Inverse design of digital nanophotonic devices using the adjoint method

A high-efficiency inverse design of "digital" subwavelength nanophotonic devices using the adjoint method is proposed. We design a single-mode 3dB power divider and a dual-mode demultiplexer to demonstrate the efficiency of the proposed inverse design approach, called the digitized adjoint method, for single-and dual-object optimization, respectively. The optimization comprises three stages:1) continuous variation for an "analog" pattern; 2) forced permittivity biasing for a "quasi-digital" pattern; and 3) a multilevel digital pattern. Compared with the conventional brute-force method, the proposed method can improve design efficiency by about five times, and the performance optimization can reach approximately the same level. The method takes advantages of adjoint sensitivity analysis and digital subwavelength structure and creates a new way for the efficient and high-performance design of compact digital subwavelength nanophotonic devices, which could overcome the efficiency bottleneck of the brute-force method, which is restricted by the number of pixels of a digital pattern,and improve the device performance by extending a conventional binary pattern to a multilevel one.