Plasma frequency and harmonic emission from fs-laser plasmas

Electromagnetic emission at the plasma frequency ω_p and its harmonics nω_p has been studied. Although such emission is well known from very low density plasmas (electron density of the order of n_e ≈ 10⁸ cm^-3) now, for the first time it has also been observed from a high density plasma (electron density of the order of n_e ≈ 10²³ cm^-3) produced by a high-intensity femto-second laser pulse. This radiation is strongly connected to jets of high energy electrons produced in such steep gradient plasmas by collective mechanisms, and probably to nonlinear Langmuir waves in the overdense plasma region. Theoretical investigations using particle-in-cell simulations have been made to pinpoint the physical origin and properties of the emission. Preliminary results show a very weak scaling of the ω_p and 2ω_p emission with intensity, which should provide clues for future theoretical analyses. Although not the main subject of the present work, also harmonics up to the 20th of the laser fundamental may have been observed close to the ω_p and 2ω_p line emission in the present experiment (PACS numbers: 52.40.Nk, 42.65.ky, 52.65.-y, 33.20.Ni, 52.25.Qt).