On-chip processing of optical orbital angular momentum

Orbital angular momentum is an important concept in optics, thus numerous researches explore the principles and applications of light beams with orbital angular momentum. This type of light beam is also called vortex beam, whose inherent infinite-dimensional feature has broad prospect in many fields like optical communications and optical computing. However, there are still a lot of problems in the propagation and processing of on-chip vortex beams. Here, with inverse design method, we design four cross-shaped waveguide structures which can support vortex beams with topological charge l = 1, 2, 3, 4 respectively. The four vortex modes are momentum matched so the coupling between every two waveguides can happen, achieving the conversion of orbital angular momentum. The longest width of each waveguide is about 2 um, so the cross-shaped waveguides are able to be integrated on a photonic chip. All the cross-shaped waveguides are silicon-on-insulator waveguide, and such silicon-based platform has the advantage of being compatible with complementary metal oxide semiconductor technology. Moreover, in order to demonstrate the application prospect, we construct an XOR logic gate with the conversion of orbital angular momentum. Therefore, our work provides a new idea for the manipulation of orbital angular momentum, expands the applications of on-chip vortex beams, and shows great potential.