This is a microscopic image of multiple electrodes on a sheet of Weyl semimetal, with red and blue arrows depicting the circular movement of the electrical current induced by either left- (blue) or right-circularly polarized light (right). Image: Zhurun Ji.Insights from quantum physics have allowed engineers to apply components used in circuit boards, optical fibers and control systems to new applications ranging from smartphones to advanced microprocessors. But even with significant progress made in recent years, researchers are still looking for new and better ways to control the uniquely powerful electronic properties of quantum materials.
A new study from researchers at the University of Pennsylvania has now found that Weyl semimetals, a class of quantum materials, have bulk quantum states whose electrical properties can be controlled using light. The study was led by Ritesh Agarwal and graduate student Zhurun Ji in the School of Engineering and Applied Science, in collaboration with Charles Kane, Eugene Mele and Andrew Rappe in the School of Arts and Sciences, and Zheng Liu from Nanyang Technological University in Singapore, together with several others. They report their findings in a paper in Nature Materials.
A hint of these unconventional photogalvanic properties, or the ability to generate electric current from light, was first reported by Agarwal in silicon. His group was able to control the movement of electrical current by changing the chirality, or the inherent symmetry of the arrangement of silicon atoms, on the surface of the material.
"At that time, we were also trying to understand the properties of topological insulators, but we could not prove that what we were seeing was coming from those unique surface states," Agarwal says.
Then, while conducting new experiments on Weyl semimetals, where the unique quantum states exist in the bulk of the material, Agarwal and Ji obtained results that didn't match any of the theories that could explain how the electrical field was moving when activated by light. Instead of the electrical current flowing in a single direction, the current moved around the semimetal in a swirling circular pattern.
Agarwal and Ji turned to Kane and Mele to help develop a new theoretical framework that could explain what they were seeing. After conducting new, extremely thorough experiments to iteratively eliminate all other possible explanations, the physicists were able to narrow the possible explanations to a single theory related to the structure of the light beam.
"When you shine light on matter, it's natural to think about a beam of light as laterally uniform," says Mele. "What made these experiments work is that the beam has a boundary, and what made the current circulate had to do with its behavior at the edge of the beam."
Using this new theoretical framework, and incorporating Rappe's insights on the electron energy levels inside the material, Ji was able to confirm the unique circular movements of the electrical current. The scientists also found that the current's direction could be controlled by changing the light beam's structure, such as changing the direction of its polarization or the frequency of the photons.
"Previously, when people did optoelectronic measurements, they always assume that light is a plane wave. But we broke that limitation and demonstrated that not only light polarization but also the spatial dispersion of light can affect the light-matter interaction process," says Ji.
This work will not only allow researchers to better observe quantum phenomena, but will also provide a way to engineer and control unique quantum properties simply by changing light beam patterns. "The idea that the modulation of light's polarization and intensity can change how an electrical charge is transported could be powerful design idea," says Mele.
Future development of ‘photonic’ and ‘spintronic’ materials that transfer digitized information based on the spin of photons or electrons respectively is also made possible thanks to these results. Agarwal hopes to expand this work to include other optical beam patterns, such as ‘twisted light’, which could be used to create new quantum computing materials that allow more information to be encoded onto a single photon of light.
"With quantum computing, all platforms are light-based, so it's the photon which is the carrier of quantum information. If we can configure our detectors on a chip, everything can be integrated, and we can read out the state of the photon directly," Agarwal says.
This story is adapted from material from the University of Pennsylvania, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.