Finite-system effects on high-harmonic generation: from atoms to solids

TL;DR

This study uses time-dependent density functional theory to analyze how high-harmonic generation (HHG) varies with system size from atoms to solids, revealing size-dependent cutoffs and the transition to bulk behavior.

Contribution

It demonstrates the size-dependent behavior of HHG cutoffs in one-dimensional structures and clarifies the transition from atomic to solid-state responses.

Findings

HHG cutoff extends linearly with system size.

Transition to bulk-like HHG response occurs around 60 nuclei.

High-energy recombination becomes possible in larger systems.

Abstract

Using time-dependent density functional theory, high-harmonic generation (HHG) is studied in one-dimensional structures of sizes from a single nucleus up to hundreds of nuclei. The HHG cutoff is observed to extent linearly with the system size from the well known atomic HHG cutoff and is found to converge into the previously observed cutoffs for bulk solids only for large systems. A change in the response from that of single atoms or small molecules is observed from system sizes of $N$ $\approx$ 6 nuclei and becomes independent of system size at $N \gtrsim 60$. The system-size dependence of the observed HHG cutoffs is found to follow the limitations, set by the finite size solid, on the classical motion of electron-hole pairs. Because of the relation between recombination energy and electron-hole propagation length in the system, high-energy recombination events are not possible in small systems but become accessible for larger systems resulting in the change of the cutoff energies with system size. When varying the field intensity we observe that the cutoffs move linearly with the intensity even for small systems of $N\gtrsim 6$ that are far from the limit of a bulk solid.