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By numerically solving the time-dependent Schrödinger equation, we study high-order harmonic generation from the asymmetric diatomic molecule HeH2+ in a corotating two-color circularly polarized laser field. Our results reveal a strong correlation between the molecule orientation and the harmonic yield. The harmonics in the plateau region can achieve an intensity modulation of one to two orders of magnitude with the change in the orientation angle. Through the time-dependent evolution of ionized electron wave packets combined with the analysis of the transition dipole moment between the continuum and bound states, the modulation of the harmonic strength may be attributed to the difference in the recollision angle of ionized electron wave packets relative to the molecules. In addition, the harmonic ellipticity is also affected by the molecular orientation. Notably, we found that the harmonic with greater ellipticity and higher intensity can be obtained with an orientation angle of 147°. These findings open up new avenues for achieving enhanced efficiency, the near-circular polarization of harmonics, and precise control over harmonic polarization states. [ABSTRACT FROM AUTHOR]

Based on the strong field approximation theory and numerical solution of Maxwell's propagation equations, the high–order harmonic is generated from a neon (Ne) atom irradiated by a high–intensity laser pulse whose central wavelength is 800 nm. In the harmonic spectrum, it is found that in addition to the odd harmonics of the driving laser, a new frequency peak appeared. By examining the time–dependent behavior of the driving laser, it is found that the symmetry of the laser field is broken. We demonstrated that these new spectrum peaks are caused by the intensity reduction and frequency blue shift of the high–intensity laser during propagation. Our results reveal that it is feasible to modulate the harmonics of the specific energy to produce high–intensity harmonic emission by changing the gas density and the position of the gas medium interacting with the laser pulse. [ABSTRACT FROM AUTHOR]

High-order harmonic generation of atoms irradiated by an ultrashort laser pulse was calculated by numerically solving the time-dependent Schrödinger equation and the Bohmian trajectory scheme. The harmonic spectra with the two schemes are quantitatively consistent. Using the wavelet behavior of the Bohmian trajectory, the spatio-temporal features of harmonic emission from different energy regions are analyzed. It is found that the spatio-temporal distribution of the harmonic well revealed the physical mechanism of harmonic generation. This method will contribute to the understanding of harmonic emission mechanisms in complex systems, which include many atoms. [ABSTRACT FROM AUTHOR]

By numerically solving the time-dependent Schrödinger equation (TDSE), the harmonic generation process of atoms irradiated by corotating laser pulses composed of a fundamental-frequency and high-frequency field is systematically studied. Compared with the harmonic generated from atoms irradiated by counter-rotating two-color circularly polarized laser pulses, the harmonic efficiency of atoms irradiated by co-rotating two-color circularly polarized (CRTCCP) laser pulses with the same laser parameters is higher. The harmonics are generated by the multiphoton radiation transition after the bound electrons undergo a multiphoton absorption transition to a higher energy level. In addition, the variation of the harmonic efficiency with the field strength of different frequency components in the driving laser pulse is also studied. The circularly polarized harmonics with higher intensity can be obtained by optimizing the field strength of the driving laser field. [ABSTRACT FROM AUTHOR]

A single attosecond pulse generation via continuum–continuum interference is investigated theoretically by exposing a single-electron atom in a femtosecond laser field with the intensity in over-the-barrier ionization regime. We show that the ground state of the atom is depleted in such intense laser field and the high-order harmonics (HHG) via continuum to continuum coherence are generated. In a few-cycle monochromatic laser field (5 fs /800 nm, 1.2×10 16 W cm −2 ), a single attosecond pulse with duration of 49 as is obtained from the HHG. With a two-color laser field combined by 1200 nm (8 fs /7.5×10 15 W cm −2 ) and 800 nm (5 fs /1.0×10 16 W cm −2 ), a shorter single pulse with duration of 29 as can further be produced by changing the relative carrier envelope phase of two laser pulses as a result of controlling the electronic quantum path in the intense electric field. Our results also show that a short single attosecond pulse can be generated in a wide range of the relative carrier envelope phase of the two laser pulses. [ABSTRACT FROM AUTHOR]

We present a high-resolution and delay-stabilized setup for attosecond spectroscopy based on high-order harmonic generation from the intense laser passing through a differential gas cell. It shows the ability to generate the tunable photons from VUV to Soft X-ray (20–180 eV) at low backing pressure of target gas. This atto-beamline consists of an HHG source, electron and XUV spectrometers, and an active stabilization system. The measurements show it can be stabilized to sub-20 as in hours, together with a home-built XUV spectrometer with an energy resolution better than E / Δ E ≥ 1200 in the wavelength range of 5–25 nm. We demonstrated its applications for producing the narrowband XUV source, extending the cutoff energy of HHG, and performing RABBITT measurements, particularly for investigating the excitation and ionization dynamics of atoms and molecules in a broad energy range by measuring the electrons and photons with attosecond temporal resolution. [Display omitted] • We designed and built a stable attosecond beamline based on strong laser-driven high-harmonic generation from a home-built differential gas cell. • Attosecond pulse trains with a 280 as duration has been produced and reconstructed from RABBITT scanning, tunable attosecond XUV pulse with energy from 20 to 180 eV are characterized. • High energy resolution electron and XUV spectrometers are built and characterized for the study of ultrafast atomic and molecular dynamics. [ABSTRACT FROM AUTHOR]

High-order harmonic generation is investigated by using time-dependent density functional theory to numerically simulate the interaction of the benzene molecule with a few-cycle circularly polarized laser pulse. Compared with the harmonics from atoms and diatomic molecules with similar ionization energy irradiated by the same laser pulse, the harmonics from benzene molecules possess higher efficiency and cut-off energy. Further research reveals that the harmonic efficiency and cut-off energy of the benzene molecule increase gradually as the wavelength of the driving laser decreases. Through the analysis of the time-dependent evolution of electronic wave packets, it is found that the harmonic generation can be attributed to the recombination of the ionized electrons and their parent ions. By optimizing the driving laser pulse, we demonstrated that high-order harmonic generation in the benzene molecule can be used to produce isolated circularly polarized attosecond pulse. [ABSTRACT FROM AUTHOR]