Medium Propagation Effects in High-Order Harmonic Generation of Ar

Abstract

We report theoretical calculations of high harmonic generation (HHG) by intense infrared lasers in atomic and molecular targets taking into account the macroscopic propagation of both fundamental and harmonic fields. On the examples of Ar and N2, we demonstrate that these ab initio calculations are capable of accurately reproducing available experimental results with isotropic and aligned target media. We further present detailed analysis of HHG intensity and phase, under various experimental conditions, in particular, as the wavelength of the driving laser changes. Most importantly, our results strongly support the factorization of HHG at the macroscopic level into a product of a returning electron wave packet and the photorecombination transition dipole, under typical experimental conditions. This implies that the single-atom/molecule structure information can be retrieved from experimentally measured HHG spectra.

Full Characterization of a Molecular Cooper Minimum Using High-Harmonic Spectroscopy

“…Overall, we found a variation of approximately 2.6 radians across~15 eV, shown in Figure 4c, consistent with previous calculations for argon [13,33,34], in which the coupled time-dependent Schrödinger equation and Maxwell's wave equations were solved [35,36] using an angular momentum-dependent pseudopotential [25]. This supports the idea that the methyl chloride HOMO largely features chlorine p characteristics, with the C-Cl bond ultimately filling the shell to complete the analogy to argon.…”

“…Overall, we found a variation of approximately 2.6 radians across ~15 eV, shown in Figure 4c, consistent with previous calculations for argon [13,33,34], in which the coupled time-dependent Schrödinger equation and Maxwell's wave equations were solved [35,36] using an angular momentum-dependent The strong-field approximation (SFA) group delay (red) is shown along with the high-harmonic contribution (τ HHG ) from the experiment. The SFA intensity was fit using methane, resulting in an intensity of 5.56 × 10 13 W/cm 2 , and then, this same intensity was applied to the methyl chloride data.…”

On the dipole, velocity and acceleration forms in high-order harmonic generation from a single atom or molecule

“…Other works have focused on these aspects and propagation effects may be very important, depending on the experimental conditions. For example substantial efforts have been put into the calculation of macroscopic effects on the HHG spectrum [5,26,28,29], the influence on the generation of attosecond pulses [30,31] and also to investigate the effects of propagation on more delicate structures such as the Cooper minimum in argon [32]. In view these works it is clear that no spectrum obtained by considering a single system can capture the full macroscopic response.…”

“…Even this description is computationally demanding, in particular if the quantum mechanical input for the polarization is obtained by solving the timedependent Schrödinger equation (TDSE) [1] and even more so if the target gas consists of molecules [2,3], where lack of spherical symmetry impedes a reduction in the dimensionality of the problem. Accordingly, to simplify the description, most theory works, aiming at an evaluation of the HHG spectrum of specific atoms and molecules, are concerned with the description of the response of a single quantum system to the external driving field, and indeed this approach has over the years accounted for many important features of the experimental data (see, for example [4,5] …”

Strong‐Field Interactions at Long Wavelengths

“…Since the laser period is proportional to λ and the cutoff energy to λ 2 , their ratio scales as λ 1 . Increasing the wavelength at a constant intensity avoids the problems of depletion and opens the route for the creation of shorter attosecond bursts centered at higher photon energies [13]. Besides these microscopic single-atom considerations, the attochirp can also be reduced, or partially compensated, by propagating the pulses through a suitable dispersive medium [14][15][16].…”