  Coherent optomechanical interaction between acoustic and optical waves known
as stimulated Brillouin scattering (SBS) can enable ultra-high resolution
signal processing and narrow linewidth lasers important for next generation
wireless communications, precision sensing, and quantum information processing.
While SBS has recently been studied extensively in integrated waveguides, many
implementations rely on complicated fabrication schemes, using suspended
waveguides, or non-standard materials such as As$_2$S$_3$. The absence of SBS
in standard and mature fabrication platforms prevents large-scale circuit
integration and severely limits the potential of this technology. Notably, SBS
in standard silicon nitride integration platform is currently rendered out of
reach due to the lack of acoustic guiding and the infinitesimal photo-elastic
response of the material. In this paper, we experimentally demonstrate advanced
control of backward SBS in multilayer silicon nitride waveguides. By optimizing
the separation between two silicon nitride layers, we unlock gigahertz acoustic
waveguiding in this platform for the first time, leading up to 15 $\times$
higher SBS gain coefficient than previously possible in silicon nitride
waveguides. Using the same principle, we experimentally demonstrate on-demand
inhibition of SBS by preventing acoustic guiding in the waveguide platform. We
utilize the enhanced SBS gain to demonstrate a microwave photonic notch filter
with high rejection (30 dB). We accomplish this in a low-loss, standard, and
versatile silicon nitride integration platform without the need of suspending
the SBS-active waveguide or hybrid integration with other materials.
