Study On The Isolated Attosecond Pulse From The Combined Pulse Laser

The realization of an attosecond(10^-18s)pulse laser,which can be used for time-precision measurement and control of the motion of electrons bound in the inner shell of matter,will lead to many major scientific and technological breakthroughs in the fields of physics,chemistry and information science.Due to its unique broadband plateau structure,the generation of high-order harmonics has become the only effective means for obtaining coherent radiation sources of table attosecond pulses.However,the low intensity of attosecond pulses obtained in current laboratory experiments has limited the widespread application of this new light source.Therefore,in this paper we conducts research on obtaining higher-intensity isolated attosecond pulses using high harmonic generation driven by combined pulse lasers.The combined pulse laser is composed of two or even three femtosecond pulse lasers with different wavelengths.Its purpose is to obtain high-intensity mid-infrared femtosecond pulse laser waveforms with few cycles or even single cycles and stable carrier-envelope phase.This overcomes the difficulties currently faced by mid-infrared femtosecond pulse laser technology in realizing this light source.The specific research contents are as follows:1)We numerically simulate high harmonic and isolated attosecond pulse generation from He atom irradiated by a two-color field,which is composed of mid-infrared femtosecond pulse lasers（12 fs/1800 nm and 8 fs/1200 nm）.It is found that when the relative phase between two pulse is set as 1.60π,a mid-infrared femtosecond laser source with high-intensity,nearly single-cycle pulse width can be obtained and further the supercontinuum high-harmonic emission spectrum with a bandwidth of 254 e V can be obtained.Compared to the harmonic emission from a single 1800 nm driving pulse,the two-color field scheme can significantly extend the cutoff position of the harmonic spectrum and generate a supercontinuum harmonic plateau structure over a wider energy range.More importantly,the radiation efficiency of the harmonic plateau region is increased by two orders of magnitude compared to the case of a single 1800 nm driving pulse.We give a detailed analysis of the characteristics of the high-harmonic spectrum in the two-color field scheme based on the electric field waveform,atomic ionization,and time-frequency analysis.Finally,by Fourier transform the supercontinuum harmonics,an isolated attosecond pulse with a duration of 157 as is obtained.2)Based on the above research,the intensity of the driving pulse laser and the emission spectrum of high harmonics are further improved by adding a suitable titanium-sapphire pulse laser,thereby optimizing the generation of isolated attosecond pulses.It is found that the addition of an 8 fs/800 nm gating pulse laser significantly enhances the strength of the electric field.This electric field is very similar to that produced by a single-cycle mid-infrared femtosecond pulse laser with a carrier-envelope phase of0.25and a wavelength of 1170 nm.Its waveform is more favorable for obtaining optimized high-order harmonic generation spectra and generating intense,isolated attosecond pulses.Through numerical simulation that the cutoff frequency of the harmonic spectrum is reduced from 635 to 515 order after the addition of a titanium sapphire pulse.However,the range of the supercontinuum spectrum with a regular plateau structure increases significantly,i.e.,the harmonics from 70 to the cutoff frequency exhibit a wideband supercontinuum spectrum structure.More importantly,the radiation efficiency of the entire plateau region increases by two orders of magnitude,ultimately resulting in the generation of isolated attosecond pulses with a high intensity of 66 as.The intensity is approximately six orders of magnitude higher than that of the two-color field scheme and two orders of magnitude higher than that of the single-cycle 1170 nm pulse.In addition,this scheme is not very sensitive to the relative phase selection between the three pulses,as the numerical simulation results are only slightly affected within a variation range of 0.3π.