Dr. Guilhem Poy (U Montpellier): Multiscale modelling and experiments of chiral liquid crystal topological solitons & defects and their interactions with light

The past 15 years have seen the emergence of powerful experimental techniques enabling the patterning, control, and optical coupling of localized birefringent structures in liquid crystal (LC) systems. More often than not, these localized patterns of orientational order have a topological nature---they can be classified with integers similar to the genus classification of surfaces---and are stabilized by chirality---the breaking of the mirror symmetry. In particular, the concept of "topological solitons", a localized field pattern that cannot be continuously deformed into the lowest energy state of the system, is especially useful to understand the structure of these patterns. Thanks to their birefringent nature, these patterns also have strong light-matter interactions, including non-linear optical effects accessible at laser powers of a few mW and a wide array of beam shaping and guiding capabilities. Modelling the structural and optical properties of these structures can however be challenging due to the wide range of scales involved, from the ~10 nm size of singular topological defects and the wavelength size of light up to the pattern size, that can be as big as 0.1-1mm. Based on recent numerical advances allowing to adresse some of the challenges behind these multiscale and multiphysics simulations, I will first discuss the relevance of singular topological defects on the optical properties of flat LC lenses---a modern and compact design of lenses with application to virtual/augmented reality. I will then discuss the waveguiding properties of nonsingular topological defects that nature offers for free in chiral liquid crystal systems, as well as optomechanical interactions between light and topological solitons. Finally, I will present recent experiments of fluctuating liquid crystal pattern analysed through the statistical properties of scattered light in a microscope.
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