Ultrafast Switching Liquid Crystals for Electro-Optic Transmissive and Reflective Displays and Microscopic Lasers
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Ultrafast Switching Liquid Crystals for Electro-Optic Transmissive and Reflective Displays and Microscopic Lasers Harry J Coles, Stephen M Morris, Flynn Castles, Philip J W Hands, Timothy D Wilkinson and Su Soek Choi. Centre of Molecular Materials for Photonics and Electronics, Electrical Engineering Division, University of Cambridge, 9 JJ Thomson Avenue, Cambridge, Cambridgeshire, CB3 0FA, UK. ABSTRACT We report on novel liquid crystals with extremely large flexoelectric coefficients in a range of ultra-fast photonic modes, namely 1) the uniform lying helix, that leads to in-plain switching, birefringence phase devices with 100 µs switching times at low fields, i.e.2-5 V/µm, and analogue or grey scale capability, 2) the uniform standing helix, using planar surface alignment and in-plane fields, with sub ms response times and optical contrasts in excess of 5000:1 with a perfect optically isotropic or black “off state”, 3) the wide temperature range blue phase that leads to field controlled reflective color, 4) chiral nematic optical reflectors electric field tunable over a wide wavelength range and 5) high slope efficiency, wide wavelength range tunable narrow linewidth microscopic liquid crystal lasers. INTRODUCTION Historically, liquid crystals are best known for their use in commercial low energy consumption, portable and lightweight displays based on 1) the Twisted Nematic (TN) mode of operation in which an optical polarization guiding effect, induced by the TN helix, is turned on or off, between crossed polarisers, by an external applied field [1]. Such displays incorporate thin film transistors (TFT) at each picture element or pixel [2], Red-Green-Blue (RGB) color filters and Polarisers to generate images or arrays of information (i.e. 1024 x 768 pixels in an XVGA monitor). Recently “In Plane Switching (IPS)” [3], in which the optical axis of the birefringent nematic is rotated in the plane of the device by an “in plane” electric field and the Vertically Aligned Nematic (VAN) modes [4] have been used. In a VAN display the director is aligned homeotropically (i.e. perpendicular to the pixel substrates) in the field off state. Between crossed polarisers the LC is then optically isotropic irrespective of the wavelength of light or temperature. Application of an electric field, for LCs with negative dielectric anisotropy, tilts the mid-plane director thus inducing a birefringence or light transmitting state. Each of these display modes has advantages and disadvantages, as a result of both the switching mode and ultimately the bulk LC properties. All of these modes rely on an applied electric field interacting with the dielectric properties of the LC medium to produce a change in the optical properties of the display devices [5]. Further these modes have led to a highly sophisticated and well understood technology platform. We recently discovered [6,7] bimesogenic nematic liquid crystals (c.f. figure 1), stable over a wide temperature range, that have very high flexoelectro-optic coupling coefficients, wherein the
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