  Temporal metamaterials have been recently exploited as a novel platform for
conceiving several electromagnetic and optical devices based on the anomalous
scattering response arising at a single or multiple sudden changes of the
material properties. However, they are difficult to implement in realistic
scenarios by switching the permittivity of a material in time, and new
strategies to achieve time interfaces in a feasible manner must be identified.
In this paper, we investigate the possibility to realize a temporal
metamaterial without acting on the material properties, but rather on the
effective refractive index and wave impedance perceived by the wave during the
propagation in an empty guiding structure by varying the boundaries in time. We
demonstrate analytically and through numerical experiments that suddenly
changing the physical distance between the metallic plates of a parallel-plate
waveguide will induce an effective temporal interface. In addition to the
standard backward and forward scattered fields at different frequencies due to
the temporal interface, we also identify the presence of a static field
necessary to satisfy the continuity of the electromagnetic field across the
interface. The proposed concept can be extended to temporally controlled
metasurfaces, opening an easier path to the design and realization of novel
devices based on time-varying metamaterials at microwave and optical
frequencies.
