Principles and Applications of Compact Refractive-Metasurface Hybrid Imaging Systems (Invited)

Significance Conventional refractive optical systems rely on phase accumulation through light propagation in bulky media, posing fundamental contradictions between miniaturization and high-performance imaging. This limitation becomes increasingly critical with the growing demand for compact optical systems across portable electronics, biomedical devices, and aerospace applications. Metasurfaces, composed of subwavelength artificial structures, enable ultra-compact wavefront control via spatially varying phase discontinuities. However, standalone metalenses face inherent trade-offs among efficiency, bandwidth, and aperture, as well as severe chromatic and monochromatic aberrations that limit their practical utility. Refractive-metasurface hybrid systems synergistically combine the stable optical power of conventional refractive elements with the precise wavefront manipulation and negative dispersion of metasurfaces, offering a compelling pathway to compact imaging systems that approach diffraction-limited performance across broad spectral ranges and large fields of view. Progress This review systematically summarizes the principles, design methodologies, aberration correction techniques, and applications of refractive-metasurface hybrid imaging systems. The physical foundations encompass four phase-modulation mechanisms: propagation phase enabled by geometric variation of meta-atoms, geometric phase through anisotropic structure rotation, resonant phase leveraging high-Q modes, and topological phase emerging from exceptional point encircling. Design methodologies have evolved from forward parametric scanning to inverse approaches employing metaheuristic algorithms and deep learning models, substantially expanding design freedom. Beyond single metasurfaces, hybrid architectures including arrays, cascaded configurations, and refractive-metasurface integrations achieve optimal balance among performance, complexity, and manufacturability. A key advantage lies in aberration correction. For chromatic aberration, artificial dispersion of metasurfaces offsets natural dispersion of refractive lenses, enabling broadband achromatic operation. For monochromatic aberrations, metasurfaces provide precise phase compensation, achieving large-aperture and wide-field-of-view lenses with near-diffraction-limited performance. Metasurfaces also exhibit superior thermal stability due to substrate-dominated thermal response, allowing athermalization over broad temperature ranges without complex compensation. Recent advances have integrated hybrid optics with computational imaging via end-to-end differentiable optimization, enabling hardware-software co-designed systems that push performance limits. These capabilities enable transformative applications across biomedical endoscopy, infrared imaging, and portable devices. In endoscopy, hybrid systems achieve significant distal optics miniaturization while maintaining high resolution. In infrared imaging, lightweight large-aperture metalenses enable compact thermography and computational vision systems. In portable devices, folded optical architectures and on-chip CMOS integration produce ultra-compact cameras with advanced functionalities including dynamic zoom and intelligent perception. Conclusions and Prospects Despite their demonstrated potential, practical deployment faces challenges in fundamental performance limits and manufacturing scalability. Future research should explore new physical mechanisms to expand design freedom, develop co-optimization frameworks accounting for manufacturing tolerances, and advance replication techniques. Promising frontiers include quantum information processing, neuromorphic photonic computing, and augmented reality displays.