All-optical switching of nonlinear-beam orbital angular momentum through crystal symmetry

Determine whether the crystal symmetry of transition-metal dichalcogenides, specifically the symmetry-dependent nonlinear response of 3R-MoS₂, can be harnessed to all-optically switch the orbital angular momentum of a nonlinear optical beam.

Background

Orbital angular momentum (OAM) control in nonlinear optics had previously been demonstrated through nonlinear frequency mixing of optical vortices in monolayer transition-metal dichalcogenides, including difference-frequency generation, sum-frequency generation, and four-wave mixing. In those experiments, the OAM of the generated field was determined by the OAM of the incident fields through OAM conservation, rather than being actively controlled by the material’s crystal-symmetry-dependent nonlinear response.

The unresolved issue is whether crystal symmetry itself can provide an all-optical control mechanism for the OAM of a nonlinear output beam. The paper addresses this question by engineering the orientation of rhombohedrally stacked (3R) MoS₂, whose C₃ᵥ symmetry produces a tensor-driven nonlinear geometric phase, and subsequently demonstrates switching among Hermite–Gaussian-like and vortex second-harmonic beams with opposite topological charges.