Researchers unlocked a new shortcut to quantum materials

That idea may sound like fantasy, but it sits at the heart of an emerging area of physics known as Floquet engineering. Researchers in this field study how repeating influences, such as carefully tuned light, can temporarily reshape the way electrons behave inside a material. When this happens, a familiar substance like a semiconductor can briefly take on unusual properties, including behaviors normally associated with superconductors.

Although the basic theory behind Floquet physics dates back to a 2009 proposal by Oka and Aoki, experimental proof has been difficult. Only a small number of experiments over the past decade have successfully demonstrated clear Floquet effects. One major limitation has been the need for extremely intense light. These high energy levels come close to destroying the material while still producing only modest changes.

Excitons Offer a More Efficient Alternative

Researchers have now identified a promising new way to achieve Floquet effects without relying on such extreme light conditions. A global team led by the Okinawa Institute of Science and Technology (OIST) and Stanford University has shown that excitons can drive these effects far more efficiently than light alone. Their findings were published in Nature Physics.

“Excitons couple much stronger to the material than photons due to the strong Coulomb interaction, particularly in 2D materials,” says Professor Keshav Dani from the Femtosecond Spectroscopy Unit at OIST, “and they can thus achieve strong Floquet effects while avoiding the challenges posed by light. With this, we have a new potential pathway to the exotic future quantum devices and materials that Floquet engineering promises.”

This approach points to a new route for controlling quantum materials while reducing the risk of damage.

How Floquet Engineering Changes Quantum Materials

Floquet engineering has long been seen as a possible way to create custom quantum materials from ordinary semiconductors. The idea is based on a familiar physical principle. When a system experiences a repeating influence, its response can become more complex than the repetition itself. A simple example is a playground swing, where timed pushes cause the swing to rise higher even though the motion remains rhythmic.

In quantum materials, electrons already experience a repeating structure because atoms are arranged in an orderly crystal lattice. This spatial repetition restricts electrons to specific energy levels, known as bands. When light with a fixed frequency interacts with the crystal, it introduces a second repeating influence that unfolds over time. As photons interact rhythmically with electrons, the allowed energy bands shift.

By carefully adjusting the light’s frequency and intensity, electrons can temporarily occupy new hybrid energy bands. These changes affect how electrons move and interact, which alters the material’s overall properties. When the light is turned off, the material returns to its original state. During the interaction, however, researchers can effectively dress materials with new quantum behaviors.

Why Light-Based Approaches Fall Short

“Until now, Floquet engineering has been synonymous with light drives,” says Xing Zhu, PhD student at OIST. “But while these systems have been instrumental to proving the existence of Floquet effects, light couple weakly to matter, meaning that very high frequencies, often at the femtosecond scale, are required to achieve hybridization. Such high energy levels tend to vaporize the material, and the effects are very short-lived. By contrast, excitonic Floquet engineering require much lower intensities.”

This challenge has slowed progress toward practical applications.

What Excitons Are and Why They Matter

Excitons form inside semiconductors when electrons absorb energy and jump from their resting state in the valence band to a higher energy state in the conduction band. This process leaves behind a positively charged hole. The electron and hole remain linked as a short-lived quasiparticle until the electron falls back and emits light.

Because excitons originate from the material’s own electrons, they interact much more strongly with the surrounding structure than external light does. They also carry oscillating energy from their initial excitation, which influences nearby electrons at adjustable frequencies.

“Excitons carry self-oscillating energy, imparted by the initial excitation, which impacts the surrounding electrons in the material at tunable frequencies. Because the excitons are created from the electrons of the material itself, they couple much more strongly with the material than light. And crucially, it takes significantly less light to create a population of excitons dense enough to serve as an effective periodic drive for hybridization – which is what we have now observed,” explains co-author Professor Gianluca Stefanucci of the University of Rome Tor Vergata.

Capturing the Effect With Advanced Spectroscopy

This advance builds on years of exciton research at OIST and the development of a powerful TR-ARPES (time- and angle-resolved photoemission spectroscopy) system.

To separate the effects of light from those of excitons, the team studied an atomically thin semiconductor. They first applied a strong optical (i.e. light) drive to directly observe changes in the electronic band structure, confirming the expected Floquet behavior. Then they reduced the light intensity by more than an order of magnitude and measured the electronic response 200 femtoseconds later. This timing allowed them to isolate the excitonic contribution.

“The experiments spoke for themselves,” says Dr. Vivek Pareek, OIST graduate who is now a Presidential Postdoctoral Fellow at the California Institute of Technology. “It took us tens of hours of data acquisition to observe Floquet replicas with light, but only around two to achieve excitonic Floquet – and with a much a stronger effect.”

Toward Practical Quantum Material Design

The results show that Floquet effects are not limited to light-based techniques. They can also be generated reliably using other bosonic particles beyond photons. Excitonic Floquet engineering requires far less energy than optical methods and opens the door to a broader set of tools.

In principle, similar effects could be achieved using phonons (using acoustic vibration), plasmons (using free-floating electrons), magnons (using magnetic fields), and other excitations. Together, these possibilities move Floquet engineering closer to practical use and the reliable creation of new quantum materials and devices.

“We’ve opened the gates to applied Floquet physics,” concludes study co-first author Dr. David Bacon, former OIST researcher now at the University College London, “to a wide variety of bosons. This is very exciting, given its strong potential for creating and directly manipulating quantum materials. We don’t have the recipe for this just yet – but we now have the spectral signature necessary for the first, practical steps.”