The bandgap, i.e. the energy gap between the highest lying valence and the lowest lying conduction band, is a defining property of insulating solids, governing how they absorb light and conduct electricity. Tracking how a bandgap changes under strong laser excitation has been a long-standing challenge, since the underlying processes unfold on femtosecond timescales and are difficult to track directly, especially for wide-bandgap dielectrics.

In a collaboration between the Max-Born-Institute, ARCNL Amsterdam, and Aarhus University, researchers have now shown that extreme ultraviolet (XUV) high-harmonic interferometry can provide direct access to such dynamics.

Using pairs of phase-locked near-infrared laser pulses, the team measured interference fringes and their intensity-dependent shift in the generated high-order harmonics from silica glass (SiO₂) and magnesium oxide (MgO).