Nowadays, complicated topological defects enable many experimental manipulations and configurational simulations of active soft matter for optical and photonic applications. Investigation of topological defects in soft anisotropic materials enables one to better understand three-dimensional orientation fields in cholesteric liquid crystals. Here, we describe optical properties of bistable bubble domain (BD) texture torons in a thin layer of cholesteric liquid crystal (CLC), frustrated by homeotropic anchoring conditions, and reliably switchable by a random process. The control of macroscopic optical density and diffraction efficiency of the BD texture is demonstrated by a selection of a confinement ratio of the CLC. Experimentally reconstructed CLC director profile reveals the topology of BD torons allowing consideration of naturally occurring BD texture for applications in optical and photonic devices, which are bistably switchable between active and transparent optical states.

Liquid crystals (LCs) form an extremely rich range of self-assembled topological structures with artificially or naturally created topological defects. Some of the main applications of LCs are various optical and photonic devices, where compared to their solid-state counterparts, soft photonic systems are fundamentally different in terms of unique properties such as self-assembly, self-healing, large tunability, sensitivity to external stimuli, and biocompatibility. Here we show that complex tunable microlasers emitting structured light can be generated from self-assembled topological LC superstructures containing topological defects inserted into a thin Fabry–Pérot microcavity. The topology and geometry of the LC superstructure determine the structuring of the emitted light by providing complex three-dimensionally varying optical axis and order parameter singularities, also affecting the topology of the light polarization. The microlaser can be switched between modes by an electric field, and its wavelength can be tuned with temperature. The proposed soft matter microlaser approach opens directions in soft matter photonics research, where structured light with specifically tailored intensity and polarization fields could be designed and implemented.

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Nematic liquid crystalline fluids are complex anisotropic fluids characterised by internal orientational order of its constituent building blocks [1,2], which ranges in scales from molecules, macromolecules like DNA, to colloidal rods or platelets. Typically, the orientational order emerges at some temperature or concentration range of building blocks as a result of the geometrical shapes of prolate or oblate building blocks. More recently, nematic order emerged also as an important characteristic of various active fluids, i.e. fluids that can self-propel. Nematic fluids are inherently soft materials, with the orientational order responding as an effective elastic medium to external perturbations, like surfaces or electromagnetic fields. And it is this soft and-optically or structure wise-strong response to external fields which makes nematic fluids potent materials in various applications, including in the fields of optics, photonics, and sensors. The broadest range of applications and experiments with nematic fluids is as at the scales of multiple building elements (which, for example, for molecular nematics, is in the micrometre regime), where mesoscopic approaches prove to be the strongest to describe the systems, as compared to molecular and effective molecular approaches [3,4], which are used at smaller scales. In view of this, this chapter will present mesoscopic approach to nematic fluids, based on continuum description of the nematic mechanisms and phenomena.

The tracheary system of plant leaves is composed of a cellulose skeleton with diverse hierarchical structures. It is built of polygonally bent helical microfilaments of cellulose-based nanostructures coated by different layers, which provide them high compression resistance, elasticity, and roughness. Their function includes the transport of water and nutrients from the roots to the leaves. Unveiling details about local interactions of tracheary elements with surrounding material, which varies between plants due to adaptation to different environments, is crucial for understanding ascending fluid transport and for tracheary mechanical strength relevant to potential applications. Here we show that plant tracheary microfilaments, collected from Agapanthus africanus and Ornithogalum thyrsoides leaves, have different surface morphologies, revealed by nematic liquid crystal droplets. This results in diverse interactions among microfilaments and with the environment; the differences translate to diverse mechanical properties of entangled microfilaments and their potential applications. The presented study also introduces routes for accurate characterization of plants’ microfilaments.