(a)–(c) The real-space picture of solid-state and atomic HHG. In (c) three-step recollision model of atomic HHG includes tunelling, acceleration and recombination whereas in (a) tunnelled electrons scattered by periodic Coulomb potential and they could collide with not only its parent ion but also neighboring ions. (b)–(d) The picture of solid-state and atomic HHG in momentum space. In panel (d) tunneled electron accelerates freely in continuum state and recombines with its parent ion whereas in panel (b) two-band model of solid-state HHG where both interband polarization and intraband current could emit high-harmonic radiation. Adapted from Ref. ^[19].