Design of 2D Periodic Ge-Based Structure for Crosstalk Mitigation in High-Density Silicon Photonics

Recently, silicon photonic integrated circuits have a significant interest due to their potential in low-cost, low-power, high-density applications. However, crosstalk (CT) between neighboring optical waveguides continues to limit integration density. This paper presents a photonic waveguide structure designed to reduce crosstalk in silicon-based waveguides. The structure features a compact, two-dimensional (2D), periodic Ge nano-unit cell arrangement embedded within a Silicon-On-Insulator (SOI) platform. The suggested structure incorporates Ge nano unit cells in a square lattice between two adjacent Si waveguides. By precisely engineering the unit cells of the periodic structure, a bandgap is formed at the designated operational wavelength of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1.55\ \mu \mathrm{m}$</tex>, which suppresses the evanescent fields of the Si waveguides and directly reduces coupling between them. Consequently, the coupling length <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\mathrm{L}_{\mathrm{c}})$</tex> is extended, thereby decreasing CT. The influence of varying the number, material, and dimensions of the unit cells on the photonic bandgap is analyzed using both the Plane Wave Expansion (PWE) and Finite Difference Time Domain (FDTD) approaches. Simulation results, based on Finite Element Method (FEM) simulations, reveal that <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{L}_{\mathrm{c}}$</tex> increases by 30 times with two rows of the Ge periodic structure and by two times with a single row, compared to cases with no periodic structures between the Si waveguides. The <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mathrm{L}_{\mathrm{c}}$</tex> is enhanced over a broad wavelength range, reaching a maximum of 0.263 m at <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$1.55\ \mu \mathrm{m}$</tex> with two rows of the Ge periodic structure. The proposed design offers a promising approach for high-density photonic devices with minimized CT, opening new pathways for more efficient silicon photonic integrated circuits.