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

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

Hybrid fluorescent layer emitting polarized light

Semiconductor nanorods have anisotropic absorption and emission properties. In this work a hybrid luminescent layer is produced based on a mixture of CdSe/CdS nanorods dispersed in a liquid crystal that is aligned by an electric field and polymerized by UV illumination. The film emits light with polarization ratio 0.6 (polarization contrast 4:1). Clusters of nanorods in liquid crystal can be avoided by applying an AC electric field with sufficient amplitude. This method can be made compatible with large-scale processing on flexible transparent substrates. Thin polarized light emitters can be used in LCD backlights or solar concentrators to increase the efficiency

Anisotropic films for optical applications based on liquid crystal and nanorods

In a push to reduce global energy consumption, it is necessary to reduce the energy needed for optical devices. In addition, there are other important issues for optical devices such as fabrication cost and miniaturization. With this point of view, in this PhD work various optical devices based on liquid crystal (LC) and nanorods (NR) are investigated and fabricated. Firstly, fabrication of thin film polarizers and color filters based on photo-polymerization of reactive LC is demonstrated. The thin film polarizer is fabricated for applications which require very small thicknesses of the final device such as electro-active contact lenses. These films are suitable to be processed in optical device manufacturing as additional films or as in-cell optical components. The third fabricated device is a widely tunable optical filter with microsecond switching time. A microsecond-range optical shutter for unpolarized light is demonstrated as a forth application using reactive LCs. On the other hand, I demonstrate full alignment of colloidal NRs in suspension by an electric field. Finally, I present four methods for the homogeneous deposition and alignment of NRs from a colloidal suspension. They may be applied in polarized emitters in OLED, polarized fluorescent sheets or polarization-selective detectors

Microsecond-range optical shutter for unpolarized light with chiral nematic liquid crystal

A fast electro-optic shutter is fabricated and demonstrated. The device works independently of the polarization state of the incoming light beam. Modulation between 3% transmission and 60% transmission is obtained within a wavelength range of 50 nm with a response time of 20 mu s. The device consists of two partly polymerized chiral nematic liquid crystal layers separated by a half wave plate. The transmission modulation is due to a 50 nm wavelength shift of the photonic band gap of the chiral liquid crystal realized by applying an electric field over a mixture of photo-polymerized LC and non-reactive nematic LC containing a chiral dopant. The shutter features high reflectivity in the photonic band gap. We investigate the influence of the amplitude of the applied voltage on the width and the depth of the reflection band. (C) 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License

Effect of UV curing conditions on polymerized tunable chiral nematic liquid crystals

Chiral nematic liquid crystals have attracted substantial interest. They spontaneously self-organize to form a helical structure with no complex fabrication procedure required and exhibit a reflection band for a certain wavelength interval. Since the photonic band gap can be tuned by applying external factors (heat, voltage, light, elasticity) chiral nematic liquid crystals are potentially interesting for large area optical filters and shutters, reflective displays and tunable lasers. In this work, a device which consists of a mixture of photo-polymerizable liquid crystal, non-reactive nematic liquid crystal and a chiral dopant is fabricated. By selecting the appropriate chiral dopant concentration, it is possible to make devices for different operation wavelengths. The influence of UV illumination on a partially polymerized chiral liquid crystal is investigated. A blue-wavelength shift of the photonic band gap is obtained as a function of power, duration time of UV illumination and the thickness of the cells. Interestingly the width and depth of the photonic band gap is unaffected by the change in UV curing conditions, which indicates that there is no degradation by the UV light

Nematic liquid crystal devices with sub-millisecond response time

Conventional nematic liquid crystal devices exhibit switching times that are in the order of several milliseconds. In this work we focus on two types of nematic liquid crystals that can overcome the limitations of conventional nematic liquid crystals and allow sub-millisecond switching times for both switching on and off: nano-pore polymer-liquid crystals and dual-frequency liquid crystals

Polarized light emission by deposition of aligned semiconductor nanorods

The ability to control the position and orientation of nanorods in a device is interesting both from a scientific and a technological point of view. Because semiconductor nanorods exhibit anisotropic absorption, and spontaneous and stimulated emission, aligning individual NRs to a preferred axis is attractive for many applications in photonics such as solar cells, light-emitting devices, optical sensors, switches, etc. Electric-field-driven deposition from colloidal suspensions has proven to be an efficient method for the controlled positioning and alignment of anisotropic particles. In this work, we present a novel technique for the homogeneous deposition and alignment of CdSe/CdS NRs on a glass substrate patterned with transparent indium tin oxide interdigitated electrodes, with a spacing of a few micrometers. This method is based on applying a strong AC electric field over the electrodes during a dip-coating procedure and subsequent evaporation of the solvent. The reproducible and homogeneous deposition on large substrates is required for large size applications such as solar cells or OLEDs. The accumulation, alignment, and polarized fluorescence of the nanorods as a function of the electrical field during deposition are investigated. A preferential alignment with an order parameter of 0.92 has been achieved