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"Guided Modes of Anisotropic van der Waals Materials Investigated by near-Field Scanning Optical Microscopy"



The estimation of refractive indexes of nanometric thick Van der Waals anisotropic materials is a challenging task for conventional methods, such as ellipsometry. A group of scientists from Harvard University and the National Institute for Material Science in Japan reported a new method using NSOM for estimation of refractive indexes of 2D anisotropic van der Waals materials and heterostructures.

Two samples, which consist of an h-BN flake on SiO2/Si and Au substrates, were prepared and studied in this work. Waveguide modes and surface plasmon polariton (SPP) were excited by supercontinuum laser in h-BN / SiO2/Si and Au/h-BN stacks respectively. The superposition of the incident laser beam and the excited guided modes in the h-BN / SiO2/Si sample, and the excited SPP modes in the Au/h-BN sample, generated interference fringes. Evanescent fringes cannot be observed in the far-field; however, they can be directly imaged by NSOM. NSOM images of interference fringes were obtained on both samples. A Fast Fourier Transform (FFT) algorithm was applied to the NSOM images, which enabled the calculation of the guided wavelength and SPP wavelength, and the refractive indexes can be extracted from these wavelength values.  Thus, refractive indexes- nx, ny and nz of 2D anisotropic Van der Waals materials can be obtained. 

Authors: Daniel Wintz, Kundan Chaudhary, Ke Wang, Luis A. Jauregui, Antonio Ambrosio, Michele Tamagnone, Alexander Y. Zhu, Robert C. Devlin, Jesse D. Crossno, Kateryna Pistunova, Kenji Watanabe, Takashi Taniguchi, Philip Kim, and Federico Capasso

Reference: ACS Photonics, 5(4), 1196-1201.

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