Efficient Terahertz Pulse Generation in Laser-Induced Gas Plasmas

With state-of-the-art amplifier laser systems one easily reaches pulse energies sufficient to ionize atoms and molecules in focused beams. If this is done in a gaseous medium at ambient pressure (e.g., in air), a plasma with a length of a few hundred μm and a diameter of up to 100 μm can be produced by a 1 kHz 150 fs Ti:sapphire laser system with laser pulse energies in the 500 μJ regime. The plasma formation process is strongly influenced by the optical non-linearity of the plasma itself which leads to strong defocusing of the beam and a limitation of the maximum plasma density to a few 10% of the particle density [1]. A CCD image of a laser-induced plasma is shown in Fig. 1. Terahertz (THz) emission from laser-induced plasmas was found for the first time by Hamster et al. in 1993 [2–4]. Their experiment was based on the acceleration of the charged electrons and ions by the ponderomotive forces exerted by the intense light pulses. Recently, we demonstrated that the emission process can be enhanced and steered to the forward direction by applying a static bias field to the plasma [5]. In following reports, Cook et al. showed that even more efficient ∗corresponding author; e-mail: t.loeffler@physik.uni-frankfurt.de