Near-circularly Polarized Attosecond Pulse Generation Via Multi-pulse Fields

From the birth of the laser technology to today,scientists have been working hard to generate shorter light pulses.Attosecond laser pulse(1 attosecond is equal to10-18 second)is the pulse with the shortest duration that humans can produce.The emergence of attosecond pulses enables scientists to observe the material world with the time scale of the attosecond level,and through combining the ultra-high time resolution of the attosecond level and the ultra-high spatial resolution of the angstrom(1 angstrom equals to 0.1 nanometer)level in ultra-fast measurements,scientists are able to detect and manipulate ultra-fast processes in the subatomic microcosm.By applying attosecond pulses to "pump-probe" technology,researchers can already study the ultra-fast dynamics of electrons in atoms and molecules.The attosecond science also becomes a new scientific field that needs to be explored.It can not only help scientists study fundamental physical problems such as the movement of electrons in atoms and molecules,but also can be applied to chemistry,biology,materials science,etc.The attosecond pulses would provide completely new research methods and experimental technology in these fields.High-order harmonics are a kind of high-order nonlinear process produced by the interaction between strong femtosecond laser and matter.The harmonic photon energy in this process is as high as tens to hundreds of times the frequency of the femtosecond laser,and it has excellent temporal and spatial coherence.At the same time,due to the generation characteristics of the radiation time being limited to half of the optical cycle,high-order harmonics generated from gas is the only path to produce attosecond pulses.In the experiment,scientists have designed many attosecond pulse generation methods based on the dependence of the high-order harmonic generation process on the parameters of the driving laser.In recent years,the shortest pulse duration obtained in experiments is 43 attoseconds,which is close to an atomic time unit;the maximum photon energy of the attosecond pulses has also reached at 400 electron volts;the pulse energy of an isolated attosecond pulse has also reached 1.3 microjoules.The electric field polarization is an important characteristic of light field as well.Giving the attosecond pulse a special polarization will bring it a broader application prospect.Therefore,how to control the ellipticity of attosecond pulses and generate fully circularly polarized attosecond pulses is one of the research hotspots in attosecond science nowadays.Focusing on the research of "Near-Circularly Polarized Attosecond Pulse Generation and Control",the contents of this thesis mainly includes:(1)We have experimentally demonstrated that the efficiency and ellipticity of high-order harmonics can be controlled with nearly orthogonal two-color(OTC)fields.On the one hand,by applying a high-intensity SH field and the proper relative phase,elliptically polarized high-order harmonics are efficiently generated with the nearly OTC field.On the other hand,high-order harmonics with controllable ellipticity can be generated through adjusting the polarization crossing angle θ and the laser intensity ratio.It is an effective way to adjust and control the ellipticity of high-order harmonics with nearly OTC fields,especially the intensity ratio of nearly OTC fields.When the intensity of the SH is comparable to or even stronger than the fundamental field,even and odd order harmonics exhibit opposite helicity.Notably,the intensity of even order harmonics can become several times stronger than that of odd order harmonics.(2)We theoretically analyze the high-order harmonic generation(HHG)in the three-color laser field for isotropic atomic media and demonstrate the possibility to generate an isolated attosecond pulse with high ellipticity.The three-color driving field is synthesized by an OTC laser field with a linearly polarized gating field.The IR gating field imposes a transversal shift for the OTC electric field in the polarization plane and extends the time interval between adjacent harmonic emittings.The gating-field induced peculiarity offers a unique gating method for multi-cycle fields to generate the circular attosecond pulses.In the simulation,by adjusting the intensity ratio between the three-color field components,we obtain an isolated attosecond pulse with a high ellipticity of 0.936.(3)We theoretically analyse the macroscopic harmonic spectrum generated by the dual-pulse field in the overdriven regime.The dual-pulse field contains a prior pulse of800-nm wavelength and a posterior pulse of 1800-nm wavelength.The prior pulse is used to create plasma in the gas target,and the posterior one is responsible for the harmonic generation.Unlike the conventional overdriven schemes,the HHG in our method is predominantly contributed by the off-axis component.Under the effect of the plasma defocusing and Gaussian focusing geometry,intense soft X-ray harmonic emissions are generated and cutoff photon energy is extended.We also demonstrate that a better phase matching is achieved in our method,rendering the growing harmonic yield and higher cutoff energy.Moreover,we investigate the HHG dependence on the intensity of the prior pulse and the gas pressure in the simulation.By adjusting the intensity of the first pulse and increasing the gas pressure,the harmonic yield can be raised by nearly 10 times than that of the conventional overdriven schemes,and harmonic cutoff energy is extended into water window region.