Nonlinear directional emission on monolayer MoS ₂ by optical orbit-orbit coupling

Controlling the direction of light emission on the subwavelength scale is a key to manipulating light and improving efficiency in optical sensing and quantum information processing. In functional metasurfaces, photonic spin angular momentum is widely used to control the propagation of light emission by geometric phase, but it is challenging to accomplish using monolayer materials because of lacking optical resonance in atomic thickness. Furthermore, the manipulation of the optical geometric phase based on spin angular momentum beams typically offers only two tunable states. Here, we report optical control of free-space routing of second harmonic generation in monolayer MoS ₂ assisted by photonic orbital angular momenta. The experimental directionality of the second harmonic light can reach 0.84. Moreover, we can dynamically route the second harmonic generation signal on the engineered monolayer MoS ₂ with four directions, showing high-order information encryption. Our findings thus provide great flexibility and more degrees of freedom in nonlinear optical modulation, integrated photonics, and dynamic flat optics.