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Considerable progress has been made in the experimental studies on laser-induced terahertz (THz) radiation in liquids. Liquid THz demonstrates many unique features different from the gas and plasma THz. For example, the liquid THz can be efficiently produced by a monochromatic laser. Its yield is maximized with a longer driving-pulse duration. It is also linearly dependent on the excitation pulse energy. In two-color laser fields, an unexpected unmodulated THz field was measured, and its energy dependence of the driving laser is completely different from that of the modulated THz waves. However, the underlying microscopic mechanism is still unclear due to the difficulties in the description of ultrafast dynamics in complex disordered liquids. Here we propose a shift-current model. The experimental observations could be reproduced by our theory successfully. In addition, our theory could be further utilized to investigate the nuclear quantum effect in the THz radiation in H2O and D2O. This work provides fundamental insights into the origin of the THz radiation in bulk liquids. [ABSTRACT FROM AUTHOR]

Molecules constituted by different isotopes are different in vibrational modes, making it possible to elucidate the mechanism of a chemical reaction via the kinetic isotope effect. However, the real-time observation of the vibrational motion of isotopic nuclei in molecules is still challenging due to its ultrashort time scale. Here we demonstrate a method to monitor the nuclear vibration of isotopic molecules with the frequency modulation of high-order harmonic generation (HHG) during the laser-molecule interaction. In the proof-of-principle experiment, we report a red shift in HHG from H2 and D2. The red shift is ascribed to dominant HHG from the stretched isotopic molecules at the trailing edge of the laser pulse. By utilizing the observed frequency shift, the laser-driven nuclear vibrations of H2 and D2 are retrieved. These findings pave an accessible route toward monitoring the ultrafast nuclear dynamics and even tracing a chemical reaction in real time. [ABSTRACT FROM AUTHOR]

High-order harmonic generation (HHG) is a nonlinear nonperturbative process in ultrashort intense laser-matter interaction. It is the main source of coherent attosecond (1 as = 10-18 s) laser pulses to investigate ultrafast electron dynamics. HHG has become an important table-top source covering a spectral range from infrared to extreme ultraviolet (XUV). One way to extend the cutoff energy of HHG is to increase the intensity of the laser pulses. A consequence of HHG in such intense short laser fields is the characteristic nonadiabatic red and blue shifts of the spectrum, which are reviewed in the present work. An example of this nonperturbative light-matter interaction is presented for the one- electron nonsymmetric molecular ion HeH2+, as molecular systems allow for the study of the laser-molecule orientation dependence of such new effects including a four-step model of MHOHG (Molecular High-order Harmonic Generation). [ABSTRACT FROM AUTHOR]

We present a systematic investigation of the depolarization properties of a supercontinuum accompanied with femtosecond laser filamentation in barium fluoride (BaF2) crystal. It is found that the depolarization of the supercontinuum depends strongly on the crystal orientations with respect to the incident laser polarization. At most crystal orientations, the depolarization of the supercontinuum rises with the increase of the input laser energies and finally saturates. While at 45°, the depolarization of the supercontinuum is not changed and keeps nearly negligible with the increase of the input laser energies. These peculiar depolarization properties of the supercontinuum can be ascribed to the orientation dependence of the cross-polarized wave (XPW) generation and ionization-induced plasma scattering in the BaF2 crystal. [ABSTRACT FROM AUTHOR]

A pseudospectral method with symplectic algorithm for the solution of time-dependent Schrödinger equations (TDSE) is introduced. The spatial part of the wavefunction is discretized into sparse grid by pseudospectral method and the time evolution is given in symplectic scheme. This method allows us to obtain a highly accurate and stable solution of TDSE. The effectiveness and efficiency of this method is demonstrated by the high-order harmonic spectra of one-dimensional atom in strong laser field as compared with previously published work. The influence of the additional static electric field is also investigated. [ABSTRACT FROM AUTHOR]