New two-dimensional materials such as graphene, boron nitride and transition metal chalcogenide belong to van der Waals crystal. Each of them has excellent mechanical, electrical and optical properties and is the basic unit for constructing a controlled van der Waal's heterojunction. Van der Waals crystals have a layered structure with strong covalent bond interactions in the layer and a weak van der Waals force in the interlayer, which determines the morphology of van der Waals' Various physical properties have natural anisotropy, of which optical anisotropy is crucial for the design and optimization of new optoelectronic devices.Because of the current problem of producing high quality van der Waals single crystal size, the traditional far-field beam-based optics It is difficult to accurately measure the optical anisotropy of van der Waals microcrystals by means of heterosexual characterization (such as end reflection and ellipsometry).
Dai Qing team first demonstrated the existence of the normal (TE) and extraordinary waveguide (TM) modes in the anisotropic van der Waals nanoflakes, and the in-plane wave vectors of the two modes are respectively in-plane and out-of-plane Then, the TE and TM waveguide modes were excited in van der Waals nanosheets by using a scattering-type near-field optical microscope (s-SNOM), and real-space near-field optical imaging was performed. Finally, Fourier analysis of the field optical images to obtain the optical anisotropy of the measured van der Waals crystals.The above method overcomes the limitation of the traditional characterization methods on the sample size and enables the optical anisotropy of the uniaxial and biaxial van der Waals crystal materials Accurate characterization.The method is also suitable for the direct characterization of the optical anisotropy of a few or even a single layer of van der Waals lens through the optimized design of the substrate material.
Relevant research results were published online in "Nature - Communication", and its characterization method has been applied for invention patents.The research has been funded by the National Natural Science Foundation of China, Ministry of Science and Technology key research and development projects and other projects.
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The National Nanocrystal Center has been progressing in the study of optical anisotropy of crystals