Ultra-compact spot size converter based on digital metamaterials

Spot size converters (SSCs) are essential optical components that connect waveguides with different modal spot sizes for adiabatic light coupling. However, previous SSCs usually suffer from large sizes or limited coupling efficiencies, which are not conducive to develop large-scale and high-density photonic integrated circuits (PICs). Herein, we theoretically study an ultra-compact SSC with a footprint of only 1x2 mu m(2). The design is based on topologically optimized digital metamaterials, which is implemented by using a genetic algorithm. The device shows a transmission of -0.11 dB for TE0-mode spot size conversion with a 1-dB bandwidth of 600 nm. Moreover, the device supports the spot size conversion for TE0-mode and TM0-mode simultaneously with a transmission of more than -2 dB. With the high transmission, wide spectral bandwidth, and ultra-compact footprint, the device is expected to open a new avenue to boost the development of PICs.