Real-time label-free microscopy with adjustable phase-contrast.

The lack of contrast represents a challenge in all imaging systems, including microscopy. This manuscript proposes the use of an azobenzene liquid crystal material as a Zernike filter in a phase-contrast configuration to enable label-free imaging. The novelty of the approach presented here is that it offers real-time adjustment of the contrast in images and prolonged-time observation. This is achieved with no SLM, any customized optical components, or mechanical elements, and voltage is not applied. Notably, the intensity level (0.95 mW/cm) is well below photodamage or phototoxicity for bioimaging, allowing extended time monitoring of cells. Additionally, due to the large LC's birefringence (Δn=0.2), it is possible not only to visualize a phase object but also to adjust the contrast of stainless samples by just rotating the polarization with a large and continuous dynamic range of phase retardation. In future work, this will enable a simple implementation of differential phase-contrast microscopy and quantitative phase imaging. Due to the low-intensity illumination required, this system can be combined with other imaging techniques, such as tomography and fluorescence microscopy.