Fig. 5: High-harmonic spectroscopy of solid materials.

a Schematics for HHS of solid-state materials in transmission geometry. Few-cycle laser pulses excite the sample at the V/Å level without damage. Attosecond pulses are produced on subcycle timescales, which interfere in the far field and produce discrete peaks in high-harmonic spectrum. b Measured crystal orientation-dependent high-harmonic spectrum from wide-bandgap cubic MgO crystal. c Real-space electron trajectory model showing how harmonic signal increases (decreases) when the electron trajectories connect (miss) neighboring atomic sites. If trajectories connect the first nearest neighbors (O–Mg), they produce the strongest signal, as seen along cubic directions. If they connect the second nearest-neighbor (O–O) secondary maxima are produced, as seen along diagonal direction of the cube. Reproduced with permission from ref. ¹⁵, Copyright 2017, Springer Nature.