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1.
The transmission spectrum of a one-dimensional photonic crystal with structural defects is investigated. An anisotropic layer of a nematic liquid crystal is considered as a defect. The possibility of effective external control of the transmission spectrum of the photonic crystal by changing either the orientational state of the defect liquid crystal layer (for example, using an electric field) or the angle of incidence of light is shown. It is established that the spectral properties of the photonic crystal change radically with a change in the optical characteristics of the defect layer in the vicinity of the temperature phase transition of the nematic liquid crystal to the isotropic state. 相似文献
2.
Spatial distortions of the director field and magnetization of a ferronematic (suspension of magnetic nanoparticles in a nematic liquid crystal) that are induced by simultaneous action of electric and magnetic fields are studied with allowance for the flexoelectric polarization of the liquid-crystalline matrix. Soft coupling of liquid crystal and magnetic particles and layer boundaries is considered. The dependence of the phase lag of the transmitted light on the external magnetic field is analyzed. 相似文献
3.
The dependence of the threshold parameters and the period of the electric-field-induced spatially periodic reorientation of the director in a flexoelectric nematic liquid crystal (NLC) on the anchoring conditions at the surface of a planar NLC cell has been studied. The threshold electric field and the corresponding wave-number of the periodic structure of the director field have been numerically calculated for arbitrary values of the anchoring energy. In the case of strong anchoring, the corresponding analytical expressions are obtained in a single-constant approximation. A decrease in the azimuthal anchoring energy leads to an increase in the intervals of possible values of the flexoelectric parameter ν and the ratio K2/K1 of the Frank elastic constants. A decrease in the polar anchoring energy leads to narrowing of these intervals as compared to the case of infinitely strong anchoring at the NLC cell surface. 相似文献
4.
The properties of localized electromagnetic modes in a one-dimensional photonic crystal with a structural defect layer were studied. The role of the defect was played by an anisotropic nematic liquid crystal layer. The frequency and the damping factor of defect modes were shown to substantially depend on the defect layer thickness and the orientation of the optical axis of the nematic. The transmission spectra of photonic crystals with one and two lattice defects were studied. Taking into account the special feature of nematic liquid crystals distinguishing them from usual crystals, namely, large permittivity anisotropy, it was shown that the transmission spectrum of the photonic crystal could be controlled by varying the orientation of the optical axis of the nematic, for instance, under the action of an external electric field. 相似文献
5.
6.
A. L. Alexe-Ionescu 《Il Nuovo Cimento D》1993,15(12):1489-1506
Summary The elastic deformations in nematic liquid crystals are analysed in one and two dimensions. The influence of the flexoelectric
polarization on the elastic properties of the medium is studied. It is shown that for one-dimensional deformations, the presence
of the flexoelectric polarization introduces a renormalization of the elastic constants. On the contrary in the bidimensional
case, the effect of the flexoelectricity is not a simple renormalization of the elastic constants. The profiles of the electric
potential and of the tilt angle are evaluated for the cases in which the substrate is an insulating or conducting material.
The validity of the calculations is discussed by considering the value of Debye's screening length of the nematic liquid crystal. 相似文献
7.
We examine director fluctuations in nematic liquid crystals in an electric field in the presence of a flexoelectric effect.
For the planar and homeotropic orientations we calculate the correlation functions and analyze these functions near the Freedericksz
transition and the flexoelectric instability threshold. For both geometries we calculate the angular dependence of the intensity
of light scattered by director fluctuations and examine its behavior at electric field strengths close to critical.
Zh. éksp. Teor. Fiz. 112, 1675–1693 (November 1997) 相似文献
8.
The electric field-induced orientational transition in a homeotropically oriented nematic liquid crystal cell is investigated.
The interaction with the field as a result of anisotropy of the permittivity and flexoelectric polarization is taken into
account. For an arbitrary energy of interaction of the nematic with the substrate simple relations are derived for determining
the threshold characteristics of the phase transition. It is shown that, in contrast with planar orientation, in fields above
a critical value a periodic structure can occur only by virtue of the flexoelectric effect. The resulting dependences for
the threshold parameters in the given geometry are exceptionally useful for determining experimentally the surface energy
and the difference in the flexoelectric coefficients.
Zh. éksp. Teor. Fiz. 116, 543–550 (August 1999) 相似文献
9.
A temperature-induced orientational transition is investigated in a mixture of nematic liquid crystals on the surface of a cleavage of a ferroelectric triglycine sulfate crystal. The transition has been observed by the variation of the polarized absorbance components of a dye introduced into the nematic matrix with increasing temperature. The reorientation of molecules in the liquid crystal volume confined by solid walls is due to competition between dispersion and polar forces at the surface and the decrease in the electric field of the substrate up to its complete disappearance at the ferroelectric Curie point. 相似文献
10.
We measure the electric-field-driven annihilation of nematic disclination pairs with strength +/- 1/2 in the 4-cyano-4'-n-pentylbiphenyl (5CB) liquid crystal. The use of a very weak azimuthal anchoring ensures a two-dimensional director field. The relaxation is governed by the formation of a pi wall connecting the two opposite charge defects. The +1/2 disclinations move almost twice as fast as the -1/2 disclinations. The simple used geometry allows a quantitative comparison with numerical studies based on the hydrodynamics of the tensorial order parameter. The simulations show that in the pi wall regime the symmetry breaking is due to the backflow and not to the elastic anisotropy. 相似文献
11.
A. L. Alexe-Ionescu 《Physics letters. A》1993,180(6):456-460
The flexoelectric properties of nematic liquid crystals are analysed. It is shown that in the frame of the usual elasticity two coefficients characterize completely the flexoelectric properties of the liquid crystal. These coefficients, in the limit of small scalar order parameter, i.e. near the clearing point, are approximately equal. More precisely they have the same linear term in the scalar order parameter, and differ for terms quadratic in this parameter. Their difference behaves, hence, as the usual nematic liquid crystal elastic constants, whereas their sum depends on the temperature, like the mixed splay-bend elastic constant. It is shown furthermore that in the frame of a second order elastic theory in the flexoelectric polarization there are no terms from the second order spatial derivatives of the nematic director, or the nematic tensor order parameter. Consequently also in the frame of the second order elasticity the flexoelectric polarization is given by the usual expression. This conclusion is important in connection with the surface polarization recently discovered in pretilted nematic liquid crystal samples. 相似文献
12.
The effect of a surface electric field produced by ionic adsorption on the molecular orientation of a nematic liquid crystal
sample is analyzed. The eigenvalue problem for a semi-infinite medium is analytically solved both for strong and weak anchoring
situations. The threshold instabilities are numerically determined and it is shown that the homeotropic pattern can be destabilized
also in the situation of strong anchoring. The dependence of the threshold field on the anchoring strength and on the surface
polarization is determined by taking into account also the coupling of the quadrupolar component of the flexoelectric coefficient
with the field gradient.
Received 4 November 1999 and Received in final form 4 April 2000 相似文献
13.
Harden J Mbanga B Eber N Fodor-Csorba K Sprunt S Gleeson JT Jákli A 《Physical review letters》2006,97(15):157802
Flexoelectricity is a coupling between orientational deformation and electric polarization. We present a direct method for measuring the flexoelectric coefficients of nematic liquid crystals (NLCs) via the electric current produced by periodic mechanical flexing of the NLC's bounding surfaces. This method is suitable for measuring the response of bent-core liquid crystals, which are expected to demonstrate a much larger flexoelectric effect than traditional, calamitic liquid crystals. Our results reveal that not only is the bend flexoelectric coefficient of bent-core NLCs gigantic (more than 3 orders of magnitude larger than in calamitics) but also it is much larger than would be expected from microscopic models based on molecular geometry. Thus, bent-core nematic materials can form the basis of a technological breakthrough for conversion between mechanical and electrical energy. 相似文献
14.
15.
Brasselet E 《Physical review letters》2012,108(8):087801
We report on the generation of tunable structured light fields endowed with various sets of phase singularities from a single topological defect in a nematic liquid crystal mesophase. The experimental demonstration relies on the use of electric field-induced nonsingular topological defects called "umbilics." 相似文献
16.
S. Faetti M. Nobili I. Raggi 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,11(3):445-453
In this paper we report an experimental investigation on the dynamics of the azimuthal director reorientation at a nematic-solid
interface. Three qualitatively different kinds of substrates have been investigated: I) intrinsically anisotropic SiO-substrates
(-evaporation), II) isotropic SiO-substrates (-evaporation) and III) rubbed PVA-substrates. In the case II), an in-plane anisotropy was induced cooling slowly the thermotropic
nematic liquid crystal (NLC) from the isotropic phase in the presence of a 0.75 T magnetic field. The reorientation dynamic
of the surface azimuthal director angle at the switching-on and off of a magnetic (or electric) field has been investigated.
All the substrates show comparable azimuthal anchoring energies and two dynamic regimes: a fast dynamic response, driven by
the bulk director reorientation and an extremely slow reorientation. The slow dynamics is explained in terms of anisotropic
adsorption of NLC molecules on the solid substrate and is well represented by a stretched exponential.
Received 7 December 1998 相似文献
17.
A macroscopic helical twist is imposed on an achiral nematic liquid crystal by controlling the azimuthal alignment directions at the two substrates. On application of an electric field the director rotates in the substrate plane. This electroclinic effect, which requires the presence of chirality, is strongest at the two substrates and increases with increasing imposed twist distortion. We present a simple model involving a trade-off among bulk elastic energy, surface anchoring energy, and deracemization entropy that suggests the large equilibrium director rotation induces a deracemization of chiral conformations in the molecules-effectively "top-down" chiral induction-quantitatively consistent with experiment. 相似文献
18.
Flexoelectric-induced voltage shift in a weak anchoring hybrid aligned nematic liquid crystal cell is investigated theoretically. Based on the elastic theory of liquid crystal and the variation method, the equations for the bulk and the boundary of the cell are derived. By computer simulation, the dependence of the
shift voltage on the sum of the flexoelectric coefficients and the anchoring energy strength is obtained. As a result, a novel method to determine the sum of the flexoelectric coefficients by measuring the shift voltage is put forward. 相似文献
19.
N. M. Shtykov M. I. Barnik L. M. Blinov B. A. Umanskii S. P. Palto 《JETP Letters》2007,85(12):602-604
The amplification of the emission of a microlaser based on a cholesteric liquid crystal by means of a uniformly oriented layer of a laser-dye-doped nematic liquid crystal is demonstrated. The nematic liquid crystal is characterized by the anisotropy of optical properties, including the amplification coefficient, which opens the possibility of creating amplifiers controlled by a low electric field. 相似文献
20.
Symmetry of a nematic liquid crystal phase is broken by an anchoring wall and also by an external field. Nematic system sandwiched
between biaxial anchoring walls is introduced as a correspondent to a bulk nematic system exposed to a couple of fields, an
electric field and a magnetic one in directions perpendicular to each other, and thermal behaviours of the system are studied.
The crossover between a homeotropic structure and homogeneous one occurs, similarly to the bulk system in the fields, in which
the anchoring condition of coexistence is shown to have the same expression as the one at the bulk. As to a characteristic
phenomenon at the sandwiched system, it is proved that an appearance of a biaxial nematic order suppresses a uniaxial nematic
order. A surface transition, i.e., a wetting phenomenon is shown to occur also in the biaxial nematics, even though the uniaxial
order is suppressed therein. 相似文献