  Interaction of electromagnetic radiation with time-variant objects is a
fundamental problem whose study involves foundational principles of classical
electrodynamics. Such study is a necessary preliminary step for delineating the
novel research field of linear time-varying metamaterials and metasurfaces. A
closer look to the literature, however, reveals that this crucial step has not
been addressed and important simplifying assumptions have been made. Before
proceeding to studies of linear time-varying metamaterials and metasurfaces
with their effective parameters, we need to rigorously describe the electric
and magnetic responses of a temporally-modulated meta-atom. Here, we introduce
a theoretical model which describes a time-variant meta-atom and its
interaction with incident electromagnetic waves in time domain. The developed
general approach is specialized for a dipole emitter/scatterer loaded with a
time-varying reactive element. We confirm the validity of the theoretical model
with full-wave simulations. Our study is of major significance also in the area
of nanophotonics and nano-optics because the optical properties of
all-dielectric and plasmonic nanoparticles can be varied in time in order to
achieve intriguing scattering phenomena.
