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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).

These fringe shifts encode transient changes of the electronic bandgap, with silica showing signatures of a shrinking bandgap, while MgO exhibits a widening.

The experiments were supported by analytical modeling and semiconductor Bloch-equation simulations, confirming that the observed phase variations are consistent with excitation-induced modifications of the electronic structure.

The work establishes interferometric HHG as a broadly applicable, all-optical probe of band-structure dynamics in solids. Beyond fundamental insight, this approach opens pathways toward ultrafast semiconductor metrology and future petahertz electro-optic technologies.

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