A 3D rendering of a pair of hands holding glowing bands of energy like a cat’s cradle. One of the bands folds inwards, reminiscent of the Mexican‑hat‑like momentum dispersion indicative of Floquet effects. The glowing orbs above the hands, one dark and the other light, represent the electron and hole that together form an exciton. Credit: Jack Featherstone

What if you could create new materials just by shining a light at them? To most, this sounds like science fiction or alchemy, but to physicists investigating the burgeoning field of Floquet engineering, this is the goal. With a periodic drive, like light, scientists can “dress up” the electronic structure of any material, altering its fundamental properties—such as turning a simple semiconductor into a superconductor.

While the theory of Floquet physics has been investigated since a bold proposal by Oka and Aoki in 2009, only a handful of experiments within the past decade have managed to demonstrate Floquet effects. And though these experiments show the feasibility of Floquet engineering, the field has been limited by the reliance on light, which requires very high intensities that almost vaporize the material while still only achieving moderate results.

But now, a diverse team of researchers from around the world, co‑led by the Okinawa Institute of Science and Technology (OIST) and Stanford University have demonstrated a powerful new alternative approach to Floquet engineering by showing that excitons can produce Floquet effects much more efficiently than light. Their results are now 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.”

Normally, the energy levels of electrons in atomically thin semiconductors form a smooth curve (or band) when plotted across crystal momentum (k) levels, with a distinct peak in the middle, as seen on the right. A key indicator of Floquet hybridization is a flattening of this peak into a Mexican‑hat‑like shape, also called a camelback—see the leftmost graph. This flattening indicates the presence of a second, overlapping band which is invisible as electrons cannot inhabit the same point in momentum space. However, these “ghost” bands influence the visible valence and conduction bands, forcing them downward in the middle. This is clearly visible in the high exciton density condition, with the strength of the effect lowering with the density of excitons. The Mexican‑hat‑like dispersal is also present, but only faintly visible, in the optically driven condition. Credit: Pareek et al., 2025.

Dressing up quantum materials with Floquet engineering

Floquet engineering has long been eyed as a path towards creating on‑demand quantum materials from regular semiconductors. The principle undergirding Floquet physics is relatively straightforward: when a system is subjected to a periodic drive—a repeating external force, like a pendulum—the overall behavior of the system can be richer than the simple repetitions of the drive. Think of a playground swing: periodically pushing the person lifts the swing to greater heights, even though the swing itself oscillates back and forth.

Floquet engineering applies this principle to the quantum world, where the lines between time and space are blurred. In crystals, such as semiconductors, electrons are already subject to one periodic potential—periodic not in time, but in space; the atoms are locked in a tight lattice formation, confining the electrons to specific energy levels, or so‑called bands, as dictated by the specific periodic atomic structure.

When light is shone at the crystal at a set frequency, a second periodic drive is introduced—now in time, as the electromagnetic photons interact rhythmically with the electrons—shifting the permitted energy bands of the electrons. By tuning the frequency and intensity of the periodic light drive, the electrons can be made to inhabit new, hybrid bands, in turn altering the electron behavior of the entire system and thus the properties of the material—like how two musical notes harmonize to form a new, third note.

The time‑ and angle‑resolved photoemission spectroscopy (TR‑ARPES) setup at OIST, here with study co‑first author Xing Zhu, Ph.D. student in the Femtosecond Spectroscopy Unit. Featuring a proprietary, table‑top extreme‑UV source emitting bursts at femtosecond intervals (1 fs = one millionth of one billionth of a second), this setup captured the first real images of excitons, helped sketch out the evolution of dark excitons, and has now proved the feasibility of excitonic Floquet engineering. Credit: Bogna Baliszewska (OIST)

As soon as the light drive is turned off, the hybridization ends and the electrons swing back into the energy bands permitted by the crystal structure. But for the duration of the “song,” researchers can “dress up” materials to exhibit entirely novel behaviors.

“Until now, Floquet engineering has been synonymous with light drives,” says Xing Zhu, Ph.D. student at OIST. “But while these systems have been instrumental in proving the existence of Floquet effects, light couples 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 requires much lower intensities.”

Excitons form in semiconductors when individual electrons are excited from their “resting” state (the valence band) to a higher energy level (the conduction band), usually by photons. The negatively charged electron leaves behind a positively charged hole in the valence band, and the electron‑hole pair forms a bosonic quasiparticle that lasts until the electron eventually drops back into the valence band, emitting light.

“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.

TR‑ARPES setup more efficient for excitonic Floquet observation

This breakthrough builds on the OIST unit’s long history of exciton research and its world‑class TR‑ARPES (time‑ and angle‑resolved photoemission spectroscopy) setup.

To investigate excitonic Floquet effects, the team first used a strong optical drive to directly observe the Floquet effect on the electronic band structure. Then they reduced the optical drive by more than an order of magnitude and measured the electron signal 200 fs later, capturing the excitonic Floquet effects separately from the optical drive.

“The experiments spoke for themselves,” says Dr. Vivek Pareek, OIST graduate now a Presidential Postdoctoral Fellow at Caltech. “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 stronger effect.”

With this, the multidisciplinary team has conclusively proven that Floquet effects are achievable not only with light but also with other bosons. Excitonic Floquet engineering is significantly less energetic than optical driving, and, in principle, similar effects could be realized with phonons (acoustic vibrations), plasmons (free‑electron oscillations), magnons (magnetic excitations), and other quasiparticles. This opens a practical pathway toward reliable creation of novel 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 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.”

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Publication details

Driving Floquet physics with excitonic fields, Nature Physics (2026). DOI: 10.1038/s41567-025-03132-z

Citation

Quantum ‘alchemy’ made feasible with excitons (2026, January 19) retrieved 19 January 2026 from https://phys.org/news/2026-01-quantum-alchemy-feasible-excitons.html