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Invited Paper

Ultrafast High Optical Contrast Flexoelectric Displays for Video Frame Rates Harry Coles, Stephen Morris, Flynn Castles, Damian Gardiner and Qasim Malik 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 describe wide temperature range bimesogenic chiral

nematic liquid crystals mixtures with high flexoelectro-optic coefficients (e/K), of the order of 2.0-4.0 CN-1 m-1, with 50100µs response times. Gray scale devices in both the ULH texture, with an optimum optical in plane switch of 45º at fields of <2Vμm-1, and the USH mode, with a unique optically isotropic “field off” black state (~0.7 nits) and contrast ratios of ~5000:1, using “in plane” electric fields driving will be described. The new materials and devices give μs level to level switching and may be used in FSC video rate displays.

1.

Introduction

At present, there is a demand for alternative liquid crystal electrooptic effects that exhibit a faster response time than those observed using current display technology which are based upon conventional nematic liquid crystals. One potential candidate is the flexoelectro-optic effect in chiral nematic liquid crystals which is a fast, in-plane deflection of the optic axis that occurs on the microsecond timescale and is linear in the applied electric field strength. The flexoelectro-optic effect has been shown to operate in two different geometries: the uniform lying helix alignment and the uniform standing helix alignment and have response times of the order of 10s to 100s of microseconds.

2.

Figure 1. Schematic of the rotation of the optic axis in a chiral nematic liquid crystal with an applied electric field. Since the tilt angle depends upon the sign of the field as well as the magnitude, the direction of the rotation of the optic axis is determined by the direction of the applied electric field.

3.

The initial reports on the flexoelectro-optic effect were only able to observe very small tilt angles (e.g. 7o) as a result of small flexoelectric coefficients [1, 2]. Extensive work was carried out by the Chalmers group in Sweden in the early 1990s [3-6] and the tilt angles were increased to 30o although impractical electric field strengths of 130 V/μm were still required. Reduction in the driving voltage was later achieved following [7-9] the development of the bimesogenic structures, below, which combined relatively strong polar groups with a configuration that was able to minimize the dielectric anisotropy through the orientation of the dipole moments. F

The Flexoelectro-Optic Effect

In 1987 Patel and Meyer showed that flexoelectric coupling between an applied electric field and a chiral nematic liquid crystal (N*LC) results in a fast-switching electro-optic effect [1]. This interaction between external stimulus and LC medium is an in-plane φ deflection of the optic axis away from the equilibrium position at zero-field, Figure 1. The main equations governing this effect are; tan  

e P E K 2

2 (1) and    P K 4 2

(2)

Where, e = (es + eb)/2, K = (K11 + K33)/2, es and eb are the flexoelectric coefficients K11 and K33 the splay and bend elastic constants, P is the pitch, τ is the response time and γ is the effective viscosity associated with the distortion of the helix. Overall, the combination of equations 1and 2 show, that to optimize the flexoelectro-optic effect, the flexoelastic ratio must be large whereas the pitch and γ must be small for a fast response time. Such a combination of material parameters is not readily available using conventional LC compounds and alternative materials are required.

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Development of Materials for the Flexoelectro-optic Effect

F

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Further research has led to the development of mixtures which exhibit very large tilt angles (greater than 80o) and, moreover, the field strength required for full intensity modulation was found to be substantially reduced to only 1 or 2 V/μm [10].

4.

Uniform Lying Helix Mode

An illustration of the uniform lying helix configuration is shown in Figure 2(a). In this case the helix axis is aligned parallel to the substrates and an electric field is applied across the sample, satisfying the requirement that the direction of the electric field is orthogonal to the helix axis. With the application of an electric field the optic axis is deflected in the plane of the device in either direction depending upon the polarity of the applied electric field. The generation of the uniform ULH texture, necessary for high optical contrast has proved problematical. Various complex methods involving combinations of mechanical, thermal and electric field cycling [1,4,9,11-13], periodic boundary conditions [14,15] involving complex lithographic processes, and electro hydrodynamic effects using materials with high dielectric anisotropy, Δε.[16], have all been used with varying degrees of success.

ISSN 0097-966X/12/4302-0544-$1.00 © 2012 SID

Invited Paper

(a)

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(b)

Figure 2. (a) The uniform lying helix configuration and (b) schematic of the electro-optic cell, comprising finger patterned in plane ITO-coated electrodes on lower glass substrates and a planar uniform ITO upper electrode, both substrates coated with planar alignment layers (rubbed parallel to electrode edges). A method of spontaneously aligning the ULH, in N*LCs optimized for the flexoelectro-optic effect, through purely electrical means at any temperature using a tri-electrode configuration, Figure 2(b) was evolved [17]. An in-plane low frequency field applied across the surface electrodes induces a uniform ULH texture. Once this is achieved, the surface electrodes are electrically connected, to be at the same potential, and using the upper electrode an electric field is applied across the cell, or optic axis, to generate the flexoelectro-optic switching observed through between crossed polarisers. In the schematic, Figure 2(b), the lower surfaces are the inter-digitated indium tin oxide (ITO) electrode arrays with a width of 4μm and electrode separation of 9 μm. The upper electrode is a uniform ITO surface and the cell gap is 5 μm. The cell surfaces were pre-treated with antiparallel-rubbed polyimide alignment layers and the direction of rubbing was parallel to the in-plane electrodes. Following the induction of the stable ULH texture, flexoelectrooptic switching was obtained by applying a common voltage to the in-plane electrodes, and addressing across the cell via the third electrode on the top substrate. Figure 3(a) shows the electro-optical response recorded through crossed polarizers, with one polarizer axis at 22.5º to the lower electrode edges, when addressing the cell in this configuration. The optical response follows the polarity of the field, as expected for ULH flexoelectro-optic switching (red line). By addressing through an in-plane field only (i.e. no field across the cell), at similar field strengths, no such modulation (blue line) was observed. In this case clearly there is no macroscopic rotation of the optic axis in the plane of the device. Figure 3(b) shows the in plane rotation of the optic axis  as a function of the applied electric field strength across the upper and lower substrates. In accordance with Eq. (1), is found to vary in a linear fashion at low tilt-angles, reaching 22.5° at E = 5.5 Vrms/m. For the mixture considered here (e/K = 1.8 C/Nm, P ≈ 320 nm at T=40ºC). Response times, for 10% to 90% (rise) and 90% to 10% (fall) changes in light transmission, for the ULH device are shown in Figure 3(c) as a function of temperature for E = 5.5 Vrms/μm the field needed to deflect the optic axis through φ = +/22.5°. The rise and decay curves were symmetrical for each pulse and the response times (rise and decay) numerically equal to each other.

Figure 3. Electro-optic characterization of the device measured at 40°C: (a) the photodiode (pd) signal recorded with application of 20 V (rms), 50 Hz signal (black line plotted on primary axis) (i) between the substrate in-plane electrodes only (blue line, plotted on the secondary axis) and (ii) addressing between the lower in-plane electrodes and the upper plane-parallel electrode (red line, plotted on secondary axis), both with crossed polarizers with one polarization axis aligned at 22.5°to the off state optic axis. (b) The switching angle of the optic axis as a function of the applied electric field. (c) The temperature dependence of the exactly superposable rise and decay response times. (d) The dependence of the transmission on the applied electric field showing full intensity modulation, through crossed polarizers and one polarization axis parallel to the electrode edge. To demonstrate intensity modulation, around the zero field “off” state, the ULH optic axis is positioned (between crossed polarizers) in the same direction as the transmission axis of one of the polarizers to give zero light transmission. The average transmission, as the optic axis switches reversibly through ±φ on field reversal is then proportional to . This is shown in Figure 3(d), where the full average intensity modulation occurs at φ = 45°, corresponding to E = 11.8 Vrms/μm. In these measurements the response time is so fast that the modulated optical response appears square wave.

Figure 4. Photographs of the electrically induced ULH aligned texture, between crossed polarizers, on a cold-cathode fluorescent backlight. (a) The “Off” state with the ULH optic axis positioned along the polarizer direction and (b) the “On” state with the application of an E = 4 V/m, 50 Hz electric field. Figure 4 shows the excellent uniform alignment and optical contrast of the tri-electrode induced ULH texture, of a large area cell even though E is lower than that required for full intensity modulation. The “zero field” alignment is fully retained after switching the sample many hundreds of times.

5.

Uniform Standing Helix Mode

An alternative geometry is to align the N*LC in the more conventional Grandjean texture and apply an electric field perpendicular to the helical axis using in-plane electrodes; this is

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referred to as the USH mode and is illustrated in Figure 5. Initially, the USH mode was considered in the context of a fastswitching phase device for telecommunications applications [18– 21] but more recently it has been considered as a potential new display mode [22, 23]. Unlike the ULH mode, the rotation of the optic is now out-of-the-plane of the device and thus a positive cplate compensation film would be required to obtain a wideviewing angle.

Figure 5. The uniform standing helix configuration where w=4μm, s= 9 μm and d=5 μm One of the key benefits of this mode over the ULH is that the alignment is facile since the Grandjean texture is the lowest energy configuration when using conventional polyimide alignment layers. A comparison of flexoelectro-optic switching in the ULH and USH modes revealed that the tilt angles are comparable for the two modes [23]. Another significant benefit of this mode is the off-state which can be optically inactive for visible wavelengths provided the pitch is very short (typically less than 200 nm). The potential of this LC mode is clear, fast response times combined with high contrast ratios, a combination not readily achievable with any other LC mode. Here we present results on a device that exhibits electro-optic switching with response times less than 100 μs for full intensity modulation, see [24] for details. The device is based upon in-plane addressing of very short pitch polymer stabilized chiral nematic LCs using a cell architecture identical to that already in use in current commercial high definition in-plane switching (IPS) display panels. Application of an electric field induces a birefringent, transmissive, on-state. Coupled with the very low transmission in the off-state, due to the short pitch chiral LC, this allows the possibility of generating extremely high contrast devices with very fast response. Figure 6(a) shows the T-E curve as a function of electric field for three different RGB wavelengths. In all cases, the square-wave (1 kHz) electric field was increased at a constant ramp rate of 0.5 V/µm per second. At low field strengths, the transmission is low, subsequently rising non-linearly with increasing field strength. The inset of Figure 6 (a) shows that, on increasing and decreasing the electric field strength, there is minimal switching hysteresis (i.e. < ±5%), furthermore the original ‘off’ state is fully recovered. The data may be approximately described by the relative transmitted intensity T through a uniaxial birefringent slab, (4) 2 2  neff d  . T  sin (2) sin  



 

Here, Δneff is the effective induced birefringence at a particular applied electric field strength, and Ψ is the angle of the induced, in-plane, optic axis with respect to the polarizer/analyzer crossed axis (Ψ = 45° in this work). Using this equation to extract Δneff, we find, for fixed λ, that Δneff is approximately proportional to the square of the applied electric field (Figure6(b)).

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Figure 6. (a) Relative transmitted intensity as a function of electric field for the polymer-stabilized uniform standing helix device measured at 436 nm (squares), 546 nm (circles) and 658 nm (triangles), respectively. (b) Plots of neff against E2 for each wavelength; (c) Response times of the USH cell as a function of electric field strength (rise time (open circles) and decay time (closed squares)); (d) The temperature dependence of the electro-optic characteristics ( = 436 nm). The rise (open circles) and decay (closed squares) times as a function of temperature for E = 17.4 Vm-1 are plotted on the primary axis. The value of Eon for the first bright state is plotted on the secondary axis (closed triangles). The quadratic dependency of the birefringence on the electric field is typical for dielectric coupling; however, in this standing helix geometry, the flexoelectric coupling is also expected to give a similar dependency at low field strengths. Previous work has shown that both dielectric and flexoelectric coupling can contribute additively to an induced optic axis [20]. By observing an isolated train of 3 bipolar pulses, the rise and decay times (each measured considering a 10% to 90% change of the transmitted intensity, respectively) as a function of the electric field strength were obtained at a constant temperature of 30oC using incident light of λ = 436 nm. These are plotted in Figure 6(c) with a typical optical response to the pulse train shown in the inset. From Figure 6(c), it is shown that the rise time reduces significantly with increasing electric field strength, decreasing from 125 µs at 13 V/µm to 50 µs at 17.4 V/µm. The field free decay time, on the other hand, increases slowly from 30 µs to 60 µs following application of fields of 13 V/µm and 17.4 V/µm, respectively. These fast response times are of considerable importance with regards to minimizing motion blur and enabling frame sequential color in a practical display. The temperature dependence of the T-E properties and response times were examined over a range from 25°C to 55°C, Figure 6(d). Eon appears to be approximately temperature independent over this range, varying by less than 10%. The rise and decay times as a function of temperature for E = 17.4 V/μm are also presented in Figure 6(d). It is shown that both response times decrease as the temperature is increased although the effect is more pronounced for the decay time. For example, the rise time decreases steadily from around 60 µs at 25°C to only 50 µs at 55°C. The decay times decrease more rapidly from 90 µs to 34 µs.

Invited Paper

40.1 / H. Coles holographic projection. Using the USH mode, full frame colour sequential 17” flat panels, TFT driven, have been fabricated in our collaboration with LG Displays.

7.

Acknowledgements

We acknowledge the support of the Engineering & Physical Sciences Research Council, LG Displays and Merck Chemicals.

8.

Figure 7 Photographs of (a) ‘Off’ and (b) ‘On’ state of the USH cell on a cold-cathode fluorescence backlight between crossed polarizers. The electrode region covers an active area of 1 cm2. The ‘Off’ (c) and ‘On’ (d) states recorded using a polarizing microscope. The dark lines in (d) represent the regions above the electrodes, which are 4 μm wide (w). The in plane electrode separation s = 9 μm. Photographs of the “Off” (zero field) and “On” states are shown in Figure 7 recorded on both a cold-cathode fluorescent light and a polarizing microscope. The images were taken after 100 repeated scans of the electric field had been carried out. There was no difference in the visual appearance of the sample before and after this cycling. Consequently, the USH device retains the high extinction “Off” state of the highly twisted chiral nematic LC. The contrast ratio, measured with a calibrated photometer at normal incidence, was in excess of 3000:1 and the off state transmission was 0.7 nits.

6.

Summary

In this paper we have two different modes of operation of the flexoelectro-optic effect: the uniform lying helix and the uniform standing helix alignments. Both modes exploit the submillisecond response time of the flexoelectro-optic effect with the gray-scale capability due to the linear response of the tilt angle on the electric field strengths. However, the two modes do have different attributes. For the uniform lying helix configuration the in-plane rotation of the optic axis leads to a wide viewing angle, negating the need for additional compensation films. We have described a new method for producing reliable alignment of the helical axis in the plane of the device. The research has made significant advancements in this area both in the materials and developing new strategies with which to achieve a high contrast ratio through control of the induced ULC alignment. In contrast to the ULH configuration, the USH mode does not have the problem of alignment and, moreover, due to the optically inactive structure for very short-pitch values between crossed polarizers potentially leads to very high contrast ratios at normal incidence. A compensation film is required due to the out-of-plane rotation of the optic axis and continued development of the materials is required to increase the flexoelectric coefficients further so as to obtain full intensity modulation at low electric field strengths. Both modes are of significant interest for next-generation fastswitching flat panel displays, telecommunications devices and

References

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