  The discovery of superconductivity in infinite-layer nickelates establishes a
new category of unconventional superconductors that share structural and
electronic similarities with cuprates. Despite exciting advances, such as the
establishment of a cuprate-like phase diagram and the observation of charge
order and short-range antiferromagnetic fluctuation, the key issues of
superconducting pairing symmetry, gap amplitude, and superconducting
fluctuation remain elusive. In this work, we utilize static and ultrafast
terahertz spectroscopy to address these outstanding problems. We demonstrate
that the equilibrium terahertz conductivity and nonequilibrium terahertz
responses of an optimally Sr-doped nickelate film ($T_c$ = 17 K) are in line
with the electrodynamics of $d$-wave superconductivity in the dirty limit. The
gap-to-$T_c$ ratio 2$\Delta/k_\mathrm{B}T_\mathrm{c}$ is extracted to be 3.4,
indicating the superconductivity falls in the weak-coupling regime. In
addition, we observed significant superconducting fluctuation near
$T_\mathrm{c}$, while it does not extend into the deep normal state as
optimally hole-doped cuprates. Our result highlights a new $d$-wave system
which closely resembles the electron-doped cuprates, expanding the family of
unconventional superconductivity in oxides.
