Such an anisotropy can be either inherent to the media (e.g., in the case of a crystal substance), or imposed externally (e.g., in the presence of magnetic field in plasma or by another laser beam).
A certain type of asymmetry can arise from within the material itself (for instance, in the case of a crystalline substance), or it can be introduced from outside (such as when a magnetic field is present in a plasma or caused by another laser beam).