Streaking of photoelectrons with optical lasers has been widely used for temporal characterization of attosecond extreme ultraviolet pulses. Recently, this technique has been adapted to characterize femtosecond x-ray pulses in free-electron lasers with the streaking imprinted by farinfrared and Terahertz (THz) pulses. Here, we report successful implementation of THz streaking for time-stamping of an ultrashort relativistic electron beam of which the energy is several orders of magnitude higher than photoelectrons. Such ability is especially important for MeV ultrafast electron diffraction (UED) applications where electron beams with a few femtosecond pulse width may be obtained with longitudinal compression while the arrival time may fluctuate at a much larger time scale. Using this laser-driven THz streaking technique, the arrival time of an ultrashort electron beam with 6 fs (rms) pulse width has been determined with 1.5 fs (rms) accuracy. Furthermore, we have proposed and demonstrated a non-invasive method for correction of the timing jitter with femtosecond accuracy through measurement of the compressed beam energy, which may allow one to advance UED towards sub-10 fs frontier far beyond the ∼100 fs (rms) jitter.

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We propose a proof-of-principle experiment to test a new scheme to produce a single-cycle radiation pulse in freeelectron lasers (FELs). Here, a few α-BBO crystals will be first used to produce an equally spaced laser pulse train. Then, the laser pulse train illuminates the cathode to produce a frequency-chirped electron bunch train in a photocathode rf gun. Finally, the frequency-chirped electron bunch train passes through a tapered undulator to produce a quasi-singlecycle THz pulse. This experiment should allow comparison and confirmation of predictive models and scaling laws, and the preliminary experimental results will also be discussed.

Significance Excitation of molecules by an ultrashort laser pulse creates rotational wave packets that lead to transient alignment of the molecules along the laser polarization direction. Here, we show that a train of ultrashort laser pulses can be used to enhance the degree of alignment to a high level such that the diffraction from precisely timed ultrashort electron beams may be used to reconstruct the structure of the isolated molecules with atomic resolution through a coherent diffraction imaging technique. Our results mark a great step toward imaging noncrystallized molecules with atomic resolution and pave the way for creation of three-dimensional “molecular movies” at the femtosecond time scale and atomic spatial scale.

Strong terahertz (THz) fields enhanced by subwavelength structures have great potential in manipulating the 6D phase space distribution of free electrons. Accurate measurement of the spatiotemporal distribution of the THz field inside the subwavelength structure is essential for understanding the physics behind such THz manipulation. Here, we demonstrate a method to directly probe the enhanced THz field inside subwavelength metallic structures in movie mode with ultrashort relativistic electrons. Information about the enhanced THz field is encoded into the deflected electron beam distribution at various time delays. Using rectangular slits as representative examples, we show that the spatiotemporal distribution of the THz field can be accurately retrieved. Furthermore, the measurement of the beam deflection demonstrates that the field can be enhanced by a factor of 20 in a rectangular slit by synthesis of a pulse train. We anticipate that this noninvasive way of probing electromagnetic fields with ultrashort electrons can find wide applications in many areas of research.