3D Photonic integration leveraging hybrid-confinement circuits

Three-dimensional (3D) photonic integration offers a pathway to overcome the fundamental scaling limitations of planar platforms by enabling enhanced routing flexibility for compact, low-loss, and highly interconnected photonic circuits. In this work, we fabricate 3D photonic circuits combining high-confinement air-clad waveguides for compact routing with low-confinement polymer-clad waveguides for robust single-mode operation within a monolithic platform. Efficient mode transition between polymer-clad and air-clad waveguides is demonstrated with a loss of 0.25 dB per interface. We also realize compact, Euler S- and U-shaped bends with minimal bending radii of 10 $\mu$m and losses as low as 0.5 dB and 0.4 dB, respectively, along with compact adiabatic air-clad splitters exhibiting a splitting loss of 0.6~dB over a length of 52 $\mu$m. Finally, full fabrication of a compact hybrid circuit is demonstrated, highlighting the feasibility and scalability of the approach. Our work represents a significant step in 3D photonic integration for applications including optical neural networks, photonic wire bonding and their potential for novel integrated photonic applications.