Nonmonotonic threshold size dependence of buried-post vertical cavity lasers

The dependence on cavity width of the threshold gain of buried-post vertical-cavity surface emitting lasers (VCSELs) is analyzed using a 2-D model. In contrast to former reports on other VCSEL structures such as the oxide-apertured structure, nonmonotonic behaviors appear clearly both in the dependencies of threshold and resonance frequency of the present structure. By considering cavity modes as a superposition of many waveguide modes propagating parallel to multilayers, it is revealed that the physical mechanism causing this nonmonotonicity is the interference effect among these component waveguide modes. The interference causes the center of gravity of the lateral energy-flow of the superimposed field to wiggle through the multilayer waveguide, and thus, depending on the cavity size, either to hit or to miss the reflecting index-step structure of the cavity sidewall which, accordingly, makes the lateral energy confinement either efficient or loose, resulting in a low- or high-loss cavity. The oscillatory threshold dependences of the fundamental and the first higher-order modes appear out of phase, which results in even larger oscillatory variation in their threshold difference. This indicates that, for stabilization of fundamental-mode oscillations in the present cavity structure, cavity size should be one of discrete optimal values rather than simply smaller. Besides the buried post structure, the oxide-apertured structure and the dielectrics-passivated post-cavity structure are also analyzed using the 2-D model and compared with experimental data.