  We propose and demonstrate a Terahertz (THz) oscilloscope for recording time
information of an ultrashort electron beam. By injecting a laser-driven THz
pulse with circular polarization into a dielectric tube, the electron beam is
swept helically such that the time information is uniformly encoded into the
angular distribution that allows one to characterize both the temporal profile
and timing jitter of an electron beam. The dynamic range of the measurement in
such a configuration is significantly increased compared to deflection with a
linearly polarized THz pulse. With this THz oscilloscope, nearly 50-fold
longitudinal compression of a relativistic electron beam to about 15 fs (rms)
is directly visualized with its arrival time determined with 3 fs accuracy.
This technique bridges the gap between streaking of photoelectrons with optical
lasers and deflection of relativistic electron beams with radio-frequency
deflectors, and should have wide applications in many ultrashort electron beam
based facilities.
