Cavity spatial mode-locking and high controllability of radial output coupling for circular/square plasmonic nano-resonator lasers

We proposed and investigated a novel output coupling scheme for a circular and a square plasmonic nano-ring laser based on a T-shaped radial coupler that is easier to realize than a tangential coupler.The amount of coupling efficiency is shown to be highly controllable from a few percent to tens of percents. This is due to the fact that the standing-wave lasingmodepattern will rotate to give the minimal cavity loss at the T-coupler's location, making the amount of output coupling surprisingly low and hence, controllable. For a non-circular cavity, other symmetry-breaking and geometryinduced scattering could result in separate mode-pattern locking. These give a few main ways to control and optimize the coupling efficiency: via widening/narrowing or rotating the T-coupler's waveguide, or, for the case of a noncircular cavity, via shifting the location of the T-coupler. We observed increased unidirectional lasing induced by either rotating the waveguide or shifting it (for non-circular cases). We simulated the coupling using Maxwell's equations based on the multi-level multi-electron FDTD (MLME-FDTD) method to realistically model the lasing and output coupling behaviors of such plasmonic semiconductor lasers.