Light Propagation in Confined Nematic Liquid Crystals and Device Applications

Liquid crystals are interesting linear and nonlinear optical materials used to make a wide variety of devices beyond flat panel displays. Liquid crystalline materials can be used either as core or as cladding of switchable/reconfigurable waveguides with either an electrical or an optical control or both. In this paper, materials and main device structures of liquid crystals confined in different waveguide geometries are presented using different substrate materials, such as silicon, soda lime or borosilicate glass and polydimethylsiloxane. Modelling of the behaviour of liquid crystal nanometric molecular reorientation and related refractive index distribution under both low-frequency electric and intense optical fields is reported considering optical anisotropy of liquid crystals. A few examples of integrated optic devices based on waveguides using liquid crystalline materials as core for optical switching and filtering are reviewed. Reported results indicate that low-power control signals represent a significant feature of photonic devices based on light propagation in liquid crystals, with performance, which are competitive with analogous integrated optic devices based on other materials for optical communications and optical sensing systems.

Incorporation of Molecular Reorientation into Modeling Surface Pressure-Area Isotherms of Langmuir Monolayers

Langmuir monolayers can be assembled from molecules that change from a low-energy orientation occupying a large cross-sectional area to a high-energy orientation of small cross-sectional area as the lateral pressure grows. Examples include cyclosporin A, amphotericin B, nystatin, certain alpha-helical peptides, cholesterol oxydation products, dumbbell-shaped amphiphiles, organic–inorganic nanoparticles and hybrid molecular films. The transition between the two orientations leads to a shoulder in the surface pressure-area isotherm. We propose a theoretical model that describes the shoulder and can be used to extract the energy cost per molecule for the reorientation. Our two-state model is based on a lattice–sublattice approximation that hosts the two orientations and a corresponding free energy expression which we minimize with respect to the orientational distribution. Inter-molecular interactions other than steric repulsion are ignored. We provide an analysis of the model, including an analytic solution for one specific lateral pressure near a point of inflection in the surface pressure-area isotherm, and an approximate solution for the entire range of the lateral pressures. We also use our model to estimate energy costs associated with orientational transitions from previously reported experimental surface pressure-area isotherms.

Photo-Aligned Nematic Liquid Crystals Enable the Modulation of Thermoplasmonic Heating

We experimentally demonstrate that the plasmonic heat delivered by a single layer of homogeneously distributed gold nanoparticles (AuNPs), immobilized on a glass substrate, can be optically tuned by taking advantage of the properties of an organic layer based on azobenzene and nematic liquid crystal (NLC) molecules. The effect, which exploits the dependence of the NLC refractive index value on the molecular director orientation, is realized using the polarization-dependent, light-induced molecular reorientation of a thin film of photo-aligning material that the NLC is in contact with. The reversibility of the optically induced molecular director reorientation of the NLC enables an active modulation of the plasmonic photo-induced heat.

Order–disorder transition of a rigid cage cation embedded in a cubic perovskite

The structure and properties of organic–inorganic hybrid perovskites are impacted by the order–disorder transition, whose driving forces from the organic cation and the inorganic framework cannot easily be disentangled. Herein, we report the design, synthesis and properties of a cage-in-framework perovskite AthMn(N3)3, where Ath+ is an organic cation 4-azatricyclo[2.2.1.02,6]heptanium. Ath+ features a rigid and spheroidal profile, such that its molecular reorientation does not alter the cubic lattice symmetry of the Mn(N3)3− host framework. This order–disorder transition is well characterized by NMR, crystallography, and calorimetry, and associated with the realignment of Ath+ dipole from antiferroelectric to paraelectric. As a result, an abrupt rise in the dielectric constant was observed during the transition. Our work introduces a family of perovskite structures and provides direct insights to the order–disorder transition of hybrid materials.

Polar in-plane surface orientation of a ferroelectric nematic liquid crystal: Polar monodomains and twisted state electro-optics

We show that surface interactions can vectorially structure the three-dimensional polarization field of a ferroelectric fluid. The contact between a ferroelectric nematic liquid crystal and a surface with in-plane polarity generates a preferred in-plane orientation of the polarization field at that interface. This is a route to the formation of fluid or glassy monodomains of high polarization without the need for electric field poling. For example, unidirectional buffing of polyimide films on planar surfaces to give quadrupolar in-plane anisotropy also induces macroscopic in-plane polar order at the surfaces, enabling the formation of a variety of azimuthal polar director structures in the cell interior, including uniform and twisted states. In a π-twist cell, obtained with antiparallel, unidirectional buffing on opposing surfaces, we demonstrate three distinct modes of ferroelectric nematic electro-optic response: intrinsic, viscosity-limited, field-induced molecular reorientation; field-induced motion of domain walls separating twisted states of opposite chirality; and propagation of polarization reorientation solitons from the cell plates to the cell center upon field reversal. Chirally doped ferroelectric nematics in antiparallel-rubbed cells produce Grandjean textures of helical twist that can be unwound via field-induced polar surface reorientation transitions. Fields required are in the 3-V/mm range, indicating an in-plane polar anchoring energy of wP ∼3 × 10−3 J/m2.

A Computational Study of Structure Development and Texture Formation in Carbonaceous Mesophase Fibers

In this thesis, thermal relaxation phenomena after the melt-extrusion of a rigid discotic uniaxial nematic mesophase pitch were studied using mathematical modeling and computer simulation. The Eriksen and Landau-de Gennes continuum theories were used to investigate the structure development and texture formation across mesophase pitch based carbon fibers. It is found that during the thermal relaxation, discotic nematic molecules stored elastic free energy decays. The distorted nematic molecular profile reoriented to release the stored elastic free energy. The difference in time scales for molecular reorientation and thermal relaxation resulted in different transverse textures. The rate at which the fibers are cooled is the main factor in controlling the structure development. A slow cooling rate would permit nemiatic discotic molecules to reorient to a well developed (radial or onion) texture. The random texture is a result of rapid quenching. The numerical results are consistent with published experimental observations.

A Computational Study of Structure Development and Texture Formation in Carbonaceous Mesophase Fibers

In this thesis, thermal relaxation phenomena after the melt-extrusion of a rigid discotic uniaxial nematic mesophase pitch were studied using mathematical modeling and computer simulation. The Eriksen and Landau-de Gennes continuum theories were used to investigate the structure development and texture formation across mesophase pitch based carbon fibers. It is found that during the thermal relaxation, discotic nematic molecules stored elastic free energy decays. The distorted nematic molecular profile reoriented to release the stored elastic free energy. The difference in time scales for molecular reorientation and thermal relaxation resulted in different transverse textures. The rate at which the fibers are cooled is the main factor in controlling the structure development. A slow cooling rate would permit nemiatic discotic molecules to reorient to a well developed (radial or onion) texture. The random texture is a result of rapid quenching. The numerical results are consistent with published experimental observations.

Compatible Ferroelectricity, Antiferroelectricity and Broadband Emission for a Multi-Functional 2D Organic-Inorganic Hybrid Perovskite

Two-dimensional (2D) organic-inorganic hybrid perovskites with multifunctional characteristics have potential applications in many fields, such as, solar cells, microlasers and light-emitting diodes (LEDs), etc. Here, a 2D organic-inorganic lead halide perovskite, [Br(CH₂)₃NH₃]₂PbBr₄ (<b>BPA-PbBr₄</b>, BPA = Br(CH₂)₃NH₃, 3-Bromopropylamine), is examined for its photophysical properties. Interestingly, <b>BPA-PbBr₄</b> reveals five successive phase transitions with decreasing temperature, including successive paraelectric-ferroelectric-antiferroelectric phases. Besides, <b>BPA-PbBr₄</b> displays ferroelectricity and antiferroelectricity throughout a wide temperature range (<376.4 K) with accompanying saturation polrization (<i>P</i>_s) values of 4.35 and 2.32 μC/cm², respectively, and energy storage efficiency of 28.2%, and also exhibits superior second harmonic generation (SHG) with maximum value accounts for 95 % of the standard KDP due to the great deformation of structure (3.2302*10^-4). In addition, the photoluminescence (PL) of the <b>BPA-PbBr₄</b> exhibits abnormal red-shift and blue-shift in different phases due to a consequence of competition between electron-phonon interaction and the lattice expansion. Further, <b>BPA-PbBr₄</b> reveals a broadband emission accompanied by bright white light at room temperature (293 K), which is supposed to be due to self-trapped excitons. In short, the versatility of <b>BPA-PbBr₄</b> originates from molecular reorientation of dynamic organic cations, as well as significant structural distortion of PbBr₆ octahedra. This work paves an avenue to design new hybrid multifunctional perovskites for potential applications in the photoelectronic field.

Compatible Ferroelectricity, Antiferroelectricity and Broadband Emission for a Multi-Functional 2D Organic-Inorganic Hybrid Perovskite

Two-dimensional (2D) organic-inorganic hybrid perovskites with multifunctional characteristics have potential applications in many fields, such as, solar cells, microlasers and light-emitting diodes (LEDs), etc. Here, a 2D organic-inorganic lead halide perovskite, [Br(CH₂)₃NH₃]₂PbBr₄ (<b>BPA-PbBr₄</b>, BPA = Br(CH₂)₃NH₃, 3-Bromopropylamine), is examined for its photophysical properties. Interestingly, <b>BPA-PbBr₄</b> reveals five successive phase transitions with decreasing temperature, including successive paraelectric-ferroelectric-antiferroelectric phases. Besides, <b>BPA-PbBr₄</b> displays ferroelectricity and antiferroelectricity throughout a wide temperature range (<376.4 K) with accompanying saturation polrization (<i>P</i>_s) values of 4.35 and 2.32 μC/cm², respectively, and energy storage efficiency of 28.2%, and also exhibits superior second harmonic generation (SHG) with maximum value accounts for 95 % of the standard KDP due to the great deformation of structure (3.2302*10^-4). In addition, the photoluminescence (PL) of the <b>BPA-PbBr₄</b> exhibits abnormal red-shift and blue-shift in different phases due to a consequence of competition between electron-phonon interaction and the lattice expansion. Further, <b>BPA-PbBr₄</b> reveals a broadband emission accompanied by bright white light at room temperature (293 K), which is supposed to be due to self-trapped excitons. In short, the versatility of <b>BPA-PbBr₄</b> originates from molecular reorientation of dynamic organic cations, as well as significant structural distortion of PbBr₆ octahedra. This work paves an avenue to design new hybrid multifunctional perovskites for potential applications in the photoelectronic field.