Ultra-broadband and ultra-compact chip-integrated logic gates based on an inverse design method

The Si-based chip-integrated OR, XOR, NOR, AND and NAND all-optical logic gates (AO-LGs) with footprints of 2 mu m x 2 mu m are inversely designed based on an intelligent method, which combines the method of moving asymptotes (MMA) with the finite element method (FEM). In this method, the substrate is divided into many small units, and the refractive index of each unit will be perturbed according to design expectation. Therefore, the optimized refractive index will be gradient, which divorces from fabrication. Binary reconstruction is the process of changing the gradient media into binary media, by filtering out the gradient virtual media, smoothing the boundaries, and refilling Si or air on the substrate to form a new structure. Numerical simulations demonstrate that all the AO-LGs designed by this method work effectively. The contrast ratio (CR) of OR/XOR gate ranges from 23.50 dB to 25.50 dB, NOR gate ranges from 6.33 dB to 7.91 dB, AND gate ranges from 4.44 dB to 6.13 dB, and NAND gate ranges from 8.85 dB to 10.07 dB. And these designed AO-LGs achieve nearly ten times more broad bandwidth than the other reported conventional gates without suffering other performances too much. The OR/XOR, and NOR gate functions well in a broad band range from 800 nm to 1600 nm, and from 850 nm to 1350 nm. AND gate functions well from 1000 nm to 1240 nm and from 1310 nm to 1400 nm. NAND gate functions well from 1360 nm to 1530 nm. It is expected that such a robust method can serve extensive applications in designing photonic devices, and the compact logic gates may contribute to broad applications of future optical computing and signal processing.