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 vertic al cavity surface emitting lasers (VCSELs). The 0 degrees polarized compone nt of the near-field always has circular or high-order rotational symmetry, indicating that all significant optical properties of the laser are isotro pic for Light of that polarization. In contrast, the simultaneously present 90 degrees polarized part of the same near-field is always symmetrical onl y upon reflection. This is evidenced by sequences of near-field images of i ncreasing 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 birefringen ce, which is induced into any electrically pumped VCSEL by the applied vert ical electrical field via the linear electro-optic effect. Thus, the optica l index becomes a function of the transverse component of the k-vector of l ight inside the cavity, which corresponds to an angular dependent index. As the functional dependence on the emission angle is of different strength f or orthogonal polarizations, the circular symmetry of the laser is broken o nly for 90 degrees polarized light, while the effect on the 0 degrees polar ization is too small to affect the near-field. The highly symmetrical near-field of the unaffected polarization shows rema rkable similarities to another physical system of circular symmetry, the st atic patterns in cellular flames. Our analysis of the symmetry properties o f the near-fields has implications for the design of VCSELs as well as for future modelling activities.