Physicists Hit a Strip of Graphene With Circularly Polarized Laser Pulses 200 Femtoseconds Long, Turning It Briefly Into a Topological Insulator That Doesn't Exist in Nature, Then Used a Second Laser at Double the Frequency to Steer the Electrons Inside It, Bending Them Sideways With No Magnet at All.

A material's electronic personality is normally fixed by its structure at rest. Hit it with intense enough light, though, and the range of energies its electrons can occupy, the band structure, is temporarily rewritten. For a decade physicists have used that trick to create so-called Floquet topological insulators, fleeting states that exist only while a periodic light field is dressing the material. Now a team from Friedrich-Alexander University Erlangen-Nürnberg, Ludwig Maximilian University of Munich, the Technion in Israel and the University of Central Florida has gone a step further: they built the state in graphene and then drove the electrons around inside it with a second beam of light.

The experiment, published in Nature Physics, used a microscopic strip of single-layer graphene grown on silicon carbide and wired to gold electrodes, held in high vacuum at room temperature. First the researchers illuminated the strip's center with circularly polarized 1,550-nanometer pulses lasting about 200 femtoseconds, or 200 millionths of a billionth of a second. "When a circularly polarized light field interacts with or 'dresses' graphene, it pushes electrons into circular orbits," said Ofer Neufeld of the Technion, co-senior author. "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 that state, the team added a second pulse at exactly twice the frequency, 775 nanometers, and measured the resulting photocurrents through the gold contacts. By changing the polarization and the relative timing of the two fields they could dial the strength and direction of the current. They saw photocurrent circular dichroism, meaning the current flipped with the rotation sense of the second beam, and an all-optical anomalous Hall effect, in which electrons were deflected sideways with no magnetic field applied. Measurements backed by calculations pointed to valley-polarized currents, with one of graphene's two electronic valleys contributing more than the other, and showed the currents responded to changes within a single cycle of the light wave.

The project grew out of a hard question at a conference. "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," said Daniel M. B. Lesko, co-first author. "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. 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."

The advance matters because most previous studies of light-driven states relied on time-averaged techniques. "That's a real challenge, because scattering and decoherence happen so fast in many materials that much of the interesting physics of the nonequilibrium state gets washed out in the averaging," said Hommelhoff, co-senior author at Erlangen. "We're hopeful this work helps bridge ideas from the cold-atom lattice community to solid-state physics."

"We think the ability to use a second optical field to control electrons in the Floquet state is an important step forward," Hommelhoff said. "This means that we can now play with electrons in a state that's a marriage of the material's state and light, allowing plenty of new ideas about how this hybrid light-matter state might be engineered to offer properties not available in a bare material, for example, topologically protected currents." The same approach should work in other two-dimensional materials, and the group is now asking whether such hybrid light-matter states could store and process information. "We're now interested in exploring how these hybrid matter-light states could be used to encode and process information, as well as studying the underlying dynamics of the electrons within them," Lesko said.