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

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 = I p + 8 U p, where I p is the ionization energy of Ne and U p 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.