Experiments confirm a quantum material's unique response to circularly polarized laser light

Laser light is typically linearly polarized. This means that it oscillates in one direction, up or down. It can be circularly polarized at right. This means that its waves spiral in the same direction as the light's movement. New research from Stanford and SLAC suggests that circularly polarized light could be used to study quantum materials in new ways. Credit: Greg Stewart/SLAC National Accelerator Laboratory
Shambhu Ghimire's research team was forced to look for another way to study an interesting research target after the COVID-19 pandemic. These quantum materials, also known as topological insulators (or TIs), conduct electricity on their surface but not through their inner parts.

Denitsa Baykusheva is a Swiss National Science Foundation Fellow who joined his Stanford PULSE Institute group two years ago with the aim of finding a way for these materials to produce high harmonic generation (or HHG) as a tool for probing the material's behavior. Laser light shining through a material causes it to shift to higher energies and higher frequencies. This is called harmonics. It's similar to pressing on a guitar string producing higher notes. Scientists would have a new tool to study these and other quantum materials if this could be achieved in TIs.

Her colleagues and she came up with a new method to generate HHG in topological insulations after the experiment was stopped midway. These results showed that circularly polarized light which spirals in the direction of the laser beam would produce unique signals from both the conductive surface and the interior of the TI they were studying. Bismuth selenide would actually enhance the signal from the surfaces.

This illustration shows how a circularly polarized laser beam (top), is used to probe topological insulation (black), which is a quantum material that conducts electricity on its surface, but not its interior. High harmonic generation is a process that causes electrons to split apart and recombine, and thus emit light (white), of higher energies and frequency. Scientists can determine the spin and momentum by analysing the emitted light. These signals have been confirmed by experiments at SLAC. Credit: Greg Stewart/SLAC National Accelerator Laboratory

Baykusheva began to test the recipe when the lab was reopened for experiments, with all safety precautions in place for covids. The research team published a paper today in Nano Letters confirming that the tests were exactly as expected and produced the first signature of the topological surface.

Ghimire, a principal investigator at PULSE, said that this material is very different from any other material. It's exciting to be able find a new material with a different optical response than any other.

Ghimire and PULSE Director David Reis have done many experiments over the past 12 years that showed that HHG could be produced in new ways. This included beaming laser light through a crystal, frozen argon gas, or an atomically-thin semiconductor material. Another study showed how HHG can be used to produce attosecond laser pulses. This can be used to control and observe electron movements by shining a laser through ordinary glasses.

The arrow pattern reflects the spin and momentum of electrons within the topological insulator's surface layer. This quantum material conducts electricity on its surface, but not its interior. Experiments at SLAC revealed that a circularly polarized laser beam couples to this spin polarization, producing a distinctive pattern of high harmonic generation which is a hallmark of topological surfaces. Credit: Denitsa Baykusheva/Stanford PULSE Institute

Quantum materials, however, had stubbornly refused to be analyzed in this manner. The split personalities of topological insulationators were a particular problem.

"When we shine laserlight on a TI both the surface as well as the interior produces harmonics," Ghimire said. Ghimire stated that it is difficult to separate them.

He explained that the team had made a key discovery that circularly polarized light interacts in fundamentally different ways with the interior and surface. This interaction boosts harmonic generation and gives the surface a unique signature. These interactions are shaped by two fundamental differences in the interior and surface: the direction in which their electron spins are polarized, orientated in a clockwise, or counterclockwise direction, and the types and symmetry of their atomic lattices.

Diagram of an experiment at SLAC's laser laboratory with high-power laser power. Scientists used circularly polarized light to probe a topological insulator, a type quantum material that conducts electricity on its surface but not its interior. High harmonic generation is a process that shifts laser light to higher energies, frequencies, or harmonics. This creates polarization patterns (arrows) in a detector that reveal spin and momentum in electrons in the conductive surface layer. It is a unique signature for the topological surface. Credit: Shambhu Ghimire/Stanford PULSE Institute

Ghimire stated that two other research groups from Germany and China published earlier this year their recipe for creating HHG using TIs. However, both experiments used linearly polarized lights so they didn't see the enhanced signal from circularly polarized. He said that this signal is unique to topological surfaces states.

The team needed to change the wavelength of their powerful titanium sapphire laser to make it 10 times longer and 10 times more energetic, because intense laser light can cause electrons in materials to become a plasma. To minimize damage to the sample they used short laser pulses. This allowed them to capture its behavior at a shutter speed equivalent to a millionthsof a billionths of seconds.

Ghimire stated that HHG has the advantage of being an ultrafast probe. "Now that we have identified this innovative approach to probing topological surfaces, we can use the technique to study other interesting materials, such as topological states inducible by strong lasers, or chemical means."

This work was done by researchers from the Stanford Institute for Materials and Energy Sciences at SLAC, Ann Arbor University, and Pohang University of Technology (POSTECH), Korea.

Explore more A new probe that uses light to examine electron behavior in topological insulators is available.

More information: Denitsa baykusheva et.al, All-Optical Probe for Three-Dimensional Topological Insulates Based on High-Harmonic Generating Circularly Polarized Laser Fields. Nano Letters (2021). Information from the Journal: Nano Letters Denitsa Baykusheva and al, All-Optical Probe for Three-Dimensional Topological Ininsulators Based on High-Harmonic Generation by Circularly Polarized laser Fields, (2021). DOI: 10.1021/acs.nanolett.1c02145