Theoretical modelling of Terahertz acoustic wave generated by a femtosecond laser pulse in a dense plasma having density gradient
Near the critical layer, a p-polarized laser that impinges on a dense plasma having density gradient at an angle to the density gradient transforms into a plasma wave. Even after the laser pulse has stopped, the plasma wave survives. The electrons close to the crucial layer are heated by the plasma wave. Increased plasma pressure and the production of ion-acoustic waves at frequencies lower than those of the ion plasma are the results of a sudden increase in plasma electron temperature. It appears to be a mechanism for ion-acoustic waves observed in an experiment (Adak et al. Phys. Rev. Lett. 114 (2015) 115001), where alternatively the plasma wave can excite ion acoustic waves in Terahertz frequency range via the decay instability. In the present work, we have theoretically demonstrated these observations.