  Ultrahigh-power terahertz (THz) radiation sources are essential for many
applications, such as nonlinear THz physics, THz-wave based compact
accelerators, etc. However, until now none of THz sources reported, whether
based upon large-scale accelerators or high power lasers, have produced THz
pulses with energies above the millijoule (mJ) barrier. Here we report on the
efficient generation of low-frequency (<3 THz) THz pulses with unprecedentedly
high energies over 50 mJ. The THz radiation is produced by coherent transition
radiation of a picosecond laser-accelerated ultra-bright bunch of relativistic
electrons from a solid target. Such high energy THz pulses can not only trigger
various nonlinear dynamics in matter, but also open up a new research field of
relativistic THz optics.
