Intrinsic dark-field Fourier ptychographic diffraction tomography under non-matched illumination

Non-interferometric optical diffraction tomography (ODT) offers a powerful route for three-dimensional (3D), label-free imaging of live biological cells. However, in high-numerical-aperture (NA) systems, its performance is fundamentally limited by the geometry of standard multi-well culture plates, which restrict oblique illumination angles and prevent the matched illumination condition required for accurate tomographic reconstruction. This mismatch causes a physical loss of low-frequency information in the recorded intensity, traditionally regarded as a major barrier to high-resolution ODT. Here, we demonstrate that such non-matched illumination can instead be harnessed as an intrinsic mechanism for dark-field-like contrast enhancement, without additional measurements or post-processing. This insight underpins our proposed method: dark-field Fourier ptychographic diffraction tomography (DF-FPDT). By selectively updating only the high-frequency components inherently encoded in intensity measurements, DF-FPDT naturally suppresses low-frequency background and enhances fine subcellular structures, mimicking the behavior of dark-field microscopy while preserving the non-interferometric, volumetric, and quantitative advantages of FPDT. Validated through simulations and experiments on 3D micro-phantom, microspheres, and COS-7 cells, DF-FPDT demonstrates robust, high-contrast 3D imaging under standard microscope configurations and culture conditions. By leveraging what was once considered a constraint, DF-FPDT offers a compact and practical solution for dynamic 3D live-cell imaging in realistic biological environments.