The class of materials investigated in the OPTOSOL project are the liquid crystals (LCs) commonly used in the displays of our mobile phones, computers and TVs. Introducing chiral molecules in liquid crystals allows the stabilization of very robust birefringent structures called topological solitons. Like balls of yarn that cannot be easily unrolled at once, topological solitons are ‘knotty’ objects that cannot be destroyed in a continuous manner. Our original motivation was to examine how topological solitons can be used to deflect and control the path of photons in chiral LCs, similar to a billiard ball deflecting the trajectory of another moving ball after collision.
The first part of our work was to examine the mechanisms of propagation of light in such chiral materials, without any topological solitons. When building miniaturized optical devices, one typical problem that you can get is that collimated beam of photons initially propagating in a single direction will spread out very quickly. To avoid this problem, one needs some kind of guiding wire to bring all the photons we need towards a target, akin to the usual wires bringing electrons to processing components in electronic devices. This can be done either with a guiding structure that forces the photons to follow a single path (like in an optical fiber) or by increasing the number of photons inside a light beam. In the latter possibility, the high number of photons will disturb the propagation material itself and can create—in specific geometries—their own guiding wire disallowing the spreading-out of a light beam. In our specific case, we discovered a fundamental (and literal!) twist: by adding chiral molecules into the LC, we showed that the light power needed to generate the soft guiding structure (aka the 'optical wire') can be significantly reduced. Indeed, the chiral molecules actually boost the creation of the optical wire by locally destabilizing the orientation of LC molecules around the guided light (see image 1). This fascinating result was published in Physical Review Letters (Poy et al., 2020) and could lead to potential applications for low-power optical devices using the energy of a laser beam to tune themselves, beyond the use as simple guiding wire in this project.
The next step of our work was to add topological solitons in the same system. We showed for the first time how these robust and localized objects can provide new means for controlling the flow of light at the microscopic level. We demonstrated a set of simple laws that can describe the behavior of laser beams incident on ball-like topological solitons, with a wide range of interaction phenomena such as lensing and deflection of light (see image 2). Furthermore, one can also change the topological solitons’ size using external electric fields, thereby controlling how light is transformed inside the chiral LC sample in real-time. This study was published in Physical Review X (Hess et al., 2020) and opens a new paradigm of interconnected optically-active devices based on topological solitons: by using multiple ball-like solitons, one can create the optical analogous of electronic logic gates, i.e. devices which can make computations based on light instead of electrons.
The purely scientific works above relied on a number of theoretical and numerical approaches developed during the fellowship allowing efficient simulations of the propagation of light in inhomogeneous LC materials. These methods were described in two papers in Soft Matter and Optics Express (Poy and Zumer, 2019; Poy and Zumer, 2020) and led to the dissemination of an open-source microscopy software allowing to bridge the gap between experimental observations of complex LC structures and theoretical model. This software, called Nemaktis (see
https://nemaktis.readthedocs.io/en/latest/intro/overview.html(s’ouvre dans une nouvelle fenêtre)) was also used for the topological identification of various classes of ring-shaped topological solitons in two collaborative publications with the photonics group of Ghent (Berteloot et al., Soft Matter 2020; Nys et al., Crystals 2020).
