In the world of solid materials, one property shapes almost everything about how they behave: the bandgap.
This is the tiny but critical energy difference between the highest occupied electronic state (the valence band) and the lowest empty state (the conduction band).
The bandgap determines whether a material is an insulator, a conductor, or a semiconductor, and it governs how the material interacts with light and electricity.
But bandgaps aren’t fixed. When a material is hit with intense light, such as a strong laser pulse, its bandgap can temporarily shrink or expand.
Tracking these ultrafast changes has long been a challenge, because the action happens on unimaginably small timescales—just femtoseconds, or millionths of a billionth of a second.
For decades, scientists lacked a direct way to watch these fleeting shifts in real time.
Now, an international team of researchers from the Max-Born Institute in Berlin, ARCNL Amsterdam, and Aarhus University has developed a powerful new method to capture bandgaps in motion.
Their findings, published recently, show that extreme ultraviolet (XUV) high-harmonic interferometry—a technique that uses precisely timed bursts of laser light—can provide a window into these rapid electronic processes.
The scientists used pairs of near-infrared laser pulses that were phase-locked, meaning they were perfectly synchronized.
When these pulses struck insulating solids like silica glass (SiO₂) and magnesium oxide (MgO), they generated high-order harmonics—special bursts of extreme ultraviolet light.
By analyzing subtle interference patterns in this emitted light, the team could see how the bandgaps shifted under the influence of the laser.
The results were striking. In silica, the bandgap temporarily shrank, while in MgO, the bandgap widened.
These differences were confirmed by advanced computer modeling and semiconductor simulations, which showed that the changes were consistent with the way the electronic structures of the materials were being modified by the laser.
This new approach is more than just a scientific breakthrough. It establishes high-harmonic interferometry as a versatile, all-optical method for probing the inner workings of solids at record speeds.
With it, researchers can now observe how electrons rearrange themselves inside materials in response to strong light fields—something that was previously hidden from view.
The implications are far-reaching.
Beyond advancing fundamental physics, this technique could pave the way for ultrafast semiconductor testing and even future technologies that operate at “petahertz” speeds—far beyond today’s electronics.
By catching electrons in the act of moving between energy states, scientists are opening the door to a new era of ultrafast science and technology.
