Optical absorption and excitation spectra of monolayer blue phosphorene

We study the optical absorption and excitation spectra of monolayer blue phosphorene with two approaches. The first is based on the [Formula: see text] approximation in conjunction with the Bethe-Salpeter equation theory. The second is based on the time-dependent density-functional theory in the adiabatic local density approximation and the random phase approximation. The spectra from the two approaches are quite similar. The optical absorption spectrum is dominated by a single peak at 4.2 eV, which originates from direct interband transitions at the [Formula: see text] point of the Brillouin zone. The excitation spectrum is dominated by a plasmon peak at 9.2 eV, which arises from collective excitations of valence electrons. The plasmon shows a positive dispersion at finite momentum transfer. The in-plane electron is responsible for the optical absorption, whereas the out-of-plane electron is responsible for the plasmon dispersion. Monolayer blue phosphorene has an indirect band gap of 2.98 eV and an exciton binding energy of 1.03 eV.