Extended transformation optics in anisotropic photonic devices: theory, design, and demonstrations

Photonic designs based on conventional transformation optics (TO) techniques often assume isotropic materials, which makes them incompatible with the anisotropic nature of emerging photonic platforms such as lithium niobate on insulator (LNOI), impeding advances in high-density and multi-functional photonic integration. In this work, we extend the application of TO to the design of anisotropic photonic devices. By incorporating topology optimization in the virtual space, our approach effectively suppresses anisotropic scattering loss and mode crosstalk, thereby reducing the constraints imposed by specific material properties. As an example, we realize a 90° fundamental transverse electric (TE) mode waveguide bend with an effective radius of 40 µm, exhibiting a low insertion loss (IL) of 0.15 dB at 1550 nm, based on the silicon nitride loaded LNOI platform. Furthermore, a 90° multimode waveguide bend with an effective radius of 60 µm, supporting TE₀ and TE₁, is also demonstrated using the proposed scheme. Experimental results for the two modes show crosstalk below -19.7 dB, and IL below 0.14 dB at 1550 nm. Both devices exhibit an ultra-broad bandwidth beyond 400 nm. Importantly, this work establishes a general TO-based design paradigm applicable to diverse material platforms and provides avenues for anisotropic device design.