Research On The Generation Of Circularly Polarized Attosecond Pulses From Plasmas Driven By Intense Lasers

The rapid development of ultrafast laser technology has greatly improved the resolv-ing ability of the pump-probe techniques.Nowadays,the ultrafast phenomena in atomics and molecular have been diagnosed by femtosecond laser pulses.But in order to capture the electron motion in atomics the pulse with attosecond duration is required.To get such pulses,radiation with a broad spectrum and frequencies lying in the extreme ultraviolet（XUV）or X-ray region is desirable.The high-order harmonic generated（HHG）from the nonlinear interaction between an intense laser and matter has been verified to be an effi-cient way to produce such unique light sources.Two main methods have been widely used including gas HHG and surface HHG from plasmas.Compared with the gas medium,the plasma is naturally ionized which can bear the irradiation of ultraintense laser,making it possible to generate an attosecond pulse of relativistic intensity or even extreme intensity.However,the circularly polarized attosecond pulses generated from such process is inef-ficient and with low ellipticity,seriously limiting their applications in material chirality and magnetic effect in attosecond time scales.In order to solve the above problems,two physical schemes are proposed which are based on a two-color co-rotating circularly polarized lasers.First,we propose a method to improve the radiation efficiency of high-order har-monics generated from an overdense plasma target.By mixing two circularly polarized lasers rotating in the same direction with different frequencies（ω₀,2ω₀）,the laser pon-deromotive force is modulated to oscillate at the fundamental frequency,so that the HHG radiation is excited successfully.In addition,the peak value of each cycle is greater than that of the monochromatic light under the same condition,which facilitates the produc-tion of HHG.Particle-in-cell simulations prove that,the harmonic radiation efficiency in the two-color laser case can be improved by orders of magnitude when the energy ratio of the second harmonic is around 0.7～0.8.Last but not least,purely circular-polarized harmonics can be generated at normal incidence and isolated attosecond pulses can be obtained through reasonable filtering.Second,a new scheme of generating a single attosecond pulse with high intensity and high ellipticity from a near-critical-density plasma is proposed.In this scheme,the inci-dent laser consists of two circularly polarized lasers with similar frequency but different pulse durations.By adjusting the time delay between the two lasers,a driving laser with higher intensity and narrower pulse width is produced.Driven by such a laser,the target surface is steadily pushed inward during the rising edge of the laser,and the charge sep-aration field gradually increases.As the laser ponderomotive force decreases,the target surface moves backward at nearly the light speed under the dragging of the strong elec-trostatic field force.Therefore,the laser is violently compressed,and a single attosecond pulse with high intensity and high ellipticity can be obtained after properly filtering.