Floquet Engineering of Quantum Materials

Floquet Engineering of Quantum Materials

Stanford scientists reveal virtual quantum states formed in new two-dimensional materials subjected to intense laser pulses. In the experiments, the mid-infrared laser beam is focused on monolayers of tungsten disulfide, where the strong electric field of the laser interacts with exciton-electron-hole pairs in it. Credit: Yuki Kobayashi.

Quantum materials are materials with unique electronic, magnetic or optical properties that are supported by the behavior of electrons at the quantum mechanical level. Studies have shown that interactions between these materials and strong laser fields can cause exotic electronic states.

In recent years, many physicists have tried to obtain and better understand these exotic states using different material platforms. One class of materials that has shown particular promise for studying some of these states is the monolayer transition metal dichalcogens.

Monolayer transition metal dichalcogens are 2D materials that consist of single layers of atoms of a transition metal (e.g. tungsten or molybdenum) and a chalcogen (e.g. sulfur or selenium) that are arranged in a crystal lattice . These materials have been found to offer exciting opportunities for Floquet engineering (a technique for manipulating material properties using lasers) of excitons (correlated electron-hole states of quasiparticles).

Researchers at SLAC National Accelerator Laboratory, Stanford University, and the University of Rochester recently demonstrated Floquet engineering of excitons driven by strong fields in a monolayer transition metal dichalcogenide. Their findings, presented in a paper in The physics of naturecould open new possibilities for the study of excitonic phenomena.

“Our group studied processes driven by strong fields, such as high-harmonic generation (HHG) in 2D crystals subjected to intense mid-infrared laser fields,” Shambhu Ghimire, one of the researchers who carried out the study, told Phys. . org.

“We are very interested in understanding the detailed mechanism of the HHG process, and 2D crystals seem to be a fascinating platform for this, as they are something between isolated atoms in the gas phase and bulk crystals. In the gas phase, the process is understood by considering the dynamics of the ionized electron in the laser field and its recombination with the parent ion.”

When exposed to strong laser fields, 2D crystals can host strongly excited excitons. In their previous research, Ghimire and his colleagues explored whether driving these quasiparticles with strong laser fields and measuring high harmonics would allow them to better understand the solid-state HHG process.

“While this previous work was the inspiration for our study, we also started measuring the change in absorption on these driven systems and learned more about the non-equilibrium state of the material itself,” Ghimire explained. “Indeed, we find that no absorption features have previously been observed that can be related to what are known in the literature as Floquet states of materials subjected to strong periodic impulses.”

In their experiments, the researchers used high-power ultrafast laser pulses in the mid-infrared to monolayer tungsten disulfide (TMD) wavelength range. Using these ultrafast pulses allowed them to avoid sample damage that typically results from strong light-matter interactions.

More precisely, the photon energy of the mid-infrared laser pulses is about 0.31 eV, which is significantly below the optical band gap of monolayer TMDs (~2 eV). Therefore, the team did not expect to observe a particularly significant generation of charge carriers.

“At the same time, the photon energy in our setup is tunable and can resonate with the exciton energies of the monolayer,” Ghimire said. “To fabricate our material samples, we collaborated with Prof. Fang Liu’s team from Stanford Chemistry. This group pioneered a new approach to fabricating millimeter-scale monolayer samples, which was also key to the success of these experiments.”

Yuki Kobayashi, a postdoctoral researcher who is the lead author of the paper, said they revealed two new mechanisms for creating virtual quantum states in monolayer TMDs. The first of these involves Floquet states, which are achieved by mixing the quantum states of materials with external photons, while the second involves the so-called Franz-Keldvsh effect.

“We found that an initially dark exciton state can be optically bright by mixing with a single photon, manifesting as a separate absorption signal below the optical band gap,” Kobayahsi said. “The second mechanism we revealed is the dynamical Franz-Keldysh effect. This is caused by the external laser field triggering the exciton pulse, which leads to a universal change in the spectral features. This effect was observed because we applied a high field. laser pulse (~0.3 V/nm) that is strong enough to break the electron-hole pair”.

By combining the two mechanisms they revealed, Kobayashi and colleagues were able to achieve energy tuning of more than 100 meV in their sample of monolayer TMDs. These remarkable results highlight the huge potential of this monolayer transition metal dichalcogen as a platform for realizing strong-field excitonic phenomena.

“One of the unanswered questions in our work is the real-time response of excitonic strong-field phenomena: how quickly can we switch virtual quantum states on and off?” added Ghimire. “We expect that by overcoming the perturbative domain, it will be possible to imprint the oscillation patterns of laser carriers in virtual quantum states, approaching the sub-petahertz regime of optical property control.”

More information:
Yuki Kobayashi et al., Floquet Engineering of strongly driven excitons in tungsten disulfide monolayer, The physics of nature (2023). DOI: 10.1038/s41567-022-01849-9

Hanzhe Liu et al., Generation of high harmonics from an atomically thin semiconductor, The physics of nature (2016). DOI: 10.1038/nphys3946

PB Corkum, The plasma perspective on high-field multiphoton ionization, Physical Review Letters (2002). DOI: 10.1103/PhysRevLett.71.1994

Shambhu Ghimire et al, Generation of higher harmonics from solids, The physics of nature (2018). DOI: 10.1038/s41567-018-0315-5

Fang Liu, Mechanical Exfoliation of 2D Large Area Materials from vdW Crystals, Advances in Surface Science (2021). DOI: 10.1016/j.progsurf.2021.100626

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Citation: Floquet Engineering of Quantum Materials (2023, January 20) retrieved January 20, 2023 from https://phys.org/news/2023-01-floquet-quantum-materials.html

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