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.