THz pulse waveforms measured by arrival-time-sorted time domain spectroscopy (TDS). (a) The THz pulse produced by the undulator set at the nominal wavelength of 155 µm (1.93 THz). The unfiltered temporal profile is shown by the blue line and the filtered by the red line, with 155 µm (1.93 THz) bandpass filter (15% bandwidth). (b) The unfiltered THz pulse at 43 µm (7 THz) (green line). (c) The corresponding THz pulse spectra.