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Van der Waals 2D materials have good mechanical and electrical properties and maintain them in layers less than 10 nm. In₂Se₃, in particular, has potential applications as photo-electric devices such as solar cells or phase-change memories. One quintuple layer (~ 1 nm) of In₂Se₃ consists of five atomic layers alternating between In and Se, and belongs to R3m symmetry group. Rotational anisotropic second-harmonic generation (RASHG) microscopy confirmed the three-fold crystal structure at room temperature.[1] We found three different stacking order samples; all layers oriented along the same direction, one layer rotated by 180 degrees with respect to the layers below, and one rotated by 90 degrees. The fundamental wavelength was tuned from visible light range to near IR, and nonlinear second-harmonic spectroscopic response of a few layers (1-3 layers) with different stacking order was measured to see the effect of interlayer coupling and the number of layers on nonlinear optical properties. We will present SHG experimental results showing four resonance peaks in this range, and relate them to the underlying nonlinear susceptibility tensor components.

[1] X. Tao and Y. Gu, Nano. Lett. 13, 3501 (2013)