Scientists Tap Excitons for Safer Quantum Material Control

Scientists from the Okinawa Institute of Science and Technology, Stanford University, and other places around the world have found a new way to change the inside workings of quantum materials. They used excitons, which are short-lived pairs of electrons and holes that form in semiconductors, to reshape how electrons move. This method works with much less light energy than before and does not harm the material. The team shared their findings this week in Nature Physics.

Background

Quantum materials are special because their electrons behave in ways that can lead to new technologies, like faster computers or better sensors. For years, researchers have wanted to control these materials by tweaking their electronic band structures. These structures decide how electrons flow and interact.

One main tool has been Floquet engineering. This technique uses periodic drives, like repeating waves of light, to mix energy states in a material. It can create effects that do not exist naturally, such as turning a normal semiconductor into something with superconductor-like traits or special topological properties.

But there was a big problem. To make Floquet effects strong enough, teams needed intense laser light. This light often damaged the materials or made the changes last only femtoseconds, which is a tiny fraction of a second. High power could heat the material too much or even vaporize parts of it. Lower power did not work well because light couples weakly to the electrons inside.

Excitons offered a different path. When light hits a semiconductor, it can knock an electron out of place, leaving a hole behind. These electron-hole pairs bind together through electric forces and form excitons. In thin, two-dimensional materials, these pairs last longer and interact more strongly with the material around them.

The idea to use excitons as the drive came from noticing their self-oscillating energy. Once created, they vibrate at tunable speeds and push on nearby electrons more effectively than plain light photons do. This strong coupling comes from Coulomb interactions, the electric pull between charged particles.

Past work focused only on light drives. Now, this team proved excitons can do the job better. They worked with atomically thin semiconductors, the kind used in modern electronics.

Key Details

The researchers started by shining a strong laser on their thin semiconductor sample. They watched the electrons' energy levels with time- and angle-resolved photoemission spectroscopy. This tool fires light at the material and measures the speed and energy of electrons that escape, mapping the band structure.

First, they saw the usual Floquet effects from the direct light drive. These showed up as replicas in the electron signals, like echoes of the main bands folded over each other. It took many hours of measurements to get clear data this way.

Then, they turned down the laser power by more than ten times. They waited 200 femtoseconds after the pulse hit. At that point, the direct light effect had faded, but a new set of replicas appeared. These came from the excitons the light had created inside the material.

The exciton-driven effects were stronger and clearer than the light ones, even with less power. Data collection took just a couple of hours instead of tens.

How Excitons Work as a Drive

Excitons form from the material's own electrons, so they fit right in. Their energy oscillations act like a gentle, repeating push on the band structure. Photons from lasers bounce off without as much grip. Excitons grab hold tighter, especially in 2D layers where forces are stronger.

The team confirmed this with direct observations. The Floquet replicas had a shape like a Mexican hat in momentum space, a sign of the hybridization they wanted.

"Excitons couple much stronger to the material than photons due to the strong Coulomb interaction, particularly in 2D materials," said 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."

Another researcher explained the process further.

"Excitons carry self-oscillating energy, imparted by the initial excitation, which impacts the surrounding electrons in the material at tunable frequencies," said Professor Gianluca Stefanucci of the University of Rome Tor Vergata. "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."

Xing Zhu, a PhD student at OIST, noted the practical side.

"Until now, Floquet engineering has been synonymous with light drives. But while these systems have been instrumental to 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."

Dr. David Bacon, a co-first author now at University College London, added that this opens doors beyond excitons.

The method works with bosonic excitations, particles that carry energy in bunches. Light uses photons, which are bosons. Now excitons join the list.

What This Means

This work shifts Floquet engineering from lab curiosities to something usable. Lower light needs mean less damage and longer-lasting changes. Materials stay intact for repeated tests or real devices.

It paves the way for custom quantum materials. Engineers could dial in properties like better conductivity or spin control without rebuilding from scratch. Think sensors that detect tiny signals, or chips that switch states in new ways.

The approach extends to other excitations. Phonons from sound waves could drive effects with vibrations. Plasmons from free electrons might work in metals. Magnons from magnetic spins open paths for spintronics. Each fits different materials and goals.

"We’ve opened the gates to applied Floquet physics to a wide variety of bosons," said Dr. David Bacon. "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."

Teams elsewhere may test these ideas soon. More labs have the tools for photoemission spectroscopy. Thin semiconductors are common now, thanks to advances in layering atoms.

For quantum tech, this means progress toward devices that mix light, electrons, and other quanta on tiny scales. It could speed up work on quantum computers or networks that use material properties directly.

The findings build on years of effort. Floquet ideas date back decades in theory, but real demos lagged. Now, with excitons proven, the field moves faster. Other groups report related work with 2D materials and light-exciton mixes, hinting at a growing toolbox.

Researchers plan follow-ups. They want to tune exciton densities for finer control. Longer-lived effects could come from better materials or cooling. Combining drives, like light plus sound, might stack benefits.

This step makes quantum material design feel closer. Labs worldwide watch, ready to build on it.