Controlling the ionization and recombination rates of an electron in preexcited ions to generate an intense isolated sub-4-as pulse in a multicycle regime

Abstract

We present a numerical study of an isolated attosecond pulse generation in a preexcited medium driven by a moderately intense multicycle laser pulse, which is synthesized by adding a chirp-free laser pulse to a chirped laser pulse. The results show that by preparing the initial state of He+ ion as a preexcited state, the electron trajectories significantly can be modulated and the supercontinuous harmonics with higher emission efficiency can be obtained. Optimizing the chirp parameter allows a multicycle field to behave like a few-cycle one due to the control of electron tunneling. Therefore, a wide 1172-eV intense supercontinuum would be achieved on the high-order-harmonic generation spectrum. The classical and time-frequency approaches reveal that the long quantum path is suppressed and only the short quantum path is left. By properly filtering the harmonics from the supercontinuum, a 34-as intense isolated pulse with clean time profile is directly obtained without any phase compensation. Our simulation shows that by superposing the total supercontinuum region with phase compensation, we can obtain the shortest sub-4-as intense isolated pulse.

High-order harmonic generation of N 2 molecule in two-color circularly polarized laser fields

“…In the past two decades, high-order harmonic generation (HHG) has been extensively studied as a powerful tool for generating isolated attosecond pulses (IAP) [1][2][3][4][5] and a coherent soft x-rays source. [6] The physical mechanism of the HHG is well established by the three step model: [7][8][9] the electron is freed by tunneling through the barrier of the atomic potential, then the ionized electron is accelerated by the laser field and captures energy, finally it may be driven back to the parent ion, emitting high energy photons.…”