Two-color high-harmonic generation in plasmas: efficiency dependence on the generating particle properties.

The high-order harmonic generation (HHG) in silver, gold, and zinc plasma plumes irradiated by orthogonally polarized two-color field is studied theoretically and experimentally. We find an increase of the HHG efficiency in comparison with the single-color case, which essentially depends on the plasma species and harmonic order. An increase of more than an order of magnitude is observed for silver plasma, whereas for gold and zinc it is lower; these results are reproduced in our calculations that include both propagation and microscopic response studies. We show that the widely used theoretical approach assuming the 1s ground state of the generating particle fails to reproduce the experimental results; the agreement is achieved in our theory using the actual quantum numbers of the outer electron of the generating particles. Moreover, our theoretical studies highlight the redistribution of the electronic density in the continuum wave packet as an important aspect of the HHG enhancement in the two-color orthogonally polarized fields with comparable intensities: in the single-color field the electronic trajectories with almost zero return energy are the most populated ones; in the two-color case the total field maximum can be shifted in time so that the trajectories with high return energies (in particular, the cut-off trajectory) become the most populated ones.

Measuring locomotor strategies of freely moving previsual rat pups.

Blind walks of previsual rat pups in the open field test were analyzed for random components and/or strategies of locomotion. Wistar infants (n = 51) on their 13th postnatal day was tracked in the open field test for 2 minutes. At this age, immature rats should rely only on non-visual modalities in their navigation. Three distinct patterns were observed. A large portion of the pups (n = 22) performed sub-diffusive localized walks in the center. A smaller cohort (n = 9) undertook almost immediate quasi-linear raids in a random direction, thus succeeded in reaching a shelter (i.e., walls' vicinity). The rest (n = 20) demonstrated a mixed strategy: localized walks interspersed with quasi-linear raids. We applied a method of space potentials for an automated segmentation of the tracks into the localized walks and quasi-linear fragments. We found that the localized walks were random only for the time scale below 2-3 seconds, but essentially non-random (sub-diffusive) for longer scales. The sub-diffusion was caused by the trajectories' re-attraction, as seen from the averaged velocity autocorrelation function. Self-odor traces can be a physical cue ensuring the trajectorial returns. Oppositely, the quasi-linear raids can be considered a primary form of an active escape response.