Twisted light in a nonlinear mirror.

Opposite directionality of the Poynting vector and the wave vector, an inherent property of negative index metamaterials (NIMs), was predicted to enable backward phase-matching condition for a second harmonic generation (SHG) process. As a result, such a nonlinear negative index slab acts as a nonlinear mirror. In this Letter, we predict that SHG with structured light carrying orbital angular momentum (OAM) and propagating in NIMs results in a possibility of generating a backward propagating beam with simultaneously doubled frequency, OAM, and reversed rotation direction of the wavefront. These results may find applications in high-dimensional communication systems, quantum information processing, and optical manipulation on a nanoscale.