Selection Of Electron Quantum Trajectories In The Process Of Macro High Harmonic Emission In Atomic Medium

The high-order harmonics generated by the interaction of ultra-short and ultra-strong laser pulses with atoms can provide a new desktop light source from extreme ultraviolet to soft X-ray.However,its weak intensity hinders its wide application in scientific research and engineering technology,and further improving the harmonic efficiency is the focus of current research.In this thesis,the single-atom induced dipole is obtained by the strong field approximation theory and the quantitative re-scattering theory,and then the macroscopic high-order harmonic spectrum can be obtained by numerically solving the Maxwell equation for the propagation of the drive laser field and the harmonic field in the medium.According to the phase matching conditions,the phase mismatch of the harmonics in the propagation process is improved by changing the gas pressure,the laser intensity,and the position of the gas target center relative to the laser focus,thereby enhancing the macroscopic emission intensity of the harmonics.The main research contents of this paper are as follows:(1)Under different gas pressure conditions,we studied the phase matching mechanism of the macroscopic high-order harmonic spectrum generated by Ne atoms irradiated by the linearly polarized infrared laser field with an intensity of 5.5×10¹⁴W/cm²,a wavelength of 1200 nm,a beam waist of 25?m,and a pulse duration of 8 fs.The length of the gas target medium is 1.00 mm,and the center of the gas target is placed at the focal point of the laser.By changing the gas pressure,it is found that the long-trajectory electron emissions are dominant at lower pressure and the short-trajectory emissions are dominant at higher and optimal pressure.Different phase matching conditions can be formed by controlling the gas pressure,and then different quantum trajectories are optimized to control harmonic emission.(2)We studied the phase matching mechanism of the macroscopic high-order harmonic spectrum generated by Ne atoms irradiated by the linearly polarized near-infrared laser field with a wavelength of 800 nm,a beam waist of 25?m,and a pulse duration of 4 fs at different driving laser intensities.The length of the gas target medium is 1.00 mm,and the center of the gas target is placed 1.00 mm in front of the laser focus.By changing the laser intensity,the phase matching conditions can be adjusted.We found that the long quantum trajectory is dominant for a lower laser intensity.At higher laser intensity,the long quantum trajectory plays an important role for lower pressure,while the short quantum trajectory is dominant at higher and optimal pressure.By adjusting the laser intensity and gas pressure,it is possible to achieve a phase matching condition that contains only short or long trajectory electrons contributed to the harmonic emission.(3)We studied the chirp behavior of harmonic spectrum near the cut-off region of Ne atoms irradiated by the linearly polarized infrared laser field with an intensity of8×10¹⁴ W/cm²,a wavelength of 1200 nm,a beam waist of 25?m,and a pulse duration of6 fs at different lengths of the gas target medium.By changing the gas pressure from 400Torr to 800 Torr,it is found that the chirp of the macroscopic high-order harmonics near the cut-off region is very small when the gas pressure is 600 Torr.The medium length of the gas target is further changed to optimize the macroscopic high-order harmonic,and the spectrum range of the small chirp emission is expanded.It is found that when the medium length of the gas target is 0.60 mm,the obtained small chirp emission has the largest spectrum range.(4)We studied the influence of the gas target density distribution and the position of the target center relative to the laser focus on the attosecond pulse.The parameters of the linearly polarized laser irradiating Ne atoms are:intensity 8×10¹⁴ W/cm²,wavelength1200 nm,beam waist 25?m,and pulse duration 6 fs.The pressure distribution inside the gas target(the length of the medium is 0.30 mm,and the end surface of the target is at the laser focus)is changed from 1000 to 300 Torr,and it is found that the cut-off position of the high-order harmonic plateau is more clearly than that under the condition of a single gas pressure.The isolated attosecond pulse of 186 as is obtained by superimposing the harmonics near the cut-off region.According to the different positions of the center of the gas target(the length of the medium is 1.00 mm)relative to the laser focus,the pressure of the gas medium is optimized to find the optimal pressure.It is found that when the center of the gas target is placed 1.00 mm before the laser focus,the harmonic emission efficiency is much higher than the harmonic efficiency observed when the center of the gas target is placed at the laser focus and 1.00 mm behind the focus.At the same time,the intensity of the isolated attosecond pulse is high one or two orders of magnitude.