Multidimensional imaging of transient events is crucial for understanding fundamental mechanisms in various fields. Real-time imaging with ultrahigh temporal resolutions is particularly important for capturing ultrashort events on picosecond timescales. While single-shot ultrafast optical imaging has advanced significantly, achieving speeds of 100 billion frames per second, its application is limited to optically transparent media. This restricts its use in studying ultrafast phenomena in materials with short optical penetration depths, such as laser ablation in ceramics, magnetization in iron films, and carrier excitations in semiconductors. Terahertz (THz) radiation, with wavelengths ranging from 30 µm to 3 mm, offers a unique advantage due to its high penetration capability in various materials, including metals, semiconductors, and dielectrics. This makes THz radiation ideal for probing thick, multilayered structures and biological tissues without causing damage. The low photon energy of THz radiation, unlike X-rays, minimizes detrimental effects on biological samples. Furthermore, THz imaging enables spectroscopic identification of substances based on their unique spectral fingerprints. Despite these advantages, single-shot ultrafast THz imaging remains underdeveloped due to the lack of high-speed THz cameras. Existing holographic approaches are theoretically proposed but experimentally challenging and limited to simple scenes. This research addresses this gap by presenting a novel single-shot ultrafast THz photography system.

Single-shot ultrafast photography techniques have significantly advanced the study of ultrafast phenomena. Recent breakthroughs in ultrafast lasers, high-speed cameras, and computational imaging have enabled the capture of two-dimensional (2D) transient scenes at extremely high frame rates. These advancements have opened new avenues in applications such as optical cloaking and the use of self-accelerating beams for imaging. However, current state-of-the-art single-shot ultrafast imaging techniques are confined to the optical window, limiting their applicability to optically opaque samples. The use of THz radiation for imaging has gained traction due to its ability to penetrate various materials. Several studies have demonstrated THz imaging techniques, including real-time near-field terahertz imaging, real-time terahertz imaging with a single-pixel detector, and ultrafast terahertz scanning tunneling microscopy. Hyperspectral terahertz microscopy has also been explored. However, single-shot ultrafast THz imaging remains largely unexplored due to technological limitations in high-speed THz cameras. Some theoretical proposals for holography-based approaches exist, but these methods are experimentally challenging and only suitable for simple scenes.

This research proposes and demonstrates a single-shot ultrafast THz photography system using the electro-optic sampling (EOS) technique for THz detection. EOS relies on an optical probe beam multiplexed in both time and spatial-frequency domains to detect THz waveforms. The THz-induced birefringence in a Pockels crystal is probed by the optical beam, and the resulting polarization changes are mapped onto a CCD camera. The key innovation is the multiplexing of the probe beam, which encodes the evolution of the ultrafast event into distinct spatial-frequency regions of the superimposed optical image. This multiplexing is achieved by splitting the probe beam into multiple sub-pulses, each delayed in time and modulated with a unique spatial-frequency pattern using Ronchi gratings. The multiplexed image is then computationally decoded to recover the individual frames of the ultrafast scene. The system utilizes an 800 nm femtosecond laser to generate both pump and probe beams. A collimated THz beam, generated from a LiNbO3 crystal, passes through the ultrafast scene and is detected using EOS in a ZnTe crystal. The probe beam is split into four sub-pulses, each delayed using optical delay lines and modulated by a Ronchi grating with a different orientation. The recombined probe beam illuminates the detection crystal, either co-propagating or counter-propagating with the THz wave. The polarization of each sub-pulse is modulated by the THz field, and the resulting image is captured by a CCD camera. The individual frames are separated in Fourier space and recovered through post-processing. The post-processing involves performing a 2D Fourier transform on the multiplexed image, isolating each frame using a band-pass filter, shifting the isolated frequency content to the origin, and applying an inverse Fourier transform to reconstruct the frame. The spatial resolution is determined by the central frequency of the THz source, while the temporal resolution is limited by the probe pulse duration. The system's flexibility allows for adjustable inter-frame time intervals and a potentially scalable number of frames.

The researchers successfully demonstrated the system's capability by imaging a THz pulse propagating through a Teflon sheet with engraved letters. The recovered frames clearly showed the temporal evolution of the THz pulse as it passed through the letters, with the inter-frame time intervals accurately reflecting the THz pulse's propagation. Further experiments involved imaging the spatiotemporal dynamics of carrier excitation in a silicon wafer upon illumination with a femtosecond laser pulse. The recovered frames illustrated the carrier excitation process across the wafer, confirming the system's ability to capture complex, transient events. The spatial resolution of the system was estimated to be around 0.56 mm, determined by the central frequency of the THz source. The temporal resolution was sub-picosecond, limited by the probe pulse duration. The inter-frame time interval was adjustable, enabling versatile capture of ultrafast dynamics. The study showcased the potential of this single-shot ultrafast THz photography system for investigating a wide range of ultrafast phenomena in various materials and structures, including 2D materials and biological matter.

The results demonstrate a significant advancement in ultrafast imaging, offering a novel approach to capturing dynamic scenes in optically opaque scenarios. The single-shot technique bypasses the need for high-speed THz cameras, a significant hurdle in previous attempts at ultrafast THz imaging. The ability to capture sub-picosecond temporal resolution with adjustable inter-frame intervals is a major achievement, broadening the scope of possible applications. The successful imaging of both a THz pulse propagating through a patterned structure and the carrier dynamics in a silicon wafer demonstrates the system's versatility. The combination of THz penetration and sub-picosecond resolution opens doors to investigate ultrafast processes previously inaccessible. Future research could focus on improving spatial resolution by employing near-field configurations, potentially leading to a single-shot ultrafast THz microscope. The system's capabilities hold immense potential for advancing the understanding of ultrafast dynamics in diverse materials and systems.

This study presents a groundbreaking single-shot ultrafast THz photography system that overcomes limitations of previous methods. The system's ability to capture complex ultrafast events in optically opaque materials with sub-picosecond resolution opens exciting possibilities for various research areas. Further improvements in spatial resolution and the exploration of near-field configurations are promising avenues for future research.

The current spatial resolution of the system is limited by the central frequency of the THz source. While sub-picosecond temporal resolution is achieved, the interference pattern resulting from closely spaced sub-pulses can create spurious frequency content, potentially affecting frame recovery. The maximum number of frames is currently limited by the experimental setup, though it is potentially scalable.