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Physical Review A

Effect of time-dependent ionization on the propagation of a few-cycle laser pulse in a two-level medium

Xiaohong Song, Shangqing Gong, Ruxin Li, Zhizhan Xu

Published Oct 26, 2005• 9 citations
Publisher

Abstract

We investigate the influence of ionization on the propagation and spectral effects of a few-cycle ultrashort laser pulse in a two-level medium. It is found that when the fractional ionization is weak, the production of higher spectral components makes no difference. However, when the two states are essentially depleted before the peak of the laser pulse, the impact of ionization on the higher spectral components is very significant.

DOI
10.1103/physreva.72.043820

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Publisher's Note: Effect of time-dependent ionization on the propagation of a few-cycle laser pulse in a two-level medium [Phys. Rev. A72, 043820 (2005)]

Xiaohong Song, Shangqing Gong, Ruxin Li +1 more

Received 2 November 2005DOI:https://doi.org/10.1103/PhysRevA.72.059904©2005 American Physical Society

Physical Review A•Nov 11, 2005•0 citations
We investigate the influence of ionization on the propagation and spectral effects of a few-cycle ultrashort laser pulse in a two-level medium. It is found that when the fractional ionization is weak, the production of higher spectral components makes no difference. However, when the two states are essentially depleted before the peak of the laser pulse, the impact of ionization on the higher spectral components is very significant.
Influence of multiphoton ionization on the propagation and spectrum of few-cycle circularly (elliptically) polarized laser pulses in an open two-level medium (two-level plus continuum model) is investigated based on the conventional two-level model proposed by Slavcheva and Hess (Phys. Rev. A 72 (2005) 053804), and the propagation dynamics of an arbitrary elliptically polarized laser pulse is reduced into that of right and left circularly polarized laser pulses. When the laser intensity is high enough to cause ionization, there are significant impacts of ionization on the pulse reshaping and on the higher order spectral components, and the impacts for the open two-level model are different from those for the closed two-level model.
Using a $\ensuremath{\omega}\ensuremath{-}2\ensuremath{\omega}$ combination scenario, we investigate the spectra of two-color ultrashort laser pulses in a two-level medium. It is found that higher spectral components can be produced even for small area pulses due to the interference effect of the two laser pulses. Moreover, these higher spectral components depend crucially on the relative phase $\ensuremath{\varphi}$ of the two laser pulses: continuum and distinct peaks can be achieved through the control of the relative phase.
By solving the full Maxwell-Bloch equations,the effects of the carrier-envelope phase(CEP)on pulse propagating and spectra characteristics of few-cycle ultra short laser pulses in a dense V type three-level atomic medium are investigated.The results show that the carrier propagation and spectrum evolution of the pulse are sensitive to its CEP and the phase sensitivity is more obvious for larger area pulse.The CEP can affect the amplitude,propagating velocity and pulse splitting case.Moreover,under the same condition,the pulse spectra are affected by the initial carrier-envelope phases due to the different coherence effect,which don't induce obvious changes of higher spectral components.When,the super continuum with wider spectral range and better continuity of high frequency components can be obtained.
The influence of ultrashort low-frequency laser pulses of different intensities and pulse lengths on the dynamics of a resonantly driven atomic system is investigated theoretically. We demonstrate the periodical dependence of the total ionization rate on the appearance time of a strong few-cycle laser pulse. By depleting the excited state, the intense short pulse not only resets the Rabi oscillations, but also introduces a time shift that is proportional to the pulse length. This time shift can be employed to determine the ultrashort pulse length. In the case of ultrashort pulses with lower intensities, we observe side-bands in the two-photon ionization spectrum, whose intensities vary periodically with the appearance time of the pulse. This can be utilized to measure the period of Rabi oscillations. We have developed an S-matrix theory of two-colour ionization which can explain some of the observations of our numerical calculations. Furthermore, we have studied the high harmonic generation spectra as well as the attosecond pulses generated from the high harmonics. We show that the Rabi oscillation can be used to alter the ionization rate within laser sub-cycles and therefore select different quantum trajectories in the high harmonics generation.