Photonics integrated circuits (PICs) have emerged as a foundational platform for next-generation optical information transmission and processing, owing to their high speed, broad bandwidth, excellent scalability, and low propagation loss. These advantages stem from the unique capability of PICs to densely integrate diverse and complex optical functionalities onto micro- and nanoscale chips. However, the efficient conversion of guided optical modes into free-space radiation propagating into the far field, together with precise and flexible manipulation of multiple optical degrees of freedom (DoFs)—including amplitude, phase, state of polarization (SoP), and orbital angular momentum (OAM)—remains a critical challenge in the field. Addressing this challenge is essential for unlocking the full potential of integrated photonic systems. In this context, the present review provides significant academic and technological value by systematically summarizing recent advances in on-chip metasurfaces for guided-mode radiation control. It fills an important gap in the existing literature by consolidating both theoretical frameworks and experimental demonstrations, while offering a comprehensive roadmap for overcoming key technical bottlenecks. By doing so, this review accelerates the translation of innovative metasurface concepts into practical PIC applications and supports the development of intelligent, high-performance optical systems.

Over the past decade, integration strategies for metasurface–waveguide architectures have evolved substantially. Early implementations typically involved placing a metasurface layer atop a conventional grating coupler. As demands for miniaturization and functional density increased, more advanced approaches emerged, most notably the direct patterning of high-aspect-ratio, high-refractive-index artificial nanostructures (meta-atoms) on waveguide surfaces. This strategy enables direct modulation of the evanescent field of guided modes, thereby facilitating comprehensive control over multiple DoFs of the radiated optical field. Recent progress in PICs and nanophotonics has further inspired innovative design paradigms, among which two representative approaches have attracted particular attention: co-designed metasurface–grating coupler hybrids and leaky-wave metasurfaces. The former offers a compact and fabrication-friendly solution, often realizable via a single etching step, in which the metasurface–grating coupler simultaneously enables efficient radiation and wavefront shaping. The latter exploits intentional symmetry breaking in photonic crystal slab waveguides to induce controlled leaky-wave radiation. Notably, both strategies can realize large surface-emitting apertures without requiring the integration of high-aspect-ratio, high-index nanostructures, making them especially attractive for scalable implementations. As a result, these two approaches are expected to play a central role in advancing the field.

Substantial progress has been achieved in manipulating guided-mode radiation using on-chip metasurfaces. Several well-established mechanisms have been proposed to control the phase of radiated wavefronts, including: 1) detour phase, determined by the lateral displacement of a meta-atom within a unit cell; 2) geometric (Pancharatnam–Berry) phase, governed by the in-plane rotation of anisotropic meta-atoms; 3) propagation phase, set by the geometric dimensions of the meta-atoms; and 4) resonant phase, arising from engineered resonances that provide strong and dispersive phase responses. Beyond phase-only modulation, interference among multiple artificial nanostructures within a unit cell enables simultaneous control of amplitude and SoP, thereby allowing full multidimensional manipulation of optical DoFs. By leveraging these mechanisms, on-chip metasurfaces have demonstrated a broad range of functionalities, including beam steering, focusing, vortex beam generation, and holographic projection. Furthermore, the inherent advantages of the on-chip platform—such as multi-port, multi-mode, and multi-wavelength excitation—enable decoupled and independent control of optical DoFs with minimal crosstalk, paving the way for advanced functional multiplexing in integrated photonic systems.

Despite these advances, several key challenges remain. First, the radiation efficiency of guided modes must be further improved. Future studies are expected to explore novel radiation-enhancement mechanisms, such as resonant architectures and multilayer composite structures, to increase energy conversion efficiency. Second, modulation precision is still constrained by current micro- and nanofabrication limitations. Advances in stacked multilayer metasurfaces, three-dimensional (3D) nanostructures, and high-precision, multi-material co-fabrication could significantly expand the available design space and improve device fidelity. Third, dynamic modulation capabilities remain limited. Most existing on-chip metasurfaces are passive and rely on static geometries, preventing adaptive responses to external stimuli such as temperature, optical intensity, or applied voltage. Future efforts may enable non-volatile reconfigurability through phase-change materials, achieve ultrafast modulation using the optoelectronic properties of two-dimensional (2D) materials, or expand modulation dimensionality via photochromic effects. Finally, current on-chip metasurfaces often lack sufficient system-level integration. Future research should therefore focus on the co-integration of metasurfaces with multi-functional photonic modules to realize intelligent, system-level optical field manipulation.

In conclusion, pioneering work on on-chip metasurfaces has significantly advanced the understanding of guided-mode radiation control and driven innovation in PICs and nanophotonics. With continued progress in nanofabrication technologies and electromagnetic modeling, on-chip metasurfaces are expected to achieve further performance improvements. As a result, metasurface-enabled control of guided-mode radiation is poised to become a cornerstone technology for next-generation PICs, with far-reaching impact across high-capacity optical communications, imaging systems, optical security, and immersive AR/VR applications.