High harmonic generation in AlN due to out-of-surface electron orbitals

High harmonic generation in solids is commonly considered as a bulk process. However, there is a controversy whether the experimentally observed effects can in their entirety be described by a pure bulk model. Our results based on beam propagation and spectral characteristics provide a clear signature of the importance of surface effects on the origin of the harmonic generation in solids. Computations performed with a time-dependent density-functional approach corroborate the experimental measurements.