Pulse-Duration-Sensitive High Harmonics and Attosecond Locally Chiral Near Fields from a Chiral Topological Weyl Semimetal.

High harmonic generation (HHG) in solids results from an interplay between intraband acceleration and electron-hole recombination driven by a high-intensity laser pulse. Here, we theoretically reveal that the driving pulse duration can play a major role in extending HHG to higher photon energies by promoting higher conduction band excitations. The effect is present in a conventional semiconductor as Si, restricted in a large-gap insulator as MgO, and most prominent in RhSi, a prototypical chiral Weyl semimetal presenting numerous band crossings. Further, we elucidate the HHG selection rules in RhSi required for the synthesis of attosecond locally chiral near fields. The chiral crystal structure enables the generation of a 3D electric field exhibiting an asymmetric instantaneous torsion on attosecond time scales. Our findings motivate future experiments in chiral Weyl semimetals to track high-energy band crossings and in situ locally chiral light, advancing prospects for compact extreme-ultraviolet sources, chiral sensing and ultrafast optoelectronics.