Fig. 4: Ultrafast imaging of the photo-excited carrier dynamics in silicon.

a The dynamic scene we imaged was the carrier generation in an undoped silicon wafer (thickness: 500 µm) pumped by a near-infrared laser pulse. The incident angle of the optical pump was ~30°, with a diameter of ~2 mm and an average power of 50 mW. b Optical pump-THz probe measurements. Note that for negative pump-probe delay values, the optical pump arrives after the THz pulse. c Multi-cycle THz pulse used for ultrafast imaging. Such a pulse was obtained by attaching a thin silicon wafer (thickness: 35 µm) onto the mirror M1. The four sub-pulses in the multiplexed probe beam were temporally aligned with the four peaks of the multi-cycle THz pulse, in turn leading to inter-frame time intervals of 0.75 ps, 0.75 ps, and 1.35 ps, respectively. d Image acquired by the CCD camera when the arrival time of the multiplexed probe beam was set to 1.25 ps after photoexcitation. e 2D Fourier transform of d, with image copies from each sub-pulse circled. f Recovered frames from the multiplexed image in d. g Recovered frames when the optical pump arrived 1.20 ps later relative to the case in f. h Recovered frames when the optical pump arrived 1.20 ps earlier relative to the case in f. The imaging contrast plotted in the frames is the modulation ratio, calculated by normalizing the individual frames with a reference frame, which was taken when the carriers within the illumination region were fully excited (at a time instant larger than 6 ps after photoexcitation).