Bragg-Berry optical elements combine the bandgap properties of a cholesteric with the spin-orbit properties of birefringent liquid crystals (LCs). By engineering an appropriate LC alignment pattern with surface angle ϕ inside a cholesteric sample, incident circular polarized light with a wavelength inside the cholesteric bandgap will be reflected by this sample and acquire the so-called Pancharatnam-Berry phase ±2ϕ. With different spatial distributions of ϕ, one can therefore obtain various reflective optical elements such as geometric phase lenses [1] or diffractive elements [2,3]. However, topological defects typically appear when ∇ϕ is not uniform, which may affect the optical quality of the realized optical elements. In this work, we study with numerical and theoretical approaches the equilibrium and optical properties of such defects inside Bragg-Berry cholesteric lenses with a rotationally invariant ϕ-pattern. We demonstrate the existence of quantified equilibrium positions for these defects, similar to Grandjean-Cano cholesteric lines in planar wedge samples, and compare this behaviour with experimental realizations of Bragg-Berry lenses done in the LC photonics group at Ghent University. We also study the impact of such defects on the reflective properties of the lens, based on finite-difference time-domain simulations and a simple point-spread-function model.[1] Stebryte, M. “Reflective optical components based on chiral liquid crystal for head-up displays”, Liquid Crystals Today 30, 36-45 (2021).[2] Stebryte, M., Nys, I., Ussembayev, Y., Beeckman, J. & Neyts, K. “Large Angle Forward Diffraction by Chiral Liquid Crystal Gratings with Inclined Helical Axis”, Crystals 10, 807 (2020).[3] Stebryte, M., Nys, I., Beeckman, J. & Neyts, K. “Chiral liquid crystal based holographic reflective lens for spectral detection”, Optics Express 30, 42829 (2022).

The interplay of soft responsive particles, such as microgels, with nanoparticles (NPs) yields highly versatile complexes that show great potential for applications, ranging from plasmonic sensing to catalysis and drug delivery. However, the microgel–NP assembly process has not been investigated so far at the microscopic level, thus hindering the possibility of designing such hybrid systems a priori. In this work, we combine state-of-the-art numerical simulations with experiments to elucidate the fundamental mechanisms taking place when microgel–NP assembly is controlled by electrostatic interactions and the associated effects on the structure of the resulting complexes. We find a general behavior where, by increasing the number of interacting NPs, the microgel deswells up to a minimum size after which a plateau behavior occurs. This occurs either when NPs are mainly adsorbed to the microgel corona via the folding of the more external chains or when NPs penetrate inside the microgel, thereby inducing a collective reorganization of the polymer network. By varying microgel properties, such as fraction of cross-linkers or charge, as well as NP size and charge, we further show that the microgel deswelling curves can be rescaled onto a single master curve, for both experiments and simulations, demonstrating that the process is entirely controlled by the charge of the whole microgel–NP complex. Our results thus have a direct relevance in fundamental materials science and offer novel tools to tailor the nanofabrication of hybrid devices of technological interest.

In this paper, we numerically and experimentally show that the director field orientation degeneracy within the Translationally Invariant Configuration (TIC) of a cholesteric liquid crystal under an electric field can be lifted by imposing a magnetic field B parallel to the electrodes. The configuration can be either parallel or perpendicular to the magnetic field depending on the values of the sample thickness, pitch, and applied voltage, with two equiprobable orientations in each case. The transition between the parallel and perpendicular orientations has hysteresis, suggesting that it is first order. When B is slightly tilted with respect to the electrode plane, the indeterminacy on the TIC orientation is removed when the TIC is directed along B.

Concentrated colloidal suspensions and emulsions are amorphous soft solids, widespread in technological and industrial applications and studied as model systems in physics and materials sciences. They are easily fluidized by applying mechanical stress, undergoing a yielding transition that still lacks a unified description. Here we investigate yielding in three classes of repulsive soft solids and find that at the microscopic level, yielding consists of a transition between two distinct dynamical states. We rationalize this by proposing a lattice model with dynamical coupling between neighbouring sites, leading to a unified state diagram for yielding. Employing the analogy with van der Waals phase diagram for real gases, we show that distance from a critical point plays a major role in the emergence of first-order-like versus second-order-like features in yielding, thereby reconciling previously contrasting observations on the nature of the transition.