The use of Liquid Crystal (LC) materials, mainly in display applications, has contributed to major advancement in liquid crystal science and technology. New and more complex phases of liquid crystals were developed to compete with conventional nematic LC displays. The challenge now is to manufacture high birefringence liquid crystal materials with low viscosity. LC is also used in many other applications, such as temperature sensors and photonics beam shaping in the form of spatial light modulators (SLM) and q-plates. The first objective of this thesis is to investigate the magic mirror effect using a SLM following Sir Michael Berry’s theory. Here, we demonstrated a simple way of producing the magic mirror effect using LC devices and aimed to use a micron-sized device to shape the phase and polarization of light with gentle phase variation. We were able to generate the magic mirror image intensity pattern, both experimentally and theoretically. This was done by computing and generating the desired phase pattern of an image on the SLM, then aligning light propagation through this phase pattern. The experimental and theoretical results showed good agreement when comparing the produced intensity patterns. In the second part of this thesis, we experimentally investigated the use of structured photons, created using q-plates, which is a birefringent liquid crystal cell of OAM and SAM coupling, in quantum key distribution (QKD) using the BB84 protocol through orbital angular momentum (OAM) maintaining optical fibres. Here, we were successful in generating a secure key between two parties with a quantum bit error rate of 8.6% which is below the security threshold of 11%. This work demonstrates the feasibility of using structured light in QKD through fibres to boost key rates and security.

The vectorial nature (polarization) of light plays a significant role in light-matter interaction that leads to a variety of optical devices. The polarization property of light has been exploited in imaging, metrology, data storage, optical communication and also extended to biological studies. Most of the past studies fully explored and dealt with the conventional polarization state of light that has spatially symmetric electrical field geometry such as linear and circular polarization. Recently, researchers have been attracted to light whose electric field vector varies in space, the so-called optical vector vortex beam (VVB). Such light is expected to further enhance and improve the efficiency of optical systems. For instance, a radially polarized light under focusing condition is capable of a tighter focus more than the general optical beams with a uniform polarization structure, which improves the resolution of the imaging system [1]. Interaction of ultrafast laser pulses with matter leads to numerous applications in material processing and biology for imaging and generation of microfluidic systems. A femtosecond pulse, with very high intensities of (10^{12} - 10^{13} W/cm^2), has the potential to trigger a phenomenon of optical breakdown at the surface and therefore induce permanent material modification. With such high intensities and taking into account the fact that most materials possess large bandgap, the interaction is completely nonlinear in nature, and the target material can be modified locally upon the surface and even further in bulk. The phenomenon of optical breakdown can be further investigated by studying the nonlinear absorption. Properties like very short pulse duration and the high irradiance of ultrashort laser pulse lead to more precise results during the laser ablation process over the long pulsed laser. The duration of femtosecond laser pulse provides a high resolution for material processing because of the significant low heat-affected zone (HAZ) beyond the desired interaction spot generated upon irradiating the material. Under certain condition, the interaction of intense ultrashort light pulses with the material gives rise to the generation of periodic surface structures with a sub-micron periodicity, i.e., much smaller than the laser wavelength. The self-oriented periodic surface structures generated by irradiating the material with multiple femtosecond laser pulses results in improving the functionality of the material's surface such as controlling wettability, improving thin film adhesion, and minimizing friction losses in automobile engines, consequently, influences positively on many implementations. In this work, we introduced a new method to study complex polarization states of light by imprinting them on a solid surface in the form of periodic nano-structures. Micro/Nanostructuring of diamond by ultrafast pulses is of extreme importance because of its potential applications in photonics and other related fields. We investigated periodic surface structures usually known as laser-induced periodic surface structures (LIPSS) formed by Gaussian beam as well as with structured light carrying orbital angular momentum (OAM), generated by a birefringent optical device called a q-plate (QP). We generated conventional nano-structures on diamond surface using linearly and circularly polarized Gaussian lights at different number of pulses and variable pulse energies. In addition, imprinting the complex polarization state of different orders of optical vector vortex beams on a solid surface was fulfilled in the form of periodic structures oriented parallel to the local electric field of optical light. We also produced a variety of unconventional surface structures by superimposing a Gaussian beam with a vector vortex beam or by superposition of different order vector vortex beams. This thesis is divided into five chapters, giving a brief description about laser-matter interaction, underlying the unique characterization of femtosecond laser over the longer pulse laser and mechanisms of material ablation under the irradiation of fs laser pulse. This chapter also presents some earlier studies reported in formation of (LIPSS) fabricated on diamond with Gaussian. The second chapter explains the properties of twisted light possessing orbital angular momentum in its wavefront, a few techniques used for OAM generation including a full explanation of the q-plate from the fabrication to the function of the q-plate, and the tool utilized to represent the polarization state of light (SoP), a Poincar'e sphere. Finally, the experimental details and results are discussed in the third and fourth chapters, respectively, following with a conclusion chapter that briefly summarizes the thesis and some potential application of our findings.

Dans ce travail nous avons résolu deux problèmes principaux de la mise en forme topologique de faisceau paraxial pour les composants plans : la modalité et le polychromatisme.Nous les résolvons en introduisant de nouveaux concepts d’éléments optiques à interaction spin orbite,à savoir la “q-plate modale” et la “q-plate Bragg-Berry”. D’un côté, la q-plate modale convertit un faisceau gaussien incident en un faisceau de Laguerre-Gauss pour un indice radial et un indice d’azimut donnés, ce qui par conséquent dépasse les capacités des q-plates conventionnelles qui ne modifient que le degré de liberté azimutal, c.à.d. le moment orbital angulaire de la lumière. À des fins expérimentales, deux approches ont été développées : une basée sur des lames de verres nanostructurées artificiellement, l’autre sur des défauts topologiques de cristaux liquides auto-organisés naturellement. D’un autre côté, la q-plate Bragg-Berry consiste en une fine couche inhomogène de cristaux liquides chiraux (cholestériques) devant un miroir, ce qui fournit une mise en forme de faisceau spin-orbite pleinement efficace sur une large bande spectrale du faisceau incident, contrairement au q-plates conventionnelles qui ne sont fabriqués que pour une longueur d’onde donnée. Par ailleurs, nous obtenons une mise en forme de faisceau spin-orbite ultra-large bande en induisant une modulation de la structure supramoléculaire torsadée des cristaux liquides cholestériques selon la direction de propagation de la lumière. Nous montrons également que la présence du miroir derrière permet un puissant contrôle spatio-temporel des propriétés vectorielles de la polarisation du champ lumineux générées par la q-plate Bragg-Berry. / It is well-known that paraxial coherent electromagnetic fields can be completelycharacterized in terms of their radial and azimuthal spatial degrees of freedom in the transverse planethat add to the polarization degree of freedom and wavelength. In this work we address two mainissues of paraxial beam shaping that are the modality and the polychromaticity in the context of flatopticsthat we address by introducing novel concepts of spin-orbit optical elements. Namely, the‘modal q-plate’ and the ‘Bragg-Berry q-plate’. On the one hand, modal q-plate converts an incidentfundamental Gaussian beam into a Laguerre-Gaussian beam of given radial and azimuthal indices,hence going beyond the capabilities of conventional q-plates that only control the azimuthal degreeof freedom, i.e. the orbital angular momentum content of light. Towards experimental realization ofmodal q-plates, two approaches are developed: one based on artificially nanostructured glasses andanother based on naturally self-organized liquid crystal topological defects. On the other hand,Bragg-Berry q-plate consist of mirror-backed inhomogeneous thin film of chiral liquid crystal(cholesteric) that provides fully efficient spin-orbit beam shaping over broad spectral range of theincident beam, in contrast to the conventional q-plates that are designed for single wavelength.Furthermore, ultra-broadband spin-orbit beam shaping is achieved by inducing an extra modulationof the supramolecular twisted structure of the cholesteric liquid crystal along the propagationdirection. We also show that the presence of a back-mirror allows a powerful spatio-temporal controlof the polarization vectorial properties of the light fields generated by Bragg-Berry q-plate.