  Time has emerged as a new degree of freedom for metamaterials, promising new
pathways in wave control. However, electromagnetism suffers from limitations in
the modulation speed of material parameters. Here we argue that these
limitations can be circumvented by introducing a traveling-wave refractive
index modulation, with the same phase velocity of the waves. We show how the
concept of "luminal grating" can yield giant nonreciprocity, achieve efficient
one-way amplification, pulse compression and frequency up-conversion, proposing
a realistic implementation in double-layer graphene.
