Measurement of the Phase and Intensity Profile of Surface Plasmon Laser Emission

We study the near- and far-field radiation patterns of surface plasmon (SP) lasers in metal hole arrays and observe radially polarized vortex-vector laser beams in both near and far field. Besides the intensity profile, also the complementary phase profile is obtained with a beam block experiment, where we block part of the beam in the near field, measure the resulting changes in the far field, and retrieve the phase using an iterative algorithm. This phase profile provides valuable information on the feedback mechanisms and coherence of the laser and shows that our SP laser operates in a phase-slip mode instead of a pure dark mode. To explain our observations, we extend the standard model for distributed feedback lasers by introducing a position dependence in the optical gain and refractive index.