High-order harmonic generation from an atom in a disordered environment

Using one-dimensional simulations analyzed through the lens of open quantum systems, we study a photoelectron's strong-field dynamics from an atom surrounded by a structurally disordered scattering environment, described by an ensemble of stochastically sampled configurations, with parameters chosen to be representative of liquid helium. We theoretically investigate high-order harmonic generation from this situation. We show that local dephasing of the photoelectron wavepacket induced by elastic scattering leads to global decoherence. This drives a transition from quantum to classical behavior, as witnessed by the localization of the photoelectron probability density around specific trajectories of the classical analog system: unstable periodic orbits. This phenomenon mirrors quantum scars traditionally observed in the eigenfunctions of time-independent systems such as quantum billiards. Here, it emerges in situ within a time-dependent framework, manifesting directly in the real-time dynamics from the ground state rather than solely through spectral analysis. We further verify that these effects are robust to the strength and sign of the scattering potential, confirming that they extend beyond the specific case of liquid helium.