Subfemtosecond, coherent, relativistic, and ballistic electron bunches generated at ω₀ and 2ω₀ in high intensity laser-matter interaction

Harmonics of the laser light have been observed from the rear side of solid targets irradiated by a laser beam at relativistic intensities. This emission evidences the acceleration of subfemtosecond electron bunches by the laser pulse in front of the target. These bunches emit coherent transition radiation (CTR) when passing through the back surface of the target. The spectral features of the signal recorded for targets of thicknesses up to several hundred microns are consistent with the electrons being accelerated by both the laser electric field-via vacuum heating and/or resonance absorption,-and the v×B component of the Lorentz force. The spatial study of the radiation shows that the relativistic electrons causing the CTR radiation are coherent and propagate ballistically through the target, originating from a source with a size of the order of the laser focal spot.