Pieter Vanbrabant

Liquid-crystal photonic applications

Liquid crystals are nowadays widely used in all types of display applications. However their unique electro-optic properties also make them a suitable material for nondisplay applications. We will focus on the use of liquid crystals in different photonic components: optical filters and switches, beam-steering devices, spatial light modulators, integrated devices based on optical waveguiding, lasers, and optical nonlinear components. Both the basic operating principles as well as the recent state-of-the art are discussed.

Calculation of Fully Anisotropic Liquid Crystal Waveguide Modes

The accurate analysis of optical waveguides is an important issue when designing devices for optical communication. Waveguides combined with liquid crystals have great potential because they allow waveguide tuning over a wide range using low voltages. In this paper, we present calculations that combine an advanced algorithm for calculating liquid crystal behavior and a finite-element mode solver that is able to incorporate the full anisotropy of the materials. Calculation examples demonstrate the validity of our program.

A finite element beam propagation method for simulation of liquid crystal devices

An efficient full-vectorial finite element beam propagation method is presented that uses higher order vector elements to calculate the wide angle propagation of an optical field through inhomogeneous, anisotropic optical materials such as liquid crystals. The full dielectric permittivity tensor is considered in solving Maxwells equations. The wide applicability of the method is illustrated with different examples: the propagation of a laser beam in a uniaxial medium, the tunability of a directional coupler based on liquid crystals and the near-field diffraction of a plane wave in a structure containing micrometer scale variations in the transverse refractive index, similar to the pixels of a spatial light modulator.

Temperature influence on the dynamics of vertically aligned liquid crystal displays

The strong influence of the complex reverse flow phenomenon on the dynamic temperature behavior of vertically aligned liquid crystal displays (VA-LCDs) has been demonstrated. Good agreement was obtained between theoretical and experimental switching profiles over a wide temperature range (25–75°C). This was achieved using the Leslie–Ericksen theory in a one-dimensional model with material viscosity coefficients obtained from an improved estimation procedure. Such accurate numerical simulations can have a large impact on further improvements of VA-LCDs (e.g., the development of temperature-compensating driving schemes).

Effect of material properties on reverse flow in nematic liquid crystal devices with homeotropic alignment

Reverse flow is undesirable in liquid crystal devices with vertical alignment. The influence of the material properties on the onset of backflow is investigated for commercially available negative dielectric liquid crystals. It is shown that the threshold voltage VBF for the occurrence of backflow is an important material characteristic. This threshold is relevant for applications and a large value is desired in devices to avoid backflow while keeping a wide applicable voltage range. Accurate finite element simulation of the liquid crystal hydrodynamics allows extraction of VBF and the unknown Miesowicz coefficients ηij. The resulting values are tabulated at 20.0 °C.

Diffraction and fringing field effects in small pixel liquid crystal devices with homeotropic alignment

Reducing the pixel dimensions of liquid crystal microdisplays in search of high resolution has a fundamental impact on their electro-optic behavior. The liquid crystal director orientation becomes distorted due to fringing fields and diffraction effects influence the optical characteristics of the device once the structure features approach the wavelength of the incident light. Three-dimensional finite element simulation of the liquid crystal dynamics with a variable order approach is combined with a full-vector beam propagation analysis to investigate how elasticity and diffraction limit the resolution as a function of the pixel size for transmissive and reflective architectures with vertical liquid crystal alignment. The key liquid crystal properties are considered and the importance of materials with high birefringence is confirmed for small pixel devices as these improve the contrast for a fixed pixel size.

Polarization Selective Wavelength Tunable Filter

In conventional color CCD cameras, each color pixel consists of different pixels that record the red, green or blue light. However, for special applications such as in astronomy, it is desirable to take pictures at a number of specific wavelengths. In this work, we present the design and fabrication of a liquid crystal wavelength tunable filter which is also able to detect the polarization state of the light. The design consists of a 4-stage Lyot–Öhman filter with an additional liquid crystal cell in front of the filter. By applying the right voltage on the first cell, one can choose the polarization component of the transmitted light.

Finding exact spatial soliton profiles in nematic liquid crystals

Finding exact analytical soliton profile solutions is only possible for certain types of non-linear media. In most cases one must resort to numerical techniques to find the soliton profile. In this work we present numerical calculations of spatial soliton profiles in nematic liquid crystals. The nonlinearity is governed by the optical-field-induced liquid crystal director reorientation, which is described by a system of coupled nonlinear partial differential equations. The soliton profile is found using an iterative scheme whereby the induced waveguide and mode profiles are calculated alternatively until convergence is achieved. In this way it is also possible to find higher order solitons. The results in this work can be used to accurately design all-optical interconnections with soliton beams.

Temperature compensated overdrive in vertically aligned liquid crystal displays

The occurrence of backflow in vertically aligned liquid crystal displays (VA-LCDs) inhibits application of conventional overdrive techniques to achieve faster switching. This (hydro)dynamic behavior is simulated accurately by using the Leslie–Ericksen theory in a one-dimensional model. Taking the limitations due to backflow into account from these simulations, we designed overdrive schemes for VA-LCDs. The temperature sensitivity of a fixed overdrive table was eliminated by adapting the scheme to the simulated temperature variations in the dynamic behavior. Experimental verification in the 25–75 °C range shows that the resulting temperature compensated overdrive leads to faster switching, which is expected to be artifact free.

Tunable quasi-homeotropic liquid crystal pretilt angle based on competing alignment layers

The liquid crystal alignment properties of a spincoated mixture of two commercial homeotropic and planar aligning polyamic acids are investigated. Baking the coated substrates induces cleaving of the long alkyl side chains of the first polyamic acid which weakens the tendency for homeotropic alignment while the imidisation ratio of both polyamic acids changes in favour of planar alignment. By detailed measurement of the retardation induced by the liquid crystal, we show that the combination of these two mechanisms in the proposed mixture yields a spatially uniform liquid crystal pretilt angle that is tunable with the baking temperature in the quasi-homeotropic range 0–10°, which is important for the Vertically Aligned Nematic technology.