16 Octobre – Thesis defense - Mikaël Ghadimi Nassiri

14 h Amphi - Building A9

Broadband topological shaping of light.

The aim of this thesis is to develop several experimental approaches for broadband beam shaping of light. Today, several beam shaping tools are available, some of them commercially, but most of them are designed for only one working wavelength. This thesis aims to develop several experimental approaches for broadband topological beam shaping of light. After the presentation of the state of the art, our work focuses on vortex shaping of polychromatic beam exploiting the spin-orbit interaction of light. Concretely, we report the development of four techniques to modulate the so-called geometric phase of polychromatic light fields. First, we describe anisotropic reflection from interfaces that involves at least one uniaxial crystal. We identify a refractive index matching criterion enabling highly pure broadband phase control. Then we discuss the use of circular Bragg reflection phenomenon inherent to the optics of cholesteric liquid crystals. This property allows the selective reflection of circularly polarized light over a bandgap whose width depends on material parameters while the reflected field acquires a geometric phase that depends on the orientation of the supramolecular helix of the material. These properties are exploited to design, fabricate and characterize structured mirrors reflecting Laguerre-Gauss optical modes to a good approximation. The last two solutions consist of vortex beam shaping using inhomogeneous anisotropic planar optical elements, namely, topological defects that spontaneously appear in homeotropic nematic liquid crystal films characterized by negative dielectric anisotropy. Such systems behave as geometric phase optical elements relying on the spin-orbit interaction of light. The first option is based on using two defects in series while the other is based of parallel processing using an array of independently controlled topological defects, each of them being dedicated to process distinct spectral channels. The latter approach can be viewed as a spatial light modulator whose pixels are inhomogeneous and potential applications are proposed in the field of super-resolution optical imaging and spatio-temporal beam shaping of ultrashort pulses.

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