The Extension Of Platform In High-Order Harmonics And Generation Of High Efficient Attosecond Pulse From An Initial Atomic Superposition States Irradiated By Chirped Pulses

The interaction of intense laser pulses with atoms, molecules, cluster, and solid can lead to high-order harmonics generation (HHG), as a consequence of highly nonlinear dynamics. Currently HHG is one of the most potential way to generate radiations in x-ray and XUV (Extreme Ultraviolet) regions. The unremitting pursuit in HHG studies is to simultaneously widen and heighten the HHG plateau on a large scale. The cutoff frequency is predicted byω_cutoff = I_p + 3.2U_p, (where I_p is the ionization potential and U_pis the pondermotive energy in pulse, it equals to e² F₀²/(4mω²),F₀ is the amplitude of the pulse). From the cutoff law, we notice that for a single atom model, taking the ion with larger ionization potential as target is one of the most direct way in extending the harmonic plateau. Unfortunately, the preparation of high-density ions with large ionization is very difficult. Another direct way is to increase ponderomotive energy. To realize the aim, it is necessary to raise the intensity of the laser pulse when laser wavelength is kept F0 constant. But an atom will be depleted completely when the laser intensity rises up to a certain threshold amount, so that the corresponding harmonic emission process terminates.In this paper, chirped pulse is used for interacting with an atom initially prepared in a superposition of the 1st-excited and ground state with equal weight. It is found that the HHG plateau is extended on a large scale by varying the chirp. The idea of preparing the initial state as a superposition state is that the HHG is a stimulated procession; the conversion efficiency is ideal as long as the populations of continuum and ground state are both considerable. We use this initial state not only to guarantee the high rate of ionization in a certain time but to maintain the considerable population of ground state. We change the instant frequency of incident laser by chirping, thus influence the dynamic behavior of electron in the field. The electron will irradiate more kinetic energy gained from the chirpy field when it is combined with the parent ion in the form of high energy photons. We achieve different power spectrums with different chirp parameters and find that the cutoff is greatly extended to 226ω₀ whenβ=10 compared with that 59ω₀ when chirp is free.One of the purposes of this paper is to rationally explain the reason that the HHG plateau is extended on a large scale with the increase of the chirp parameter. The numerical calculation of HHG contains two parts at least: firstly, we need to calculate the single atom respondence under the field in terms of seeking the average value of dipole moment or induced dipole moment, and then getting the spectra via Fourier Transforms; secondly, the propagation effect in macroscopical medium. We only pay attention to the first one in the work. A simple semiclassical model which is proposed by Corkum et al. is used. As the atom is perturbed, it ionizes with zero initial velocity in the laser field. The means of ionization depend sensitively on the intensity of the field, it can be multiphoton ionization, tunnel ionization or over the barrier ionization. The ionized electron trajectories depend on the phase of the field at the time of their birth;Some ones will never come back and others will return to combine with the nucleus. HHG occurs when some electrons recombine with the parent ion by emitting high energy photons.The above law is used by analyzing the reason for the widen HHG. We can get the time of recombination and the recombined energy which corresponds to each time of ionization by Newtonian equation. It is found that the bigger chirp parameter of the laser pulse, the larger kinetic energy will be gained by the electron from the field. The physical reason is that the electron is ionized by a laser field whose instant frequency is higher, and then drawn by a field whose instant frequency is lower and lower. The ionized electron will gain much more pondermotive energy in such a chirpy field. The cutoff will extend to much shorter length region. The effect will be intensified as the growth of chirp parameter. So the HHG plateau is widen largely as the chirp parameter increases.In attosecond science, it is favorable for high efficient attosecond pulse with short FWHM (full width with half magnitude) as much as possible. At present, the dominant scheme of attosecond pulse's generation is called"two-color field"which makes good winning in shorten pulse's FWHM. But the scheme intensely weakens the efficiency simultaneously which is a fatal objection. Our focus is on the generation of high efficient attosecond laser pulse. We enhance the platform of harmonics by superposition states, and thus realize the concentrative launch and recombination of the electron. We enhance the attosecond's efficiency by a facor of as well as shorten the pulse width greatly by right of chirped pulse on the basis of such an initial state. Our scheme works out the predicament of low efficiency in principle at least and opens a new means for a high efficient attosecond pulse's generation. 1 .5×10²The two-color field scheme makes use of harmonics that are low efficiency but, as luck would have it, burst synchronously in its raw state. The scheme devotes supercontinnum spectrum's extension to shorten FWHM but weaken efficiency intensively. Although one can superimpose many more harmonics by extended supercontinnum spectrum, it is trivial for the enhancement of the emit efficiency for those harmonics'efficiency is much lower than the case of single field.On the basis of superposition states, our scheme makes use of harmonics which are high efficient but burst in different time in its raw state to make a single-cycle attosecond pulse. The problem of"burst in different time"is the puzzle bewildered us. We find that the distribution of high efficient region in time-frequency diagram depends on the chirp parameter. When the parameter is appropriate, that is, the chirped field is"good", some or other"high efficient region"is isolated in time-frequency diagram (the emission channel of long trajectory is restrained at the time). A single and high efficient attosecond pulse is generated by intercepting the"high efficient region".