An international team of physicists has discovered a new way to change the properties of materials at the quantum level without using extremely powerful lasers. Instead, they used excitons, which are bound pairs of an electron and a “hole” in semiconductors. The results were published in the journal Nature Physics.

From lasers to excitons

Floquet engineering is a method in which a periodic external influence, usually light, changes the electronic structure of a crystal and creates new energy states. In theory, this makes it possible to temporarily turn an ordinary material into a superconductor or give it other exotic properties. In practice, however, such experiments required lasers so powerful that the samples quickly heated up and were destroyed.

The authors of the new study proposed an alternative approach by using excitons instead of photons. These quasiparticles interact much more strongly with the crystal lattice because they are directly linked to the electrons in the material. This makes it possible to achieve pronounced Floquet effects with much lower energy input.

The experiment and the result

The researchers worked with an atomically thin semiconductor. First, they observed classical Floquet replicas of electronic bands under strong optical excitation. Then they reduced the light intensity by more than an order of magnitude and carried out measurements after 200 femtoseconds, at the moment when excitons dominated the system. The effect not only remained but turned out to be noticeably stronger.

This demonstrates that Floquet engineering is no longer limited to photons. In principle, similar changes can be induced by other quasiparticles, such as phonons, which are lattice vibrations, magnons, which are spin waves, or plasmons.

The co author David Bacon from University College London stated that this work opened the way to applied Floquet physics in which a wide variety of bosons could be used. He also explained that although there is still no universal recipe for creating a quantum material with any desired property, researchers now have a clear spectral “fingerprint” that can serve as a starting point.

Prospects for applications

If the technology can be scaled, it may lead to the creation of controllable quantum materials. Such materials would be able to change their properties on demand, for example becoming superconductors at room temperature, ultra sensitive sensors, or platforms for quantum computers. Energy efficient electronics, ultra fast computation, and new foundations for quantum technologies now appear closer to reality.

In brief

Physicists have used excitons for Floquet engineering for the first time, enabling temporary changes in material properties at the quantum level. This made it possible to achieve strong effects with much lower energy than when using lasers. Experiments with an atomically thin semiconductor showed stable Floquet replicas even under weak excitation. The discovery opens the way to using other quasiparticles and to creating materials with controllable properties, from superconductors to platforms for quantum computing.