Varifocal Metalens for Compact and Accurate Quantitative Phase Imaging

The transport of intensity equation (TIE), a noninterferometric method for quantitative phase imaging (QPI), enables the characterization of transparent objects and is widely applied in fields such as biomedicine, materials science, and optical metrology. Traditional TIE methods require the mechanical movement of detectors to capture multiple images, which limits integration, stability, and speed of the system. In this work, an approach is introduced that replaces mechanical movement with the alteration of the properties of incident light. By combining a specially designed polarization multiplexed metasurface with the rotation of the light source's polarization state, a varifocal metalens is realized. This approach allows the acquisition of multiple defocused images without mechanical movement. Coupled with the multiple-frame TIE algorithm, it yields a compact, stable, and accurate phase-imaging technique. Compared to a dual-channel polarization multiplexed metalens, phase reconstruction based on a varifocal metalens exhibits superior robustness and enhanced accuracy. The experimental results indicate that our method can accurately reconstruct the phase of objects with different phase gradients using partially coherent light illumination, achieving an average percentage error of less than 2.7%. This dynamic illumination approach, combined with specific metasurfaces, holds potential applications in realizing lightweight, compact, and multifunctional imaging systems.