When intense light interacts with an atomic gas, recollision between an ionizing electron and its parent ion1 creates high-order harmonics of the fundamental laser frequency2. This sub-cycle effect generates coherent soft X-rays3 and attosecond pulses4, and provides a means to image molecular orbitals5. Recently, high harmonics have been generated from bulk crystals6, 7, but what mechanism8, 9, 10, 11, 12 dominates the emission remains uncertain. To resolve this issue, we adapt measurement methods from gas-phase research13, 14 to solid zinc oxide driven by mid-infrared laser fields of 0.25 volts per ångström. We find that when we alter the generation process with a second-harmonic beam, the modified harmonic spectrum bears the signature of a generalized recollision between an electron and its associated hole11. In addition, we find that solid-state high harmonics are perturbed by fields so weak that they are present in conventional electronic circuits, thus opening a route to integrate electronics with attosecond and high-harmonic technology. Future experiments will permit the band structure of a solid15 to be tomographically reconstructed.
