Generation of non-integer high-order harmonics and significant enhancement of harmonic intensity

High-order harmonics from helium atom in the orthogonally two-color(OTC) laser field are investigated by solving the two-dimensional time-dependent Schrodinger equation.Non-integer high-order harmonics are obtained in some ratio of frequencies of two components.Pure odd and even harmonics from atoms could be separated in two components by adjusting the ratio of frequencies in OTC scheme,and the resolution of harmonics is improved at the same time.The physical mechanism is explained by the periodicity of dipole.With the same intensity of the incident laser,the intensity of the high-order harmonics from the OTC field scheme is improved by three orders of magnitude compared to the monochromatic laser field scheme.A theoretical scheme is provided for experimentally achieving improving energy resolution and separation of pure odd and even harmonics in atoms.Also,we provide a means for improving harmonic intensity.

Altitude and intensity characteristics of parametric instability excited by an HF pump wave near the fifth electron harmonic

An ionospheric heating experiment involving an O mode pump wave was carried out at European Incoherent Scatter Scientific Association site in Troms？. The observation of the ultra high frequency radar illustrates the systematic variations of the enhanced ion line and plasma line in altitude and intensity as a function of the pump frequency. The analysis shows that those altitude variations are due to the thermal effect, and the intensity variations of the enhanced ion line are dependent on whether or not the enhanced ion acoustic wave satisfy the Bragg condition of radar. Moreover, a prediction that if the enhancement in electron temperature is suppressed,those systematic variations will be absent, is given.

High-order harmonic generation of CO_2 with different vibrational modes in an intense laser field

We apply the strong-field Lewenstein model to demonstrate the high-order harmonic generation of CO2 with three vibrational modes（balance vibration,bending vibration,and stretching vibration） driven by an intense laser field.The results show that the intensity of harmonic spectra is sensitive to molecular vibrational modes,and the high harmonic efficiency with stretching vibrational mode is the strongest.The underlying physical mechanism of the harmonic emission can be well explained by the corresponding ionization yield and the time-frequency analysis.Finally,we demonstrate the attosecond pulse generation with different vibrational modes and an isolated attosecond pulse with a duration of about 112 as is generated.

Focusing of Intense Laser via Parabolic Plasma Concave Surface

Since laser intensity plays an important role in laser plasma interactions,a method of increasing laser intensity- focusing of an intense laser via a parabolic plasma concave surface- is proposed and investigated by three-dimensional particle-in-cell simulations.The geometric focusing via a parabolic concave surface and the temporal compression of high harmonics increased the peak intensity of the laser pulse by about two orders of magnitude.Compared with the improvement via laser optics approaches,this scheme is much more economic and appropriate for most femtosecond laser facilities.

Intense ke V IAP generation by orthogonally polarized multicycle midinfrared two-color laser fields

We theoretically investigate the attosecond pulse generation in an orthogonal multicycle midinfrared two-color laser field. It is demonstrated that multiple continuum-like humps, which consist of about twenty orders of harmonics and an intensity of about one order higher than the adjacent normal harmonics, are generated when longer wavelength driving fields are used. By filtering these humps, intense isolated attosecond pulses（IAPs） are directly generated without any phase compensation. Our proposal provides a simple technique to generate intense IAPs with various central photon energies covering the multi-ke V spectral regime by using multicycle midinfrared driving pulses with high pump energy in the experiment.