Two states hydrogenlike model for high-order harmonic generation and an isolated attosecond pulse generation in a He+ ion

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

“…Despite the large amount of the ground-state population, the harmonic signal for ions has been shown to be very weak because the ionization rate is very low. As pointed out by Mohebbi et al, the dipole transition is induced between the continuum and the ground state, where only the ground state is responsible for the ionization which is directly proportional to the ionization rate [31]. If we use the rare-gas atoms, the large ionization of the ground state populates the continuum while depleting the ground state.…”

“…As a result, the conversion efficiency is decreased. This suggests that a high efficiency of the harmonic signal could be obtained if both the ground and continuum states have the large populations [31]. We are interested in producing high-energy-harmonic photons with high conversion efficiency.…”

“…On the other hand, it is much easier to promote electrons into the continuum from the excited states. Thus, a suitable method of enhancing the harmonic efficiency is to employ an excited state in the preexcited scheme [31]. Figure 1(b) shows the population probabilities of the ground and excited states as a function of time when the initial state is a coherent superposition of the ground and excited states with equal population of 0.5.…”

“…For example, it has been suggested that the conversion efficiency of HHG can be meaningfully enhanced by proper application of an additional harmonics to populate the excited states [24], This harmonic pulse plays a fundamental role in the transition to excited states or to a nearby virtual state. Avetissian et al pro posed the efficient generation of moderately high harmonics by bound-bound transitions [25][26][27], Another fascinating way to produce a plateau with high energy and high conversion efficiency is to prepare a coherent superposition state of two bound states [28][29][30][31], In this way, part of the electron wave packet in an initial state populates an excited bound state. Many ways were suggested for a population transfer from an initial state to a bound state.…”

Electron path control of high-order harmonic generation by a spatially inhomogeneous field

“…The production of isolated attosecond pulses can be experimentally achieved either with the technique of HHG in gas from a few-cycle driving pulse [10,11] or with the technique of temporal confinement of the HHG by polarization gating [12]. Moreover, theoretically, the two-colour control mechanisms [13][14][15][16], the coherent superposition state of the target technique [17][18][19] and the frequency-chirping method [20][21][22][23][24][25][26][27][28], have been introduced. To have easy way to create and control a focused wavepacket of the laser field, we employ the frequencychirping of driving laser pulse method this in article.…”