M. Muhammad Waqar IQBAL soutiendra publiquement ses travaux de thèse intitulés : “Tailoring Conical Diffraction Vector Beams in Linear and Nonlinear Regime”, le vendredi 10 novembre 2023 à 10h00 dans l’Amphithéâtre de CentraleSupélec.
Membres du jury :
Patricia SEGOND, Rapporteu, Professeure, Université Grenoble Alpes, France
Eugenio FAZIO, Rapporteur, Professeur, Sapienza Università di Roma, Italy
Sylvain LECLER, Examinateur, Professeur, Université de Strasbourg, France
Miguel ALONSO, Examinateur, Professeur, Centrale Méditerranée, France and University of Rochester, USA
Virginie CODA, Examinateur, Maître de conférences, Université de Lorraine, France
Germano MONTEMEZZANI, Directeur de thèse, Professeur, Université de Lorraine, France
Nicolas MARSAL, Co-directeur de thèse, Maître de conférences, CentraleSupélec, France
Abstract :
A specific type of vector beams can be created naturally via the conical diffraction (also known as conical refraction) phenomenon, which occurs as a tightly focused beam is incident along one of the optic axis of a biaxial crystal. In the case of a homogeneously polarized input wave, conical diffraction (CD) gives rise to double circular rings, where every two diametrically opposite points on the rings possess orthogonal linear polarization. In this thesis research work, we perform fundamental new investigations on the tailoring of CD vector waves in linear and nonlinear biaxial crystals (BC). The properties being affected are shape, intensity, polarization and orbital angular momentum (OAM). Complex beam shaping is obtained in conjugate cascades of BCs. This breaks the usual circular symmetry characteristics typical of CD and leads to complex non-circular CD patterns with peculiar properties. The conjugate cascade involves at least two BCs with aligned optic axis and two crossed cylindrical lenses between adjacent BCs to perform intermediate wave-vector space manipulation. A modified paraxial diffraction theory of CD, which incorporates the effects of this manipulation reproduces well the experimentally observed features. Besides the exact theoretical treatment, specific rules of thumb established via newly introduced structure parameters help to predict the shape and polarization distribution of the non-circular CD beams. In strong contrast to conventional cascades, which result in generating multiple concentric rings with uniform intensity and homogeneously varying polarization, the conjugate cascades results in the creation of patterns that consist of non-circular individual structures possessing convex or concave curvatures, with non-homogeneously varying intensity and polarization distribution. Extended conjugate cascade of three or more biaxial crystals results in patterns, where individual structures can cross each other or merge locally to give interference. While for conventional cascades the polarization on the rings keep being linear, in our case elliptical or circular polarizations can be observed locally at structure crossing or merging regions. By looking at the azimuthal phase of the non-circular CD beams, it turns out that they possess single charged vortices and phase discontinuity lines. Their fractional OAM values can be scaled by adapting the conjugate cascade parameters. All in all, the non-circular CD beams lead to an increased vector pattern richness and versatility as compared to the circular rings of conventional CD.
In this thesis we also combine for the first time the linear optical CD effect with the nonlinear recording of dynamic holograms in a photorefractive (PR) biaxial crystal, with all processes taking place within the same medium. Even though the hologram recording is driven by the intensity distribution as in conventional holography, it is observed that efficient recording of CD object beams can be achieved with any combination of the input polarizations of the object and reference waves, including the case of orthogonal linear or circular polarizations. Similarly for a two-wave mixing (TWM) experiments, it is observed that different regions of CD object beam can be amplified by varying the polarization of the pump beam. A simplified theoretical model that considers the physics of the CD phenomenon as well as of the photorefractive grating recording and read-out (in a complex oblique orientation inside the crystal) verifies qualitatively the experimental observations. These investigations might open interesting perspectives for CD holography and TWM to be utilized for multichannel polarization multiplexing of vector beams and for intensity reshaping and equalization within the vector beam.