Realizing optical frequency conversion function with high operability using topological materials – ResOU

Figure: Schematic showing switching of optical frequency conversion function at Weyl Semimetal.

The research group is led by graduate student Kentaro Shuriki, Assistant Professor Yoshihiro Okamura, graduate student Keigo Morishi, and Associate Professor Yutaro Takahashi from the University of Tokyo Graduate School of Engineering, and led by Yoshinori Tokura from the RIKEN Emergency Center. Material Science: In collaboration with a research group that includes the Center Director, Associate Professor Hiroshi Murakawa, Associate Professor Hideaki Sakai, and Professor Noriaki Hanasaki from the Graduate School of Science, Osaka University, Associate Professor Kohei Yokoi from the Faculty of Science, Gakushuin University, and Associate Professor Hidetomo Usui from College of Science and Technology, Shimane University. In Weyl semimetals with effective electromagnetism and polarization, second harmonics occur (SHG, a phenomenon that doubles the frequency of light), which is one of the nonlinear optical effects. With very high efficiency, we have demonstrated that the strength of the SHG can be switched depending on the direction of light transmission and the direction of magnetization. .

Materials and physical phenomena described using a mathematical concept called topology are attracting worldwide interest, and Weyl metals are a representative example. Although topological properties are expected to be used as new functions of materials, examples are still limited. The semi-metallic PrAlGe, which this research group focused on, has effective electrical polarization, so when it is irradiated with light, SHG, a nonlinear optical effect, occurs. As a result of actually measuring the SHG generation density, we found that the nonlinear susceptibility, which is an indicator of generation efficiency, is very large compared to known materials. Furthermore, we discovered that different optical functions are expressed due to the interference effect with the SHG derived from the magnetization occurring at the same time. For example, it has been observed that the intensity of SHG generation is modified depending on the direction of magnetization, and that switching of SHG intensity occurs when the direction of light incidence is reversed. These properties will form the basis of future optical switching devices. It can be said that this result demonstrates the function of materials combining topological and multiferroic materials, and paves the way for the development of various linear and nonlinear electromagnetic responses expected in the future.

The results of this research will be published in the online version of the US scientific journal 'Proceedings of the National Academy of Sciences of the United States of America' on March 14, 2024 (Eastern Daylight Time).

In crystalline solids, one of the basic design guidelines for producing functional responses is symmetry breaking. The breaking of the spatial reversal symmetry represented by electric polarization and the time reversal symmetry resulting from magnetization appear in ferroelectric and magnetic materials, respectively, and are used as functional materials that support modern society. Materials that have both of these properties are called multiferroics, and many electromagnetic (ME) effects that differ from normal electromagnetic phenomena have been discovered in static electromagnetic fields and optical responses. On the other hand, Weyl metalloids, which have topological properties, require either time-reversal symmetry breaking or space-reversal symmetry breaking as a condition for their manifestation, but topological materials such as ferrites have both properties and little is known about their functions.

It has been revealed that many giant electromagnetic responses occur in Weyl semimetals due to the quantum geometric phase effect of a pair of electronic state singularities called Weyl nodes (Figure 1(a)). In fact, the largest nonlinear optical response and magneto-optical response in the infrared region have ever been achieved in Weyl semimetals. Therefore, in Weyl metals where the spatiotemporal reflection symmetry is broken, a variety of optical phenomena can be expected to occur due to the synergistic effect of general nonlinear optical effects and ME effects.

This research group focused on PrAlGe, a Weyl quasi-metal whose spatiotemporal reversible symmetry is broken by magnetization and effective electrical polarization, and conducted research aimed at observing nonlinear optical effects and developing its functionalities. SHG, a kind of nonlinear optical effect, is a phenomenon of frequency doubling of light, widely used in familiar optical devices. By irradiating PrAlGe with light and observing the density of SHG generated on the crystal surface, it was revealed that the conversion to SHG occurred with extremely high efficiency. In fact, we found that nonlinearity, an indicator of conversion efficiency, is one of the highest among materials observed to date. We also find that the observed SHG contains a component due to magnetism. By interfering the magnetically induced SHG with the nonmagnetic SHG, we achieved an optical function that enables switching of the SHG intensity. Indeed, we demonstrated that the SHG intensity is modulated by reversing the sign of magnetization, and SHG intensity switching occurs by reversing the direction of light propagation (Figure 1(b)). Moreover, it is clear that the magnetically induced nonlinear susceptibility, which is the origin of this optical function, shows a very large value, about 30 times that of Cr2O3, a typical antiferromagnetic material showing ME response. This indicates that in Weyl metals where the space-time reversal symmetries are broken simultaneously, the quantum geometric phase of the topological band structure essentially produces strong ME coupling in nonlinear optical phenomena.

Figure 1. (a) Band structure in Weyl semi-metals. (b) Schematic diagram of SHG generation. Top row: Light fell from the left side of the crystal, and the SHG intensity generated on the crystal surface was measured. Bottom row: Arrangement of light entering from the right side. It has become clear that there is a significant difference in the intensity of the SHG generated depending on the direction of incident light.

In this study, we reveal that a huge nonlinear optical ME response occurs in Weyl metals as a phenomenon resulting from the fusion of multiple ferroelectric materials and topological materials. This is not only important because it broadens the characteristic electromagnetic response of topological materials, but is also expected to lead to the development of future optical functional devices such as nonlinear optical effect switching devices using topological metalloids.