  Recently a stable monolayer of antimony in buckled honeycomb structure called
antimonene was successfully grown on 3D topological insulator Bi$_2$Te$_3$ and
Sb$_2$Te$_3$, which displays semiconducting properties. By first principle
calculations, we systematically investigate the phononic, electronic and
optical properties of $\alpha-$ and $\beta-$ allotropes of monolayer
arsenene/antimonene. We investigate the dynamical stabilities of these four
materials by considering the phonon dispersions. The obtained electronic
structures reveal the direct band gap of monolayer $\alpha-$As/Sb and indirect
band gap of $\beta-$As/Sb. Significant absorption is observed in $\alpha-$Sb,
which can be used as a broad saturable absorber.
