A new study from Penn researchers found that Weyl semimetals, a class of quantum materials, have bulk quantum states whose electrical properties can be controlled using light. The project 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 M. Rappe in the School of Arts and Sciences, along with Zheng Liu from Nanyang Technological University. Penn's Zachariah Addison, Gerui Liu, Wenjing Liu, and Heng Gao, and Nanyang's Peng Yu, also contributed to the work. Their findings were published in Nature Materials .
A hint of these unconventional photogalvanic properties, or the ability to generate electric current using light, was first reported by Ritesh 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", Ritesh Agarwal explained.
Then, while conducting new experiments on Weyl semimetals, where the unique quantum states exist in the bulk of the material, Ritesh Agarwal and Zhurun Ji got results that didn't match any 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.
Ritesh Agarwal and Zhurun Ji turned to Charles Kane and Eugene 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", stated Eugene 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 Andrew M. Rappe's insights on the electron energy levels inside the material, Zhurun 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", stated Zhurun Ji.
This work allows researchers to not only better observe quantum phenomena, but it provides 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", stated Eugene 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. Ritesh 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", Ritesh Agarwal stated.
Ritesh Agarwal and Eugene Mele emphasized the "heroic" effort made by Zhurun Ji, including an additional year's measurements made while running an entirely new set of experiments that were crucial to the interpretation of the study. "I've rarely seen a graduate student faced with that challenge who was able not only to rise to it but to master it. She had the initiative to do something new, and she got it done", stated Eugene Mele.