Higher Harmonics Generation: Relativistic ionization dynamics

Relativistic ionization dynamics

Frequency spectrum of the Lienard-Wiechert radiation field produced at the retarded time due to an initially bound electron, in the ground hydrogen state, moving under the influence of a traveling laser field is displayed. The direction of the observation (z) is parallel to the magnetic component and perpendicular to the electric component (x) of the laser field. The relativistic but classical equations of motion of the electron has been solved numerically. The spectrum is computed at 200 peak electric field strengths and displayed with increasing values (indicated by the figure as well as the moving color bar near top of the frame). Even though the electron is initially bound ionization would occur for all the peak laser fields chosen in this movie. Spectrum is seen to grow with laser intensity and peaks are well matched by the formula w(L) = L w_l^s with L = integer. Here w_l^s is related to the laser frequency w_l via w_l^s = (1-a) w_l with a = E0^2 / (E0^2 + 4*w_l^2*c^2). Both even and odd order harmonic peaks are present due to relativistic electron motion. Redshift in the laser frequency here may be attributed laboratory manifestation of the Doppler effect by an ExB drifted electron. Computation parameters associated with this movie are as follows. Laser field strength is varied linearly within 1 a.u. < E0 < 40 a.u., laser frequency w_l = 0.15 a.u., laser pulse is turned on and off in 2 laser cycles and maintained at its peak value for 30 cycles. The initial position of the electron is at origin with Vx0 = 0.1 a.u. bound by the screened Coulomb potential V(r) = -1 / [r^2+1]^(1/2).