THz Gun

Ultrashort electron beams with a narrow energy spread, high charge, and low jitter are essential for resolving phase transitions in metals, semiconductors, and molecular crystals. These accelerated beams, produced by phototriggered electron guns, are also injected into accelerators for x-ray light sources. The achievable resolution of these time-resolved electron diffraction or X-ray experiments has been hindered by surface field and timing jitter limitations in conventional RF guns, which thus far are <200  MV/m and >96  fs, respectively. A gun driven by optically generated single-cycle terahertz (THz) pulses provides a practical solution for enabling not only GV/m surface fields but also absolute timing stability, since the pulses are generated by the same laser as the phototrigger.

We have demonstrated an all-optical THz gun yielding a peak electron energy approaching 1 keV, accelerated by >300  MV/m THz fields in a micrometer-scale waveguide structure. We also achieve quasi-monoenergetic, sub-kiloelectron volt bunches with 32 fC of charge, which can already be used for time-resolved low-energy electron diffraction. Such ultracompact, easy-to-implement guns — driven by intrinsically synchronized THz pulses that are pumped by an amplified arm of the already-present photoinjector laser — provide a new tool with the potential to transform accelerator-based science.

 

THz gun concept. (a) Photograph of the THz gun; (b) a single-cycle THz pulse, generated via optical rectification in lithium niobate (LN), is coupled into the THz gun, which takes the form of a parallel-plate waveguide for field confinement. A UV-backilluminated photocathode emits an electron bunch, which is accelerated by the THz field. The bunch exits through a slit on the top plate, and a retarding field analyzer (RFA) measures its energy spectrum. (c) Cross section of the gun, showing the UV-photoemitted electrons accelerated by the THz field and exiting through the slit.

THz scaling at the delays 𝜏1 and 𝜏2 between UV and THz pulse. (a) and (b) Energy gain plotted on a spectrogram to highlight its scaling as a function of accelerating THz energy or THz field. Error bar radius is equal to the absolute RMS energy spread. (c) and (d) Relative RMS energy spread, 𝜎𝑈, of the accelerated bunch. (e) and (f) Total detected bunch charge exiting the gun, 𝑄. Error bar radius is equal to the RMS instrument noise.