Digital metamaterials with nanoscale silicon/Sb2Se3 pixels for reconfigurable integrated mode converters

Waveguide-integrated reconfigurable nanophotonic devices are of importance for on-chip all-optical connection and flexible photonic circuits. In this paper, we propose inversely designed reconfigurable TE0-to-TE1 and TE0-to-TE2 mode converters based on digital metamaterials consisting of nanoscale silicon and non-loss phase change material Sb2Se3 pixels. The design strategy combines finite element method, density method and method of moving asymptotes. Only 73 and 56 optimization iterations are taken to obtain the optimal digital metamaterials for targeted mode conversions. The footprints of ultra-compact functional regions are 0.92 and 1.04 square center wavelengths, respectively. TE0-to-TE1 and TE0-to-TE2 mode conversions with high transmission efficiency and output mode purity are realized with crystalline Sb2Se3, while TE0 mode directly pass through with amorphous Sb2Se3. Meanwhile, by cascading these two mode converters, TE1-to-TE2 mode conversion is achieved as well. Moreover, the robustness of these designs is demonstrated by introducing random wrong pixels.