S. A. Jewell

Tuneable Fabry–Perot etalon for terahertz radiation

An indium tin oxide-clad liquid crystal filled Fabry-Perot etalon structure has been shown to act as an effective narrow-band filter at terahertz frequencies. An applied voltage, which controls the alignment of the nematic liquid crystal allows the refractive index of the core to be tuned. Transmission spectra show well-defined resonant peaks which shift in position when the alignment is changed from planar to homeotropic. The measured transmission spectra agree well with the results of a multilayer optics model and the birefringence of the liquid crystals over this frequency range are determined as 1n = 0.15(±0.01) and 1n = 0.08(±0.01) for E7 and ZLI 2293, respectively.

Fully leaky guided mode study of the flexoelectric effect and surface polarization in hybrid aligned nematic cells

The fully leaky guided-mode optical-waveguide technique has been used to characterize the tilt profiles in two E7-filled hybrid-aligned nematic cells. The director distortion was measured with ac and dc voltages applied perpendicular to the substrates. By comparing the tilt profiles with model profiles produced using a free-energy minimization program, the flexoelectric coefficient of E7 at 20 °C has been measured as +1.5±0.2×10−11 C m. The effect of surface polarization in the cells has also been observed. This surface polarization is attributed to the influence of the homeotropic surface. The dipole alignment imbalance of the liquid crystal at the homeotropic surface has then been estimated.

Living systems and liquid crystals

The chemical, physical and electrical properties of naturally occurring liquid crystals are fundamentally linked to the functions of certain biological systems. This review focuses primarily on the liquid crystalline properties of cell membranes and the two-dimensional smectic A model used to describe the lipid bilayer structure that it forms. The composition of mammalian cells is considered along with the effects that disease has on the changes to the molecular composition, and the elastic and electrical properties of the cell membrane. Particular emphasis is given to the role that the flexoelectric effect in a two-dimensional lipid bilayer plays in mechanotransduction (the conversion of a mechanical stimulus to an electrical signal) in living systems. The intrinsic electrical properties arising from the membrane composition itself in terms of the transmembrane, surface and membrane dipole potential are considered in relation to the liquid crystalline structure of the cell membrane and the latest measurement techniques for the measurement of membrane electrical potentials are presented.

Polarization rotator using a hybrid aligned nematic liquid crystal cell

Using both direct mathematical analysis and numerical modeling based on the predictions by Jones [1] it is shown that if the director in a liquid crystal cell is in a plane which lies at 45 degrees to the incident polarization, then, for normally incident light, the transmission signal which conserves polarization will always have a phase difference of pi/2 from the transmission signal of the orthogonal polarization. This is independent of the director profile in the plane, the cell thickness, the anisotropy of the liquid crystal refractive index and the optical parameters of other isotropic layers in the cell. Based on this realization a hybrid aligned nematic liquid crystal cell has been tested as a thresholdless voltage-controlled polarization rotator. By using a quarter-wave plate to compensate for the phase difference between the two orthogonal output polarizations a simple liquid crystal spatial light modulator has been realized.

Rapid switching in a dual-frequency hybrid aligned nematic liquid crystal cell

We report the optical characterization of a dual-frequency hybrid aligned nematic (HAN) liquid crystal cell driven between two states using a multiple-frequency sinusoidal pulse. The complex dynamic director structure formed during the fast switching process is resolved in unprecedented detail on a submillisecond time scale. The results reveal backflow effects and a total switching time that is substantially faster than that achievable with conventional HAN cells.

Optical characterization of a dual-frequency hybrid aligned nematic liquid crystal cell

The dielectric anisotropy of a highly dispersive dual-frequency nematic liquid crystal (MDA-00-3969 (Merck KGa)) has been determined using the optical fully-leaky guided-mode technique. A 4Vrms sinusoidal voltage was applied across a 5microm hybrid aligned nematic (HAN) cell at various frequencies in both the positive and negative dielectric anisotropy regime. Optical data was collected at each frequency enabling the director profile in each case to be determined using a multi-layer optics model in combination with a liquid crystal free-energy minimization routine. The thresholdless response of the HAN cell combined with the extreme sensitivity of the optical characterization technique has allowed subtle changes in dielectric permittivity with frequency to be observed. The resulting measured dispersion shows excellent agreement with a single Debye-type relaxation model.

Backflow in the relaxation of a hybrid aligned nematic cell

The optical convergent-beam technique has been used to measure the changing director profile in a 4.6 μm ZLI-2293 filled hybrid aligned nematic cell when a 7 Vrms ac voltage was removed. The relaxation process has been recorded in 0.3 ms time steps allowing the detailed director backflow occurring in the initial 9 ms of the reorientation process to be quantified. The measured tilt profiles over the 60 ms total relaxation period were compared to model tilt profiles produced using the Leslie–Eriksen–Parodi theory, and excellent agreement was found. Further analysis shows that the backflow is dominated by the viscosity coefficient η1 and the overall relaxation is governed by the coefficient γ1.

Resonant Absorption of THz Radiation Using Nematic Liquid Crystals

The wavelength-dependent reflectivity of linearly-polarised Terahertz (THz) radiation has been recorded from a metal grating spaced above a metal mirror by a homogeneously aligned liquid crystal layer. Clearly defined minima were observed in the spectra measured in the range 0.1–3 THz. On application of a voltage across the liquid crystal layer these resonances shift in position by up to 30 GHz due to the resulting change in the sensed refractive index. The minima are attributed to resonant absorption through the excitation of standing waves generated both across and along the liquid crystal. The mechanism responsible for this phenomenon and methods for optimising the device are discussed.

Observation of backflow in the switch-on dynamics of a hybrid aligned nematic

The optical convergent-beam technique is used to measure, in 0.3 ms steps, the response of the director in a 4.6-μm-thick ZLI-2293 filled hybrid aligned nematic cell when a 10 kHz, 7 Vrms ac voltage is applied to the cell. The total time taken for the reorientation process is 2.4 ms, with backflow observed during the first 1.5 ms after the application of the voltage. The measured director profiles show excellent agreement with theoretical profiles produced from the Leslie–Eriksen–Parodi theory using typical values for the viscosity coefficients. Fluid velocity profiles within the cell are also modeled.

FULLY-LEAKY GUIDED MODE MEASUREMENT OF THE FLEXOELECTRIC CONSTANT (e11+e33) OF A NEMATIC LIQUID CRYSTAL

The response to applied voltages of the director profile within an E7-filled Hybrid Aligned Nematic liquid crystal cell has been determined by using the optical wave-guide method known as the Fully-Leaky Guided Mode Technique. Data was collected with applied AC and DC voltages and the measured director profiles were compared with model profiles generated through a minimisation-of-free-energy routine. The sum of the splay and bend flexoelectric coefficients (e11+e33 of E7 was measured as 1.5×10−11Cm−1. Results also show the presence of a DC offset voltage of +0.64 V in the cell.