Metalens for passive generation of a droplet Bessel beam.

Extending the depth of field (DOF) is essential for large-volume imaging in biological research, particularly in thick tissue environments. Bessel beams and their variants are widely used due to their simplicity and have been successfully applied to in vivo imaging. A recent advancement demonstrated the application of droplet Bessel beams (DBBs) for multi-photon microscopy, enabling functional imaging in live mouse brains. However, DBB generation inevitably requires active phase switching devices such as spatial light modulators, which reduce imaging speed and increase system complexity. This study introduces a droplet Bessel beam metalens (DBBM) that passively generates DBBs without phase switching by employing rectangular meta-atoms for orthogonal polarization modulation and X-shaped meta-atoms for amplitude control. Optical simulations identify optimal DBBM parameters that maximize the point spread function (PSF) aspect ratio while minimizing energy leakage into side lobes. Furthermore, the fabricated DBBM produces PSFs consistent with simulations. Imaging simulations based on three-dimensional confocal images of expansion microscopy-treated organoids demonstrated that the DBBM maintains superior performance even in the presence of aberrations. These findings establish the DBBM as a compact and passive solution for extended DOF imaging without the need for beam-shaping devices. Metalens technology is anticipated to have broad applications in real-time volumetric bioimaging and enable simplified optical system designs.