Large Exciton-Driven Linear and Nonlinear Optical Processes in Monolayers of Nitrogen Arsenide and Nitrogen Antimonide

We demonstrate that atomically thin nitrogen-based binary group V wide band gap indirect semiconductors (β-NX (X = As, Sb)) embody 3-fold valley degenerated band structures and strong linear and nonlinear optical activities. Contrary to NAs, the fundamental optical absorption in NSb is quite resilient toward the temperature variations between 0 and 450 K. In the absence of lattice vibrations, exciton in NSb is, however, less strongly bound (1.30 eV) compared to the tighter case in NAs (1.62 eV), leading to a more delocalized Mott–Wannier-type texture. The inhomogeneous excitonic line widths for both monolayers within these temperatures are within the 100–400 meV range. The most significant nonlinear second harmonic optical coefficients (2ω resonances) in NAs and NSb monolayers are obtained as ∼636 and 270 pm/V at 3.2 eV (387 nm) and 2.6 eV (477 nm), respectively. We also present a detailed analysis of in-plane biaxial strain on these structures and found that tensile strain dynamically stabilizes structures with no loss in absorbance spectra (and does not influence exciton binding energy in NAs). The nonlinear coefficient also significantly improves at the two most important 810 and 1560 nm wavelengths compared to the cases offered by the monolayer transitional metal dichalcogenides. Our analyses originate from a fully abinitio G0W0+Bethe-Salpeter excited-state theory. The temperature-dependent linear absorption spectra are evaluated by including the electron–phonon self-energies, whereas the nonlinear spectra are treated using the modern theory of polarization within the same perturbative approach. Our reference findings provide important advanced characteristics for applications of two-dimensional β-NX (X = As, Sb) materials and should motivate further theoretical and experimental investigations of these interesting materials.本研究表明，原子级薄层氮基二元第V族宽带隙间接半导体（β-NX，X=As、Sb）具备三重谷简并能带结构，且展现出优异的线性与非线性光学活性。与NAs不同，NSb的本征光吸收对0至450 K区间内的温度变化具有良好耐受性。在无晶格振动的条件下，NSb中的激子束缚能（1.30 eV）低于NAs中的激子束缚能（1.62 eV），进而形成离域程度更高的莫特-万尼尔（Mott–Wannier）型激子结构。两种单层材料在该温度区间内的非均匀激子线宽均处于100~400 meV范围内。NAs与NSb单层的最显著非线性二次谐波光学系数（2ω共振峰）分别在3.2 eV（387 nm）与2.6 eV（477 nm）处达到约636 pm/V和270 pm/V。本研究还详细分析了面内双轴应变对该类结构的影响，发现拉伸应变可实现结构的动态稳定，且不会削弱吸收光谱（也不会改变NAs中的激子束缚能）。相较于单层过渡金属二硫族化合物，该材料在810 nm与1560 nm这两个关键波长下的非线性光学系数也得到了显著提升。本研究的分析基于全从头算G0W0+贝塞-萨尔佩特（Bethe-Salpeter）激发态理论。温度相关的线性吸收光谱通过引入电子-声子自能进行计算，而非线性光谱则通过同一微扰框架下的现代极化理论进行处理。本研究所得的参考结果为二维β-NX（X=As、Sb）材料的应用提供了重要的先进特性参考，有望推动这类极具研究价值的材料开展进一步的理论与实验研究。