Fig. 3: Ultrafast imaging of a THz pulse propagating through a patterned structure.

a The dynamic scene consists of a collimated THz beam traveling through a Teflon sheet (thickness: 2 mm) with four engraved letters (‘I’, ‘N’, ‘R’, and ‘S’). b Detailed geometries of the letters shown in an ‘X-ray’ view. The stroke width of the letters is ~2.1 mm. The engraving depths of the four letters, ‘I’, ‘N’, ‘R’, and ‘S’, are 2 mm, 1.5 mm, 1.0 mm, and 0.5 mm, respectively. As a result, the letter ‘I’ is engraved throughout the sample along the depth direction. c THz waveforms acquired via the EOS technique with and without the sample. The arrival times of the four sub-pulses for imaging are highlighted with red dots. The inter-frame time intervals are all equal and set to 0.75 ps. d Multiplexed image acquired by the camera in real space. e 2D Fourier transform of the image in d. The ‘image copies’ of the four frames to be extracted are indicated by circles. f Recovered frames from the multiplexed image in d. g Recovered frames captured when the probe beam arrived 0.70 ps earlier relative to the case in f. h Recovered frames captured when the probe beam arrived 0.70 ps later relative to the case in f. The image contrast of the frames is given by the amplitude of the THz electric field.