Watching bandgaps in motion - attosecond interferometry of solids

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-Insitute, 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 (see Fig. 1 for experimental setup), 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 [Fig. 2(a) and (b)] encode transient changes of the electronic bandgap, with silica showing signatures of a shrinking bandgap [Fig. 2(c)], while MgO exhibits a widening [Fig. 2(d)].

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.