Conical terahertz radiation from femtosecond laser created filaments

The mechanism of sub-THz emission with conical structure by laser and plasma interaction since the discovery of Yugami {\it et. al.} in 2006 has left unresolved.\footnote{N. Yugami {\it et.al}, Jpn. J. Appl. Phys. {\bf 45} L1051 (2006).} One tried to explain by the wakefield oscillation by laser pulse, however, it is impossible to generate sufficeint electric field for sub THz radiation due to high nutral gas pressure and low intense laser pulse. Furthermore, the radiation frequency is not identical to and much lower than the plasma frequency which is estimated by the initial gas density and laser intensity. In this presentation, we will present recent experimental data and explain the radiation generation mechanism by 2DPIC code by taking plasma density gradient and the electron current behind the laser propagatin into account.\footnote{H.-C.Wu {\it et.al}, Phys. Rev. E, {\bf 83}, 036407 (2011).} The calculation shows the existence of the radiation with lower than 1 THz, which is much lower than the local plasma freqency and distribution structure around focal point of the laser.