TM-Polarization-Enhanced Stimulated Brillouin Scattering in Fully Oxide-Cladded Double-Stripe Silicon Nitride Waveguides

Silicon nitride (Si3N4) has emerged as a low-loss, CMOS-compatible photonic platform with strong potential for large-scale integrated optical systems. However, achieving strong stimulated Brillouin scattering (SBS) in fully oxide-cladded Si3N4 waveguides remains challenging because of weak acoustic confinement and limited optomechanical coupling. Here, we investigate backward SBS in fully oxide-cladded symmetric doublestripe Si3N4 waveguides and reveal optical polarization as a key design degree of freedom for enhancing Brillouin interactions in this geometry. By systematically exploring the waveguide geometry for both fundamental polarizations, we identify a pronounced TM-favored regime in which the TM-like mode can exhibit several-fold larger Brillouin gain than the corresponding TE-like mode. The optimized TM configuration reaches a peak gain above 7 m−1 W−1, representing an approximately sixfold enhancement over the highest previously reported simulated Brillouin-gain value of about 1.2 m−1 W−1 for comparable fully oxide-cladded double-stripe Si3N4 waveguides. Previous studies of comparable fully oxide-cladded Si3N4 structures generally neglected radiation-pressure contributions because the modest Si3N4/SiO2 refractive-index contrast was expected to make boundary forces weak. We further show that radiation pressure, although weak when considered independently, coherently interferes with electrostriction and produces a substantial correction to the overall SBS response, accounting for nearly 20% of the peak gain in the optimized TM regime. These results establish TM polarization as an effective route for substantially strengthening SBS in fully oxide-cladded double-stripe Si3N4 waveguides and open a path toward enhanced Brillouin functionalities in low-loss, CMOS-compatible silicon nitride photonic circuits.