  Controlling amplitude, phase and polarization of electromagnetic waves is key
for a full manipulation of wave-matter interactions. The Brewster angle is one
of the important features in this context. Here, we exploit metamaterial
concepts with a time-modulated permittivity to propose the temporal equivalent
of the spatial Brewster angle, a concept we call temporal Brewster angle. We
consider temporal boundaries (as the temporal equivalent of the spatial
boundaries between two media) by rapidly changing the permittivity of the
medium, where a wave travels, from isotropic to an anisotropic permittivity
tensor. It is theoretically shown that when the incidence angle coincides with
that of the temporal Brewster angle a forward (temporal transmission) wave is
produced while the backward (temporal reflection) is eliminated. We provide a
closed-form analytical expression of the temporal Brewster angle and
demonstrate its performance both theoretically and numerically. Our findings
may provide a fresh view on how to control electromagnetic wave propagation and
wave-matter interactions in real time using temporal metamaterials.
