Optical gears in a nanophotonic directional coupler

Abstract

Gears are rotating machines, meshing with each other by teeth to transmit torque. Interestingly, the rotating directions of two meshing gears are opposite, clockwise and counterclockwise. Although this opposite handedness motion has been widely investigated in machinery science, the analogue behavior of light remains undiscovered. Here, we present a simple nanophotonic directional coupler structure which can generate two light beams with opposite handedness of polarization states-optical gears. Due to the abrupt phase shift effect and birefringence effect, the angular momentum (AM) states of photons vary with the propagation distance in two adjacent waveguides of the coupler. Thus, by the choice of coupling length, it is able to obtain two light beams with opposite handedness of polarization, confirming the appearance of optical gears. The full control in the handedness of output beams is achieved via tuning the relative phase between two orthogonal modes at the input port. Optical gears thus offer the possibility of exploring light-matter interactions in nanoscale, opening up new avenues in fields of integrated quantum computing and nanoscale bio-sensing of chiral molecules.

On chip chirality-distinguishing beamsplitter

“…In particular, there is an abrupt π phase jump as the energy of one mode is fully coupled to the other. This is because there are two times of π/2 accumulations when the energy of any dominant linear component is coupled totally from one mode to the other [35]. Meanwhile, one should note that since the e-exponential function is a periodic function with a period of 2π, there is essentially no difference between the relative phase jumps of π or -π in Figs.…”

“…for a quasi-RCP one. The word "quasi-" means that in this non-paraxial case, the fundamental 0th order eigenmodes (quasi-TE and -TM modes) in the nanophotonic waveguides are not purely transverse and inevitably have some longitudinal electric field component (E z ) [35].…”