Second harmonic generation from relativistically quivering electrons

Summary form only given. Second harmonic generation without stimulated Raman process at a/sub 0//spl sim/1 is studied. Here, a/sub 0/ is the normalized vector potential field, which is given by a/sub 0/=0.86/spl times/10/sup -9//spl lambda/[mm](I/sub 0/[W/cm/sup 2/]) 1/2 , /spl lambda/ is the laser wavelength and I/sub 0/ is the laser intensity. In order to conform the long oscillation period of electron plasma wave (EPW), we measured the temporal oscillation of EPW using a frequency domain interferometry technique. The experimentally obtained temporal oscillation of EPW is shown. In this measurement, the He gas pressure of 0.8 Torr was used, then the electron plasma density n/sub e/ is estimated to be 5.3/spl times/10/sup 16/ cm/sup -3/. The oscillation period Tp/sub e/ of EPW are evaluated to be 480 fs. The peak intensity of pump pulse at the focal plane was estimated to be 7/spl times/10/sup 17/ W/cm/sup 2/, then a/sub 0/ was 0.5. For the pulse width of the pump pulse was measured to be 110 fs, the oscillation period of EPW is long enough even in 0.8 Torr. In the measurement of the second harmonic forward emission, the emission signal from plasma was imaged on the CCD camera. The emission area of the second harmonics was measured to be /spl sim/10 /spl mu/m, which was almost the same size as that of the focal spot of the pump pulse. The polarization of the second harmonic was also the same as that of pump pulse. Although the size of the plasma was measured to be 100 /spl mu/m. We think that those second harmonics were generated by the spatially asymmetric nonlinear motion of electrons, which are expelled from the focal spot by ponderomotive force.