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The quiver motion of an electron in a high-intensity laser field becomes relativistic (β= v/c ~ 1) at high-field strengths. This relativistic motion can lead to the emission of second and higher harmonics of the laser field1 and to forward ponderomotive acceleration of the electrons emerging from the laser focus. We will present the first observations of forward ponderomotive acceleration of the electrons born by field ionization at the center of the laser focus.

Production of laser harmonics occurs when a high-intensity laser interacts with a non-linear medium. At sufficiently high intensities any medium will show non-linear effects. We have observed high-order odd harmonic generation using a l-µm, 1-ps laser pulse at intensities between 1013 and 1016 W/cm2, and we have measured the spatial profiles of the harmonics. The existence of high order odd harmonics has previously been demonstrated1 in noble gases using intensities between 1013 and 1014 W/cm2.