Linking high harmonics from gases and solids

When intense light interacts with an atomic gas, recollision between an ionizing electron and its parent ion(1) creates high-order harmonics of the fundamental laser frequency(2). This sub-cycle effect generates coherent soft X-rays(3) and attosecond pulses(4), and provides a means to image molecular orbitals(5). Recently, high harmonics have been generated from bulk crystals(6,7), but what mechanism(8-12) dominates the emission remains uncertain. To resolve this issue, we adapt measurement methods from gas-phase research(13,14) to solid zinc oxide driven by mid-infrared laser fields of 0.25 volts per angstrom. 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 hole(11). 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 solid(15) to be tomographically reconstructed.