  We propose a method to generate isolated relativistic terahertz (THz) pulses
using a high-power laser irradiating a mirco-plasma-waveguide (MPW). When the
laser pulse enters the MPW, high-charge electron bunches are produced and
accelerated to ~ 100 MeV by the transverse magnetic modes. A substantial part
of the electron energy is transferred to THz emission through coherent
diffraction radiation as the electron bunches exit the MPW. We demonstrate this
process with three-dimensional particle-in-cell simulations. The frequency of
the radiation is determined by the incident laser duration, and the radiated
energy is found to be strongly correlated to the charge of the electron
bunches, which can be controlled by the laser intensity and micro-engineering
of the MPW target. Our simulations indicate that 100-mJ level
relativistic-intense THz pulses with tunable frequency can be generated at
existing laser facilities, and the overall efficiency reaches 1%.
