Light with enhanced optical chirality

Tang and Cohen [Phys. Rev. Lett.104, 163901 (2010)] recently demonstrated a scheme to enhance the chiral response of molecules, which relies on the use of circularly polarized light in a standing wave configuration. Here we show a new type of light that possesses orbital angular momentum and enhanced chiral response. In the locations where the beams show enhanced optical chirality, only the longitudinal components of the electric and magnetic fields survive, which has unexpectedly shown what we believe is a new way to yield an enhanced optical chiral response.     

Incoherent-to-coherent optical conversion by means of azobenzene liquid-crystalline films

Received: 20 July 1998/Revised version: 21 September 1998     

Nonlinear optical propagation and self-limiting effect in liquid-crystalline fibers

The nonlinear optical properties of the isotropic phase of liquid crystals induced by nanosecond laser pulses are analyzed in the context of nonlinear multi-mode propagation in a liquid-crystal-cored fiber. The negative thermal density nonlinearity of the core gives rise to an intensity-dependent loss in the core-guided transmission and optical action. Experiments conducted with such liquid-crystal-cored fibers show that the optical limiting threshold for nanosecond laser pulses can be as low as 0.09 J/cm², which is one of the lowest among known nonlinear optical materials and structures, including bulk liquid-crystal films.     

Nonlinear optical properties of a series of azobenzene liquid-crystalline materials

The nonlinear optical properties of a series of azobenzene liquid-crystalline materials, which have different side-chain lengths in their molecular structure from one to another, were investigated using Z-scan method under picosecond pulse laser at 532 nm, 1064 nm and CW 488 nm excitation. The mechanism accounting for the process of nonlinear refraction was discussed under different laser excitations. The polymer films possess very large nonlinear refraction at all the three different laser excitations. Especially, the nonlinear refractive index becomes larger as the length of side-chain, where azobenzene group is contained, increases under pulse excitation at 1064 nm.     

Monodomain and polydomain helicoids in chiral liquid-crystalline phases and their biological analogues

Many natural composites exhibit an architecture known as twisted plywood which imparts to them a superior set of physical properties. The origin of this structure is complex and not yet understood. However, it is thought to involve a lyotropic chiral nematic liquid-crystalline mesophase. Indeed, striking structural similarities have been observed and reported between biological fibrous composites and ordered fluids. In this work, a mathematical model based on the Landau-de Gennes theory has been developed to investigate the role played by constraining surfaces in the structural development of a composite material that experiences a liquid-crystalline state during the early steps of its morphogenesis. The goal of this study is to verify the need for an initial constraining surface in the formation of monodomain twisted plywoods as hypothesized by Neville (Tissue & Cell 20, 133 (1988); Biology of Fibrous Composites (Cambridge University Press, 1993)). The numerical simulations qualitatively confirm this theory and highlight the important role that modelling of liquid-crystalline self-assembly plays in the study of tissue morphogenesis.Received: 15 September 2003, Published online: 11 November 2003PACS: 61.30.-v Liquid crystals - 61.30.Dk Continuum models and theories of liquid crystal structure - 61.30.Mp Blue phases and other defect-phases - 61.30.St Lyotropic phases     

Theoretically predicted picosecond optical switching of spin chirality in multiferroics

We show theoretically with an accurate spin Hamiltonian describing the multiferroic Mn perovskites that the application of the picosecond optical pulse with a terahertz frequency can switch the spin chirality through intensely exciting the electromagnons. There are four states with different spin chiralities, i.e., clockwise and counterclockwise ab/bc-plane spin spirals, and by tuning the strength, shape and length of the pulse, the switching among these states can be controlled at will. Dynamical pattern formation during the switching is also discussed.     

On the emergence of mysterious liquid crystal phases: chirality and non-chirality issues

The structure–property correlation has been systematically investigated by the modification of the central core part of the dichiral materials so that the chiral and/or non-chiral issue having an effect on the formation of the layered or inter-connected structure can be studied. The central phenyl core of the dichiral compound showing the inter-connected ordered smectic Q phase has been modified in the following three ways: introduction of the substituent in the central phenyl core, introduction of the alkyl spacers between the central phenyl core and the peripheral mesogenic groups, and replacement of the central phenyl ring by the alkyl spacer. It has been found that (1) a symmetric structure stabilizes the tilted molecular ordering and the inter-connected structure, whereas the non-symmetric structure stabilizes the smectic A ordering; (2) a flexible molecular structure stabilizes the layered phase rather than the inter-connected; (3) increase in the degree of the segregation in the molecular assemblies stabilizes the anticlinic layered ordering; (4) increase in the local liquid-crystallinity stabilizes the layered structure; and (5) the ordered inter-connected structure is more sensitive to the chirality of the system than the disordered inter-connected structure.     

Improvement of the chirality near avoided resonance crossing in optical microcavity

Chirality is one of the important phenomena at the vicinity of exceptional point(EP). The conventional understanding is that the chirality is determined by asymmetrical scattering efficiency(?), which reaches to zero only when the resonance approaches EP. Here we study the possibility to enhance the chirality in open systems with a more robust mechanism. By combining chirality with avoided resonance crossing, we show that the chirality and ? can be dramatically modified. Taking a spiral shaped annular cavity as an example, we show that the chirality of optical resonances can be significantly improved when two sets of chiral states approach each other. The imbalance between counterclockwise(CCW) components and clockwise(CW) components has been enhanced by more than an order of magnitude. Our research provides a new route to tailor and control the chirality in open systems.     

Nonlinear optical properties and photoinduced anisotropy of an azobenzene ionic liquid-crystalline polymer

The nonlinear optical properties and photoinduced anisotropy of an azobenzene ionic liquid-crystalline polymer were investigated. The single beam Z-scan measurement showed the polymer film possessed a value of nonlinear refractive index n₂ = −1.07 × 10⁻⁹ cm²/W under a picosecond 532 nm excitation. Photoinduced anisotropy in the polymer was studied through dichroism and photoinduced birefringence. A photoinduced birefringence value Δn ∼ 10⁻² was achieved in the polymer film. The mechanism for the nonlinear optical response and the physical process of photoinduced anisotropy in the polymer were discussed.     

Quantum phases of dipolar bosons in optical lattices

The ground state of dipolar bosons placed in an optical lattice is analyzed. We show that the modification of experimentally accessible parameters can lead to the realization and control of different quantum phases, including superfluid, supersolid, Mott insulator, checkerboard, and collapse phases.     

NMR relaxation study of molecular dynamics in columnar and smectic phases of a PAMAM liquid-crystalline co-dendrimer

We present the first results obtained by proton (¹H) nuclear magnetic relaxation studies of molecular dynamics in a supermolecular liquid-crystal dendrimer exhibiting columnar rectangular and smectic-A phases. The ¹H spin-lattice relaxation time (T₁) dispersions are interpreted using two relaxation mechanisms associated with collective motions and local molecular reorientations of the dendritic segments in the low- and high-frequency ranges, respectively. The T₁ values show a drop around 2.3 MHz that is attributed to a contribution coming from cross-relaxation between ¹H and nitrogen nuclear spins. In the high-frequency range the motions appear to be of similar nature in both mesophases and are ascribed to reorientations of dendritic segments (belonging to the core and/or to the mesogenic units) characterized by two correlation times. Notable differences in the dynamics between the columnar and layered phases are observed in the low-frequency range. Depending on the mesophase they are discussed in terms of elastic deformations of the columns and layer undulations. In this study we find that the dendritic core influences the dynamics of the mesogenic units both for local and collective motions. These results can be understood in terms of spatial constraints imposed by the dendritic architecture and by the supermolecular arrangement in the mesophases.     

Field-induced phase transitions and reversible field-induced inversion of chirality in tilted smectic phases of bent-core mesogens

Three homologous achiral five-ring bent-core mesogens are presented where 4-chlororesorcinol is the central core and the aromatic rings are linked by ester groups. These compounds form smectic phases with a tilted arrangement of the molecules (tilt angle ≈ 45^°). On cooling the isotropic liquid this phase adopts a fan-like texture which shows for two homologues at relatively high electric fields ( 25-35V μm^-1) an antiferroelectric electro-optical response based on the collective rotation of the molecules around their long axes. At lower temperature the application of a sufficiently high electric field leads to a continuous transition into a non-birefringent texture which exhibits randomly distributed domains of opposite handedness. These domains can be reversibly switched into a state of opposite chirality by reversal of the field polarity. This switching is bistable and shows a current response typical for a ferroelectric ground state. The possible mechanism of the field-induced phase transition, of the ferroelectric switching and of the field-induced inversion of the chirality is discussed on the base of XRD, ¹³C- and ¹H-NMR investigations, dielectric and electro-optical measurements.     

Visualizing electronic chirality and Berry phases in graphene systems using photoemission with circularly polarized light

Electronic chirality near the Dirac point is a key property of graphene systems, which is revealed by the spectral intensity patterns as measured by angle-resolved photoemission spectroscopy under various polarization conditions. Specifically, the strongly modulated circular patterns for monolayer (bilayer) graphene rotate by ±90° (±45°) in changing from linearly to circularly polarized light; these angles are directly related to the phases of the wave functions and thus visually confirm the Berry's phase of π (2π) around the Dirac point. The details are verified by calculations.     

Nonlinear optical properties of polymer-dispersed liquid-crystalline systems based on fullerene-doped 2-cyclooctylamino-5-nitropyridine

Holographic recording of thin diffraction gratings by nano-and picosecond laser pulses λ=532 nm) is studied in polymerdispersed liquid-crystalline structures based on photosensitive molecules of 2-cycloocty-lamino-5-nitropyridine sensitized with C₇₀ fullerene. Using experimental data, nonlinear refraction n ₂ and nonlinear third-order susceptibility χ⁽³⁾ are calculated for the first time. The obained values determine the potential of the studied media for the modulation and frequency generation of laser radiation in the visible spectral range.     

Superfluid and insulating phases of fermion mixtures in optical lattices

The ground state phase diagram of fermion mixtures in optical lattices is analyzed as a function of interaction strength, fermion filling factor, and tunneling parameters. In addition to standard superfluid, phase-separated or coexisting superfluid -- excess-fermion phases found in homogeneous or harmonically trapped systems, fermions in optical lattices have several insulating phases, including a molecular Bose-Mott insulator (BMI), a Fermi-Pauli (band) insulator (FPI), a phase-separated BMI-FPI mixture or a Bose-Fermi checkerboard (BFC). The molecular BMI phase is the fermion mixture counterpart of the atomic BMI found in atomic Bose systems, the BFC or BMI-FPI phases exist in Bose-Fermi mixtures, and lastly the FPI phase is particular to the Fermi nature of the constituent atoms of the mixture.     

Novel quantum phases of dipolar bose gases in optical lattices

We investigate the quantum phases of polarized dipolar bosons loaded into a two-dimensional square and three-dimensional cubic optical lattices. We show that the long-range and anisotropic nature of the dipole-dipole interaction induces a rich variety of quantum phases, including the supersolid and striped supersolid phases in two-dimensional lattices, and the layered supersolid phase in three-dimensional lattices.     

Effect of disorders on topological phases in one-dimensional optical superlattices

In a recent paper, Lang et al. proposed that edge states and topological phases can be observed in one-dimensional optical superlattices. They showed that the topological phases can be revealed by observing the density profile of a trapped fermion system, which displays plateaus with their positions. However, disorders are not considered in their model. To study the effect of disorders on the topological phases, we introduce random potentials to the model for optical superlattcies.Our calculations show that edge states are robust against the disorders. We find the edge states are very sensitive to the number of the sites in the optical superlattice and we propose a simple rule to describe the relationship between the edge states and the number of sites. The density plateaus are also robust against weak disorders provided that the average density is calculated over a long interval. The widths of the plateaus are proportional to the widths of the bulk energy gaps when there are disorders. The disorders can diminish the bulk energy gaps. So the widths of the plateaus decrease with the increase of disorders and the density plateaus disappear when disorders are too strong. The results in our paper can be used to guide the experimental detection of topological phases in one-dimensional systems.