Clearing the Way for Floquet-Bloch States

Estimated reading time: 6 minutes

Interest in graphene and topological insulators such as bismuth selenide triggered a quest for ways to artificially stimulate a bandgap in them so they could be used for electronic devices. Gedik’s recent work focuses on using periodic light bursts to accomplish this. Although the electrons don’t absorb the light, they react to the electric field by forming a hybrid, or dressed, state between electrons and photons. This effect only occurs when both electrons and photons are present together, and it changes the electronic energy levels inside the material. Changing electronic properties through chemical processes, such as inserting another element into the solid, are usually permanent, but changing the properties of electrons with light excitation is reversible and controllable. “You can just change the intensity of the light, the frequency of the light, or polarization of the light,” Gedik says.

The electronic states are graphically represented by a Dirac cone, which plots energy versus momentum for electrons. Like ripples from a stone dropped in water or echoes of sound, stimulation by laser pulses results in these repeated cones, which are the hybrid photon-electron states. Gaps appear in the energy spectrum where these bands cross.

Lee, who contributed to the theoretical work in the new paper, says, “This work represents an important experimental progress in creating novel laser-driven electronic states, called Floquet states, on the surface of materials. We found that the traditional interpretation of the data was not adequate because of extrinsic effects not considered previously. Once this was understood, we found a tunable way to manipulate these Floquet states. The result may aid in the design of light-controllable electronic devices.”

Experimental challenges

To generate the light-driven state, Mahmood used mid-IR light pulses that diverge and lose power rapidly as they propagate. Guiding these pulses onto a small sample in a vacuum chamber while maximizing their power required special optical setups developed in the Gedik Lab. The time-of-flight analyzer is also relatively new requiring careful alignment. “We had to make sure that our beam was focused in such a way to get the best resolution out of our instrument,” Mahmood says.

The results show that the interference between Floquet-Bloch and Volkov states depends strongly on the polarization of the light. This polarization determines the orientation of the electric field with respect to the sample plane. Since the Volkov state is primarily activated by an out-of-plane electric field, switching the light polarization to one with only an in-plane electric field eliminates the Volkov state.

“This is a very challenging experiment, as it requires performing detailed photoelectron spectroscopy measurements on a transient photo-driven state that lasts merely fractions of a picosecond,” says David Hsieh, assistant professor of physics at Caltech. “The Gedik group has succeeded not only in directly visualizing the spectrum of this transient state, but also in quantifying and controlling the electromagnetic coupling between different energy bands of the transient state. This provides an interesting opportunity to use optical excitation as a means to engineer a desired materials response.”

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