Transition of the generation mechanism of high-order harmonics in an extended neon system

摘要

Using a time-dependent density functional theory method,we perform a systematic numerical study of the transition of high-order harmonic generation in neon(Ne)systems from an isolated Ne atom to an extended Ne system of solid density.We show that ionized electrons wander in such extended systems until they meet a nearby ion and collide with it.The maximum energy edge for the main feature of the high-order harmonic spectrum in this“wandering electron”picture is determined as Eedge=Ip+8Up,where Ip is the ionization energy of Ne and Up is the ponderomotive energy delivered by the driving laser.The factor of 8 comes from the maximum kinetic energy of an ionized electron in the driving laser field.Beyond the atomic limit of high-order harmonic spectra,a multiplatform feature is observed,corresponding to re-collisions of ionized electrons with their nearby ions.It is also shown that a Ne simple cubic lattice of appropriate size provides a selection condition for the direction of polarization of high-order harmonics beyond the atomic limit,which may be further used to manipulate the emitted radiation.