Short-pulse terahertz radiation from high-intensity-laser-produced plasmas

The interaction between high-intensity, ultrashort laser pulses and plasmas leads to the emission of coherent, short-pulse radiation at terahertz frequencies. In this work we discuss a model for this effect and its experimental realization. Our measurements constitute the direct observation of laser-induced wake fields. From gas-density targets, resonant enhancement of the terahertz emission is observed if the plasma frequency is close to the inverse pulse length of the exciting laser pulse. At higher plasma densities, the emission of subpicosecond, unipolar electromagnetic pulses is observed. With the use of solid density targets, emission of more than 0.5 μJ of far-infrared-radiation energy was measured. Simultaneous emission of MeV x rays and 0.6-MeV electrons was observed and correlated with the terahertz emission. This indicates that the radiative processes in such plasmas are driven by ponderomotively induced space-charge fields in excess of ${10}^8$ V/cm.