Weyl Semimetals for Photonic Devices

Insights from quantum physics have allowed engineers to incor­porate components used in circuit boards, optical fibers, and control systems in 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. Now, 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.

A microscopic image of multiple electrodes on a sheet of Weyl semimetal to provide circular movement of the light-induced electrical current by either left- or right-circularly polarized light. (Source: Z. Ji)

A hint of these uncon­ventional photo­galvanic properties, or the ability to generate electric current using 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 explains.

Then, while conducting new experiments on Weyl semimetals, where the unique quantum states exist in the bulk of the material, Agarwal and 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. 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 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,” 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. (Source: U. Penn)

Reference: Z. Ji et al.: Spatially dispersive circular photogalvanic effect in a Weyl semimetal, Nat. Mat., online 15 July 2019; DOI: 10.1038/s41563-019-0421-5

Link: Nanoscale Phase-Change and Photonics (R. Agarwal), University of Pennsylvania, Philadelphia, USA

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