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1. Low Dielectric Medium for Hyperbolic Phonon Polariton Waveguide in van der Waals Heterostructures

   https://doi.org/10.3390/nano14161344  

   Noh, Byung-Il; Reza, Salvio; Hardy, Cassie; Li, Jiahan; Taba, Adib; Mahjouri-Samani, Masoud; Edgar, James H; Dai, Siyuan (August 2024, Nanomaterials)

   Polar van der Waals (vdW) crystals, composed of atomic layers held together by vdW forces, can host phonon polaritons—quasiparticles arising from the interaction between photons in free-space light and lattice vibrations in polar materials. These crystals offer advantages such as easy fabrication, low Ohmic loss, and optical confinement. Recently, hexagonal boron nitride (hBN), known for having hyperbolicity in the mid-infrared range, has been used to explore multiple modes with high optical confinement. This opens possibilities for practical polaritonic nanodevices with subdiffractional resolution. However, polariton waves still face exposure to the surrounding environment, leading to significant energy losses. In this work, we propose a simple approach to inducing a hyperbolic phonon polariton (HPhP) waveguide in hBN by incorporating a low dielectric medium, ZrS2. The low dielectric medium serves a dual purpose—it acts as a pathway for polariton propagation, while inducing high optical confinement. We establish the criteria for the HPhP waveguide in vdW heterostructures with various thicknesses of ZrS2 through scattering-type scanning near-field optical microscopy (s-SNOM) and by conducting numerical electromagnetic simulations. Our work presents a feasible and straightforward method for developing practical nanophotonic devices with low optical loss and high confinement, with potential applications such as energy transfer, nano-optical integrated circuits, light trapping, etc. 

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2. Strongly Chiral Liquid Crystals in Nanoemulsions

   https://doi.org/10.1002/smll.202105835  

   Yang, Yu; Palacio‐Betancur, Viviana; Wang, Xin; de Pablo, Juan J.; Abbott, Nicholas L. (January 2022, Small)

   Abstract Liquid crystal (LC) emulsions represent a class of confined soft matter that exhibit exotic internal organizations and size‐dependent properties, including responses to chemical and physical stimuli. Past studies have explored micrometer‐scale LC emulsion droplets but little is known about LC ordering within submicrometer‐sized droplets. This paper reports experiments and simulations that unmask the consequences of confinement in nanoemulsions on strongly chiral LCs that form bulk cholesteric and blue phases (BPs). A method based on light scattering is developed to characterize phase transitions of LCs within the nanodroplets. For droplets with a radius to the pitch ratio (R_v/p₀) as small as 2/3, the BP‐to‐cholesteric transition is substantially suppressed, leading to a threefold increase of the BP temperature interval relative to bulk behavior. Complementary simulations align with experimental findings and reveal the dominant role of chiral elastic energy. ForR_v/p₀ ≈1/3, a single LC phase forms below the clearing point, with simulations revealing the new configuration to contain a τ^−1/2disclination that extends across the nanodroplet. These findings are discussed in the context of mechanisms by which polymer networks stabilize BPs and, more broadly, for the design of nanoconfined soft matter. 

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3. Prolate and oblate chiral liquid crystal spheroids

   https://doi.org/10.1126/sciadv.aba6728  

   Sadati, Monirosadat; Martinez-Gonzalez, Jose A.; Zhou, Ye; Qazvini, Nader Taheri; Kurtenbach, Khia; Li, Xiao; Bukusoglu, Emre; Zhang, Rui; Abbott, Nicholas L.; Hernandez-Ortiz, Juan Pablo; et al (July 2020, Science Advances)

   Liquid crystals are known to exhibit intriguing textures and color patterns, with applications in display and optical technologies. This work focuses on chiral materials and examines the palette of morphologies that arises when microdroplets are deformed into nonspherical shapes in a controllable manner. Specifically, geometrical confinement and mechanical strain are used to manipulate orientational order, phase transitions, and topological defects that arise in chiral liquid crystal droplets. Inspired by processes encountered in nature, where insects and animals often rely on strain and temperature to alter the optical appearance of dispersed liquid crystalline elements, chiral droplets are dispersed in polymer films and deformation induced by uniaxial or biaxial stretching. Our measurements are interpreted by resorting to simulations of the corresponding systems, thereby providing an in-depth understanding of the morphologies that arise in these materials. The reported structures and assemblies offer potential for applications in smart coatings, smart fabrics, and wearable sensors. 

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4. Geometric transformation and three-dimensional hopping of Hopf solitons

   https://doi.org/10.1038/s41467-022-30494-2  

   Tai, Jung-Shen B.; Wu, Jin-Sheng; Smalyukh, Ivan I. (May 2022, Nature Communications)

   Abstract Arising in many branches of physics, Hopf solitons are three-dimensional particle-like field distortions with nontrivial topology described by the Hopf map. Despite their recent discovery in colloids and liquid crystals, the requirement of applied fields or confinement for stability impedes their utility in technological applications. Here we demonstrate stable Hopf solitons in a liquid crystal material without these requirements as a result of enhanced stability by tuning anisotropy of parameters that describe energetic costs of different gradient components in the molecular alignment field. Nevertheless, electric fields allow for inter-transformation of Hopf solitons between different geometric embodiments, as well as for their three-dimensional hopping-like dynamics in response to electric pulses. Numerical modelling reproduces both the equilibrium structure and topology-preserving out-of-equilibrium evolution of the soliton during switching and motions. Our findings may enable myriads of solitonic condensed matter phases and active matter systems, as well as their technological applications. 

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5. Structure and dynamics of tail-free discotic liquid crystals: Simulations of fluorinated triphenylene

   https://doi.org/10.1063/5.0106722  

   Powers, M.; Twieg, R. J.; Portman, J.; Ellman, B. (October 2022, The Journal of Chemical Physics)

   Recently, a large family of at least 14 discotic liquid crystals was discovered that are exceptions to the conventional paradigm that discotic mesogens tend to feature long, flexible tails on their periphery. To understand why these materials are liquid crystals, as well as the structural determinants of discotic phase behavior, we studied a group of closely related small tail-free disk-like molecules, including both mesogenic and non-mesogenic compounds differing only in the position of a single fluorine substituent. The rigidity and structural simplicity of these molecules make them well suited to for study by large, fully all-atom simulations. Using a combination of static and dynamic metrics, we were able to identify several key features of the columnar mesophase and, thereby, conclusively identify a columnar liquid crystalline mesophase present in a subset of our systems. Our simulations feature molecules hopping between columns in the columnar mesophase and distinctive molecular rotations in 60° steps about the columnar axis. The ability to create and characterize columnar mesophases in silico provides a potent tool for untangling the structural determinants of liquid crystalline behavior in these and other tail-free discotic liquid crystals. 

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