Movie-mode visualization of terahertz fields inside subwavelength metallic structures using ultrashort relativistic electrons

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. (C) 2022 Author(s). All