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."
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 space; the atoms are locked in a tight lattice formation, confining the electrons to a specific energy level, or so-called band, 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. 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, 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."
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.
Lowering the bar with world-class TR-ARPES setup
This breakthrough is the culmination of the OIST unit's history of exciton research and the world-class TR-ARPES (time- and angle-resolved photoemission spectroscopy) setup they have built in tandem.
To investigate excitonic Floquet effects, the team excited an atomically thin semiconductor with an optical (i.e. light) drive and recorded the energy levels of the electrons. First, they used a strong optical drive to directly observe the Floquet effect on the electronic band structure, itself an important achievement. Next, they dialed the optical drive down by more than an order of magnitude and measured the electron signal 200 femtoseconds later, to capture the excitonic Floquet effects separately from the optical. "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."
With this, the multidisciplinary team have conclusively proven that not only are Floquet effects achievable in general, and not only with light, but that these effects can be reliably generated with other bosons than just photons, which have dominated the field until now. Excitonic Floquet engineering is significantly less energetic than optical, and theoretically, the same effect should be achievable with other -ons created through a broad palette of excitation – like phonons (using acoustic vibration), plasmons (using free-floating electrons), magnons (using magnetic fields), and others. As such, the researchers have laid the foundation for 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 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."
