Metalenses, due to their lightweight and planar characteristics, have shown great promise for miniaturizing imaging systems. The flexible light field modulation capabilities further enable the imaging system of multidimensional light field perception. Recently, multiple researchers have introduced metalenses in quantitative phase imaging microscopy (QPI)—a technique to harvest contrast from transparent samples—to enhance system compactness and accelerate image acquisition. Our group, led by Prof. Tao Li at Nanjing University, has previously utilized the large dispersion of the metalens in a transport-of-intensity-equation-based (TIE) QPI process. This natural dispersion facilitates focal tuning without requiring the movement of the object or image planes, thereby eliminating the need for bulky and expensive mechanical translation stages and resulting in a more compact and stable system (npj Nanophotonics, 1, 4 (2024)).
Despite the progress, current metasurface-based QPI research remains proof-of-concept demonstrations, falling short of practical application standards, particularly in terms of lateral resolution and object-side field of view (FoV). Two fundamental factors underlie the challenges:
- Aberrations: Singlet metalenses suffer from severe off-axis aberrations that worsen with increasing numerical aperture (NA), consequently degrading phase accuracy and resolution.
- Phase retrieval limitations: The employed phase retrieval methods in previous work are constrained by the paraxial approximation or the need for high spatial coherence, thereby limiting the lateral resolution.
To address the aforementioned challenges in resolution and FoV, we proposed three key strategies:
- We designed and fabricated a monolithically integrated metalens-doublet, termed a plan meta-objective (PMO), to correct the off-axis aberrations and expand the FoV. This design achieves a plan FoV (~90% of lens aperture) with minimal distortion and curvature. The monolithically integrated doublet configuration, where one metalens was fabricated on each side of the substrate, gives a total track length of less than 6 mm, ensuring both compactness and mechanical stability.
- We introduced partially coherent illumination into the PMO system to increase the effective numerical aperture, thereby fundamentally improving the resolution.
- We employed the mixed transfer function method (MTFM) to recover the phase with high accuracy and high resolution under partially coherent illumination.
The schematics of the proposed PMO system are illustrated in Fig. 1a. We utilized the natural dispersion of the PMO to obtain two symmetrically defocused images, from which we recovered the phase using MTFM. The proposed method achieved the currently best-reported performance in metalens-based QPI with a half-pitch resolution of 488 nm and a diffraction-limited FoV of 200 μm, as shown in Fig. 1b.
Figure 2 demonstrates the imaging results of amplitude and phase resolution charts. PMO system clearly distinguishes group 10 element 1 (10-1 line pairs) in the amplitude resolution chart, confirming a resolution of 1024 lp∙mm-1 (half-pitch 488 nm). The phase resolution chart shows less obvious intensity contrast when observed in the bright field. More distinct structure information (line pair details and optical thickness) can be obtained after phase retrieval, and the MTFM-based phase image demonstrates obvious improvement compared to the TIE-based phase. In the MTFM-based phase image, 10-1 line pairs are also distinguished, confirming the phase resolution similar to the amplitude resolution.
Applications and prospects
QPI recovers the phase of light after traversing the transparent samples. The quantitative phase value not only generates label-free imaging contrast for transparent samples, but also relates to the optical thickness of the samples or the dry mass of the cells. The PMO system provides a miniaturized platform suitable for large-scale deployment and portable applications. We showcase its potential in biomedical research by imaging primary human cervical epithelial and cancer cells cultured in vitro, as shown in Fig. 3. By comparing the retrieved phase results, the primary cervical cells cultured in vitro exhibit a large nuclear-cytoplasmic ratio, and the cancer cell is much larger than the normal cells in size and optical thickness. These insights, which are difficult to extract from intensity images (shown in the inset), can be quantitatively assessed using the phase information. By analyzing parameters such as dry mass, nuclear-cytoplasmic ratio, and cell size from a large sample set, potential diagnostic thresholds can be established to differentiate between normal and cancerous cervical cells, thereby accelerating the diagnostic process in cytopathology.
In summary, this work demonstrated a significant advancement in metasurface-based QPI by successfully combining partially coherent illumination with a meta-objective, enabling high-quality QPI with a compact and stable system. As a result, we achieved sub-micron resolution and a wide effective FoV in an ultra-compact manner. The reported method shows great promise for developing compact, stable, and practical QPI platforms, facilitating broader adoption of QPI systems in diverse applications, including cell culture, digital pathology, point-of-care diagnostics, and industrial metrology.
For more information, please refer to our paper published in Light: Science & Applications, “Plan meta-objective for sub-micron quantitative phase imaging”.
https://doi.org/10.1038/s41377-025-02099-z