Smectic dynamic scattering in laser-addressed liquid crystal projection displays

Smectic dynamic scattering in laser-addressed liquid crystal projection displays C J WALKER AND W A CROSSLAND

Two viable methods of producing high-uniformity scattering, suitable for laser-scanned highresolution light-on-dark contrast liquid crystal projection displays, are reported. The advantages of dynamic scattering over heat-pulse-induced scattering are outlLued in terms of power consumption, addressability, uniformity and thermal m a n a g e m e n t .

Keywords: liquid crystal displays, scattering, smectics, lasers

Recent technical advances in computer graphics and data processing systems have led to the need for displays with high information content. For some applications, it is also desirable to project optically the information written on a laser-addressed liquid crystal display. Early work on laser-written cholesteric devices ~showed serious limitations in performance, and the advantages of using smectic liquid crystal materials were recognized in 19732. The aligned state of these devices is homeotropic and completely transparent, and the scattering centres formed by the thermal writing process are smaller and more dense 3, leading to a much higher contrast ratio. Laser-written smectic devices also have a selective erase capability. Incident laser energy is absorbed by conducting layers of indium tin oxide (ITO) or metal 4 on the glass face of the cell or, alternatively, by a dye 5 which is dissolved in the liquid crystal and has a peak absorption at the wavelength of the laser used. The absorbed energy locally heats the liquid crystal layer into the isotropic phase. In the absence of an aligning field, rapid cooling into the smectic phase freezes the liquid crystal into a light-scattering disordered focal conic texture. Thus scattering lines are written on a clear panel by the scanning laser beam. A repeat scan, followed by rapid cooling into the smectic phase in the presence of an aligning field, leads to transparent homeotropic alignment of the liquid crystal and thus selective erasure. Bulk erasure can be achieved by field-assisted thermal

Standard Telecommunication Laboratories Ltd, London Road, Harlow, Essex CMI 7 9NA, UK DISPLAYS,OCTOBER 1 9 8 5

erase as described by Kahn 2 or by electric field only, which requires high voltages and is the method we have used. In the application reported, the smectic display is coprojected with a map to give an overlay of additional information. For this purpose, a light-on-dark contrast mode is required; and, if the conventional scattering-on-clear display is to be used, complex schlieren projection optics are necessary which are highly inefficient. There are two methods of achieving the light-on-dark contrast for direct projection by a clear-on-scattering mode of display. The first uses thermal scattering of the entire display area by applying, along one surface only, a direct current pulse across the ITO conductor to heat the liquid crystal into the isotropic phase. Rapid cooling in the absence of an applied field results in a light-scattering focal conic texture, and this method has been used by several previous authors 6'7 to achieve a light-on-dark contrast. The second method uses dynamic scattering of the smectic liquid crystal to form the overall scattering texture. In both cases, clear lines are written by locally heating areas with a scanned laser beam and then cooling them in the presence of an applied field. EXPERIMENTAL ARRANGEMENT

Laser addressing system The optical system used is shown in Fig. 1. A nominally 7 mW He-Ne laser is directed into the mini-mirror sys-

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Scanning ] electronic t driver I

Table 1. Phase-transition temperatures

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~ - - TemperaTurecontrolled enclosure Fig. 1 Experimental arrangement (F, focal length of scanning lens) tem (designed and supplied by Laserscan Laboratories

Ltd). The mini-mirror block incorporates two deflecting galvanometers fitted with 3.6 mm diameter mirrors, a complex prism arrangement and a focusing lens. For test purposes, the galvanometers are controlled by an electronic drive unit which produces two triangular waveforms, 90* out of phase, to deflect the beam in a square pattern at adjustable speeds and amplitudes. This provides a facility for writing a series of squares, at constant angular rate but with varying amplitude and at different writing speeds. The laser and the mini-mirror block are mounted on a standard V-type horizontal optical bench. The cell is positioned in a thermostatically controlled enclosure with the liquid crystal layer at the focus of the scanning lens. The laser power incident onto the enclosure is approximately 3 roW, and there are further losses due to the window on the enclosure and the finite efficiency of absorption in the liquid crystal cell. Liquid crystal cells The cells used for both types of device are made of ITO-coated glass panels spaced apart by 12 iam glass fibres, with an active display area 40 mm square. Homeotropic alignment was used. For heat-pulse scattering, commercially available smectic $2 (BDH Chemicals Ltd), which contains alkyl and alkoxy cyanobiphenyls, doped with 2%CEI and 2%DC5, is used. This mixture displays a narrow nematic phase. Phase-transition temperatures are given in Table 1. The addition of 2%CEI eholesteric (BDH Chemicals Ltd) enhances the scattering. DC5 is a proprietary anthraquinone dye with an absorption band of 600650nm, centred close to the wavelength (630 nm) of the He-Ne laser used to address the cell, DC5 has an order parameter (S) of 0.53 in the nematic phase of ZLII32 (E Merk Damstadt), and an extinction coefficient (e) of 1.3 x l& L mol-~ cm-~. Cells are constructed from glass coated with 12 f~/IS] ITO. For dynamic scattering, the liquid crystal used is the commerically available smectic $4, which contains alkyi and alkoxy cyanobiphenyls, plus a blue proprietary anthraquinone dye DCI56, plus dopants. DCI56 has an 208

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absorption band of 590-650 nm, and an absorbance of 1.2 absorption units. The dye has an order parameter (S) of 0.7, and a slightly higher extinction coefficient than DCb. The liquid crystal mixture has a narrow monotropic nematic phase, and phase-transition temperatures are given in Table 1. Conventional ITO coatings with higher surface resistance of the order of 200 f~/[2 are used for dynamic scattering displays.

MODE OF OPERATION

Heat-pulse scattering Scattering is induced by the passage of a single directcurrent pulse of up to 10 A for 2-1000 ms along the front electrode of the cell. The action of this current over the whole area is to heat a bulk thickness into the isotropic phase. Rapid cooling of the liquid crystal (from the isotropic through the cholesteric to the raised smectic phase thermostatted above ambient temperature) freezes it into a light-scattering focal conic texture. The raised ambient is close to the smectic-to-cholesteric phase-transition temperature, Owing to the rise in temperature of the cell substrate after the scatter pulse has been initiated, a dwell time between scatter and writing is required to allow the liquid crystal to reach equilibrium. Writing is achieved by scanning the focused laser beam across the cell and then cooling the locally heated areas into the isotropic in the presence of a lowlevel 2 kHz bias voltage (//bias). The bias voltage is chosen to be just below the bulk erase threshold voltage measured over the temperature range considered. Thus the fastest attainable writing rates can be achieved. Scatter, erase and writing waveforms are produced by a heat-pulse drive unit. This is a single-output highcurrent driver used for single-shot operation, suitable for load resistances of 10-50 f~ with a voltage range 0-300 V and a maximum current of 10 A. The main scattering pulse written is variable from 2 to 60 ms with a voltage range 0--300 V. Heat pulse widths of 25-1000 ms are also available with a voltage range 0-100 V. The 2 kHz waveform is variable from 0 to 200 V and can be either continuous, for writing purposes, or pulsed variable from 10 to 2500 ms, suitable for high-voltage erase. For the erasure of strong bulk scattering, a preselected 2 kHz waveform can be applied at the instant the heating pulse finishes. This aligns the liquid crystal on rapid cooling from the isotropic to the smectic phase. DISPLAYS,OCTOBER1985

threshold voltages considered.

Dynamic scattering Dynamic scattering is a form of electrohydrodynamic instability which occurs as a result of ion migration. High voltages are pulsed across the cell to destabilize the smectic liquid crystal which is aligned homeotropically. The liquid crystal is doped to produce controlled electrohydrodynamic instability. At a selected raised ambient temperature, the scattering voltage is increased until uniform scattering is achieved. Bulk scattering is achieved by applying a direct-current pulse of about 300 V across the cell for 10-50 ms.

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Heat-pulse s c a t t e r i n g Typical results for line width and writing speed are shown in Fig. 3. Scattering pulse energies are in the range 100-1000 kJ m--'s-~ with pulse widths in the range 10-40 ms. There is no discernible difference in scattering texture over these ranges. Good resolution has been obtained with typical line widths of 25 lam at 8 cm sand 40°C, and 30 lam at 13 cm s -~ and 42°C in $2 + 2% DC5 + 2%CEl. Typical power consumption figures for heat-scatter pulses are 150 V and 10 A for 10-40 ms. Fig. 4b shows the typical magnitude and density of scattering centres for heat-pulse scattering. The sample shown is $2 + 2%DC5 + 2%CEI at 38°C. Thermal management of the display is difficult. Typical scatter pulses result in a rapid rise in the temperature of the device. Thermal fluctuations in the device are of the order of 2°C. This results in the display contrast being below the saturation contrast.

Writing speeds and line widths have been measured for a range of ambient temperature between 29°C and 460C. The results are shown in Fig. 2. The bias voltage was chosen on the basis of measurements of the bulk erase

Dynamic scattering Typical results for line widths and writing speeds are

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RESULTS

Unlike heat-pulse scattering, no measurable temperature rise of the outside cell substrate occurs and laser writing may follow immediately. Writing is achieved by scanning the focused laser beam across the cell, causing localized heating into the isotropic phase, and cooling in the presence of a low bias voltage at high frequency. The bias voltage is chosen to be just below the threshold voltage for bulk erase. Rapid cooling through the instantaneous nematic, under the influence of the highfrequency signal, aligns the liquid crystal (which has positive dielectric anisotropy) parallel to the field and thus in line with the homeotropic alignment. On further cooling into the smectic phase, the liquid crystal remains in a clear ordered state.

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DISPLAYS, OCTOBER 1985

209

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Fig. 3 Line speed vs line width in heat-pulse-scattered liquid crystal cells for different hosts and temperatures (cell thickness, 15 ~tm) shown in Fig. 2. Scatter voltage pulses are in the range 250-300 V and are of 25-50 ms duration. Erase pulses are in the range 125-175 V at 29 ms duration. Line widths of about 13 Ixm at 1 cm s -I and 25 Ixm at 5 cm s- 1have been achieved at 29"C and 40"C respectively. Typical power consumption figures for dynamic scatter pulses are 250 V and 0.4 mA for 30~0 ms. The anomalous results for the linewidth at low writing speeds do not appear to be an experimental artefact, but we cannot offer an explanation of these changes. Dynamic scattering gives full contrast and uniformity over the range investigated. The density of scattering centres and texture of the liquid crystal is shown in Fig. 4a. Selective erase has been demonstrated for both types of display. The overwrite, scattering over clear, is achieved by scanning in the absence of an applied field. It should be possible to achieve perfect selective erase by controlling the speed of the laser overwrite. Although a difference in contrast between laser-overwrite scattering and bulk scattering can be seen on a microscopic scale, it is not noticeable on projection. DISCUSSION The results shown in Figs 2 and 3 indicate the range of conditions under which laser writing was carried out on liquid crystal layers. Scattering was induced by thermal pulsing or dynamic scattering. A direct

210

comparison between these two methods is difficult because different hosts, dyes and dopants, displaying different transition temperatures, are required to optimize the two phenomena. In the particular systems studied, the electrically induced dynamic scattering resulted in better contrast; this appears to be related to the higher density of scattering centres that results from this method. Heat-pulse scattering probably cannot achieve saturation contrast (Fig. 4) owing to the slow rate of cooling, near the baseline temperature, which is a result of the current heating of the device substrates. Useful laser writing performance (Fig. 2) is reported for the first time on a dynamically scattered liquid crystal layer, although maximum writing speeds were below those achieved on thermally scattered layers. It is not clear whether this is fundamentally related to the differences in scattering texture between the two particular liquid crystal systems studied. The major advantages of electrically, induced dynamic scattering appear to be in power consumption and thermal management. Heat-pulse-addressed devices have high power consumption and difficult thermal management problems as a result of the large currents required during scattering to raise the liquid crystal temperature into the isotropic phase. Harter et al. s have recently reported on these problems. Dynamic scattering does not suffer from these disadvantages, since it relies on ion migration within the liquid crystal to achieve bulk scattering.

DISPLAYS, OCTOBER 1985

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On balance, we suggest that dynamic scattering appears to offer the more elegant solution to the problem of providing a practical light-on-dark laser-addressed display for direct projection.

tion liquid crystal light valves' Appl. Phys. Lett. Vol 21 (1972) p 392

Fig. 4a Texture of dynamically scattered $4 2.5%DC156 at T = 38°C

ACKNOWI.,EDGEMENTS The authors thank STL Ltd and Laserscan Laboratories Ltd for permission to publish this paper. The work has been supported by the Procurement Executive, UK Ministry of Defence (Directorate of Components. Valves and Devices), and sponsored from the Signals Research Establishment. The authors also thank BDH Ltd for providing the host smectics, A B Davey and D Coates for their help in providing working mixtures, and P Gunn for assistance in cell making. The helpful discussions and contributions of D McDonnell, A J Hughes and P Bonnett of RSRE are acknowledged.

REFERENCES 1 Melchoir, H, Kahn, F J, Maydan, D and Fraser, D B 'Thermally addressed electrically erased high-resolu-

DISPLAYS,OCTOBER1985

2 Kahn, F J 'IR laser addressed thermo-optic smectic liquid crystal storage displays' Appl. Phys. Lett. Vol 22 (1973) p I 1 l 3 Taylor, G N and Kahn, F J 'Materials aspects of thermally addressed smectic and cholesteric liquid crystal storage displays' J. Appl. Phys. Vol 45 (1974) p 4330

4 Hareng, M, Le Berre, S and Thirant, L 'Electric field effects on biphenyl smectic A liquid crystals' Appl. Phys. Lett. Vol 25 (1974) p 683 5 Tani, C and Ueno, T 'New electrothermo-optic effect in a certain smectic liquid crystal with a pleochroic dye added' Appl. Phys. Lett. Vol 33 (1978) p 275 6 Chirkov, V N, Aliev, D F, Radshabov, G M and Zeinally, A Kh 'Stimulation of electrohydrodynamic instability in the smectic A phase' Mol. Cryst. Liq. Cryst. Lett. Vol 49 (1979) p 293 7 Coates, D, Crossland, W A, Morrissy, J H and Needham, B 'Electrically induced scattering textures in smectic A phases and their electrical reverse' J. Phys. D. Appl. Phys. Vol I i (1978) p 2025 8 Harter, W, Lu, S, Ho, S, Basler, B, Newman, W and Otaguro, C 'An 80-character by 25-line liquid crystal display system' SID Digest Vol 15 (1984) p 196

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