Complete image Mueller polarimeter

The polarimetric images shown in this work (Figs. ​(Figs.1,1, ​,22 and Supplementary Figs. ^S1 and ^S2) are gathered from the experimental Mueller matrices of the studied samples, which are acquired by means of a complete image Mueller polarimeter. By taking advantage of the wide spectral response of the light source, which actually covers the visible spectrum (from 400 to 700 nm approx.), we use three different illuminating wavelengths (625 nm, 530 nm and 470 nm) for the consequently inspection of the sample at different depths⁴⁷. The polarimeter consists of two independent optical systems based on Parallel Aligned Liquid Crystals (PA-LC) retarders, mounted into two compact mobile arms respectively. The Polarization State Generator (PSG) optical design leads to generate any fully polarized state. It is composed by a linear polarizer oriented at 0° with respect to the laboratory vertical, followed by two PA-LC at 45° and 0°. Equivalently, the Polarization State Analyzer (PSA) is capable to detect any state of polarization. The PSA uses 6 optimized polarization analysis states⁵⁵. In our instrument, both the PSG and the PSA consist of a linear polarizer followed by two Parallel-Aligned Liquid Crystal cells externally controlled by sending different voltages. The combination of both PSG and PSA, are used to record 36 intensity images which are used to deduce the Mueller matrix⁵⁵. Regarding internal optical set-up, PSA has the same optical elements as PSG but arranged in reverse order. For the acquisition of sample intensity, a CCD camera is placed on the PSA system.

To perform the Mueller matrix measurements of biological samples, two main optical configurations are used. By tilting by 34° the PSG with respect to the horizontal laboratory reference and maintaining the PSA at 0° to avoid the ballistic reflection, we perform what we call scattering measurements. Complementary, by placing both PSG and PSA at 90° we perform transmission measurements. In both configurations, we selected from the whole sample, a region of interest (ROI) of 512 × 512 pixels, which corresponds to an area of 1.1 × 1.1 cm². The detailed information about optical components and the visualization of measurement configurations (Figs. ^S5 and ^S6) is shown in Supplementary document.