Mater. Res. Soc. Symp. Proc. Vol. 1293 © 2011 Materials Research Society DOI: 10.1557/opl.2011.23

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 applied electric field couples with the flexoelectric polarization

properties of the liquid crystals [8] to generate new LC device modes beyond those used in classical displays. Herein we will describe five different operating modes for these materials; 1) the Uniform Lying Helix (ULH), 2) the Grandjean or Uniform Standing Helix (USH), 3) the Blue Phase (BP), 4) Reflective Chiral Nematic (N*) and 5) Laser Devices.

Figure 1. Generic nematic bimesogenic structures used in flexoelectrooptic mixture formulation.

1) THE FLEXOELECTRO-OPTIC EFFECT – UNIFORM LYING HELIX MODE The flexoelectro-optic effect was first observed by Patel and Meyer[8], in chiral nematic N* liquid crystals, in which the helix axis was constrained to lie in the plane of a simple device with conventional top and bottom electrodes to give transverse electric fields, (Fig 2). On application of an external electric field across the cell, a periodic splay-bend deformation was induced which resulted in a macroscopic deflection of the optic axis in the plane of the device. This deflection was linear [8-13] in the applied field and led to wide viewing angles, gray scale capability [11, 12] and switching times [10] of tens of microseconds, albeit at very high fields (> 100 Vμm-1). We have shown [14], through molecular engineering, that these fields may be reduced to less than 5Vμm-1. The flexoelectro-optic effect, in N* materials, is described [8] by two fundamental relationships. Firstly the field induced deflection of the optic axis ф, on application of a field E (Fig 2), from its equilibrium position (E = 0) is given, to a first approximation, by tan ф = e p E / 2π K, where e and K are the effective flexoelectric and elastic constants, respectively and e= (es+ eb )/2 and K = (K11 + K33 )/2. The parameters K11 and es represent splay deformations, K33 and eb

the bend deformations of the director field and p is the pitch of the helix in the N* phase. The electro-optic response time τ is given by, τ = (γp2)/K 4π, where γ is the effective viscosity (of the same order as twist or splay viscosity) associated with the distortion of the helix. Note there is no E dependent term in the definition of τ, which means that response times are dominated predominantly by the helix pitch and the visco-elastic material’s properties. There is one further relationship that is important, for devices, and that is the critical field Ec for a twist Freedericksz transition. The threshold field for helix unwinding is given by Ec = π2/p2 ((4π K22)/Δε)), where K22 is the twist elastic constant and Δε (=ε║ - ε┴) is the usual low frequency dielectric anisotropy. These equations set the design parameters for good flexoelectro-optic materials and ultimately device performance. There are several remarkable features of the flexoelectro-optic effect. Firstly φ is virtually independent of temperature, since e and K have similar temperature dependencies, and tan φ varies linearly with the applied field thus leading to the gray scale capability. Secondly, to a first approximation, the response time τ is independent of applied field, depending only on the macroscopic properties. In our mixtures, c.f. figure 1, we vary the pitch p by adding small quantities (~1-2%) of high twisting power (HTP) chiral dopants to the nematic mixtures. This allows us to conserve the bulk macroscopic properties of the nematic materials i.e. K, e, γ etc. It is preferable to use short pitch materials since τ is quadratically dependent on p and for p<<λ, where λ is the wavelength of light, the diffraction losses due to the helical or banded texture, viewed through crossed polariser, are negligible. Thus the polarisation state of the light within the device does not undergo optical rotation. Further, the critical field Ec, for helix unwinding is also increased due to its inverse quadratic dependence on p. Although the tilt angle or switching angle (2 φ) is concomitantly reduced this may be compensated for by designing molecular materials with high values of e/K. The key question is then how to maximise e/K, which implies large molecular dipoles, whilst at the same time minimising the dielectric anisotropy to prevent helix unwinding?

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Figure 2. Schematic of ULH device: (a) “off” state E=0, (b),( c) switched states with “out of plane” field E for a RH helix and two opposite polarities and (d) a representation of the device layout including polarisers. It is initially interesting to recall that the flexoelectricity arises from a distortion of shape anisotropic materials, i.e. “pear or banana” shaped molecules. Indeed molecular systems, such as the alkyl or alkyloxy cyano-biphenyls, through the bulk of their aromatic cores, relative to the

cross section of their alkyl chains should exhibit flexoelectro-optic effects. Indeed the heptylcyanobiphenyl homologue, 7 CB has a tilt angle of up to 8º which is linear in field at low fields, secondly there is a critical field, Ec= 4V/µm, above which the helix unwinds, and thirdly the response times are ~ 100µs and independent of temperature [15]. Further both ф and τ were independent of temperature and field, respectively. For these room temperature liquid crystals the pitch, p, was adjusted to be 450nm, on addition of high twisting power (HTP) chiral dopants to the nematic material, Δε= +10 and e/K was measured to be 0.5CN-1m-1. In order to overcome the large dielectric anisotropy we developed a molecular design paradigm to synthesise bimesogenic structures that have two polar mesogens (with large outboard dipoles) connected by a flexible alkyl spacer. Initially we synthesised symmetric bimesogens with outboard cyano biphenyl terminal groups, shown below.

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These materials had a very low dielectric anisotropy and, for a pitch of 700nm, gave tilt angles linear in the applied field of up to 23o, switching times of 40-60µs, a critical helix unwinding field of 18Vµm-1, as compared with 4Vµm-1 for 7CB, and e/K was measured to be 0.6CN-1m-1. These materials also allowed us to establish that the electro-optic switching gave a “V” shaped response for an applied triangular wave, and that whilst the electro-optic response, to a square wave, was also “square” the field free decay was equally fast, i.e. ~ 200µs, Figures 3a) and b). Clearly the bimesogenic concept is a practical way of reducing Ec, whilst at the same time maintaining high switching angles and microsecond response times. However, concomitant with the extended molecular length the Isotropic to Nematic temperature was high; ~205oC and the measurements reported in Figure 3 were carried out at 183oC. To reduce these temperatures we developed the laterally fluorinated analogues of those structures shown in figure1.

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Figure 3. Electro-Optic response to an Electric field for (a) bipolar and (b) a monopolar pulse. To reiterate our molecular design paradigm has been to synthesise bimesogenic structures (Fig. 1), and related mixtures, that have two polar mesogens (with large outboard dipoles) connected by a flexible alkyl spacer. The net dielectric anisotropy of the bimesogens is low (Δε << 3), preventing helix unwinding, whilst strong dipole moments are still present [14] to give

large flexoelectric switching with low applied fields, E. Typical flexoelectro-optic responses are shown in Figure 3. These data show a number of interesting features for phase devices. Firstly the tilt angle φ is linear in the applied field up to the angles of 35º - 40º and above these values there is only a slight curvature, Fig 4a). The highest applied field was 10Vμm-1 and a tilt angle of 22.5º (i.e. the ideal birefringence switching angle of 45º) was achieved with fields of < 5V/μm. At such fields the response times were ~1 ms at Ts = 40ºC reducing to ~ 50μs at Ts = 10ºC, Fig 4b), where Ts = Tc – T and the clearing temperature, Tc, for this mixture was 85ºC. There is a nematic to smectic phase transition at 33ºC which causes this divergence. Mixtures have been prepared that eliminate this effect. This also helps to reduce the slight temperature dependence of φ for a fixed field, Fig 4c. The data shown demonstrates that for a short pitch system (p ~ 300 nm) the electric field needed to give a 22.5º tilt (45º switch) decreases with decreasing temperature and at 35ºC a field of 4Vμm-1 gives a 45º switch. For the longer pitch system (p ~ 600 nm) the fields required are approximately halved. For both systems helix unwinding did not occur up to fields of 20Vμm-1. The birefringence of the achiral mixture was ~ 0.25 which, averaged across the tightly wound helix in the N* phase and viewed orthogonally, (Fig 2c) reduces to ~ 0.13 and the material is negatively uniaxial. Cells of 4μm path length led to a π phase change at a wavelength of 500 nm. Thus for the light transmission, It,, in a birefringence device, i.e. It = Io sin2 (2ψ) sin2 (πΔn d/λ) where Io is the incident intensity and ψ = 2φ it is possible to gain maximum contrast, between crossed polarisers with φ = 22.5º and d ~ 2μm at λ = 500 nm. With typical applied fields of 4V/μm-1 being needed to give φ = 22.5, in the short pitch system, this implies low voltage drivers and the devices have been used in conventional transmissive displays as well as in reflection using Liquid Crystal Over Silicon (LCoS) devices[16 ]where the cell thickness is halved. Further, for φ = 45º the materials may be used in dichroic or fluorescent dye guest host geometries, but still at low driving voltages. The current test cells were aligned using rubbed polyimide alignment layers and by applying an electric field during cooling and a weak shear field [14]. This alignment was then set optically [16], using 3% w/w of reactive mesogens (RM257 from Merck NB-C) and UV light. The cells then may be taken through temperature excursions into the isotropic or crystalline phases without losing their ULH alignment when returned to the N* phase. Typical contrast ratios were ~ 300:1.

Figure 4. (a) Flexoelectro-optic tilt angle; b) response time both as a function of field and temperature; c) optimum fields for 45º switching for a long pitch( ~600nm, ○) and short pitch ( ~300nm, ■) system as a function of temperature.

2) THE FLEXOELECTRO-OPTIC EFFECT – UNIFORM STANDING HELIX MODE In the Uniform Standing Helix or Grandjean texture mode the helix or optical axis is constrained to be orthogonal to the display cell surfaces, figure 5. In this flexoelectro-optic mode, viewed in the light transmission direction, for a short pitch helix, the twisting ensures that the cell is optically isotropic, thus giving a perfect “black” dark state whatever the orientation of the crossed polarizer-analyzer pair, figure 6. Application of an “in plane” field then causes the optical axis to tilt around the field direction, as described in the previous section. In this case the system switches from optically isotropic to positive uniaxial, with the induced major optical axis always along the field direction, thus producing a phase device [16-20] and the response is again linear in the applied field leading to an analogue phase device. We have probed the subtle orientation-reorientation effects using these materials, doped with fluorescent dyes, as chiral nematic laser cavities [21], where the induced birefringence alters the laser wavelength output as a function of applied field [21], c.f. Section 5 below. In this geometry switching times of 20μs were recorded in comparison with 120μs, under the same temperature and field conditions as recorded for the ULH texture, and threshold fields were of the order of 2.5Vμm-1. In these initial test devices the optical path length was defined by the electrode thickness, 10μm, with an electrode spacing of 50μm. It is noteworthy that the electro-optic response is as fast as typical ferroelectric or antiferroelectric materials (i.e. 10’s of μs) but, using nematic mixtures and conventional nematic alignment layers, to give a perfect optically isotropic or “black” off state and a linear electro-optic response. Since the major optic axis, whatever the applied field, is in the direction of E, this facilitates device construction with respect to setting the crossed polariseranalyser directions (i.e. at 45º) but, with all of the other flexoelectro-optic advantages described above. Figure 6 illustrates some of the advantages of this switching mode whereby for short pitch lengths a pure black state is observed below a pitch of ~200nm, depending on the mixture and cell thickness, whereas Fig 6(b) shows the optical transmission through crossed polarisers. With typical extinction ratios of >5000:1, these materials, in the USH texture, offer a new route to ultra fast, high contrast and complexity, field sequential color display devices, that can exceed the performance of IPS or VAN displays[22,28] but lead interestingly to new optical phase devices, potentially for holographic/LCoS applications.

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Figure 5. Schematic of (a) the USH flexoelectro-optic switching, from the optically isotropic state (E-0) to uniaxial birefringent state E>0(the field E is out of plan as drawn) and (b) the behavior of the refractive index indicatrix.

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Figure 6. (a) “Off” state optical contrast as a function of helical pitch and (b) transmission as a function of applied field through crossed polarisers of sample flexoelectric liquid crystals in the USH mode. 3) THE WIDE TEMPERATURE RANGE BLUE PHASE We recently showed [23] that the bimesogenic materials of the type described herein could be used to produce true Blue Phase I materials stable over a 40º - 50ºC wide temperature range. Such materials give very narrow band bright reflections, Figure 7, that may be continuously and reversibly tuned in external electric fields, ~ 10-15Vμm-1, from green to blue, red to green etc without the use of external polarisers, colour filters or surface alignment layers. The zero field reflections arise from “optical” Bragg reflection caused by a body centred cubic lattice of disclination lines arising from the unique double twist cylinder structure of these phases [24]. Application of an electric field, through electrostriction, causes a distortion of this lattice and a change in the wavelength of the reflected light con, figure 7. As for the flexoelectro-optic effect, the initial pitch of the system is altered by changing the concentration of HTP chiral additive to give the required “off” state reflected colour and the maximum reflected intensity for one circularly polarised state of light approaches 50%. Thus double cells can be used to give specularly bright reflection or, λ/4 plates and mirrored surfaces can be used to give over 90% reflections. Any of the optical configurations used for reflecting chiral nematics [25] may be applied to these BPI devices. One significant difference for the BP devices is that their switching times are relatively fast. “On” times are controlled by the applied field whilst “off” times are controlled by the lattice elasticity and typical off times are of the order of 1 – 10ms. Unlike the N* or Cholesteric devices these reflecting textures are mono-stable and so, as for the twisted nematic (TN) device, the “on” state is held by an applied field being kept “on”. Contrary to this, it is possible to apply a high enough field to induce a different “stable” lattice structures to give a different reflected colour, through electrostriction, and then the colour or reverse switching mechanisms may be obtained using “in plane” fields of the type used in the USH mode to reverse this colour. We are currently developing simple, colour filter and polariser/analyser free, alpha-numeric displays based on these BP materials utilising these mechanisms for bistable colour switching. Many of the application areas that currently use conventional N* or Cholesteric displays could benefit from this new “Blue Phase” based technology and we currently are studying temperature stable colour switchable Blue Phase mixtures, stable between -20 and +80oC, and their application in hand held mobile information devices.

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Figure 7. RGB Blue Phase Reflected Textures over large areas, their spectral properties and electric field response. 4) COMMAND SWITCHABLE REFLECTING CHIRAL NEMATICS

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Figure 8. Reflection bands of chiral nematic films. Selective visible light reflection bands as a function of wavelength for different applied fields and (b) the dependence of the long wave photonic band edge as a function of applied field and its frequency using ferroelectric command surfaces. Chiral nematics (N*) are well known for their reflection of one circular polarisation of light and numerous devices have been realised due to the relative ease of construction and the brightness of the specular reflection [25]. In most of these devices electric fields are applied across homogeneous N* thin films to induce scattering focal conic or homeotropic optical textures. Such textures are bistable. Herein we will report briefly on two methods used to control electrically the selective reflection from thin N* films where the selective reflection band may be selected and controlled at will as a function of the applied field. In the first kind of device the surface alignment layers are composed of switchable ferroelectric layers [26]. In the second kind of device a ferroelectric material is added to the bulk of the N* material [27]. Using a switchable ferroelectric command surface the spectral reflection band can be shifted by ~80 – 90 nm in moderate fields, figure 8. It is noticeable that the quality of the reflection band remains the same for all fields and that the switching is fully reversible and the amplitude remains identical. Thus we observe a perfectly reproducible and switchable Photonic Band Gap (PBG). Secondly the frequency of the applied electric field may be fine tuned to increase the position of the Photonic Band Edge of the switched band gap, figure 8(b). The maximum response time for the complete

spectral switch was ~100ms [26]. In the case of a volume switchable reflection band an insoluble ferroelectric liquid crystal was added to the chiral nematic host [27] and the spectral response observed as a function of applied field, figure 9. Here again wide shifts in spectral band edges were observed ~120nm again with maintenance of the quality of the photonic band gap. The switching effects were fully reversible and again switching times were of the order of a few 100ms. In figure 9a & b these effects are clearly illustrated whilst figure 9c demonstrates the large area optical appearance of the switched thin (10µm) films. Both techniques offer fine control of the reflected colors and lend themselves to interesting photonic applications.

Figure 9. Selective reflection, at room temperature, from a chiral nematic thin film controlled by the inclusion of ferroelectric liquid crystals in the bulk of the film as a function of applied field, in (a) 1-5 correspond to fields of 0, 15.9, 21.5, 26.1, and 31.2 Vμm-1 respectively, (b) as a function of applied field frequency and (c) the film’s visual appearance. 5) LIQUID CRYSTAL LASERS We have recently reviewed the enormous progress made in the understanding and production of low threshold and high slope efficiency Liquid Crystal lasers [29] using chiral nematic structures. Such lasers can produce light emission between the near UV and IR spectral regions, and these wavelengths may be chosen, almost at will. Using the bimesogens described above, and with suitable choice of the materials parameters, it has been possible to achieve laser slope efficiencies of > 60%. Quasi-continuous lasers have been demonstrated with average powers of 5mW at a repetition rate of 3 kHz and such lasers are suitable for holographic image projection. In figure 10 we show the color gamut of typical LC lasers. The review illustrates the potential for such devices.

Figure 10. Monochromatic Red, Green and Blue High efficiency N*LC laser array emissions compared with the color gamut of a Cathode Ray Tube. SUMMARY In this paper we have presented some of our recent work on bimesogenic liquid crystals that lead to new phase and reflective devices suitable for displays and indeed telecommunications devices. The materials have broad temperature ranges, very high flexoelectro-optic coefficients and very low dielectric anisotropy. The materials may be aligned in the ULH or the USH (Grandjean) texture. Both modes give rise to sub ms to μs switching times, i.e. as fast as ferro- or anti-ferroelectric LCs, at fields of a few V/μm-1. The ULH mode may be used in either transmissive or reflective devices with inherent gray-scale capability. In the USH texture the applied electric field is “in plane” for relatively short helix pitches, and the helix or optic axis is normal to the substrates. For this mode the off state is perfectly black since the refractive index indicatrix is circular. The applied field induces a uniaxial ellipsoid indicatrix and again these devices have similar switching properties to the ULH device. Conventional surface alignment techniques may be used and both devices may be stabilized through the use of a few percentages of reactive mesogens. This opens a possible route to flexible displays. In the third mode of operation we have demonstrated wide temperature range BP materials that are highly reflective (or indeed transmissive through crossed polarisers) and in which the reflected color may be selected using electric fields with ms switching times without the need for polarisers or color filters. A fourth use demonstrated is in the use of visible reflective films in which their reflection spectra may be controlled by both command surfaces and volume effects. A fifth use of these bimesogenic materials is in chiral nematic lasers where high optical slope efficiencies (>60%) were obtained.

The performance, properties and potential of liquid crystal lasers were reviewed in a special volume of Nature Photonics [29]. What is becoming increasingly clear is that, despite their ubiquitous use in displays, liquid crystals, as a naturally self assembling molecular system, are presenting new photonic opportunities well beyond displays. ACKNOWLEDGEMENTS We thank the EPSRC for financial support under the BTRG COSMOS Research Grant EP/D 04894X/1.

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