  We report a plasma-based strong THz source generated by using intense
femtosecond laser pulses to irradiate solid targets at relativistic intensity
>10^18W/cm2. Energies up to 50 microJ/sr per THz pulse is observed in the
specular direction when the laser pulses are incident onto a copper foil at
67.5 degree. The source appears to be linearly polarized. The temporal,
spectral properties of the THz are measured by a single shot, electro-optic
sampling method with a chirped laser pulse. The THz radiation is attributed to
the self-organized transient fast electron currents formed along the target
surface. Such a strong THz source allows potential applications in THz
nonlinear physics.
