Abstract
Electromagnetic emission at the plasma frequency ωp and its harmonics nωp has been studied. Although such emission is well known from very low density plasmas (electron density of the order of ne ≈ 108 cm−3) now, for the first time it has also been observed from a high density plasma (electron density of the order of ne ≈ 1023 cm−3) produced by a high-intensity femtosecond 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 ωp and 2ωp 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 ωp and 2ωp line emission in the present experiment (PACS numbers: 52.40.Nk, 42.65.ky, 52.65.-y, 33.20.Ni, 52.25.Qt).
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