Study On Ultrafast Absorption Spectroscopy With Attosecond Ultrashort Pulses

The thesis is concerned with the generation,measurement and application of attosecond pulses.Firstly,the mechanism of high order harmonics generation is introduced by the three-step-model which is proposed by Corkum,and the relationship between the conversion efficiency of high order harmonics and the laser ellipticity is discussed.The result proves that the conversion efficiency of high order harmonics rapidly decreases with the increasing of the laser ellipticity.When the ellipticity is 0.1,the conversion efficiency of high order harmonics reduces by more than 50%.So,people propose a variety of gating technologies to obtain an isolated attosecond pulse by controlling the laser ellipticity.We discuss the polarization gating,double optical gating and generalized double optical gating in detail.The double optical gating is friendly in the laser pulse duration than polarization gating.The 10 fs laser can be used in the experiment.The pulse duration is matched with the output of the most of commercial femtosecond lasers.The double optical gating consisting of a BBO crystal and two wave plates are simpler in apparatus and easier in the location adjustments of components than the generalized double optical gating.So in our experiment,we select the double optical gating based on the laboratory conditions.Secondly,we present the attosecond streak camera technology that is used to measure the attosecond pulses with the photoelectron energy spectrum.The devices used to collecting the photoelectrons are magnetic bottle TOF apparatus that are larger in acceptance angle and higher in energy resolution than the normal ones.We introduce two algorithms of FROG CRAB in detail,which are the PCGPA and the LSGPA.The former is mostly used in the femtosecond pulses measurements.When the algorithm is applied in the attosecond pulses,there are many disadvantages,for example interpolation along the delay axis in the spectrogram and periodic boundary conditions of the gate pulse,the LSGPA isn't limited by the aforesaid conditions.The achievements of this thesis mainly include the following points:1.In the measurement of attosecond pulses,we set up the experimental devices and design the system using a feedback loop with the Lab VIEW to control the delay between the femtosecond pulses and the attosecond pulses within 20 as RMS.2.We design the experimental scheme that is used to measure the attosecond pulses by the attosecond streak camera and the simulation software of the photoelectron energy spectrum.The simulation results provide an important reference for the setting and optimization the system parameters of the attosecond pulses generation.3.When the atoms are under action of the NIR pulse and the XUV pulse,we use the density matrix method and wave function series expansion method to calculate the time evolution of the atomic polarization response in the three-level-model.The result can explain the reasons of the Autler-Townes splitting and sub-cycle modulated adsorption features in the transient absorption spectroscopy.