ABSTRACT
The approach in the calculation of attosecond HHG is similar to that described in previous works [7'8]. We calculated the dipole acceleration by solving numerically the time-dependent Schrõdinger equation (TDSE). The synthesized laser pulse in the calculation is described by E(t) = Ei(t) cos (cot + cpi) + E2(t-t' ) cos [co(t-t')+ cp2], where co is the frequency of the laser field and cp, the absolute phase, Ez(t) = E/exp(-t2 / T,2) denotes the envelope of the driving electric field, where E, is the peak amplitude of the laser pulse and (21n2)1/2T, is its duration, / = 1, 2. t' is the delay between two pulses. The wavelength of the laser pulse is 800nm. The dipole acceleration is then Fourier transformed to obtain the spectrum, and a time-frequency analysis (the Gabor analysis[9]) finally provides the temporal profile of the harmonics. We calculated the HHG in helium atoms and the attosecond pulses are generated at the cutoff regime of the HHG
Fig. 1 The temporal profile of attosecond x-ray pulses generated by single few-cycle laser pulse (0.15a.u./5fs, solid line) and synthesized pulse scheme (0.05a.u./5fs+0.1a.u./50fs, dashed line). The inset shows the temporal profile of the attosecond x-ray pulse by the single laser pulse of 0.05a.u./5fs in the synthesized pulse. Fig. 2 The electric field curves of the single 5fs laser pulse (solid line) and the synthesized (50fs + 5fs) pulse (dashed line) used in obtaining the results shown in Fig. 1.
