  Brillouin enhanced four wave mixing in the form of a Brillouin dynamic
grating (BDG) enables a uniquely tunable filter, whose properties can be tuned
by purely optical means. This makes the BDG a valuable tool in microwave
photonics (MWP). BDGs have been studied extensively in fibers, but the only
observation in an integrated platform required exotic materials. Unlocking BDG
in a standard and mature platform will enable its integration into large-scale
circuits. Here we demonstrate the first observation of a BDG in a silicon
nitride (Si$_3$N$_4$) waveguide. We also present a new, optimized design, which
will enhance the BDG response of the waveguide, unlocking a path to large-scale
integration into MWP circuits.
