This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: The electronic properties of materials are typically determined by their structure under normal, undisturbed conditions, when they are in a state known as equilibrium. Intense light beams, however, can temporarily reshape a material's electronic band structure (i.e., the range of energy states available to electrons), potentially giving rise to new electronic behaviors.
Researchers at Friedrich-Alexander University Erlangen-Nürnberg, Ludwig Maximilian University of Munich, the Technion—Israel Institute of Technology, and the University of Central Florida recently demonstrated that illuminating graphene with a specific type of light temporarily modifies its electron states, prompting the emergence of a so-called Floquet topological insulator. This is a transient, out-of-equilibrium state created when a periodically oscillating field reshapes a material's electronic structure, resulting in topological properties that are absent at equilibrium. In this study, a fundamental light field created the state, while a second laser field with a doubled frequency was used to control electrons within it.
The team's paper, published in Nature Physics, builds on recent demonstrations of how bicircular two-color light fields can alter the electronic properties of two-dimensional (2D) materials. "After presenting some early results at a conference, Ofer Neufeld asked several tough questions during the Q&A about the topological properties of the system," Daniel M. Lesko, co-first author of the paper, told Phys.org.
"Over the following months, Peter Hommelhoff, Tobias Weitz, and I worked through numerous simulations and derivations, as well as ab initio simulations with Ofer, and eventually connected several phenomena that were missing from experimental observations of Floquet states to our two-color-driven system. It was a unique position to be in. "The moment we made that connection, we realized that many of the unusual measurements we'd taken were directly tied to theoretical predictions that hadn't yet been observed experimentally." In their experiments, Lesko, Hommelhoff, Weitz, Neufeld and their colleagues used a microscopic strip of monolayer graphene that was grown on a silicon carbide substrate and connected to gold electrodes.
This graphene strip was placed under high vacuum at room temperature while the team directed laser pulses onto its center. First, the sample was illuminated with circularly polarized 1,550-nm laser pulses, each about 200 femtoseconds long. This light field temporarily modified the electronic band structure of the graphene strip.
"When a circularly polarized light field interacts with or 'dresses' graphene, it pushes electrons into circular orbits," explained Neufeld, co-senior author of the paper. "Because these orbits repeat periodically, they generate a new time-periodic state called a Floquet state, which has different properties from the material's equilibrium (non-driven) state. To control electrons within this Floquet state, we use a harmonic of the dressing field, which lets us take full advantage of the system's periodicity." The second-harmonic field applied to the sample had double the frequency of the original light used by the team, resulting in 775-nm pulses with twice the photon energy of the first pulses.
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