  In view of momentum continuity at a temporal slab, it is shown that
instantaneous switching of an isotropic medium to an anisotropic medium offers
the incident frequency a directional property- a counterintuitive process which
is called frequency vectoralization. By expressing the dispersion diagrams
before and after the temporal transition, a general analogy between spatial and
temporal interfaces is given and the concept of frequency birefringence, i.e.
double frequency jump, will be explained. Furthermore, it will be shown that an
anisotropic temporal interface diffracts a monochromatic beam in both the
spatial and spectral domains.
