  The amplitude of a pulse that propagates in a homogeneous material whose
properties are instantaneously changed periodically in time will undergo an
exponential increase, due to the interference between the reflected and
transmitted pulses generated at each sudden switch. Here we resolve the issue
by designing suitable reciprocal PT-symmetric space-time microstructures both
in the one-dimensional and two-dimensional case, so that the interference
between the scattered waves is such that the overall amplitude of the wave will
be constant in time in each constituent material. Remarkably, for the
geometries here proposed, a pulse will propagate with constant amplitude
regardless of the impedance between the constituent materials, and for some,
regardless of the wave speed mismatch. Given that the energy associated with
the wave will increase exponentially in time, this creates the possibility to
exploit the stable propagation of the pulse to accumulate energy for
harvesting.
