  We show in theory and experiment that in periodically patterned spintronic
THz emitters (STE), charge dynamics can modify the emission spectrum in a
well-controlled way. Characterization of sub-wavelength patterned STE at
frequencies up to 30 THz shows that the STE's emission spectrum systematically
changes with emitter size. The spectral intensity exhibits significant
reductions at frequencies below 4 THz, accompanied by pronounced dips at around
15 THz and 24 THz. While reducing the STE size enhances the modulation of all
features, it does not alter the dip frequencies. The effect originates from the
charging of the structure's edges by THz currents, causing a backflow that
interferes with the primary current pulse. An analytical model quantitatively
reproduces these results and agrees well with control experiments. Our findings
enable a detailed investigation of the charge dynamics in STE and provide
additional means for controlled shaping of STE emission spectra by nano
patterning.
