Strong-field propagation effects in high harmonic generation from MgO

High harmonic generation in crystals is a strong-field process that can be used to probe the electronic structure and dynamics of solids and promises a route to attosecond control in the condensed phase. Here, we investigate the complex spatiospectral structures in high harmonics generated from MgO crystals in transmission geometry. We explore the patterns within individual harmonics as a function of laser intensity. Using z-scan analysis, we retrieve the nonlinear refractive index and employ propagation calculations to examine the transverse spatial coherence model, consistent with the interband dominated harmonic emission, as the source of these patterns. We find that the inclusion of the nonlinear propagation effects explains the structures well, without requiring a multi-trajectory interference or changing dephasing times.