Silicon-based surface gratings for efficient fiber-chip and free-space beam coupling

Silicon photonics has established itself as a key integration platform, leveraging high-quality materials and large-scale manufacturing using mastered toolsets of complementary metal-oxide-semiconductor (CMOS) foundries. Chip-scale photonics offer unique promises for dense integration of versatile optical functions through compact and high-performance building blocks. Integrated photonics is now competing technology for many applications, spanning from telecom/datacom and interconnects up to quantum sciences and light detection and ranging (LIDAR) systems, among others. However, the lack of low-loss input/output chip interfaces can be prohibitive to successfully deploy multi-diverse device applications. Low coupling loss is essential in reducing overall power budget in photonic systems, impacting on-chip integration level. The light coupling from an off-chip environment into the planar waveguide platforms has always been a challenging research problem since the early years of integrated photonics. Optical interfaces formed on a photonic chip surface, rather than implemented on a chip edge, have been widely used to access photonic circuits with optical fibers or enabling free-space coupling of light beams. Surface gratings can be positioned at arbitrary locations and/or arranged in pre-defined patterns on the chip, facilitating wafer-scale testing and optical packaging. In this work, we present our recent progress in the development of silicon-based surface gratings for use in fiber-to-chip and free-space beam coupling. In particular, we discuss prospective design approaches to develop low-loss surface grating couplers implemented on silicon-on-insulator (SOI), silicon nitride (SiN), and hybrid silicon-silicon nitride (Si-SiN) platforms, allowing to approach a coupling loss below -1 dB. Among these, we also cover contemporary advances in compact silicon metamaterial nano-antennas for dense optical phased arrays, obtaining high a diffraction performance (> 90%) and wideband operation (> 200 nm) simultaneously.