Helically twisted nonlinear photonic crystals.

Nonlinear photonic crystals with a helical structure in the second-order nonlinear coefficient (χ) are fabricated using infrared femtosecond laser poling in ferroelectric SrBaNbO crystals. The quasi-orbital angular momentum of the helical χ structure can be imprinted on the interacting photons during nonlinear optical processes, allowing the topological charge of the generated photons at new frequencies to be controlled. Here we study the case of a double-helix nonlinear photonic structure for the generation of a second-harmonic vortex beam from a Gaussian pump beam without phase singularity. The conservation law for orbital angular momentum in the second-harmonic process is also verified, with the topological charge of the pump photons being fully compensated by the double-helix structure. The flexible control of light carrying orbital angular momentum (OAM) at new frequencies will find important applications in both classical and quantum photonics, such as nonlinear wavefront shaping and multidimensional entanglement of photons.