Dynamically Tunable Optofluidic Multifocal Microlens Arrays by 3D Printing

Microlens arrays (MLAs) are key components in 3D integrated imaging optical systems, particularly the multifocal MLAs, which provide a new strategy to break through the depth-of-field limitations for 3D imaging. However, the focal lengths of most existing multifocal MLAs that are produced by solid materials are fixed, making it difficult to meet dynamic imaging requirements with a large depth of field. In this article, we innovatively propose dynamically tunable multifocal MLAs using fluid as the lens material, which is integrated into a three-dimensional optofluidic chip fabricated by two-photon 3D printing technology. The fluid multifocal MLAs are realized by filling a microcavity array with flow streams of a gradient refractive index (RI) distribution, which is formed through convection and diffusion between miscible liquids of different RIs. By changing the flow rates, the RI distribution in the microcavity array can be readily regulated; thus, the optical characteristics of the MLAs can be dynamically tuned. The modulation mechanism is revealed by combining theoretical analysis, numerical simulations, and experimental observations. Thanks to the excellent regulatability of fluids by optofluidics, the present fluid MLA offers a wide adjustment range for the focal length, numerical aperture, and focal spot intensity. Especially, it possesses the ability to rapidly switch between different focal planes (flat, concave, and multiple-curved focal planes). Furthermore, the imaging applications of fluid MLAs are demonstrated using fluorescent microparticles and fluorescence-stained cells as samples, which exhibit enhanced magnification and improved clarity. This adaptability supports dynamic sample observation, highlighting the great potential of optofluidic multifocal MLAs for applications requiring large depth-of-field imaging.