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National Nano Centers and others have made progress in the study of crystal optical anisotropy
Recently, a research team from the National Center for Nanoscience and Technology, led by Dai Qing, in collaboration with Professor Liu Mengkun from Stony Brook University in the U.S., has made significant progress in characterizing the optical anisotropy of van der Waals crystals. By employing near-field optical techniques, they overcame the challenges posed by the small size of these materials and successfully measured the dielectric tensor of boron nitride and molybdenum disulfide. This breakthrough has led to the development of a novel method for analyzing optical anisotropy in two-dimensional crystals.
Two-dimensional materials such as graphene, boron nitride, and transition metal dichalcogenides are all classified as van der Waals crystals. These materials exhibit exceptional mechanical, electrical, and optical properties, making them essential building blocks for constructing functional van der Waals heterostructures. They are also considered key candidates for next-generation optoelectronic devices. The layered structure of van der Waals crystals, held together by strong covalent bonds within layers and weak van der Waals forces between them, leads to inherent anisotropy in their physical properties—especially optical anisotropy, which plays a crucial role in device design.
However, traditional methods for measuring optical anisotropy, such as far-field reflection techniques like ellipsometry, face limitations when applied to small or micro-sized van der Waals crystals due to challenges in obtaining high-quality single crystals. To address this, the research team introduced a new approach using scattering-type scanning near-field optical microscopy (s-SNOM).
In their study, the team first demonstrated the presence of both transverse electric (TE) and transverse magnetic (TM) waveguide modes in anisotropic van der Waals nanosheets. These modes correspond to in-plane and out-of-plane wavevectors, respectively, and are closely related to the dielectric properties of the material. By exciting these modes with s-SNOM, they captured real-space near-field optical images and performed Fourier analysis on the resulting data. This allowed them to accurately determine the optical anisotropy of the material.
This innovative technique not only overcomes the size constraints of conventional characterization methods but also enables precise measurement of optical anisotropy in both uniaxial and biaxial van der Waals crystals. Moreover, the method can be adapted for studying few-layer or even monolayer van der Waals materials by optimizing the substrate design.
The findings were published online in *Nature Communications*, and the team has filed for invention patents. The research was supported by the National Natural Science Foundation of China and the Ministry of Science and Technology’s key R&D programs. This advancement opens new possibilities for the study and application of two-dimensional materials in advanced optoelectronic systems.