11 research outputs found
Stretchable liquid-crystal blue-phase gels
Liquid crystalline polymers are materials of considerable scientific interest
and technological value to society [1-3]. An important subset of such materials
exhibit rubber-like elasticity; these can combine the remarkable optical
properties of liquid crystals with the favourable mechanical properties of
rubber and, further, exhibit behaviour not seen in either type of material
independently [2]. Many of their properties depend crucially on the particular
mesophase employed. Stretchable liquid crystalline polymers have previously
been demonstrated in the nematic, chiral nematic, and smectic mesophases [2,4].
Here were report the fabrication of a stretchable gel of blue phase I, which
forms a self-assembled, three-dimensional photonic crystal that may have its
optical properties manipulated by an applied strain and, further, remains
electro-optically switchable under a moderate applied voltage. We find that,
unlike its undistorted counterpart, a mechanically deformed blue phase exhibits
a Pockels electro-optic effect, which sets out new theoretical challenges and
new possibilities for low-voltage electro-optic devices.Comment: 15 pages, 6 figures, additional data and discussion included.
Supplementary videos available from F. Castles on reques
Transverse and longitudinal correlation functions in the Intergalactic Medium from 32 close pairs of high-redshift quasars
We present the transverse flux correlation function of the Lyman-alpha forest
in quasar absorption spectra at z~2.1 from VLT-FORS and VLT-UVES observations
of a total of 32 pairs of quasars; 26 pairs with separations in the range
0.6<theta<4 arcmin and 6 pairs with 4<theta<10 arcmin. Correlation is detected
at the 3sigma level up to separations of the order of ~4 arcmin (or ~4.4 h^{-1}
Mpc comoving at z=2.1 for Omega_{m}=0.3 and Omega_{Lambda}=0.7). We have,
furthermore, measured the longitudinal correlation function at a somewhat
higher mean redshift (z=2.39) from 20 lines of sight observed with high
spectral resolution and high signal-to-noise ratio with VLT-UVES. We compare
the observed transverse and longitudinal correlation functions to that obtained
from numerical simulations and illustrate the effect of spectral resolution,
thermal broadening and peculiar motions. The shape and correlation length of
the correlation functions are in good agreement with those expected from
absorption by the filamentary and sheet-like structures in the photoionized
warm intergalactic medium predicted in CDM-like models for structures
formation. Using a sample of 139 C_{iv} systems detected along the lines of
sight toward the pairs of quasars we also investigate the transverse
correlation of metals on the same scales. The observed transverse correlation
function of intervening C_{iv} absorption systems is consistent with that of a
randomly distributed population of absorbers. This is likely due to the small
number of pairs with separation less than 2 arcmin. We detect, however, a
significant overdensity of systems in the sightlines towards the quartet
Q0103-294A&B, Q0102-2931 and Q0102-293 which extends over the redshift range
1.5<z<2.2 and an angular scale larger than 10 arcmin.Comment: accepted for publication in MNRAS, Appendix B and C will be published
online onl
Frequency-dependent dielectric contribution of flexoelectricity allowing control of state switching in helicoidal liquid crystals.
The contribution of flexoelectric polarization to the dielectric susceptibility in helicoidal liquid crystals is formulated for the static equilibrium case, and further in the case of a time-varying field. A dispersion of the dielectric permittivity due to the frequency response of flexoelectric switching is described. The special case of a negative dielectric-anisotropy nematic material is considered and experimentally shown to agree with the analytical theory. It is further demonstrated how relaxation of the flexoelectric contribution to the dielectric tensor in this special case can be exploited to switch between states in cholesteric liquid crystal structures by altering the applied time-dependent field amplitude, if Δε<0 and (e(1)-e(3))(2)/(K(1)+K(3))>-Δεε(0). Consequentially, a versatile mechanism for driving between states in liquid crystal systems has been demonstrated and its implications for technology are suggested, and include dual-mode, bistable, and transflective displays
A Novel Method for Measuring the Difference in Flexoelectric Coefficients of Nematic Liquid Crystals
Flexoelectric and dielectric in-plane switching behaviour of Grandjean liquid-crystal structures
A key requirement for the exploitation of the chiral-flexoelectrooptic effect in technology is the development of materials with large flexoelectric response and small dielectric anisotropy. Consequently, techniques to understand details of the switching of chiral systems used in such technologies require development. Here, analytical and numerical models of dielectric and flexoelectric switching in cholesteric liquid-crystal structures with in-plane switching (IPS) electric fields are employed to analyse angle-of-incidence–dependent transmission of Grandjean structures. Adoption of a perturbative driving regime, and measurement of both 1st- and 2nd-harmonic transmission amplitudes, allow the detailed and simultaneous measurement of distortions arising from both flexoelectric and dielectric interactions, at electric fields as little as 0.01 Vμm−1. The method is applied to measure the flexoelectric parameter (e1 − e3)/(K1 + K3) and the dielectric distortion parameter Δε/K2 in E7 liquid crystal and a bimesogen mixture MDA-1245 optimised for the chiral-flexoelectrooptic effect