Large second harmonic generation in a penta-CdO2 sheet exfoliated from its bulk phase

Abstract

Two-dimensional (2D) materials composed solely of pentagonal motifs are of particular interest due to their unique geometries and novel properties. Especially, the broken centrosymmetry and in-plane mirror symmetry in penta-graphene-like materials result in both in-plane and out-of-plane second harmonic generation (SHG). Transition metal cations with a filled d¹⁰ shell (d¹⁰-TM) can significantly enhance the SHG response as found in many previously studied bulk materials. Here, based on first-principles calculations combined with independent particle approximation, we show that large SHG can also exist in some 2D d¹⁰-TM oxides, such as penta-CdO₂, which can be chemically exfoliated from its bulk phase with its dynamical, thermal, and mechanical stability intact. We further find that penta-CdO₂ possesses extraordinary in-plane and out-of-plane SHG responses with large static SHG susceptibilities of χ^sheet₁₄(0) = χ^sheet₂₅(0) = χ^sheet₃₆(0) = 8.86 pm² V⁻¹ due to the phase-matching between fundamental and second-harmonic light. In addition, the low thermal expansion and a large optical band gap of 3.24 eV endow the penta-CdO₂ sheet with a high laser-induced damage threshold. Discussions are also made on the penta-ZnO₂ sheet. This study expands the family of 2D materials with outstanding SHG performance.