Lentille de Fresnel | EPFL Graph Search

vignette|Une lentille de Fresnel et deux ampoules au phare d' (île de Suðuroy) dans les îles Féroé. La lentille de Fresnel ou lentille à échelons est un type de lentille inventé par Augustin Fresnel en 1822 pour remplacer les miroirs utilisés dans l'éclairage des phares de signalisation marine qui absorbaient jusqu'à 50 % du flux lumineux. C'est une lentille plan-convexe découpée de sections annulaires concentriques optimisées pour alléger l'élément. Sa conception lui permet d'obtenir une courte distance focale pour un large diamètre, sans la masse et le volume nécessaires à une lentille standard. Historique Phare#Historique vignette|Lentille de Fresnel du phare de Cordouan, la première lentille qui y fut installée fut démontée et remplacée par une lentille de Fresnel du premier ordre en 1856. vignette| Une lentille de Fresnel exposée au Musée national de la Marine (Paris). Jusqu'au et au les phares se sont heurtés à de gros écueils en ce qui concerne la luminosité de l

Regularity of solutions for a fourth-order elliptic system via Conservation law

Changyu Guo

In this paper, we obtain interior Holder continuity for solutions of the fourth-order elliptic system Delta(2)u = Delta(V center dot del u) + div(w del u) + W center dot del u formulated by Lamm and Riviere [Comm. Partial Differential Equations 33 (2008) 245-262]. Boundary continuity is also obtained under a standard Dirichlet or Navier boundary condition. We also use conservation law to establish a weak compactness result which generalizes a result of Riviere for the second-order problem.

2019

Application of Adaptive Optics to Focusing and Imaging

Daniel Iwaniuk

Adaptive optics is used to abate aberrations with a wavefront correction. It is widely used in astronomy and is starting to expand into applications of laser focusing and imaging with high numerical apertures, e.g. microscopy. The physical background of focusing laser light and imaging is the same, so we can use the same methods. In both cases, light is propagating through a lens; either it becomes focused into a small region or light from a small region becomes imaged on a camera. Modulations applied on the lens to implement wavefront corrections behave similarly in both applications. A powerful simulation tool was created to characterize the impact of those modulations. As an example, we validated a design for a Fresnel lens produced on a glass fibre tip to focus its emitting light. We have developed solutions mainly for three different problems. First, a high depth of focus enables keeping a laser beam focused within a larger length or imaging objects from different positions simultaneously. In photography, this can be attained by stopping down the aperture, which introduces a huge loss of light. State-of-the-art for focusing into a line segment also shows an inefficient performance. We present an elegant lens design, which enables highly efficient elongation of the depth of focus. Preliminary studies have shown that it might be a feasible alternative for current intra-ocular lens implants in ophthalmology or for 3D visualization in imaging and microscopy. A high depth of focus also has a large potential in optical lithography and data storage, because the focal position is enlarged and does not have to be adjusted precisely to obtain a useful spot. Second, specimen-induced aberrations can affect even a perfectly adjusted, diffraction limited lens system. A planar refraction index mismatch introduces spherical aberrations, degrading the optical resolution. As a first step to correct them, we were able to characterize simultaneously the refractive index and the thickness of an unknown medium that is placed between the lens and its focal region. This was done by clever manipulation of the beam angles with the same adaptive optics element in the focusing and in the imaging system. Finally, the medium-induced spherical aberrations were corrected based on the characterization results. The point spread function degradation of a focused laser beam was completely removed, which might be useful in optical tweezers or in laser processing of biological samples. While imaging through the planar refractive index mismatch, the bending of the object field was corrected and the diffraction limited performance restored.

EPFL2012

Phase resolved characterization of nanoscale features in optical microstructures

Michail Symeonidis

In this study, we apply interferometric microscopy to study the phase, alongside the intensity, of the light field transmitted through a wide variety of samples. Additionally, we conduct those interferometric measurements at different wavelengths within the visible spectrum, probing the spectral evolution of the optical effects that the studied samples create. Using a priori knowledge about the samples, we identify specific features and trends they imprint on the parameters of the transmitted field, thus we optimally analyze the recorded intensity and phase data. We demonstrate that phase maps include useful information that reveal the features of the samples. The wide applicability of the spectrally resolved interference microscopy is the major novelty contributed by the present work. The phase seldom recorded, therefore we first present and explain the tools that we use throughout this work, both in terms of set-up and data post-processing. Next, we show the acquired results, starting from samples with typical dimensions in the few hundreds of microns and concluding with particles of few hundreds of nanometers. More specifically, we study microlenses of high numerical aperture that are difficult to characterize with conventional methods. We propose a novel approach to extract the surface profile of such microlens, providing useful feedback for manufacturing purposes. We do that by using phase measurements at a plane that lies between the surface of the lens and its focal plane. The next sample we study is a phase element, which is naturally easier to discern in the phase domain. We localize specific points in intensity and phase profiles that are correlated to the position of the walls of the actual sample. We also demonstrate the existence of phase singularities in phase measurements that can be proved an invaluable tool in high precision characterization processes. Next, we investigate the photonic nanojet phenomenon, which is the bright spot created on the shadow side of a dielectric sphere. The creation of this bright spot is not achieved through propagation inside the material, like conventional lenses, it is the combination of the light scattered and diffracted by the sphere. Therefore, we study its spectral evolution using intensity measurements and outline the size of the sphere that indicates the crossing from the dominant refractive regime to the diffractive one; combining intensity and phase information, we attempt to identify the size at which the behavior of the dielectric spheres changes again from diffractive to scattering. In the last experiments chapter, we study individual nano-sized particles, which are either simple spheres (dielectric / metallic) or the more complex structures of the core-shell meta-atom. We show that the spectral information of their response in intensity and phase can be used to identify the particle itself (simple dielectric / metal or meta-atom) and assess its responses with the respect to the engineered one (for the meta-atom case). Those examples validate the claimed benefits of the phase exploration. Still, there is ample room for further study; we debate about those prospects in the concluding chapter of this work.

EPFL2019