We demonstrate experimentally how an external mechanical stress applied to a VCSEL wafer results in an in-plane anisotropic strain, which dramatically alters the polarization behavior of our VCSELs. In the presence of in-plane strain, the VCSEL still emits linearly polarized light but its direction strongly depends on the magnitude and the orientation of the strain (with respect to the crystal co-ordinate system). The latter behavior can be understood by taking into account the elasto-optic effect and the proper strain distribution. Furthermore, for a specific range in magnitude and orientation of the externally induced strain, current induced polarization switching between the two fundamental modes (with orthogonal linear polarization) is present in a reproducible way. The current at which switching occurs strongly depends on the magnitude of the external stress and can be tuned in the whole region of single-mode operation.
These effects can be explained by accounting for the anisotropy of the valence band. The latter is induced by the in-plane uniaxial strain, leading to a modification of density of states and effective masses for different directions in the plane of the QW, and henceforth to a gain anisotropy and a different gain curve for each of the two polarization modes. Furthermore, the frequency splitting between the two cavity eigenmodes (also a result of the stress via the elasto-optic effect) has to be taken into account. We will discuss how the gain anisotropy changes with current, lattice temperature and carrier density, and how all these determine the polarization behaviour of VCSELs.