  We refine the method towards extraction of sub-cycle transients in the 0.1-1
THz frequency (mm-wavelength) range from optical rectification in lithium
niobate using tilted pulse fronts. Our scheme exploits previously unexplored
spatio-temporal shaping of the pump pulses, resulting in highly efficient and
near diffraction-limited sub-THz transients reaching 0.2 GV/m electric field
strengths in free-space using only mJ-level optical ps-pulses. We address
experimentally and theoretically the means to producing above-GV/m relativistic
sub-THz transients with proper beam confinement and under moderate pumping
conditions, thereby bringing widespread access to strong-field and nonlinear
terahertz applications and devices.
