  We study the observational signatures from particle production of a $U(1)$
gauge field kinetically coupled to an inflaton. Regarding the form of gauge
kinetic function, we consider the possibility that it becomes stabilized at a
certain time, which makes the growing power of the gauge field evolve
non-monotonically with a sharp transition. Remarkably, the copious production
of the gauge field occurs on super-horizon scales at the late stage of
inflation and perturbations are enhanced on the intermediate scales during
inflation. We find that it can predict a bumpy shape of the curvature power
spectrum which leads to the generation of primordial black holes as a dark
matter after inflation. We also estimate two types of tensor modes sourced by
the gauge field: the primordial gravitational waves generated during inflation
and the induced gravitational waves provided by the enhanced curvature
perturbation after inflation. We show that both of them are potentially
testable with the future space-based gravitational wave interferometers.
