Due to the COVID-19 pandemic lockdown, experiments at the Department of Energy’s SLAC National Accelerator Laboratory came to a halt early last year. Shambhu Ghimire's research group had to figure out another way to study a fascinating research target: quantum materials termed as topological insulators (Tis), which carry electric current on their surfaces but not via their interiors.
An illustration shows how circularly polarized laser light (top) is used to probe a topological insulator (black), a quantum material that conducts electric current on its surface but not through its interior. The light causes electrons in the material to fly apart, recombine and emit light (white) of higher energies and frequencies through a process called high harmonic generation. By analyzing this emitted light, scientists can measure the spin and momentum of electrons in the material. Experiments at SLAC confirm that these signals are a unique signature of topological surfaces. Image Credit: Greg Stewart/SLAC National Accelerator Laboratory.
Denitsa Baykusheva, a Swiss National Science Foundation Fellow, had joined Ghimire's group at the Stanford PULSE Institute two years ago with the aim of discovering a way to produce high harmonic generation (HHG) in these materials as a tool for studying their behavior. In HHG, laser light shining through a material shifts to higher energies and higher frequencies, known as harmonics, quite similar to pressing on a guitar string creates higher notes.
If this could be achieved in TIs, which are potential building blocks for technologies such as quantum sensing, spintronics and quantum computing, it would offer researchers a new tool for probing these and other quantum materials.
With the experiment closed before completion, she and her colleagues sought the use of theory and computer simulations to create a new recipe for producing HHG in topological insulators.
The results recommended that circularly polarized light, which spirals along the course of the laser beam, would yield clear, distinctive signals from both the conductive surfaces and the interior of the TI they were examining, bismuth selenide — and would actually improve the signal transmitting from the surfaces.
After the lab reopened for experiments with COVID safety measures in place, Baykusheva was able to test the recipe for the first time. In an article published recently in Nano Letters, the researchers explained that those tests took place exactly as predicted, generating the first exclusive signature from the topological surface.
This material looks very different than any other material we’ve tried. It’s really exciting being able to find a new class of materials that has a very different optical response than anything else.
Shambhu Ghimire, PULSE Principal Investigator, SLAC National Accelerator Laboratory
In the last 12 years, Ghimire had performed a series of experiments with PULSE Director David Reis, demonstrating that HHG can be generated in ways that were formerly thought far-fetched or even impossible: by directing laser light into a frozen argon gas, a crystal or an atomically thin semiconductor material.
Another study illustrated how to use HHG to produce attosecond laser pulses, which can be used to analyze and control the movements of electrons, by directing a laser through a standard glass.
But quantum materials had persistently resisted being examined in this manner, and the split personalities of topological insulators posed a specific problem.
When we shine laser light on a TI, both the surface and the interior produce harmonics. The challenge is to separate them.
Shambhu Ghimire, PULSE Principal Investigator, SLAC National Accelerator Laboratory
The team’s main discovery, he described, was that circularly polarized light interacts with the surface and the interior in extremely diverse ways that improve high harmonic generation emanating from the surface and also give it a characteristic signature.
Those interactions, consecutively, are shaped by two major differences between the surface and the interior: the degree to which their electron spins are polarized — oriented in a counterclockwise or clockwise direction, for example — and the types of symmetry seen in their atomic lattices.
Since the group published their recipe for realizing HHG in TIs at the beginning of this year, two other research teams in China and Germany have stated that they have developed HHG in a topological insulator, Ghimire said. However, both of those experiments used linearly polarized light, so they did not see the superior signal created by circularly polarized light. That signal, he said, is an exceptional feature of topological surface states.
Since intense laser light can convert electrons in a material into a soup of electrons — a plasma — the scientists had to discover a method to modify the wavelength of their high-powered titanium sapphire laser so it was 10 times longer, and consequently 10 times less energetic.
They also employed very short laser pulses to reduce damage to the sample, which had the extra benefit of allowing them to capture the behavior of the material with the equivalent of a shutter speed of millionths of a billionth of a second.
The advantage of using HHG is that it’s an ultrafast probe. Now that we’ve identified this novel approach to probing the topological surface states, we can use it to study other interesting materials, including topological states induced by strong lasers or by chemical means.
Shambhu Ghimire, PULSE Principal Investigator, SLAC National Accelerator Laboratory
Scientists from the Stanford Institute for Materials and Energy Sciences (SIMES) at SLAC, the University of Michigan, Ann Arbor, and the Pohang University of Science and Technology (POSTECH) in Korea contributed to this study. A large portion of the funding was provided by the DOE Office of Science, including an Early Career Research Program award to Shambhu Ghimire, and from the Swiss National Science Foundation.
Journal Reference:
Baykusheva, D., et al. (2021) All-Optical Probe of Three-Dimensional Topological Insulators Based on High-Harmonic Generation by Circularly Polarized Laser Fields. Nano Letters. doi.org/10.1021/acs.nanolett.1c02145.
Source: https://www6.slac.stanford.edu