Visualization and selective manipulation of sub-terahertz whispering gallery modes

The development of technologies based on electromagnetic resonant structures usually relies on precise understanding of the spatial form of the supported modes. In millimeter-sized resonators that support millimeter-wave modes, this is often done numerically. However, simulations often fail to accurately predict the resonance frequencies and mode numbers. Such discrepancies can arise from uncertainties in the dielectric properties of the materials or inaccuracies in the geometric characterization of the structures. Here, we experimentally map the spatial distribution of terahertz modes in a silicon disk resonator by perturbing the mode with a metal needle scanned across the disk. This allows us to identify the modes, and hence to design a perturbative periodic structure to selectively manipulate specific modes. This technique could be useful for targeted tuning of such modes for spectral engineering.