exciton, 2D materials, optical properties, ab initio calculations

Large linear and nonlinear optical responses can be obtained from two-dimensional semiconductors with an indirect electronic gap. Here we demonstrate exceptionally large exciton-driven responses such as the fundamental exciton binding energy (2 eV), zero-point band-gap renormalization (200 meV), and nonlinear second and third harmonic coefficients (800 pm/V and 1.4x10^{-18} m^2/V^2 respectively) from an atomically thin binary group-V NP semiconductor. The influence of lattice vibrations on the absorption spectra unfolds strong electronic couplings to both LA and ZO-TO phonon modes. All the linear process analyses were computed using a fully ab-initio based G0W0+Bethe-Salpeter equation approach that also includes the electron-phonon self-energies. The nonlinear processes were instead obtained using a real-time ab-initio process flow after creating a coupling between the time-dependent external electric field and the correlated electrons within the modern theory of polarization and the same electron-hole interaction level.