Second-harmonic and linear spectroscopy of $\textit{α}$-In2Se3

We report spectroscopic measurements of optical transmission, ellipsometry, and second-harmonic generation (SHG) from as-grown vapor-deposited $\textit{α}$-In2Se3 nanoflakes ranging in thickness from a single quintuple layer (QL) to bulk material. In this work, we compare these measurements with ab initio calculations of structural and optical properties. Linear optical measurements yield thickness-dependent band gaps and dielectric functions, while SHG diagnoses microscopic film structure and ferroelectric polarization. The rotational anisotropy of SHG for s-polarized incident photons of energy 1.4 eV reveals crystalline symmetry and orientation of individual QLs. Peak SHG intensity increases from one to three QLs, then decreases for larger numbers, tracking thickness-dependent trends in the second-order susceptibility components $χ^{xxx}$ and $χ^{zxx}$. Comparison of measured and calculated SHG spectra for s-polarized photons of energies between 1.2 and 1.7 eV incident upon two QL samples discriminates among candidate stacking arrangements, and favors an arrangement with electric polarization vectors of the two QLs pointing outward in opposite directions.