurements of the impedance of the probe tip at THz frequencies (0.3 THz). A key feature is the on-chip balanced hybrid coupler that serves as an interferometer for passive signal cancellation to increase the readout circuit sensitivity despite extreme impedance mismatch at the tip. We observe distinct changes in the reflection coefficient of the tip when brought into contact with different dielectric (Si, SrTiO₃) and metallic samples (Au). By comparing finite element simulations, we determine the sensitivity of our THz probe to be well below 0.25 fF. The cantilever further allows for topography imaging in a conventional atomic force microscope mode. Our THz cantilever removes several critical technology challenges and thus enables a shielded cantilever based THz near field microscope.