Lenses with tunable focal lengths play important roles in nature as well as modern technologies. In recent years, the demand for electrically tunable lenses and lens arrays has grown, driven by the increasing interest in augmented and virtual reality, as well as sensing applications. In this paper, we present a novel type of electrically tunable microlens utilizing polymer-stabilized chiral ferroelectric nematic liquid crystal. The lens offers a fast response time (5ms) and the focal length can be tuned by applying an in-plane electric field. The electrically induced change in the lens shape, facilitated by the remarkable sensitivity of the chiral ferroelectric nematic to electric fields, enables the tunable focal length capability. The achieved performance of this lens represents a significant advancement compared to electrowetting-based liquid lenses and opens exciting prospects in various fields, including biomimetic optics, security printing, solar energy concentration, and AR/VR devices.
Nematic liquid crystals of achiral molecules or racemic mixtures of chiral ones form flat films and show uniform textures between circular polarizers when suspended in sub-millimeter size grids and submersed under water. Recently it was shown that on addition of chiral dopants to the liquid crystal, the films exhibit optical textures with concentric ring patterns with radial variation of the birefringence color, while the films become biconvex. The curved shape together with degenerate planar anchoring leads to a radial variation of the optical axis along the plane of the film, providing a Pancharatnam-Berry type phase lens that dominates the imaging. Here we describe preliminary results of nematic liquid crystal microlenses formed by the addition of chiral nanoparticles. It is found that the helical twisting power of the nanoparticles, the key factor to form the lens, is an order of magnitude greater than that of the strongest molecular chiral dopants. From the observations we present here, we were able to estimate the shape and the geometric focal length of the lens and demonstrated its performance as an optical device. The use of chiral nanoparticles to make microlenses may allow tuning by light that the nanoparticles absorb or, for magnetic NPs, by magnetic fields. Further, the measurement of focal length at known NP concentration offers a new method to measure the helical twisting power of chiral nanoparticles.
In this paper, we present a new photoswitchable viscoelastic material with liquid crystalline natures, which exhibits two types of nematic liquid crystalline phases and a crystal phase beneath the isotropic liquid. We present its light-triggered phase behaviors and photo-switchability of the viscoelastic properties.
Topological defects determine many static and dynamic properties of liquid crystals. They are mainly studied by
optical microscopy, which cannot reveal the detailed structure of the defect core, where the deformations are too
strong to sustain the usual type of order. The size of the core in most of liquid crystals is in the range of 1-10
nanometers, which calls for imaging techniques with resolution much higher than the optical one. Here we
summarize and discuss results of Transmission Electron Microscopy (TEM) nanoscale imaging of defects in
several layered liquid crystals built of rod- and bent-shaped molecules. We will present and analyze structures of
edge and screw dislocations, twist and tilt grain boundaries of smectic layers. Topological defects have large
impact on optical properties of the LCs and understanding their nanoscale properties will help us structuring
them for optical applications.
Recent cryo-TEM studies in the helical nanofilament (HNF) phase of several bent-core liquid crystal materials having blue structural color showed that in subsequent layers the nanofilaments twist with respect to each other by about 37º angle, leading to a secondary helical structure that can explain their opal appearance. In this paper, after summarizing these observations, we show additional features that help understanding why the same bent-core homologs P-n-O-PIMB with n ≥ 9 do not show structural color.
The difficulty of aligning bent-core liquid crystals at a surface is addressed from three directions: We form Langmuir
monolayers of bent core molecules at the air-water interface, and explore their orientation and packing. We transfer these
films by Langmuir-Schaefer techniques to a solid surface, and test them for the alignment of bulk liquid crystal. We use
atomistic molecular dynamics simulations to directly probe possible molecular orientation at the water surface, for
comparison with experiments. We find that relatively small changes in the bent-core molecule affect both the stability of
the films and their ability to promote alignment within liquid crystal cells.
We review electro-optical properties and discuss possible applications of bent-core liquid crystals. There
are three different electro-optical switching modes in the polar tilted smectic (SmCP) phase, relying on
chirality, birefringence and induced biaxiality. During switching from transparent to scattering state a
racemic antiferroelectric structure is driven to a ferroelectric state, whereas while switching from a
transparent to a racemic state a chiral antiferroelectric state is driven to a ferroelectric one. Importantly
the racemic and chiral states can be interchanged after applying electric fields with proper waveforms.
The anticlinic (racemic ferroelectric and chiral antiferroelectric) states have low birefringence (in certain
anticlinic states it can even be zero) whereas the synclinic (racemic antiferroeectric and chiral
ferroelectric) states have large birefringences, which offers switching between optically isotropic and
birefringent states. This type of switching can also be achieved in samples with vertical alignment and in
plane electrodes. This latter effect is a field induced polar or dielectric switching between uniaxial and
biaxial smectic structures.
We briefly review systematic and comprehensive studies on several chlorine-substituted bent-core liquid crystal materials
in their nematic phases. The results, in comparison to rod-shaped molecules, are both extraordinary and technologically
significant. Specifically:
a) Electrohydrodynamic instabilities provide unique patterns including well defined, periodic stripes and optically isotropic
structures.
b) Rheological measurements using different probe techniques (dynamic light scattering, pulsed magnetic field, electrorotation)
reveal that the ratio of the flow and rotational viscosities are over two orders of magnitudes larger in bentcore
than in calamitic materials which proves that the molecule shape and not its size is responsible for this behaviour.
c) Giant flexoelectric response, as measured by dynamic light scattering and by directly probing the induced current
when the material is subject to oscillatory bend deformation, turns out to be more than three orders of magnitude larger
than in calamitics and 50 times larger than molecular shape considerations alone would predict. The magnitude of this
effect renders these materials as promising candidates for efficient conversion between mechanical and electrical energy.
d) The converse of this effect when the bent-core material sandwiched between plastic substrates 4 times thicker than the
liquid crystal material provided displacements in the range of 100nm that is sensitive to the polarity of the applied
field thus suggesting applications as beam steering and precision motion controls.
The uniqueness of liquid crystals (LCs) lies in the large anisotropies in their properties, which can be utilized to generate high electromechanical responses. In a properly oriented liquid crystal polymer system, an external electric field can induce re-orientation of the mesogenic units possessing a dielectric anisotropy, which, when coupled with the shape anisotrophy of the mesogenic units, can in turn produce large mechanical strain. Anisotropic liquid crystal gels, which can be obtained by in situ photopolymerization of the reactive LC molecules in the presence of non-reactive LC molecules in an oriented state, are an example of such liquid crystal polymer systems. It has been shown that a homeotropically aligned liquid crystal gel in its nematic phase exhibits high electrically induced strain (>2%) with an elastic modulus of 100MPa and a high electromechanical conversion efficiency (75%) under an electric field of 25 MV/m. These anisotropic LC polymeric materials could provide a technologically compatible system for such applications as artificial muscles and as micro-electromechanical devices.
Novel scattering-type displays using antiferroelectric smectic phases of liquid crystals of bent-shape molecules are reviewed and discussed. There can be two distinct states racemic and chiral that work in opposite ways. The racemic structure is scattering in the OFF state and is optically clear under sufficiently large (E~4-6V/m) electric fields. The chiral structure is transparent at zero fields and scattering in the field ON state. These two structures may be reversibly interchanged implying their use in devices that consume energy only during switching from one stable state to the other. After summarizing the previous results on the film thickness, driving voltage and temperature dependences of the light shutters, new results will be presented on a banana smectic material, which has an optically isotropic transparent antiferroelectric OFF state. We show that the optically isotropic and transparent OFF state can be reversibly switched to birefringent and scattering ferroelectric states in less than hundred microseconds.
An unusual depression of phase transitions was observed in a polymer composite system, when 4,4'-bis(2- methylpropenoyloxy)-biphenyl (BMB) was UV polymerized in a cholesteryl propionate matrix. Our analysis indicates that different chemical processes take place simultaneously during the photo polymerization. One is the polymerization of BMB leading to the formation of a polymer network. Independently from this, cholesteryl propionate suffered some chemical changes and, as main product, 7-hydroxy- cholesteryl propionate formed. This compound was isolated and its structure established by IR, NMR and MS spectroscopy.
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