Single-shot ultrafast terahertz photography

Abstract

Multidimensional imaging of transient events has proven pivotal in unveiling many fundamental mechanisms in physics, chemistry, and biology. In particular, real-time imaging modalities with ultrahigh temporal resolutions are required for capturing ultrashort events on picosecond timescales. Despite recent approaches witnessing a dramatic boost in high-speed photography, current single-shot ultrafast imaging schemes operate only at conventional optical wavelengths, being suitable solely within an optically-transparent framework. Here, leveraging on the unique penetration capability of terahertz radiation, we demonstrate a single-shot ultrafast terahertz photography system that can capture multiple frames of a complex ultrafast scene in non-transparent media with sub-picosecond temporal resolution. By multiplexing an optical probe beam in both the time and spatial-frequency domains, we encode the terahertz-captured three-dimensional dynamics into distinct spatial-frequency regions of a superimposed optical image, which is then computationally decoded and reconstructed. Our approach opens up the investigation of non-repeatable or destructive events that occur in optically-opaque scenarios.

Fig. 1. Concept of probe-beam multiplexing in the spatial-frequency and time domains.

a An object (here, an apple) is uniformly illuminated in real space. The frequency content of the resulting image is band-limited and is mainly localized in the central region of Fourier space. b Illuminating the same object with a sinusoidal intensity modulation produces two ‘image copies’ of the object in two unexploited regions, symmetrically located in Fourier space with respect to the origin. Their actual locations (i.e. the distance from the origin) depend on the spatial-frequency k of the modulation pattern. c By varying the pattern orientations, ‘image copies’ can be moved to different non-overlapping regions in the Fourier domain, thus allowing for the implementation of a multiple-illumination scheme without any mutual interference. d By judiciously implementing multiple spatially modulated and temporally delayed patterns, a dynamic scene can be imaged. Despite being overlapped in the multiplexed image captured by the camera, these frames are well separated in Fourier space, and can be extracted and recovered computationally.

Fig. 2. Schematics of the proposed single-shot ultrafast THz photography system.

Details about the experimental setup are introduced in the Method section. The EOS technique can be operated in either a co-propagating or counter-propagating configuration, depending on the specific experiment requirements. In the counter-propagating configuration (see inset), the multiplexed probe beam is sent to the rear of the ZnTe crystal, traveling in the opposite direction to the THz beam. After being reflected from both surfaces of the crystal, the multiplexed probe beam then co-propagates with the THz pulse. The beam portion reflected from the left surface of the crystal is modulated by the THz electric field, and thus, carries the 2D information of the ultrafast scene. BS1-BS6 beam-splitter, ODL1-ODL4 optical delay lines, RG1-RG4 Ronchi gratings, L1-L4, lens; M1-M2 mirrors, PM parabolic mirror, TPX1-TPX3 THz lens, WP quarter-wave plate.

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.

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).