Anisotropic metamaterials for scalable photonic integrated circuits: a review on subwavelength gratings for high-density integration.

Photonic integrated circuits (PICs) are transforming optical technology by miniaturizing complex photonic elements and systems onto single chips. However, scaling PICs to higher densities is constrained by optical crosstalk and device separation requirements, limiting both performance and size. Recent advancements in anisotropic metamaterials, particularly subwavelength gratings (SWGs), address these challenges by providing unprecedented control over evanescent fields and anisotropic perturbations in PICs. Here we review the role of anisotropic SWG metamaterials in enhancing integration density, detailing two foundational mechanisms - skin depth engineering and anisotropic perturbation - that mitigate crosstalk and enable advanced modal controls. We summarize their applications within four key functions: confinement manipulation, hetero-anisotropy and zero-birefringence, adiabatic mode conversion, and group velocity and dispersion control, showing how each benefits from distinct SWG properties. Finally, we discuss current limitations and future directions to expand the full potentials of anisotropic SWG metamaterials, toward highly dense and scalable PICs.