Polarization selective symmetry breaking in the near-fields of vertical cavity surface emitting lasers

We study symmetries in multi-transverse-mode near-fields of circular vertical cavity surface emitting lasers (VCSELs). The 0◦ polarized component of the near-field always has circular or high-order rotational symmetry, indicating that all significant optical properties of the laser are isotropic for light of that polarization. In contrast, the simultaneously present 90◦ polarized part of the same near-field is always symmetrical only upon reflection. This is evidenced by sequences of near-field images of increasing complexity for increasing pump current. The presence of a preferred direction of the symmetry axis is evidence for a symmetry-breaking anisotropy. We attribute this anisotropy to birefringence, which is induced into any electrically pumped VCSEL by the applied vertical electrical field via the linear electro-optic effect. Thus, the optical index becomes a function of the transverse component of the k-vector of light inside the cavity, which corresponds to an angular dependent index. As the functional dependence on the emission angle is of different strength for orthogonal polarizations, the circular symmetry of the laser is broken only for 90◦ polarized light, while the effect on the 0◦ polarization is too small to affect the near-field. The highly symmetrical near-field of the unaffected polarization shows remarkable similarities to another physical system of circular symmetry, the static patterns in cellular flames. Our analysis of the symmetry properties of the near-fields has implications for the design of VCSELs as well as for future modelling activities.