Inverse design of 3D polymer integrated optics compatible with multi-photon lithography.

Integrated optics is primarily based on planar designs due to the availability of mature lithographic manufacturing and optical confinement constraints. These 2D designs with finite thickness in the third dimension are often referred to as 2.5D. Full 3D photonic architectures, with refractive-index variations along the three dimensions, hold the promise of higher integration density and novel, to the best of our knowledge, light control capabilities, but require advanced multi-layer stacking techniques with precise alignment and planarization. Nanoscale 3D printing techniques, such as multi-photon lithography, can address these challenges, and are gaining momentum thanks to their cost-effectiveness and rapid prototyping capabilities compared to silicon foundries. Despite this potential, the exploration of freeform polymer optics at the nanoscale remains limited due to challenges associated with low-index materials and a lack of design tools. Here, we address these limitations by applying a multi-layered inverse design approach for polymer-based integrated optics. We systematically compare 3D with 2.5D designs (all simulations are conducted in 3D), for the task of demultiplexing two wavelengths with spectral spacing from 100 nm to 20 nm. Our numerical results show that fully 3D polymer designs consistently outperform their 2.5D counterparts, achieving higher efficiencies at equal footprint. These findings propel the advancement of a next generation of miniaturized 3D devices for polymer-based integrated optics.