Attosecond high-harmonic interferometry probes orbital- and band-dependent dipole phase in magnesium oxide

Control over the spatial coherence, wavefront, and focusability of emitted light relies on understanding the intrinsic phase of the emission process, and vice versa; measuring the phase can provide insights about microscopic generation mechanisms. Using attosecond interferometry with phase-locked extreme ultraviolet (XUV) pulses, we directly measure the phase differences of XUV pulses emitted from two separate foci on the same solid sample that generate high harmonics at different driving intensities, allowing us to assess the intensity- and frequency-dependent dipole phase. Our results are supported by analytical, two-band, and full-band numerical theoretical models. The analytical approach benefits future solid-state high-harmonic generation studies, while the full numerical model details orbital- and band-resolved current contributions to the dipole phase. This research delivers combined quantitative measurement and rigorous theoretical description of the harmonic emission phase in solids.