Quantitative phase microscopies (QPMs), that are microscopy techniques capable of mapping the phase of light, have been increasingly used in biology this last decade, and more recently for applications in nanophotonics by the principal investigator (PI). They represent a powerful strategy to non-invasively characterize micro and nanoscale systems, and are complementary to more conventional fluorescence-based techniques in biology.
In particular, quadriwave lateral shearing interferometry (QLSI) is a wavefront sensing technique that can be used as a high-resolution, highly sensitive QPM technique. It is based on the association of a camera sensor, and a 2-dimensional (2D) diffraction grating, separated by a millimetrer distance from each other.
Problem 1:
QPMs do not capture the whole information a light beam can contain, in terms of wavelength and polarization, as they are usually based on a monochromatic, scalar description of the electromagnetic field. Moreover, for some samples, this assumption can yield misleading measurements, or data that are difficult to interpret.
Problem 2:
QPMs and fluorescence microscopy techniques can give complementary information, especially in biology. Unfortunately, implementing fluorescence and phase microscopy in parallel on the same microscope is cumbersome: Multimodal phase/fluo correlation microscopy normally requires two camera sensors, which complexifies the setup and the image registration. To fix this problem, a single sensor can be used, with different optical filters that can be flipped. However, in this case, phase and fluorescence images cannot be acquired at the exact same time, preventing the study of dynamic systems.
Solution:
The project aimed to solve both these problems at once with a single ground-breaking innovation: coupling QLSI systems with more advanced camera sensors, sensitive to wavelength or polarisation. This multimodal QLSI (multiQLSI) approach provides 4 different phase images from a single camera acquisition. Depending on the camera sensor, these 4 images provide either (i) the missing polarization information, or (ii) phase images at various wavelengths. The former implementation, polar-QLSI, is intended to map the vectorial electromagnetic field at the sample plane (intensity and two phase profiles along x and y), no longer an approximated scalar field. The latter imaging mode, colour-QLSI, is intended to (i) achieve coloured phase microscopy, a valuable concept for dispersive samples, or for the study of Mie or plasmon resonances in nanophotonics, and also (ii) to perform phase/fluorescence correlation microscopy in biology, in parallel, with the same camera sensor, a new paradigm.
