Director fluctuations in nematic LCs : simulations, issues and opportunities

C

Thin film polarizer and color filter based on photo-polymerizable nematic liquid crystal

We present a method to fabricate a thin film color filter based on a mixture of photo-polymerizable liquid crystal and chiral dopant. A chiral nematic liquid crystal layer reflects light for a certain wavelength interval ∆λ (= ∆n.P) with the period and ∆n the birefringence of the liquid crystal. The reflection band is determined by the chiral dopant concentration. The bandwidth is limited to 80nm and the reflectance is at most 50% for unpolarized incident light. The thin color filter is interesting for innovative applications like polarizer-free reflective displays, polarization-independent devices, stealth technologies, or smart switchable reflective windows to control solar light and heat. The reflected light has strong color saturation without absorption because of the sharp band edges. A thin film polarizer is developed by using a mixture of photo-polymerizable liquid crystal and color-neutral dye. The fabricated thin film absorbs light that is polarized parallel to the c axis of the LC. The obtained polarization ratio is 80 % for a film of only 12 µm. The thin film polarizer and the color filter feature excellent film characteristics without domains and can be detached from the substrate which is useful for e.g. flexible substrates

Full-2D simulation of in-plane liquid crystal lasers

Lasing in liquid crystals has been demonstrated in numerous configurations and material systems. In most systems the laser light is emitted perpendicularly to the liquid crystal layer, but in the last few years also in-plane lasers have been demonstrated [1]. Such cheap in-plane tunable lasers could be combined in an opto-fluidic device, allowing to build fully integrated platforms for biological sensing applications. The accurate modelling of light generation in in-plane liquid crystal laser is difficult because the structure is two-dimensional and the optical properties are anisotropic. Moreover, 2D simulations of the liquid crystal orientation in such layers is necessary because the lying helix structure, which is often used for such lasers, exhibits defects. These defects appear because typical planar or homeotropic alignment is not compatible with the lying helix structure. Quite a lot of theoretical and numerical work has been carried out for perpendicularly emitting LC lasers. A one-dimensional plane wave expansion method was previously applied for the analysis of light emission from OLEDs. The extension to anisotropic materials and to simulation of lasing threshold makes it suitable for the simulation of LC lasing characteristics. Good agreement between simulations and experiments was found [2]. For the simulation of in-plane lasers we rely on finite-element calculations of the optical modes in periodic two-dimensional structures [3]. The optical modes in a lying-helix configuration are calculated including the band diagram. The band diagram reveals at which wavelength lasing can occur while the optical mode profile gives information about the electric field profile and the polarization state. Additionally the laser mode of the complete structure can also be calculated. The figure below gives an example of the field profile of the laser mode in a lying helix liquid crystal. The structure consists of a number of periods, terminated by an air layer at both sides

Fast widely tunable chiral nematic liquid crystal filter

Chiral nematic liquid crystals (CLCs) can spontaneously arrange into helical structures with periodicities of a few hundred nanometers with a certain pitch (P) and corresponding periodic refractive index profile. As such they exhibit a reflection band for a certain wavelength interval ∆λ (= ∆nP) with P and ∆n the birefringence. Since the photonic band gap (PBG) can be controlled by external stimuli (electricity, heat, light, elasticity), CLCs are potentially interesting in order to enable new applications: photonic information technology, lab-on-a-chip devices and switchable optical devices such as biosensors, reflectors, polarizers, reflective displays and tunable lasers.[1] In this work, a wavelength shift of the photonic band gap of 141 nm is obtained by electrical switching of a partially polymerized chiral liquid crystal with response times of 50 µs and 20 µs for switching on and off. The method features high stability and reflectivity in the photonic band gap without any noticeable degradation or disruption. The device consists of a mixture of photo-polymerizable liquid crystal, non-reactive nematic liquid crystal and a chiral dopant that has been polymerized with UV light. The influence of the amplitude and the frequency of the applied voltage on the width and the depth of the reflection band are investigated. By selecting the appropriate chiral dopant concentration, it is possible to make devices for different operation wavelengths. Compared to previously reported work, we have drastically improved the contrast and the switching speed of the device and the tuning range of the photonic bandgap