Optical Phased Arrays on Photonic Integrated Circuit Platforms: Challenges, Opportunities, and Future Applications.

On-chip optical phased arrays (OPAs) have emerged as a core technology in photonic integrated circuits (PICs), enabling compact, solid-state, and digitally reconfigurable beam steering with high scalability. By eliminating mechanical motion and leveraging semiconductor fabrication, OPAs provide a versatile platform for large-aperture and high-speed optical wavefront control in applications including LiDAR, free-space optical communication, imaging, and holographic displays. Recent progress across silicon, silicon nitride, hybrid Si- Si 3 N 4 , III-V, and lithium niobate-based platforms has advanced OPAs from proof-of-concept demonstrations to highly integrated systems with sub-milliradian resolution, wide fields of view, and megahertz-to-gigahertz update rates. This review presents a system-level overview of on-chip OPAs, covering fundamental beam-steering principles, array theory, and key performance metrics, as well as the essential building blocks of laser sources, phase shifters, and grating antennas. Particular emphasis is placed on aperiodic array architectures, system-level scalability challenges, and emerging electro-optic platforms such as lithium niobate on insulator (LNOI), which are redefining the speed-power trade space. Finally, we discuss future directions toward programmable, large-scale optical wavefront engines enabled by heterogeneous integration and intelligent control.