Conference Agenda

Light-based in-vivo brain imaging relies on light transport through highly scattering tissues over long distances. As scattering gradually reduces imaging contrast and resolution, visualising structures at greater depths becomes challenging, even when using multi-photon techniques. To overcome this limitation, minimally invasive endo-microscopy techniques have been developed that typically use graded-index rod lenses. A recently proposed alternative involves the exploitation of holographic control of light transport through multimode optical fibres [1], which promises superior imaging performance with less traumatic application [2]. Following the review of the fundamental and technological bases, the talk will introduce a 110µm thin laser-scanning endo-microscope, which enables volumetric imaging of the entire depth of the mouse brain in vivo [3]. The system is equipped with multi-wavelength detection and three-dimensional random-access options, and it has a lateral resolution of less than 1µm. Various modes of its application will be presented including the observations of fluorescently labelled neurons, their processes, and blood vessels. Finally, the use of the instrument for monitoring calcium signalling and measurements of blood flow in individual vessels at high speeds will be discussed.

[1] Nature communications 3.1, 1027 (2012).

[2] Light-Science & Applications 7, 92 (2018).

[3] Nature Communications, 14, 1897 (2023).

Optical sensors are limited to measuring intensity. For this reason, wavefront sensors need to convert phase information into intensity modulations. One method to achieve this involves using a Hartmann mask positioned near a camera sensor. This technique is compatible with low-coherence illumination and has been implemented using various encoding optical elements, such as arrays of holes or microlens arrays. For instance, high-resolution and quantitative phase imaging has been demonstrated using a diffraction grating, a method known as lateral shearing interferometry (LSI).

In this presentation, we will illustrate how LSI can also measure broadband speckle wavefields generated through multiple scattering media, enabling digital fluorescence phase conjugation through tissues. Additionally, we will present a generalization of LSI using a birefringent diffraction grating to perform polarimetric LSI of vector beams, which is relevant for optical metrology and polarization-resolved fluorescence microscopy. Finally, we will demonstrate that this generalized principle can be applied to single-shot hyperspectral wavefront sensing, leveraging the spectral dispersion of thin scattering media, with applications in the metrology of ultrashort lasers.

Optical fields’ polarimetry follows well-known principles in paraxial conditions where light polarization is manipulated in transverse planes. In non-paraxial optics such as microscopy imaging, complex media propagation or nanophotonics, it is however still a challenge that requires formalisms appropriate to 3D polarized fields. In this work, we present a few illustrations of how the analysis and control of polarized wavefronts in high numerical aperture microscopy provides access to 3D field’s properties at the nanoscale. We show that the point spread functions (PSFs) of dipole’s radiation through a microscope can be treated similarly to radiation sources possessing 3D-Stokes extensions to 2D paraxial Stokes parameters. By engineering phase and polarization through the detection path of a microscope, it is possible to encode, in a PSF, quantitative information on the 3D orientation of fluorescent single molecules, together with the knowledge of their averaged angular fluctuations (wobbling) and their 3D spatial localization at 10’s nm precision. We demonstrate an extension of PSF engineering to 3D nano-polarimetry to monitor polarization states in 3D (including spin and depolarization), scattered from metal nanoparticles. We finally show the advantages of general 3D Stokes decompositions to analyze polarization properties of light sources through an imaging system.