Meta-Optics Enabled Compact and Scan-Free In-Line Digital Holographic Microscopy for 3D Tracking
Three-dimensional (3D) tracking of microscopic objects is essential for probing dynamic processes across biological, microfluidic, and nanoscale systems. In-line digital holographic microscopy (DHM) provides a powerful framework for this purpose by encoding volumetric information into a single intensity measurement, enabling scan-free reconstruction through numerical wave propagation. However, despite its conceptual simplicity, the practical implementation of in-line DHM remains fundamentally constrained by its reliance on bulky relay optics, which limit system compactness, alignment robustness, and integration with emerging miniaturized platforms. Here, we rearchitect in-line DHM using planar meta-optics by replacing conventional refractive relay optics with metalenses, thereby establishing a compact single-path holographic imaging platform. This architecture preserves the intrinsic advantages of in-line holography, including single-shot volumetric encoding and numerically propagated reconstruction, while substantially reducing optical complexity and system footprint. We validate the metasurface-enabled in-line DHM experimentally using resolution targets, microspheres, and live microorganisms and demonstrate scan-free 3D tracking in a compact optical configuration. Our results establish planar meta-optics not only as a substitute for refractive components but also as an effective system-level strategy for miniaturizing holographic microscopy and advancing integrable volumetric imaging platforms.