We present an original electromagnetic solver for three-dimensional photonic structures with a two-dimensional periodicity. It is based on Fourier expansions in the two periodic directions and an elegant yet efficient combination of finite-difference discretization and modal treatment in the propagation direction. The Fourier Finite Difference Method (FFDM) is cast into an easy-to-handle, agile, and efficient matrix formalism. Our approach uses matrix inversions instead of eigenmodes computations, a strategy that confers a clear advantage in terms of computation time for the large Fourier truncation orders that are typically used to deal with micro and nanophotonic structures of current interest. Therefore, it is particularly well suited for the analysis of generalized grating structures and metasurfaces. The computational gain increases with the Fourier truncation order. It is maximal (10-30) for coarse finite-difference steps, which are nevertheless sufficient to achieve a relative accuracy of approximately 1%. A second important advantage of the proposed method is the possibility of using model-order reduction techniques. We use Proper Orthogonal Decomposition (POD), in conjunction with the snapshot technique, to further speed up the computation of diffraction spectra as a function of the frequency, the incident angle, or any geometric parameter.
Obesity is a devastating worldwide metabolic disease, with the highest prevalence in children and adolescents. Obesity impacts neuronal function but the fate of functional hyperemia, a vital mechanism making possible cerebral blood supply to active brain areas, is unknown in organisms fed a high-caloric Western Diet (WD) since adolescence. We mapped changes in cerebral blood volume (CBV) in the somatosensory cortex in response to whisker stimulation in adolescent, adult, and middle-aged mice fed a WD since adolescence. To this aim, we used non-invasive and high-resolution functional ultrasound imaging (fUS). We efficiently mimicked the metabolic syndrome of adolescents in young mice with early weight gain, dysfunctional glucose homeostasis, and insulinemia. Functional hyperemia is compromised as early as 3 weeks of WD and remains impaired after that in adolescent mice. These findings highlight the cerebrovascular vulnerability to WD during adolescence. In WD, ω-6:ω-3 polyunsaturated fatty acids (PUFAs) ratio is unbalanced towards proinflammatory ω-6. A balanced ω-6:ω-3 PUFAs ratio in WD achieved by docosahexaenoic acid supplementation efficiently restores glucose homeostasis and functional hyperemia in adults. WD triggers a rapid impairment in cerebrovascular activity in adolescence, which is maintained at older ages, and can be rescued by a PUFA-based nutraceutical approach.
Recent progress in the design and fabrication of thermal metasurfaces allows a broad control of the properties of light emission, including its polarization state. Stokes polarimetry is a key approach to accurately characterize partially polarized light. The quality of a Stokes polarimeter made of retarders and polarizers can be evaluated by use of metrics such as the equally weighted variance or the condition number of the matrix representing the polarimeter. Although specific instrument configurations are used to maximize polarimeter performance at a given wavelength, such optimal solutions are not spectrally robust because of the wavelength dependence of retardance. This becomes an issue in characterizing broadband thermal sources in the infrared. We report a Stokes polarimeter making use of five polarization analysis states and consisting of two simple and common optical elements-a crystalline waveplate and a linear polarizer. We combine this setup with a Fourier transform infrared spectrometer to measure accurately in a single set of acquisitions without requiring any spectral filtering, and to measure the polarization state with accuracy over a broad range of wavelengths. Such a Stokes polarimeter allows for close to optimal noise in the data reduction process in the mid-wave infrared spectral range from 2.5 to 5 mu m.
Forces inside cells play a fundamental role in cell behavior, for example in cancer cell migration. We focus on the vinculin protein which is involved in the stabilization of cell adhesion. Through fluorescence transfer (FRET), forces within vinculin can be measured with picoNewton sensitivity. We measure these internal forces while applying a calibrated external force with a laser-based optical tweezer via a microbead attached to the cell. Our most recent results using fibroblast cells show that the force applied with the optical tweezer induces the recruitment of vinculin and the formation of focal adhesions on the bead within a few minutes. Once the bead is attached to the cell, we record its trajectory and infer the force exerted by the cell. We correlate this force with the FRET efficiency of the force sensor.
