Generation of Sub-Picosecond Terahertz Radiation by Laser-Produced Plasmas

The interaction between high-intensity, sub-picosecond laser pulses and plasmas at both gas and solid densities leads to the emission of coherent, short-pulse far-infrared radiation (FIR) at terahertz frequencies. In this work, such an effect was predicted, modeled and experimentally characterized for the first time. It also constitutes the first direct observation of wake fields produced by laser-plasma interaction. The terahertz emission was generated by 120 fs laser pulses with pulse energies of up to 0.5 J at intensities of up to 10^{19} W/cm ^2. The thesis discusses some performance and design aspects of the laser systems generating such pulses. A model based on the linearized hydrodynamic cold fluid equations was used to calculate the emitted FIR and is in reasonable agreement with observations at low plasma densities. From gas targets, strong resonant enhancement of the FIR emission is observed if the plasma frequency is close to the inverse pulse length of the exciting laser pulse. The emission is then found to be oscillatory in nature with a frequency close to the plasma frequency. The frequency of the radiation was found to be tunable by changing the plasma density. For plasma densities above 10 ^{18} cm^{ -3}, the emission of sub-picosecond, unipolar electromagnetic pulses was observed. From solid targets, the emission of more than 0.5 muJ of FIR was measured. Simultaneously, the 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. These are the strongest fields that have ever been reported in experiments involving laser-plasma interactions. The generation of 0.1 muJ of FIR energy was demonstrated in an experiment using nonlinear frequency down-mixing in LiNbO_3. This is the highest energy yield ever to be produced with such a method. A 200 times higher conversion efficiency was demonstrated with this scheme in the new nonlinear organic crystal DAST.