Theoretical study of terahertz generation from atoms and aligned molecules driven by two-color laser fields

Abstract

We study the generation of terahertz radiation from atoms and molecules driven by an ultrashort fundamental laser and its second harmonic field by solving time-dependent Schrödinger equation (TDSE). The comparisons between one-, two-, and three-dimensional TDSE numerical simulations show that initial ionized wave-packet and its subsequent acceleration in the laser field and rescattering with long-range Coulomb potential play key roles. We also present the dependence of the optimum phase delay and yield of terahertz radiation on the laser intensity, wavelength, duration, and the ratio of two-color laser components. Terahertz wave generation from model hydrogen molecules are further investigated by comparing with high harmonic emission. It is found that the terahertz yield is following the alignment dependence of ionization rate, while the optimal two-color phase delays varies by a small amount when the alignment angle changes from 0 to 90 degrees, which reflects alignment dependence of attosecond electron dynamics. Finally we show that terahertz emission might be used to clarify the origin of interference in high harmonic generation from aligned molecules by coincidently measuring the angle-resolved THz yields.

Analysis of THz generation through the asymmetry of photoelectron angular distributions

“…(16). Thus the peak of the THz is not maximum at ϕ = 1.0 π , but rather at ϕ = 0.9 π for example, and will be intensity dependent [17,18]. Similar analysis can be applied to THz and 2D electron momentum spectra generated for atomic hydrogen by a two-color pulse where the intensity of the fundamental is raised to 2.26 × 10 14 W/cm 2 .…”

“…So far, the evidence supports the photocurrent model, in that ionization of the medium is indispensable [13,14], yet the mechanism behind the generation of THz remains unclear. A lot of theoretical calculations have been carried out in the past ten years by solving the time-dependent Schrödinger equation (TDSE) [8,[15][16][17][18] to study the THz signals versus the phase between the two colors of the laser, the intensity of the fundamental and of the second harmonic [17][18][19]. These TDSE calculations have shown consistent results among the studies, but the underlying mechanism remains unclear.…”

High-Harmonic and Terahertz Spectroscopy (HATS): Methods and Applications

“…Slow electrons for THz output should be modeled differently from the ones for the plateau or cut-off of high harmonics even though they are driven by the same laser fields, because the slow electron motion could be strongly distorted by the Coulomb potential. This Coulomb field effect has been identified by tracing the phases of THz wave with the help of high harmonics [17,18], and by analyzing the underlying dynamics via classical [19] and quantum calculations [20]. Therefore, the simultaneous detection of the high-harmonic and THz spectroscopy (HATS) gives us an opportunity to illustrate more aspects of the ultrafast dynamics of electrons with different ionization times, especially for the electrons contributing to the low-energy THz wave generation.…”

Strong-field ionization and gauge dependence of nonlocal potentials

“…Study of strong field ultra-short pulse laser gas interactions, including as THz frequency radiation generation [1][2][3], high harmonic generation [4], and the growing field of attosecond atom-field dynamics [5], relies on numerical modeling of laser-gas interaction. This is often done in two different regimes: A "macroscopic" simulation of laser-pulse evolution over distances of millimeters or centimeters, where the gas is treated as a medium that includes the linear field response, nonlinear field response, including the possibly rotational field response for a diatomic gas, field ionization, and free electron response [6,7].…”