ECAMP 15

Light can be used to modify and control properties of quantum systems in many areas of physics. The excitation and ionization of atoms and molecules are at the heart of strong-field physics and ultrafast nonlinear optics, playing a central role in various phenomena such as light-induced electron diffraction (LIED), high harmonic generation (HHG) and spectroscopy (HHS), atomic stabilization in intense fields, femtosecond (fs) laser filamentation or air lasing.
Standard methods [1,2] for describing the propagation of intense, short light pulses in extended media usually lack an ab initio description of the (laser-dressed) multi-level structure of the system, as the computational cost of an accurate first-principle treatment of the local response quickly becomes prohibitively expensive. Hence, excitations of (laser-dressed) bound states during strong-field propagation have received far less attention than ionization, despite promising application possibilities, e.g., in light amplification [3] and coherent control [4].
To elucidate the effect of bound state population dynamics during fs laser filamentation, we have developed a first-principle model [5,6] for the accurate simulation of spatio-temporal light pulse dynamics based on the solution of a (2+1)D unidirectional pulse propagation equation (UPPE). At each propagation step, the system's full microscopic response is provided by the solution of the 3D time-dependent Schrödinger equation (TDSE). Our model thus goes beyond the standard SFA-like approximation and takes into account the multi-level structure of the system, including both the bound and free states, the subcycle response of the system to the field, and the feedback of the generated light to the IR driver.
We demonstrate its performance using the example of spatiotemporal pulse propagation dynamics in atomic hydrogen gas [5]. Notably, our novel approach can be applied to any problem where the accurate description of unidirectional, non-relativistic propagation requires a first-principle account of the light-matter interaction, including propagation in liquids and solids, provided that the TDSE is modified accordingly.
Our fs laser filamentation simulations reveal [7] the generation of electronic population inversion in the hydrogen gas that persists throughout the pulse propagation distance and manifests in a new emission peak in the spectrum. We find characteristic inversion dynamics in both the multiphoton and tunneling regimes.
Our findings open the way to visualizing and controlling bound electron dynamics during the propagation of strong laser fields and reexamining its role in various strong-field processes. In addition, advanced pulse shaping techniques may provide a way to generate bound state dynamics specifically tailored to create and optimize new ultrafast gain mechanisms during laser filamentation.

References
[1] A. Couairon and A. Mysyrowicz, Phys. Reports 441, 47 (2007)
[2] M. Kolesik and J. V. Moloney, Phys. Rev. E 70, 036604 (2004)
[3] M. Matthews et al., Nature Physics 14, 695 (2018)
[4] M. Fushitani et al., Nature Photonics 10, 102 (2016)
[5] M. Richter et al., Opt. Express 31, 39941 (2023)
[6] F. Morales et al., Opt. Express 29, 29128 (2021)
[7] M. Richter et al., in preparation