Research On Generation And Phase Matching Of High-order Harmonic Radiation Source Driven By Strong Laser

With the rapid development of ultra-short and ultra-intense laser technology,the focusing intensity of laser pulse has been greatly improved and reached to 10²²W/cm².Such a high-intense laser pulse has far exceeded the field intensity inside the atom.Even for the current commercial high repetition rate laser,the output pulse focusing intensity can easily exceed 10¹⁴W/cm².When such a laser pulse interacts with matter,it will produce many new nonlinear physical phenomena,such as tunneling ionization,backscattering,non-sequential double ionization,and high-order harmonic generation.As an important strong field nonlinear phenomenon,high-order harmonic is expected to realize sub-femtosecond ultrafast dynamic diagnosis of transient process because of its wide radiation frequency band,short time-domain pulse duration,and tunable wavelength.Therefore,it has very important research significance and application value.However,at present,the conversion efficiency of high-order harmonics is mostly low(10^-6?10^-7),which limits its practical application.The main reason for the low efficiency of high-order harmonic conversion is the phase mismatching during its propagation process.Therefore,this thesis mainly investigated the phase matching technology of the high-order harmonics to improve the output intensity and increase the conversion efficiency.The main contents and innovations of this thesis are as follows:(1)The phase mismatching of high-order harmonics in the propagation were investigated.The effects of different laser parameters and gas back-pressure parameters on neutral atomic dispersion phase mismatching,plasma dispersion phase mismatching,geometric focusing phase mismatching and atomic dipole moment phase mismatching were studied.The applicable conditions for complete phase-matching technology and quasi-phase-matching technology were obtained.(2)The complete phase matching of low-order harmonics with low laser intensity was realized by optimizing the gas back pressure,which greatly enhanced the output intensity.At a specific laser intensity(5×10¹⁴W/cm²)and beam waist radius(35?m),the axial total phase mismatching of 56^th harmonic(87 e V,14.2 nm)was decreased to zero by optimizing the gas back pressure of He.By numerical simulation,the growth of the harmonic intensity in the case of complete phase matching was obtained.It was found that the harmonic intensity increased 110 times compared with the case without gas pressure optimization.(3)At higher light intensity,the quasi-phase matching of higher order harmonics was realized by periodic gas-density modulation,which greatly enhanced the intensity of harmonics.Under the laser intensity condition of 8.5×10¹⁴W/cm²,the quasi-phase matching of 120^th(186.11 e V,6.67 nm)harmonic was realized through the periodic modulation of pressure in He(the critical light intensity is 7.5×10¹⁴ W/cm²).The simulation showed that the modulated waveform is square,the correction factor of the best modulation mode is 0.97(modulation period length of 262?m),and the modulation depth is 1(complete modulation),the growth intensity of 120^th harmonic was the largest,which was 20.94 times higher than that without modulation.The harmonic intensity of the whole modulated harmonic spectrum increased in varying degrees from 79^th to 120^th,but the harmonic intensity below 79^th decreased in varying degrees.