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 specific type of asymmetry can emerge from the inherent properties of the medium (for example, in the case of a crystalline substance), or it can be introduced through external factors (such as a magnetic field present in a plasma or caused by another laser beam).