van der Waals (vdW) layered semiconductors exhibiting large nonlinear-optical (NLO) effects have substantial potential for developing nanoscale quantum optical devices. ${\mathrm{NbOCl}}_2$, 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-µm lasers. In this paper, we found that ${M\mathrm{O}}_2{X}_2$ $(M=\mathrm{W}, \mathrm{Mo}; X=\mathrm{Cl}, \mathrm{Br})$ effectively widens the bandgap by using ${\mathrm{W}}^{6+}$ or $\mathrm{M}{\mathrm{o}}^{6+}$ instead of $\mathrm{N}{\mathrm{b}}^{4+}$ to eliminate in-gap nonbonding orbitals. Meanwhile, the octahedral $\left[{M\mathrm{O}}_4{X}_2\right]$ motifs display polar Jahn-Teller distortions as large as $\left[{\mathrm{NbO}}_2{X}_4\right]$ to demonstrate comparable NLO polarization effects. Based on first-principles calculations, ${\mathrm{WO}}_2{\mathrm{Cl}}_2$ and ${\mathrm{MoO}}_2{\mathrm{Br}}_2$ 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 $\mathrm{NbO}{X}_2$. These findings provide strong incentives to explore new vdW materials with superior NLO properties.

中文翻译：

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具有大双折射、大二次谐波和铁电光伏效应的极性和层状宽带隙半导体WO2Cl2和MoO2Br2

范德华（vdW）层状半导体表现出巨大的非线性光学（NLO）效应，具有开发纳米级量子光学器件的巨大潜力。${\mathrm{氯化铌}}_2$一种超薄量子光源，最近表现出巨大的二次谐波效应。然而，其带隙较小（~1.8 eV）阻碍了宽光谱区域的透明光转换，特别是在使用实用的 1 µm 激光器时。在这篇论文中，我们发现${\mathrm{中号}\mathrm{氧}}_2{X}_2$ $（\mathrm{中号}=\mathrm{瓦}, 莫; X=氯, 溴）$通过使用有效地加宽带隙${\mathrm{瓦}}^{6+}$或者$\mathrm{中号}{\mathrm{哦}}^{6+}$代替$\mathrm{氮}{\mathrm{乙}}^{4+}$消除带隙内的非键合轨道。同时，八面体$\left[{\mathrm{中号}\mathrm{氧}}_4{X}_2\right]$图案显示极性 Jahn-Teller 扭曲高达$\left[{\mathrm{氧化铌}}_2{X}_4\right]$展示可比较的 NLO 偏振效应。根据第一性原理计算，${\mathrm{沃}}_2{\mathrm{氯}}_2$和${\mathrm{氧化钼}}_2{\mathrm{溴}}_2$预计具有更宽的带隙（2.9 和 2.2 eV）、巨大的双折射（0.5-0.7）以及显着的二次谐波和铁电光伏效应，可与$\mathrm{氧化铌}{X}_2$。这些发现为探索具有优异 NLO 性能的新型 vdW 材料提供了强有力的动力。