Integrated silicon nitride devices via inverse design

Integrated photonic devices made of silicon nitride, which can be integrated with silicon-on-insulator and III-V platforms, are expected to drive the expansion of silicon photonics technology. However, the relatively low refractive index contrast of silicon nitride is often considered a limitation for creating compact and efficient devices. Here, we present three freeform silicon nitride devices-a coarse wavelength-division multiplexer, a five-mode mode division multiplexer, and a polarization beam splitter-while systematically benchmarking both the design capability and the fabrication repeatability and robustness of inverse-designed components. We demonstrate up to a 1200× reduction in footprint while maintaining relatively large minimum feature sizes of up to 160 nm, showing that inverse-designed silicon nitride devices can be as compact as their silicon counterparts. These results enable high-density integration in silicon nitride photonics and pave the way for multidimensional data transmission and quantum applications, as the inverse design technique can be applied to different silicon nitride thicknesses and is potentially extendable to other low- and mid-index contrast platforms.