The Characteristics Of High Harmonic Generation From Polyatomic Molecules And Solids

In recent two decades,with the development of ultrafast laser technology,the researches on the interaction between the strong laser field and matter have made remarkable progress.The ultrafast laser pulse provides a powerful tool to study the material structure in the microworld and to explore the physical process under extreme conditions.As a high order nonlinear and nonperturbed radiation produced by the interaction between femtosecond laser pulse and matter,high order harmonic(HHG)is the first choice to obtain the high brightness extreme ultra-violet(EUV)coherent light source and to synthesize the attosecond pulse.By decoding the high order harmonic signals,the electronic structure information of atoms and molecules and the attosecond dynamic process of electrons in the strong laser fields can be detected effectively.At present,the research on high order harmonics in gases mainly focuses on atoms and small molecules with simple structure due to the limitation of theoretical methods and experimental conditions.There are only a few researches involving molecular HHG from polyatomic molecules with complex structure.In recent years,the experimental observation and theoretical prediction of HHG in solids have further extended the target media to more complex crystal materials.HHG from solids has become a hot topic in the areas of strong field physics.Based on the above research situation,we mostly study some characteristics of HHG from the complex molecules and solid systems in this paper.The main contents in this paper include the following aspects:(1)Based on the rotational symmetry of the target molecule and the laser electric field,the“greatest common divisor rule”is proposed to determine the selection rule of HHG in gases.This study first extends the researches of selection rules to stereomolecules.It is points out that the rotational symmetry of the stereomolecules is determined by the projection of the stereomolecules on the plane of laser polarization.In addition,it is found that the rotational symmetry of laser electric field is affected by the rotational direction of laser electric field.Our study provides the theoretical basis to reveal the geometric structures of molecules by the HHG spectroscopy.(2)The π-π~*transitions in conjugated molecule are observed in high harmonic spectrum.The π-π~*transition processe is a typical characteristic in the conjugate systems.It is found that the π-π~*transitions can be observed by high harmonics.The energy gap between π and π~*orbitals corresponds to a radiation peak in the harmonic spectrum from the conjugated molecule.Using time-frequency analysis of the harmonic spectrum,the transitions of electrons in conjugated molecules between π and π~*orbitals can be observed in real time.(3)The time-dependent population image(TDPI)is proposed to map the dynamic process of electrons in HHG from solids.This study provides an intuitive picture for the HHG from solids.Using the TDPI,the real-time radiation energy and cutoff energy of HHG from solids can be well predicted.Likewise the gas HHG,the concepts of the long and short orbitals in solid HHG are built by the TDPI.We study the modulation effect of the carrier envelope phase(CEP)on harmonic spectra by the TDPI.(4)The wavelength dependences of the cutoff energies of intraband and interband harmonics are studied systematically.By separating the contributions of intraband and interband harmonics,it is found that the cutoff energy of intraband harmonics has nothing to do with the driving wavelength,whereas the cutoff energy of interband harmonics shows a linear dependence with the driving wavelength.The wavelength dependence of the cutoff energy of the observed harmonic signal depends on which mechanism(intraband or interband)is dominated near the cutoff region.Our results can uniformly interpret two different existing conclusions on wavelength dependences of cutoff energies of HHG from solids.(5)The wavelength scaling law of HHG efficiency from solids is studied systematically.It is found that the wavelength scaling law of solid HHG is still determined by the wave-packet spreading for the overall yield and the energy distribution effect due to the increase of the cutoff.This is very similar to the case of gas HHG.In addition,the fine oscillation of the HHG efficiency on the wavelength is also found.The oscillation period of the fine oscillation is determined by the band structure of the solid target.