Enhanced optical gradient forces between coupled graphene sheets.

Optical gradient forces between monolayer infinite-width graphene sheets as well as single-mode graphene nanoribbon pairs of graphene surface plasmons (GSPs) at mid-infrared frequencies were theoretically investigated. Although owing to the strongly enhanced optical field, the normalized optical force, fn, can reach 50 nN/μm/mW, which is the largest fn as we know, the propagation loss is also large. But we found that by changing the chemical potential of graphene, fn and the optical propagation loss can be balanced. The total optical force acted on the nanoribbon waveguides can thus enhance more than 1 order of magnitude than that in metallic surface plasmons (MSPs) waveguides with the same length and the loss can be lower. Owing to the enhanced optical force and the significant neff tuning by varying the chemical potential of graphene, we also propose an ultra-compact phase shifter.