Directional Coupling of Emitters into Waveguides: A Symmetry Perspective

Abstract

Recent experiments demonstrate strongly directional coupling of light into waveguide modes. Here, the symmetry mechanisms behind this effect are studied, and it is shown that the analysis of the symmetries and symmetry-breakings of the emitter-waveguide system allows to qualitatively understand directional coupling in several situations. The authors consider emitters either centered in a median plane of the waveguide, or displaced from it, and whose emissions have a well-defined angular momentum in either one of the two different axis typically chosen experimentally, which are called transverse and vertical. These insights are matched by simulations, and previous experimental measurements. It is shown that handedness plays a secondary role in directional coupling. The spin-momentum locking concept is generalized to an exponentially strong locking between the transverse angular momentum and the preferential coupling direction. A new selection rule is obtained that controls the coupling of electric(magnetic) multipolar emissions into waveguide modes. An experiment is proposed featuring a transverse magnetic bias that aggregates the directional emissions from many quantum dots on top of waveguides, in contrast to the typically used vertical bias, which effectively restricts experiments to using a single quantum dot. Finally, the Huygens' dipole is analyzed and the symmetries that enable its directional behavior revealed.