We studied the optical properties of a recently discovered, electric field induced stable lattice of topological defects in a nematic liquid crystal [1,2]. The square grid pattern with a unit cell containing two types of defects with opposite topological charge were studied by quantitative polarimetry, laser diffraction, and interferometry.

We present the tunable generation of optical vortices in this system by two distinct mechanisms. First, individual topological defects can convert circularly polarized light into a vortex beam with opposite handedness. Second, the grid structure containing a dislocation can be used as a fork grating; then diffracted laser beams carry different optical vorticity (and orbital angular momenta) depending on the diffraction order. In both cases, the vortex generation efficiency was found to be tunable by the applied voltage. The experimental results were confirmed by optical simulations.

[1] Y. Sasaki, V.S.R. Jampani, C. Tanaka, N. Sakurai, S. Sakane, K.V. Le, F. Araoka, H. Orihara, Nature Communications, 7, 13238 (2016).
[2] Y. Sasaki, M. Ueda, K.V. Le, R. Amano, S. Sakane, S. Fujii, F. Araoka, H. Orihara, Advanced Materials 29, 1703054 (2017).
[3] P. Salamon, N. Éber, Y. Sasaki, H. Orihara, Á. Buka, F. Araoka, Phys. Rev. Applied 10, 044008 (2018)