  Spintronic emitters are a unique class of terahertz (THz) sources due to
their quasi-two-dimensional geometry and thereby their capability to couple to
resonator near fields. Global excitation of the emitters often obstructs the
intricate details of the coupling mechanisms between local THz dipoles and the
individual modes of resonator structures. Here, we demonstrate the spatial
mapping of the coupling strength between a local terahertz source on a
spintronic emitter and far-field light mediated by a structured metallic
environment. For a bow-tie geometry, experimental results are reproduced by a
numerical model providing insights into the microscopic coupling mechanisms.
The broad applicability of the technique is showcased by extracting the THz
mode structure in split-ring resonator metasurfaces and linear arrays. With
these developments, planar THz sources with tailored spectral and angular
emission profiles are accessible.
