Single-shot ultrafast direct dark-field imaging based on spatial encoding

To meet the demand for high-fidelity observation of non-repeatable transient events, this paper proposes a single-shot ultrafast direct dark-field framing imaging technique based on spatial encoding. The system employs 25 fs laser pulses to synchronously pump multiple non-collinear optical parametric amplification channels, achieving a maximum effective frame rate of 40 Tfps, a temporal resolution of 25 fs, and a spatial resolution of 28.5 lp/mm. Using ultrafast pulse laser-induced air plasma as the observation target, the experiment verified that the system successfully captured 8 frames of images in a single exposure, with a two-dimensional space-bandwidth product exceeding 65,500 over a 10×10 mm2 field of view. Combined with dark-field imaging, the system gains the capability to image phase objects while effectively eliminating background noise from idler images. The experiments clearly captured fine structures, including the two-dimensional intensity distribution and the spatiotemporal evolution of electron density during the early stage of plasma evolution (0–1.2 ps). This technology provides an effective observation tool for studying non-repeatable dynamic processes in fields such as laser processing, plasma diagnostics, and biomedicine.