| It is well-known that the interaction between intense laser field and atoms or molecules can produce many nonlinear optical phenomena,such as above-threshold ionization(ATI),high-order harmonic generation(HHG),non-sequential double ionization(NSDI)and so on.Among them,HHG has been considered as the important subject,we can get intense isolated attosecond pulses from HHG.There are many ways to get an isolated attosecond pulses,for example,by using polarization gating,double optical gating(DOG),two-color laser field,three-color laser field,a combination of a chirped laser and electrostatic field,a combination of a chirped laser and a high-frequency field and so on.The molecular HHG has been studied extensively for few decades.It is a new research field that generating isolated attosecond pulses from molecular HHG.For the oriented polar molecule(such as CO molecule),the ones can get intense isolated attosecond pulses by using the ionization gate(so-called system-induced gating).By using mid-infrared chirped laser and an unipolar pulse,we theoretically investigate HHG and isolated attosecond pulses from the oriented polar molecule CO.In the second chapter,we review Lewenstein model(or strong field approximation)for studying the high-order harmonic behavior of atoms or molecules exposed to an intense laser field.The third chapter provides an effective method which gets a single attosecond pulse.We theoretically create the 6 keV(~0.2nm)soft X-ray high-order harmonics from the oriented CO molecule exposed to the combination of an intense mid-infrared chirped laser and an unipolar pulse.We show that the harmonic plateau can be broadened significantly using the chirped laser field and the cutoff of harmonic spectra is dramatically extended by adding an unipolar pulse.An ultrabroad(5.4 keV)supercontinuum spectra in the plateau is generated by controlling the effective quantum path.More importantly,supercontinuum harmonic spectra is obtained in a very wide energy range.At the same time,we found that the plateau of harmonic spectra can be further extended by increasing the mid-infrared wavelength and the shorter isolated attosecond pulses are obtained.When the wavelength is 3000 nm,by superposing some supercontinuum harmonics centered at several different central frequencies,we can directly produce the isolated attosecond(as)pulses in which the shortest one is 26 as.Combined with the classical trajectory simulation and the quantum time-frequency analysis,we demonstrate how to efficiently control the quantum paths by introducing the laser chirp and an unipolar pulse. |
