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Journal of Atomic and Molecular Physics

The enhancement of the attosecond pulse intensity by an intense short laser pulse irradiating an atom with coherent superposition state

Chen Ji

Published Jan 1, 2008• 0 citations
OpenAlex

Abstract

The scheme of the attosecond(asec) pulse intensity enhancement by an intense short laser pulse(duration of the pulse is several cycles) irradiating an atom with coherent superposition state is proposed.Our numerical results show that,this scheme also could generate an isolated asec pulse,furthermore the intensity of which is prominently enhanced,compared with the asec pulse generation of an intense short laser pulse irradiating an atom with ground state.

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The scheme of the attosecond(asec) pulse intensity enhancement by an intense short laser pulse(duration of the pulse is several cycles) irradiating an atom with coherent superposition state is proposed.Our numerical results show that,this scheme also could generate an isolated asec pulse,furthermore the intensity of which is prominently enhanced,compared with the asec pulse generation of an intense short laser pulse irradiating an atom with ground state.
Using a linearly polarized, phase-stabilized 3-fs driving pulse of 800 nm central wavelength shape-optimized on its ascending edge by its an amplitude-reduced pulse irradiating on a superposition state of the helium atom, we demonstrate theoretically the generation of a super strong isolated 176-attosecond pulse in the spectral region of 93–124 eV. The unusually high intensity of this attosecond pulse is marked by the Rabi-like oscillations emerging in the time-dependent populations of the ground state and the continuum during the occurrence of the electron recombination, which is for the first time observed in this work.
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.
High-order harmonic spectra and attosecond pulse generation from Rydberg atom (He[Formula: see text]) driven by the spatially inhomogeneous field have been theoretically investigated. (i) Firstly, with an electron initially in a single excited Rydberg state (nth), the harmonic yield can be enhanced due to the decreased ionization potential, and a maximum enhancement can be obtained when the initial state is prepared as the third excited state (n = 3). However, the low cutoff energy from the excited state is unbeneficial to the generation of the higher photon pulse. Thus, with the further introduction of the laser chirp, not only the harmonic cutoff is extended, but also the harmonic modulation is reduced. As a result, five super-bandwidths from 63 eV to 267 eV can be found. (ii) Secondly, by preparing the initial state as a coherent superposition of excited state, the harmonic yield can be further enhanced, especially for the coherent superposition of the first and the third (n = 1 + 3) and the second and the fourth (n = 2 + 4) excited states, the harmonic yield is enhanced by 4–8 orders of magnitude compared with the case of the single ground initial state. Furthermore, by properly adding the laser pulse into the spatially inhomogeneous region (gap center [Formula: see text] a.u.) from left [Formula: see text] to right [Formula: see text], much higher cutoff energies can be obtained in the left region. As a consequence, two super-bandwidths of 248 eV and 496 eV can be obtained. Finally, by properly superposing the harmonics, a series of sub-25-as pulses with intensity enhancement of 5–8 orders of magnitude can be produced.
An optical coherent control scheme has been proposed and theoretically investigated where an extreme ultraviolet single attosecond pulse (SAP) propagates through a dense helium gas dressed by a time-delayed femtosecond laser pulse. The laser pulse couples the 2s2p([superscript]1P) and 2s[superscript]2([superscript]1S) autoionizing states when the SAP excites the 2s2p state. After going through the gas, the spectral and temporal profiles of the SAP are strongly distorted. A narrowed but enhanced spike in the spectrum shows up for specific intensities and time delays of the laser, which exemplifies the control of a broadband photon wave packet by an ultrashort dressing field for the first time. We analyze the photon and electron dynamics and determine the dressing condition that maximizes this enhancement. The result demonstrates new possibilities of attosecond optical control.
The interaction of a short, few-cycle light pulse and an atom which is prepared initially in a superposition of two stationary states is shown to exhibit strong signatures of atomic coherence. For a given waveform of the laser pulse, appropriate quantum mechanical relative phase between the constituents of the initial superposition can increase the ionization probability by a factor of three. A similarly strong effect can be observed when the waveform of the ionizing pulse is changed. These results allow for intuitive explanations, which are in agreement with the numerical integration of the time dependent Schr ¨odinger equation.