Asymmetry of Raman scattering by structure variation in space

We report on the discovery of asymmetries of Raman scattering along one scanning direction, between two scanning directions, and by structure variation of the sample in space. Asymmetry of Raman shift along the x direction, and the asymmetry of Raman shift and linewidth between the two scanning directions (x and y) are found for a 1210 nm diameter silica particle. The observed asymmetries are confirmed by further 2D Raman scanning of the same particle. To further explore the asymmetry of Raman scattering, glass fibers of three diameters (0.53, 1.00, and 3.20 μm) are scanned along two directions. The asymmetry of Raman shift along each direction, the asymmetry of linewidth along the y direction, and the asymmetry of Raman shift and linewidth between the two scanning directions are discovered. Additionally, 11 nm-thick MoSe₂ nanosheets on silicon are used to discover whether an asymmetry of Raman scattering exists at the edge of the nanosheets. One edge of the nanosheet is scanned in four directions and the asymmetry of Raman scattering caused by the step variation is also detected. All the observed Raman scattering asymmetries are explained soundly by the Raman signal diffraction and image shift on the CCD detector arrays of the Raman spectrometer. In practice, to use scanning Raman for surface structure study, great measure has to be taken to consider the structure-induced asymmetries to uncover the real Raman wave number variation by intrinsic material structure. We propose a signal processing method by averaging the scanning points along four directions to eliminate the interference of the edge. This method works well to significantly suppress the asymmetries of Raman properties and uncover the real Raman signal change by structure variation.