An apparatus (100) for controlling propagation of incident electromagnetic radiation (110) is described, comprising a composite material (102) having electromagnetically reactive cells (106) of small dimension relative to a wavelength of the incident electromagnetic radiation (110). At least one of a capacitive and inductive property of at least one of the electromagnetically reactive cells (106) is temporally controllable to allow temporal control of an associated effective refactive index encountered by the incident electromagnetic radiation (110) while propagating through the composite material (106).