New Tutorial Connects Atoms and Molecules to Solids in High Harmonic Generation

A new tutorial by Shambhu Ghimire and David Reis of Stanford PULSE Institute brings together decades of research on high harmonic generation (HHG), unifying how scientists understand the phenomenon in atoms, molecules, and condensed matters. HHG occurs when intense ultrafast laser pulses drive electrons in materials to emit light at multiples of the laser frequency, producing attosecond bursts of radiation that can probe matter on the fastest timescales - the natural time scale of the electron motion. First established in atomic and molecular gases through the foundational work that led up to 2023 Nobel prize in physics, the field has expanded dramatically in recent years to include condensed matter systems such as solids and two-dimensional crystals. In solids, HHG provides a powerful new window into electronic band structure, symmetry and strong-field electron motion—turning tabletop laser systems into probes of quantum materials. The tutorial draws clear conceptual connections between the three-step model developed for atoms and the interband and intraband dynamics that govern harmonic generation in crystals.

“High harmonic generation gives us a way to watch electrons move in real time,” Ghimire said. “By connecting the atomic and solid-state pictures, we hope to provide a common framework that helps researchers across disciplines push the field forward.”

The article serves as both an accessible entry point for newcomers and a reference for experts working at the frontiers of ultrafast and strong-field science. By bridging atomic physics and condensed matter, it highlights HHG’s growing role in creating “quantum movies” of electrons and exploring next-generation materials.