van der Waals (vdW) layered semiconductors exhibiting large nonlinear-optical (NLO) effects have substantial potential for developing nanoscale quantum optical devices. NbOCl₂, an ultrathin quantum light source, has recently displayed a large second harmonic effect. However, its small bandgap ({~}1.8 eV) impedes transparent light conversion across wide spectral regions, especially when utilizing practical 1-{}m lasers. In this paper, we found that MO₂X₂ (M=W, Mo; X=Cl, Br) effectively widens the bandgap by using W⁶⁺ or Mo⁶⁺ instead of Nb⁴⁺ to eliminate in-gap nonbonding orbitals. Meanwhile, the octahedral [MO₄X₂] motifs display polar Jahn-Teller distortions as large as [NbO₂X₄] to demonstrate comparable NLO polarization effects. Based on first-principles calculations, WO₂Cl₂ and MoO₂Br₂ are predicted to possess wider bandgaps (2.9 and 2.2 eV), giant birefringence (0.5--0.7), and significant second harmonic and ferroelectric photovoltaic effects competitive with NbOX₂. These findings provide strong incentives to explore new vdW materials with superior NLO properties.
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Huo et al. (2024) studied this question.
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