Generation of strong short coherent terahertz pulses in Ladder-Lambda   and Double-Lambda systems
  We show that coherently driven atomic or molecular media potentially yield strong controllable short pulses of THz radiation. The method is based on excitation of maximal quantum coherence in a gas medium by optical pulses and coherent scattering of infra-red radiation to produce pulses of THz radiation. The pulses have the energies range from several nJ to $\mu$J and time durations from several fs to ns at room temperature. 
Nikolai G. Kalugin, Yuri V. Rostovtsev, and Marlan O. Scully