What if you could create new materials just by shining a light at them? While this concept might seem like science fiction or alchemy, it is becoming a reality for physicists in the burgeoning field of Floquet engineering. By using a periodic drive, such as light, scientists have aimed to alter the fundamental properties of materials, potentially transforming a simple semiconductor into a superconductor. However, recent advancements suggest that excitons, rather than light, might hold the key to achieving these transformative effects more efficiently.
A diverse team of researchers from around the world, co-led by the Okinawa Institute of Science and Technology (OIST) and Stanford University, has demonstrated a new approach to Floquet engineering. Their groundbreaking study, published in Nature Physics, reveals that excitons can produce Floquet effects much more effectively than light. “Excitons couple much stronger to the material than photons due to the strong Coulomb interaction, particularly in 2D materials,” explains Professor Keshav Dani from the Femtosecond Spectroscopy Unit at OIST. “This allows for strong Floquet effects while avoiding the challenges posed by light.”
Understanding Floquet Engineering
Floquet engineering has been considered a promising path toward creating on-demand quantum materials from regular semiconductors. The principle is relatively straightforward: when a system is subjected to a periodic drive—a repeating external force—the system’s behavior can become richer than the simple repetitions of the drive. This concept is akin to a playground swing, where periodic pushes can lift the swing to greater heights.
In the quantum realm, Floquet engineering applies this principle to materials like semiconductors. Electrons in these materials are confined to specific energy levels due to the periodic atomic structure. By introducing a second periodic drive, such as light, researchers can shift these energy bands, altering the material’s properties. However, this method has limitations, as light couples weakly to matter, requiring high intensities that can damage the material.
The Role of Excitons
Excitons form in semiconductors when electrons are excited from their resting state to a higher energy level, usually by photons. The electron-hole pair that results forms a bosonic quasiparticle, which can strongly interact with the material. “Excitons carry self-oscillating energy, impacting surrounding electrons at tunable frequencies,” explains Professor Gianluca Stefanucci of the University of Rome Tor Vergata. “This strong coupling allows for effective periodic drives for hybridization with significantly less energy than light.”
Breakthrough with TR-ARPES Technology
This breakthrough is the culmination of OIST’s extensive research on excitons and their world-class TR-ARPES (time- and angle-resolved photoemission spectroscopy) setup. The team excited an atomically thin semiconductor with an optical drive and recorded the energy levels of the electrons. They observed the Floquet effect on the electronic band structure, a significant achievement in itself. By reducing the optical drive intensity, they captured excitonic Floquet effects, demonstrating a much stronger impact with less energy.
“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,” says Dr. Vivek Pareek, an OIST graduate now at the California Institute of Technology.
Implications for Quantum Material Development
The team’s findings conclusively prove that Floquet effects can be achieved not only with light but also with other bosons, such as excitons. This discovery opens the door to practical Floquet engineering, which holds great promise for the reliable creation of novel quantum materials and devices. “We’ve opened the gates to applied Floquet physics,” concludes Dr. David Bacon, co-first author of the study and former OIST researcher now at University College London. “This is very exciting, given its strong potential for creating and directly manipulating quantum materials.”
The implications of this research are vast. Excitonic Floquet engineering is significantly less energetic than optical methods, and theoretically, similar effects could be achieved with other particles, such as phonons, plasmons, and magnons. This flexibility could lead to a new era of quantum material development, paving the way for innovative technologies and applications.
As researchers continue to explore the potential of excitonic Floquet engineering, the scientific community eagerly anticipates the next steps. While the recipe for creating these exotic quantum materials is not yet fully developed, the foundational work has been laid, providing a spectral signature necessary for practical advancements in the field.
