Polarimetric imaging, as a novel optical imaging technique, is widely used in fields such as biomedicine, object recognition, polarimetric remote sensing, and 3D imaging. In the field of biomedicine, polarimetric imaging has two unique advantages compared to traditional optical imaging: 1) By analyzing the polarization properties of light interacting with biospecimens, microscopic-level information about the composition and structure can be obtained without requiring labeling agents. For example, collagen fibers in connective tissues can alter the polarization state of light passing through them. Measuring this alteration provides information about collagen fiber orientation and density, which are associated with various diseases such as cancer and fibrosis. 2) By selectively filtering out light waves with certain polarization states, it is possible to enhance image contrast and improve the visibility of certain structures or components based on the birefringence maps.

Conventional lens-based polarization light microscopy utilizes a polarizer-analyzer pair to measure the changes in the polarization state of light induced by the sample. Despite widespread use in different application settings, there are several limitations of these lens-based systems. One significant drawback is the trade-off between the spatial resolution and the imaging field of view. One can obtain high resolution with a small field of view or low resolution with a large field of view, but not both. To obtain a high-resolution whole slide image of the sample, integration of the polarization optics with a whole slide imaging system is necessary for data acquisition. However, this integration remains elusive as whole slide scanners are often dedicated and costly tools. Additionally, high numerical aperture (NA) objective lenses imply a small depth of field, typically on the micrometer scale, which presents a challenge when tracking the axial topography variations of biospecimen.

To address these limitations, Professor Xiaopeng Shao's team from Xidian University, in collaboration with Professor Guoan Zheng's team from the University of Connecticut, propose a lensless polarimetric coded ptychography (pol-CP) technique that enables high-resolution, high-throughput gigapixel birefringence imaging on a chip. Relevant research results were recently published in Photonics Research, Volume 11, No. 12, 2023. [Liming Yang, Ruihai Wang, Qianhao Zhao, Pengming Song, Shaowei Jiang, Tianbo Wang, Xiaopeng Shao, Chengfei Guo, Rishikesh Pandey, Guoan Zheng. Lensless polarimetric coded ptychography for high-resolution, high-throughput gigapixel birefringence imaging on a chip[J]. Photonics Research, 2023, 11(12): 2242].

The method proposed by the research team represents an important development of the traditional ptychography technique. Ptychography is initially proposed to solve the phase problem encountered in electron crystallography. The process can be described as follows: the sample is placed on a scanning stage and illuminated with a spatially confined probing beam (Probe). The sample is then translated to different lateral positions, and the corresponding diffraction patterns are acquired in the far field. The sample's complex wavefront is then retrieved utilizing the acquired diffraction patterns. The lateral scanning of the sample in ptychography extends the imaging field of view and provides phase diversity measurements.

Unlike traditional ptychography, in pol-CP implementation, the research team illuminate the specimen with circularly polarized laser light and attached an ultrathin polarizer film to a blood-coated sensor for image acquisition. The integrated coded sensor was then mounted on a compact, customized stage, enabling control over both rotational and translational motions. By analyzing four different polarization states with Jones calculus, the team retrieved the quantitative birefringence retardance and orientation information of the specimen.

The fabrication process of the integrated polarimetric coded sensor is illustrated in Figures 1(c) and (d). First, the goat blood was smeared onto the sensor's coverglass, forming a thin and dense blood-cell layer that acts as a high-performance scattering lens with a theoretically unlimited field of view. Second, an ultrathin polarizing film (80 µm thick) was adhered to the blood-cell layer using PDMS, forming an integrated polarimetric coded image sensor. The use of PDMS ensures tight adhesion between the polarizing film and the blood-cell layer, preventing the formation of interference patterns in the acquired diffraction patterns due to air gaps, which could affect the quality of retrieved images.

Figure 1 Schematic diagram and operation of the lensless polarimetric coded ptychography (pol-CP) platform.(a) A circular polarizer transforms linearly polarized light from the laser diode into circularly polarized light, which then interacts with the specimen. The exit wave is recorded by the integrated polarimetric coded image sensor. (b) Illustration of changes in the polarization state within the pol-CP system. (c) A 2 μL sample of goat blood is smeared onto the sensor's coverglass, forming a thin and dense blood-cell layer that acts as a high-performance scattering lens with a theoretically unlimited field of view. (d) An ultrathin polarizing film is adhered to the blood-cell layer, forming an integrated polarimetric coded image sensor. (e) The pol-CP prototype features a polarimetric coded sensor mounted on programmable stages, allowing control over both rotational and translational motions.

The research team validated the pol-CP prototype with various samples, such as potato starch granules, corn stem, and so on. To demonstrate its biomedical applications, they perform high-throughput imaging of malaria-infected blood smears, locating parasites using birefringence contrast. They also generate birefringence maps of label-free thyroid smears to identify thyroid follicles. Prof. Guoan Zheng commented that, "The reported approach provides a turnkey and portable solution for lensless polarimetric analysis on a chip, with promising applications in disease diagnosis, crystal screening, and label-free chemical imaging, particularly in resource-constrained environments." Prof. Xiaopeng Shao remarked that, "In comparison to traditional optical imaging techniques, the approach reported allows for birefringence imaging at high resolution, high throughput, and at low cost, thereby, promoting the advancement of computational imaging."