Mise en forme de faisceau par un milieu homogène anisotrope chiral ou achiral

par Mushegh Rafayelyan

Projet de thèse en Lasers, Matière et Nanosciences

Sous la direction de Etienne Brasselet.

Thèses en préparation à Bordeaux , dans le cadre de École doctorale des sciences physiques et de l’ingénieur (Talence, Gironde) , en partenariat avec Laboratoire Ondes et Matière d'Aquitaine (laboratoire) et de Photonique et matériaux (equipe de recherche) depuis le 01-09-2013 .

  • Titre traduit

    Beam shaping from chiral or achiral anisotropic inhomogeneaus media

  • Résumé

    Controlling the topology of light, and more specifically its orbital angular momentum content, constitutes an exciting contemporary challenge for future photonic technologies. One option to do it consists to transfer the topology of a material system to a light field that passes throughout, which is possible by exploiting the coupling between the polarization and spatial degrees of freedom of light. Once tailored, the latter spin-orbit interaction of light can be used to shape the topology of light at will. From a material point of view this implies to structure matter and two-dimensional spatial distribution of the optical anisotropy is a prime choice. In general, spin-orbit photonic elements based on space-variant anisotropic systems are fabricated in the transmission mode and there is a stringent condition on the birefringent phase retardation of the element to ensure appropriate spin-controlled operation. Importantly, such a requirement adds to the technological difficulty to structure the material at small spatial scale. In our submitted article to Physical review letters, entitled by “Reflective Spin-Orbit Geometric Phase from Chiral Anisotropic Optical Media”, we demonstrate that the two latter limitations can be removed by exploiting the exquisite self-organized properties of anisotropic soft matter: liquid crystals. Indeed, by unveiling that helicity-preserving circular Bragg reflection phenomenon occurring in chiral liquid crystals are endowed with the geometric (Berry) phase, we suggest and demonstrate experimentally a novel kind of formally 100%-pure reflective spin-orbit optical elements. This is illustrated in practice by the demonstration of spin-orbit optical vortex generation from spontaneously formed chiral nematic liquid crystal droplets in the Bragg regime. As such, our results thus establish a new kind of spin-optical elements that we naturally call ‘Bragg-Berry chiral metasurfaces'. From a fundamental point of view, we also unveil both spin-orbit consequences of Poincaré-Hopf theorem and the role of the curvature of 3D metasurfaces in the framework of topological shaping of light. For all these reasons, I believe that the experimental realization of flat mirrors enabling the broadband generation of optical vortices upon reflection is very actual and doable. The effect will be based on the geometric Berry phase associated with the circular Bragg reflection phenomenon from chiral uniaxial media. We plan to show the reflective optical vortex generation from both diffractive and nondiffractive paraxial light beams using spatially patterned chiral liquid crystal films. The intrinsic spectrally broadband character of spin-orbit generation of optical phase singularities also will be demonstrated over the full visible domain. Importantly, our results will not rely on any birefringent retardation requirement and consequently foster the development of a novel generation of robust optical elements for spin-orbit photonic technologies. The further moves will be in broadband vortex generation of patterned cholesteric liquid crystal in transmission regime, which will be based on well know Bragg reflection and transmission phenomenon. Finally, we plan also to initiate the direct optomechanical observations of spin orbit torque exerted from the incident light on cholesteric droplet and based on above mentioned geometrical phase effect endowed with helicity-preserving circular Bragg reflection. We also studied the optical medium which possesses certain type of phase mask, which can work as a modal q-plate. This modal q-plate transforms the incident plane wave to a Laguerre-Gaussian mode with radial index p and azimuthal index l (LG¬p,l). Experimental and theoretical results of transmitted LG0.2 like beam through common q-plate was compared with modal q-plate transmitted beam which retardation possesses l=2 and p=0 characteristics. The correlation method was used for numerical comparison and it was shown that LG02 mode generated by modal q-plate is better compared with the output of the simple q-plate. Moreover, unlike common q-plate, which generates this certain mode in the case of the impinging Gaussian beam, modal q-plate works in the case of incident plane waves, i.e. there is no need to focus the beam on the sample. Few new ambitious projects also are developed and planned to carry out during this year. Some of them directly associated with q-plates technology and its improvement. Others can be deeply applied in new generation display technologies. The research work I will perform during my remaining PhD studies at Université Bordeaux will contribute to reinforce the scientific expertise of Laboratoire Ondes et Matière d'Aquitaine on a subject with high application