Aperture-scanning-assisted ABCD technique. The drilled mirror has been covered by an ITO plate. (a) Evolution of THz amplitude as a function of frequency and pinhole position. (b) Integrated spectra for different spectral bandwidths. (c) Amplitude of the spectra at three different positions of the pinhole, as indicated by the dashed line in (a). “Center” refers to 0 mm (spectrum at the center of the THz beam), “Ring” refers to −2.5 mm (spectrum along the emission cone emitted at high frequency, above 12 THz), “Edge” refers to −7 mm (spectrum at the edge of the emission cone).