We elucidate that coherent phonon can trigger nonzero nonlinear optical injection current otherwise silent systems. By performing density functional theory calculations in black phosphorene monolayer, we show that infrared active phonon could trigger unidirectional photocurrent, which wiggles in magnitude but propagates along one direction. For more details, please see our recent Phys. Rev. B paper at https://journals.aps.org/prb/abstract/10.1103/PhysRevB.104.155146.