Selective Crystal Growth and Optical Responses of 2H and 3R Polytypes in van der Waals Polar Insulator α‐In 2 Se 3

Abstract The selective growth, unambiguous identification, and optical characterization of 2H and 3R polytypes in bulk single crystals of the van der Waals polar insulator α‐In 2 Se 3 are reported. High‐quality single crystals are obtained by optimizing chemical vapor transport (2H) and horizontal Bridgman (3R) methods, and their phases are reliably and efficiently verified by X‐ray Laue back‐reflection. Optical transmission spectra show the 2H phase has a slightly smaller band gap and steeper absorption edge than the 3R phase, consistent with first‐principles predictions of their electronic structures. Furthermore, absolute reflectance measurements unveil rich peak‐valley structures reflecting distinct van Hove singularities in their joint density of states, providing a physical characteristic that more clearly distinguishes the polytypes than the band‐gap feature. The good agreement with DFT simulations confirms that absolute reflectivity provides crucial insights into fine electronic structures beyond the band‐gap region. The results highlight pathways to further explore polytype‐dependent functional properties of α‐In 2 Se 3, such as nonlinear optical responses.