Compact infrared varifocal optics enabled by hybrid refractive lenses and freeform cascade metalenses

Compact and lightweight varifocal imaging systems are increasingly important for emerging applications such as portable sensing, wearable optics, and aerospace instrumentation. However, conventional varifocal architectures rely on mechanically complex axial or lateral translations of optical components, limiting their miniaturization and integration. Here, we present a hybrid refractive-metalens varifocal system that enables focal tuning through rotational modulation of cascaded freeform metalenses rather than component displacement. The proposed architecture combines the high imaging capability of refractive optics with the wavefront-engineering flexibility of metalenses, enabling continuous varifocal operation within a significantly reduced system footprint. To enable the practical design of such hybrid systems, a real-ray-tracing-based co-design strategy is developed, together with a multi-configuration optimization framework that suppresses parasitic zeroth-order diffraction while maintaining imaging performance across the full varifocal range. A fabricated prototype demonstrates continuous 2× zoom operation with stable aberration control and low distortion, showing good agreement between experimental measurements and theoretical predictions. These results validate both the proposed hybrid refractive-metalens architecture for compact translation-free varifocal imaging and the accompanying system-level co-design methodology, providing a practical framework for future hybrid refractive-metasurface optical systems in infrared sensing, AR/VR, aerospace instrumentation, compact cameras, and portable intelligent devices.