From the session Tomography and Computational Imaging (JSII_1) (invited) 4D Digital Holographic Microscopy (4D-DHM) is a new technique capable of performing 3D imaging of moving objects. 3D reconstruction is made from a single hologram, but is restricted to sparse objects that occupy only small part of the volume. For a sequence of holograms, 4D-DHM therefore performs 4D (x; y; z and t) imaging. 4D-DHM is well suited to image micro circulation, and has been validated imaging red blood cells circulating in a zebrafish embryo in vivo [1-3]. Fig. 1 (a) Holographic setup. (b, c and d) Images of blood flow within a zebrafish embryo: blood vessel (purple) and RBCs (green). The displayed 2D images correspond to the projections along z of the 3D images rotated along the y direction by-30° (b), 0° (c) and +30° (d). Bar is 50 m, The principle of the 4D DHM is illustrated by Fig. 1 (a). The setup is an upright microscope, which has been modified to perform Digital Holographic Microscopy (DHM) in transmission. The white light illumination is replaced by a laser, which is split in two beams of different directions. To perform holographic detection a reference arm (ELO field) is added on the detection side. The field E scattered by the object interferes with ELO , and a camera records sequences of frames. These frames are used to calculate the holograms, and to reconstruct in 3D the field scattered by the sample. By a proper analysis of this information, 3D images of the blood vessels, and 4D images of the moving red blood cells (RBCs) can be obtained see Fig,1 (b) to (d). To get these images, we have proceeded by the following way [2,3]. First, we calculate the holograms which differences of frames, and select by the way the signal from the moving components of the object (i.e.) the RBCs signal. We have then extract from the data, the holograms corresponding to the two illumination. For each RBC, and each illumination, the field that can be reconstructed by holography as a narrow cone of light (narrow because RCBs are much larger than the wavelength), which is angularly tilted like the illumination. The location of RBCs is then obtained by considering the intersection of the two cones. To prevent the signals of the different RBCs to mix together, the positions of the RBCs that give the largest signals are calculated first. The contributions of these RBCs are then removed from the hologram, and the positions of the following RBCs are calculated from the hologram thus cleaned. The reconstruction by cleaning is highly non-linear (unlike conventional holographic reconstructions), since it is the algorithm that decides to place a RBC in a plane and to remove the field diffused by this RBC from the other planes. In order to converge successively it requires to image sparses objects, that occupy only small part of the holographic reconstructed volume. 4D-DHM is thus capable of performing 3D reconstruction is made from a single hologram, but is restricted to sparse objects that occupy only small part of the volume.

Open channels are peculiar incidents modes of the optical field that are transmitted by a highly diffusing medium with high efficiency (100 % in the ideal case). The existence of these modes has been predicted by Dorokhov et al. [1] more than 25 years ago. Although many experiments were made to prepare these modes [2,3], the maxima of transmission obtained in experiments, are ever much lower than 100 %. To test the theory, we have considered here another Dorokhov's prediction: the average transmission of the modes that sustain the transmitted field must be TT=66%, where TT is defined by N2 TT= N1 T (where T is the averaged transmission of energy, and N1 and N2 the number of incident and transmitted modes i.e. the size of the incident and transmitted mode basis). Fig.1 (a) Holographic experimental setup. (b) Residual correlation of the transmitted field. The horizonal axis is SQ'. The sample transverse size is L=70 µm red, 50 µm green, 35 µm blue.... The horizontal grey line is the residual correlation expected for N1=N2. To verify this prediction, we have measured N2 on a slab sample of ZNO powder (transverse size L<=70 µm, thickness 20 µm) [4], by assuming that N1 is given by the sample geometry. The experimental setup is seen in Fig. 1(a). The incident fields are random (there are scrambled by a diffusing liquid DL) and digital holography is used to measure the complex transmitted field E(x,y) for the two polarisations. N2 is measured by analysing the residual correlations of the transmitted field measured for uncorrelated incident fields. Since the average transmission is T=1/25, we expect N2=N1/16. To get this optimal result, the transverse size of sample L must be infinite and the defocusing SQ' of the microscope objective MO2 must be zero (where S is the sample outgoing surface and Q' the plane imaged by MO2). Figure 1 (b) shows the residual correlation (which is proportional to 1/N2) as a function these two parameters. We get N2=N1/12 without extrapolation, and N2=N1/16 with extrapolation to L infinite. Here, the open channel effect is huge (factor 16) and the aggreement with the Dorokhov's prediction is excellent. We acknowledge Labex Numev (convention ANR-10-LABX-20) for funding. References 1. O.N. Dorokhov. On the coexistence of localized and extended electronic states in the metallic phase. Solid state communications, 51(6): 381-384, (1984).

Core-shell microgels studied by ellipsometry, DLS, and AFM.

A gel is a dispersed system which consists of at least two different components: a solid or flexible mesh and a fluid. If the fluid is water the gel is called a hydrogel. Microgels are gels smaller than 10 μm and can be used in a wide range of applications like drug delivery and smart surface coatings. If the microgel consists of acrylamides like N-isopropylmethacrylamide (NIPMAM) or N-n-propylacrylamid (NNPAM) as network component, they show a volume phase transition (VPT) at a certain temperature, the volume phase transition temperature (VPTT). An increase in temperature above the VPTT leads to an abrupt decrease in size and a decrease in temperature leads to an abrupt increase in size. The VPTT is specific for each monomer. To use microgels in sensors or for nanoactuators the thermoresponse has to be precise and well known. That is why we investigated microgels with a complex architecture containing NIPMAM and NNPAM. These particles show a tunable linear change in size between the two VPTTs of NNPAM (22 °C) and NIPMAM (43 °C). Furthermore, we deposited these microgels on surfaces and investigated the properties of the microgel coating. The properties of these particles (like phase transition behavior or particle size) can be adjusted by selecting specific synthesis conditions.