Harmonic Generation by an Intense Picosecond Laser in an Underdense Plasma

Modest power lasers interacting with a neutral gas have been observed to produce coherent harmonic radiation (up to the 61 st harmonic) at odd multiples of the fundamental laser frequency[1]. This is a result of the laser field causing the bound electrons to oscillate in the anharmonic atomic potential. The process is limited to relatively low powers since increasing the fundamental laser power leads to ionization of the gas and to the production of unbound electrons. A very high-power laser interacting with a fully ionized plasma, however, may lead to the generation of large levels of coherent radiation at high harmonics (including even harmonics) of the incident laser frequency based on an entirely new mechanism. If the laser pulse is sufficiently intense, the plasma electron mass becomes modulated because of nonlinear relativistic effects [2]. We describe in this paper the theory and some experiments in which we observe signatures of this mechanism.