Effect of anisotropic MoS2 nanoparticles on the blue phase range of a chiral liquid crystal

Nanocomposite Photoanisotropic Materials for Applications in Polarization Holography and Photonics

Photoanisotropic materials, in particular azodyes and azopolymers, have attracted significant research interest in the last decades. This is due to their applications in polarization holography and 4G optics, enabling polarization-selective diffractive optical elements with unique properties, including circular polarization beam-splitters, polarization-selective bifocal lenses, and many others. Numerous methods have been applied to increase the photoinduced birefringence of these materials, and as a result, to obtain polarization holographic elements with a high diffraction efficiency. Recently, a new approach has emerged that has been extensively studied by many research groups, namely doping azobenzene-containing materials with nanoparticles with various compositions, sizes, and morphologies. The resulting nanocomposites have shown significant enhancement in their photoanisotropic response, including increased photoinduced birefringence, leading to a higher diffraction efficiency and a larger surface relief modulation in the case of polarization holographic recordings. This review aims to cover the most important achievements in this new but fast-growing field of research and to present an extensive comparative analysis of the result, reported by many research groups during the last two decades. Different hypotheses to explain the mechanism of photoanisotropy enhancement in these nanocomposites are also discussed. Finally, we present our vision for the future development of this scientific field and outline its potential applications in advanced photonics technologies.

History-dependent phase transition character

We consider history-dependent behavior in domain-type configurations in orientational order that are formed in configurations reached via continuous symmetry-breaking phase transitions. In equilibrium, these systems exhibit in absence of impurities a spatially homogeneous order. We focus on cases where domains are formed via (i) Kibble-Zurek mechanism in fast enough quenches or by (ii) Kibble mechanism in strongly supercooled phases. In both cases, domains could be arrested due to pinned topological defects that are formed at domain walls. In systems exhibiting polar or quadrupolar order, point and line defects (disclinations) dominate, respectively. In particular, the disclinations could form complex entangled structures and are more efficient in stabilizing domains. Domain patterns formed by fast quenches could be arrested by impurities imposing a strong enough random-field type disorder, as suggested by the Imry-Ma theorem. On the other hand, domains formed in supercooled systems could be also formed if large enough energy barriers arresting domains are established due to large enough systems' stiffness. The resulting effective interactions in established domain-type patterns could be described by random matrices. The resulting eigenvectors reveal expected structural excitations formed in such structures. The most important role is commonly played by the random matrix largest eigenvector. Qualitatively different behavior is expected if this eigenvector exhibits a localized or extended character. In the former case, one expects a gradual, non-critical-type transition into a glass-type structure. However, in the latter case, a critical-like phase behavior could be observed.

Experimental Advances in Nanoparticle-Driven Stabilization of Liquid-Crystalline Blue Phases and Twist-Grain Boundary Phases

Recent advances in experimental studies of nanoparticle-driven stabilization of chiral liquid-crystalline phases are highlighted. The stabilization is achieved via the nanoparticles' assembly in the defect lattices of the soft liquid-crystalline hosts. This is of significant importance for understanding the interactions of nanoparticles with topological defects and for envisioned technological applications. We demonstrate that blue phases are stabilized and twist-grain boundary phases are induced by dispersing surface-functionalized CdSSe quantum dots, spherical Au nanoparticles, as well as MoS2 nanoplatelets and reduced-graphene oxide nanosheets in chiral liquid crystals. Phase diagrams are shown based on calorimetric and optical measurements. Our findings related to the role of the nanoparticle core composition, size, shape, and surface coating on the stabilization effect are presented, followed by an overview of and comparison with other related studies in the literature. Moreover, the key points of the underlying mechanisms are summarized and prospects in the field are briefly discussed.

Nickel nanoparticle-stabilized room-temperature blue-phase liquid crystals

Blue-phase liquid crystals (BPLCs) are regarded as a potential candidate for the next generation of optical devices, but they appear in a narrow temperature range, which restricts their applicability. The nanoparticle (NP) stabilization of BPLCs, is usually about either simple-cubic BP (BPII) or body-centered-cubic BP (BPI). Hence, NP-stabilized BPIIs and BPIs possessing wide temperature ranges have been scarcely reported. Here, nickel nanoparticles (Ni NPs) were synthesized and introduced into a BPLC. The concentration-dependent temperature range effects of Ni NPs on the BPLC were investigated. A trace amount of Ni NPs could stabilize the BPLC and increase the temperature ranges of BPII and BPI; this is attributable to the Ni NPs tending to be trapped at the joints of the disclination lines, as a result of the elastic interaction, stabilizing the overall lattice structure of the BPLC. When doped with 0.05 wt% Ni NPs, the mixture shows a wider range with 7.7 °C than the range with 5.9 °C of the BPLC without Ni NPs. Meanwhile, the phase sequence and range of the aforementioned phases are reproducible upon heating and cooling, which shows that the BPLCs doped with Ni NPs are thermodynamically stable.

All-inorganic perovskite quantum dots stabilized blue phase liquid crystals

All-inorganic perovskite quantum dots (PQDs) have been effectively incorporated in the three-dimensional ordered structure of blue phase liquid crystals (BPLCs) to stabilize the BPLCs. Uniform dispersion, reduced phase transition temperature, widened BP temperature range, dynamic and fast electro-optical response and static optical display of selective reflection mode and photoluminescence mode have been confirmed with a given concentration of PQDs. Such a novel strategy of assembling all-inorganic PQDs in BPLCs shows favorable prospects for wide-range and near room temperature BPLCs, responsive BPLCs, multifunctional display materials and tunable bandgap lasers.

Effects of cell gap on the optoelectronic properties of pure blue-phase liquid crystal devices: estimating the Kerr constant

In this study, the Kerr constant of pure blue-phase liquid crystal (BPLC) without polymer doping at room temperature and the optoelectronic properties dependent on the cell thickness are explored. The relation between the phase and the voltage in oblique incident light was measured via a reasonable vertical electric field for different thicknesses of BPLC cells. It was found that the Kerr constant formula can be amended with the functions related to the cell gap. This study demonstrates a method to estimate the Kerr constant, especially for cells within a small electrical field, which will benefit optoelectronic applications.

The phase behavior of rigid rods in an anisotropic mean field with applications to carbon nanotubes in nematic liquid crystals

The phenomenological model [van der Schoot et al., J. Phys. Chem. B 112, 4512 (2008)] for predicting the alignment of carbon nanotube (CNT) dispersions in thermotropic liquid crystals is extended to include the attractive interactions between CNTs. The influence of the attractive forces (described by a spatially uniform mean field) on the phase behavior and orientational properties of the mixture are analyzed.

Nanoparticle-induced twist-grain boundary phase

By means of high-resolution ac calorimetry and polarizing optical microscopy, it is demonstrated that surface-functionalized spherical CdSSe nanoparticles induce a twist-grain boundary phase when dispersed in a chiral liquid crystal. These nanoparticles can effectively stabilize the one-dimensional lattice of screw dislocations, thus establishing the twist-grain boundary order between the cholesteric and the smectic-A phases. A Landau-de Gennes-Ginzburg model is used to analyze the impact of nanoparticles on widening the temperature range of molecular organizations possessing a lattice of screw dislocations. We show that in addition to the defect-core-replacement mechanism, the saddle-splay elasticity may also play a significant role.

Hybrid organic-inorganic materials for novel photonic applications

This novel joint feature issue on "Hybrid organic-inorganic materials for photonic applications" in Applied Optics and Optics Materials Express comprises 14 papers on liquid crystals, polymers, photoconductive materials, and gratings and filters. It is hoped that this feature issue encourages and stimulates further research into hybrid materials with enhanced linear and nonlinear optical properties, their mechanisms of operation, and their applications.