Polarization And Spectral Characteristics Manipulation Of High Harmonics Via Two-color Fields

High-order harmonic generation(HHG)is a highly nonlinear physical process in which matter interacts with an intense laser field.Thanks to the high brightness and coherence of high-order harmonics,generating extreme ultraviolet and soft X-ray radiation has become an important application of HHG.The generated attosecond pulses(APs)based on HHG provide unprecedented temporal resolution for probing ultrafast processes in physics and chemistry.In recent years,two-color driving fields have been widely utilized in HHG to achieve bright extreme ultraviolet(XUV)attosecond light sources.However,most of the existing two-color fields limit the flexibility of manipulation because the two composed light fields possess the same polarization state and integer frequency ratio,which results in significant room for improvement in terms of brightness,spectral width and ellipticity.Besides,in the existing two-color driving field scheme,one can only manipulate the harmonic characteristics in the spectral domain.However,it is difficult to control the harmonic characteristics in the temporal domain.Since the two-color fields contain versatile combinations,we apply a series of brand-new two-color fields to realize the multi-directional manipulation of the harmonic polarization and spectral characteristics,giving full play to the flexibility and high freedom of the manipulation.Thus,the generated harmonics are significantly improved in terms of spectral width and ellipticity while ensuring brightness.Furthermore,the ultrafast modulation of the harmonic polarization state in the temporal domain can be realized.Our schemes provide an effective way to realize ultra-short APs with high brightness,adjustable ellipticity and ultrafast helicity modulation.Based on the above research status,we have carried out the following studies in turn around the theme of manipulating the harmonic polarization and spectral characteristics in the two-color driving fields.(1)We propose and theoretically demonstrate a scheme to generate high-order harmonics and APs with adjustable ellipticities.Specifically,we use a combined laser field composed of a strong linearly polarized component with a weaker circularly polarized assistant component(LPCP field).Through numerical calculation and theoretical analysis,it is found that the large harmonic ellipticity is attributed to the manipulation of the electron trajectory by the LPCP field and the molecular asymmetric structure.Thus,the bright XUV APs with large ellipticities can be synthesised by selecting the appropriate harmonics.Because of the difference of harmonic ellipticities contributed from the long and short trajectories,an even larger harmonic ellipticity can be obtained by filtering the long trajectory.(2)We propose and theoretically demonstrate an idea to generate high-order harmonics and AP trains with ultrafast helicity modulation in the temporal domain using detuned twocolor driving fields.When a detuned two-color field is used,the waveform of the detuned field changes periodically with time.Since the polarization state of the AP is very sensitive to the waveform of the two-color driving field,the polarization state of the AP will change periodically in the time domain.It is worth mentioning that our scheme is simple and general.For other driving fields,as long as the harmonic polarization can be tuned by adjusting the relative phase of the field,it is possible to generate APs with ultrafast helicity modulation in the temporal domain by using a proper frequency detuning.Furthermore,the frequency of the modulation is continuously tunable in a wide range at the sub-PHz level by adjusting the frequency detuning.The idea provides a powerful tool to control or detect ultrafast dynamical processes that are sensitive to the helicity.(3)We propose and theoretically demonstrate a scheme to achieve a dramatic cutoff extension of high-order harmonics using a two-color field with a large frequency ratio.The scheme takes advantage of the preacceleration of electrons and introduces a new way for electron to enter the conduction band,i.e.,the resonant electron injection channel(REIC),which modifies electron dynamics in HHG from semiconductors.In our scheme,even the electrons far from the top of the valence band can be preaccelerated to the position of the REIC and effectively injected into the conduction band via the REIC.Then the electrons in the conduction band can move further away from the Γ point driven by the mid-infrared light,and finally recombine with associated holes in the valence band,generating high-order harmonics with higher photon energy.In addition,by adjusting the frequency of the ultraviolet field,one can adjust the position of the REIC so that the cutoff of the HHG can be tuned in a wide range.It is worth mentioning that the scheme is not only applicable to trivial semiconductors,but also remains applicable to topological semiconductors.For topological semiconductors,one can achieve the cutoff extension with a lower frequency of the ultraviolet light because of the emergences of extra energy levels in the middle of the band gap.