  We demonstrate synthesis of super-octave-spanning and phase-locked transients
in the multi-terahertz frequency range with amplitudes exceeding 13 MV/cm.
Sub-cycle polar asymmetry of the electric field is adjusted by changing the
relative phase between superposed fundamental and second harmonic components.
The resultant broken symmetry of the field profile is directly resolved via
ultrabroadband electro-optic sampling. Access to such waveforms provides a
direct route for control of low-energy degrees of freedom in condensed matter
as well as non-perturbative light-matter interactions.
