104 research outputs found
Inverse scattering for reflection intensity phase microscopy
Reflection phase imaging provides label-free, high-resolution characterization of biological samples, typically using interferometric-based techniques. Here, we investigate reflection phase microscopy from intensity-only measurements under diverse illumination. We evaluate the forward and inverse scattering model based on the first Born approximation for imaging scattering objects above a glass slide. Under this design, the measured field combines linear forward-scattering and height-dependent nonlinear back-scattering from the object that complicates object phase recovery. Using only the forward-scattering, we derive a linear inverse scattering model and evaluate this model's validity range in simulation and experiment using a standard reflection microscope modified with a programmable light source. Our method provides enhanced contrast of thin, weakly scattering samples that complement transmission techniques. This model provides a promising development for creating simplified intensity-based reflection quantitative phase imaging systems easily adoptable for biological research.https://arxiv.org/abs/1912.07709Accepted manuscrip
Isotropic diffraction-limited focusing using a single objective lens
International audienceFocusing a light beam through a lens produces an anisotropic spot elongated along the optical axis, because the light comes from only one side of the focal point. Using the time-reversal concept, we show that isotropic focusing can be realized by placing a mirror after the focal point and shaping the incident beam. This idea is applied to confocal microscopy and brings about a dramatic improvement of the axial resolution
Generalization of the coupled dipole method to periodic structures
We present a generalization of the coupled dipole method to the scattering of
light by arbitrary periodic structures. This new formulation of the coupled
dipole method relies on the same direct-space discretization scheme that is
widely used to study the scattering of light by finite objects. Therefore, all
the knowledge acquired previously for finite systems can be transposed to the
study of periodic structures.Comment: 5 pages, 2 figures, and 1 tabl
Selective nanomanipulation using optical forces
We present a detailed theoretical study of the recent proposal for selective
nanomanipulation of nanometric particles above a substrate using near-field
optical forces [Chaumet {\it et al.} Phys. Rev. Lett. {\bf 88}, 123601 (2002)].
Evanescent light scattering at the apex of an apertureless near-field probe is
used to create an optical trap. The position of the trap is controlled on a
nanometric scale via the probe and small objects can be selectively trapped and
manipulated. We discuss the influence of the geometry of the particles and the
probe on the efficiency of the trap. We also consider the influence of multiple
scattering among the particles on the substrate and its effect on the
robustness of the trap.Comment: 12 pages, 17 figure
Isotropic Single Objective (ISO) microscopy : Theory and Experiment
International audienceIsotropic single-objective (ISO) microscopy is a recently proposed imaging technique that can theoretically exhibit the same axial and transverse resolutions as 4Pi microscopy while using a classical single-objective confocal microscope. This achievement is obtained by placing the sample on a mirror and shaping the illumination beam so that the interference of the incident and mirror-reflected fields yields a quasi-spherical spot. In this work, we model the image formation in the ISO fluorescence microscope and simulate its point spread function. Then, we describe the experimental implementation and discuss its practical difficulties
Two-photon fluorescence isotropic-single-objective microscopy
International audienceTwo-photon excitation provides efficient optical sectioning in three-dimensional fluorescence microscopy, independently of a confocal detection. In two-photon laser-scanning microscopy, the image resolution is governed by the volume of the excitation light spot, which is obtained by focusing the incident laser beam through the objective lens of the microscope. The light spot being strongly elongated along the optical axis, the axial resolution is much lower than the transverse one. In this Letter we show that it is possible to strongly reduce the axial size of the excitation spot by shaping the incident beam and using a mirror in place of a standard glass slide to support the sample. Provided that the contribution of sidelobes can be removed through deconvolution procedures, this approach should allow us to achieve similar axial and lateral resolution
Numerical approach for reducing out-of-focus light in bright-field fluorescence microscopy and superresolution speckle microscopy
International audienceThe standard two-dimensional (2D) image recorded in bright-field fluorescence microscopy is rigorously modeled by a convolution process involving a three-dimensional (3D) sample and a 3D point spread function. We show on synthetic and experimental data that deconvolving the 2D image using the appropriate 3D point spread function reduces the contribution of the out-of-focus fluorescence, resulting in a better image contrast and resolution. This approach is particularly interesting for superresolution speckle microscopy, in which the resolution gain stems directly from the efficiency of the deconvolution of each speckle image
Coherent anti-Stokes Raman Fourier ptychography
International audienceWe present a theoretical and numerical study of coherent anti-Stokes Raman scattering Fourier ptychography microscopy (CARS-FPM), a scheme that has not been considered so far in the previously reported CARS wide-field imaging schemes. In this approach, the distribution of the Raman scatterer density of the sample is reconstructed numerically from CARS images obtained under various angles of incidences of the pump or Stokes beam. Our inversion procedure is based on an accurate vectorial model linking the CARS image to the sample and yields both the real and imaginary parts of the susceptibility, the latter giving access to the Raman information, with an improved resolution
Discrete dipole approximation for the study of radiation dynamics in a magnetodielectric environment
A cross-sectional study assessing the contributions of body fat mass and fat-free mass to body mass index scores in male youth rugby players
Background. In some sports such as rugby, a large body size is an advantage, and the desire to gain weight can bring young players to become overweight or obese. The aim of this study was to evaluate the prevalence of overweight and obesity and the contribution of body fat mass index (BFMI) and fat-free mass index (FFMI) to body mass index (BMI) changes among young male rugby players (15-a-side rugby). Methods. The criteria of the International Obesity Task Force were used to define overweight and obesity from BMI. The method of skinfold thickness was used to assess percentage of body fat (%BF), BFMI, and FFMI. Excess body fat was defined by using BFMI and %BF above the 75th percentile. Data were grouped according to the age categories of the French Rugby Federation (U11, under 11 years; U13, under 13 years; U15, under 15 years) and to BMI status (NW normal-weight versus OW/OB overweight/obese). Results. Overall, 32.8% of the young players were overweight, and 13.8% were obese. However, 53% of young players classified as obese and overweight by BMI had an excess body fat by using BFMI above the 75th percentile. FFMI increased significantly between U11 and U13 in both groups, without significant change in BMI and BFMI. Both groups had similar significant gains in BMI and FFMI between U13 and U15, while BFMI only increased significantly in OW/OB (+ 18.5%). The strong correlations between BMI and %BF were systematically lower than those between BMI and BFMI. FFMI was strongly or moderately associated with BFMI. Conclusions. Chart analysis of BFMI and FFMI could be used to distinguish changes in body composition across age categories in young male rugby players classified as normal-weight, overweight, and obese by BMI
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