  Refractory metal nitrides have recently gained attention in various fields of
modern photonics due to their cheap and robust production technology,
silicon-technology compatibility, high thermal and mechanical resistance, and
competitive optical characteristics in comparison to typical plasmonic
materials like gold and silver. In this work, we demonstrate that by varying
the stoichiometry of sputtered nitride films, both static and ultrafast optical
responses of refractory metal nitrides can efficiently be controlled. We
further prove that the spectral changes in ultrafast transient response are
directly related to the position of the epsilon-near-zero region. At the same
time, the analysis of the temporal dynamics allows us to identify three time
components - the "fast" femtosecond one, the "moderate" picosecond one, and the
"slow" at the nanosecond time scale. We also find out that the
non-stoichiometry does not significantly decrease the recovery time of the
reflectance value. Our results show the strong electron-phonon coupling and
reveal the importance of both the electron and lattice temperature-induced
changes in the permittivity near the ENZ region and the thermal origin of the
long tail in the transient optical response of refractory nitrides.
