NEW APPLICATIONS OF LIQUID CRYSTALS

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Submitted on 1 Jan 1975

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NEW APPLICATIONS OF LIQUID CRYSTALS

J. Borel, G. Labrunie, J. Robert

To cite this version:

J. Borel, G. Labrunie, J. Robert. NEW APPLICATIONS OF LIQUID CRYSTALS. Journal de Physique Colloques, 1975, 36 (C1), pp.C1-215-C1-230. �10.1051/jphyscol:1975140�. �jpa-00216219�

JOURNAL DE PHYSIQUE Cotloque C1, suppldment au.no 3, Tome 36, Mars 1975, page C1-215

Classification Physics Abstracts

7.130 - 0.640

NEW APPLICATIONS OF LIQUID CRYSTALS

J. BOREL, G. LABRUNIE and J. ROBERT Laboratoire d'Electronique et de Technologie de l'Informatique, Laboratoire de MicroClectronique, C. E. A.-C. E. N. G., Grenoble, France

R6sum6. - Les cristaux liquides ont BtB Ctudiks tout d'abord essentiellement en raison de leurs applications potentielles dans le domaine de l'affichage. De nombreux phknom6nes physiques peu- vent &tre mis en ceuvre pour cette application : la diffusion dynamique, l'effet de champ, la transition de phase, le changement de structure ...

On pensait en gknkral que les limitations fondamentales Btaient likes B la stabilitk, la vitesse, la facilitk d'adressage. Actuellement, la plupart de ces limitations ont Ct6 surmontkes, et de nouveaux domaines d'applications apparaissent, qui sont nBs soit de nouveaux effets physiques, soit .d'une comprehension plus complete du comportement electro-optique.

A titre Cexemple, un dispositif d'affichage en temps rQ1, adressC en ^x - y (128 x 128 points), est pr6sentB : c'est ulie premiere &ape vers 1'Ccran plat de telkvision. D'autres applications sont passBes en revue dans le domaine du traitement optique du signal, de l'optique intCgree ...

~bstract. - Liquid crystals were,.at first; studied mainly for their potential applications in the display field. Various physical phenomena can be used fofsucli an application : dynamic scattering, field effect, phase change, structure change ... ,The general feeling was that basic limitations were related mainly, to stability, speed, ease of addressing. Most of these limitations are now overcome and new fields of applications appear, related either to newcbasic physical effects or increased under- standing of electfo-optical behaviour. As an example a real time x - y addressed display (128 x 128) is presented, an. early 'stage to a flat TV screen. Other applications in the field of optical signal processing, integrated optics.. . are foreseen.

Introduction. - For a very long time liquid crystals emphasis on the driving problems and solutions. Then have been investigated as laborittory curiosities and we present a class o f electrooptic devices which can no application has been emerging from the various be used in optical signal processing. At last some studies. From 1888 to 1915 [I-21 the basic work led other peculiar applications are given.

to no practical appli&tion. Since thst ddate a lbt of efforts were, made to 'control surface alignments [3]

and material quality, and physical studies were made more practical. Among the three main classes of liquid crystals [4] : nematics, smectics and cholesterics, the last ones were used, for the first time, around 1950 for thermal pattern measurements. Most promising appli- cations issued mainly from 'research work done a t RCA from 1967 to 1971 [5-6-74-91 and much of the basic understanding came from this period [lo t o 271 dealing mainly with nematics. Smectics are now being studied [28] and will probably lead tb original appli- cations.

We restrict ourselves to nematic and cholesteric liquid crystals 'applications and- even ih this field it is hard to be exhaustive due to the great variety of devices we can built. Non destructive testing using thermal properties of cholesterics is not considered [lo- 29-30] because much of the work'has already been presented a t this conference in 1970 [31].

In a first part we describe the main techniques using liquid crystal cells for displays with a particular

1. Present applications of liquid crystal cells : .the display field. - Generally we are dealing with electronic displays, that is to say, we convert electrical information ilito visual information. There are two main problems to be solved ; first to use the m,osf efficient way to apply the electrical power to each element of the display (this is the addressing technique) then to 'di,splay optically the data (either using the electrical power delivered by the addressing circuitry to produce light or using ambient light). We are used to saying that the cost of a display is the cost of the driving circuitry. This is often true and care must be taken to use economical techniques minimizing, for example, the number of interconnections in the display.

As far as liquid crystals are concerned they use ambient light and are fairly'readable at high ambient light levels, but not readable in the dark [37].

1.1 WHY USE LIQUID CRYSTALS FOR DISPLAYS ? - First, we can ask the question : why use liquid crystal cells to display data ? The answer is in the specific properties of such a display, figure 1 :

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1975140

C1-216 J. BOREL, G . LABRUNIE AND J. ROBERT

-EASY TO BUILD (LOW COST) 1 IONS MOTION~FIELD EFFECT -LARGE RANGE OF CI-IARACTER SIZES

NEMATICS -GOOD CONTRAST

L.C. DISPLAYS

-SMALL POWER CONSUMPTION

CHOLESTERICS -TRANS~~IISSIVE OR REFLECTIVE MODES

FIG. 1. - Why use L. C. cells to display data ?

D. S.M.

D.S.M

- It is very easy to build and use cheap materials requiring less care than semiconductor devices for the technology.

- A large range of character sizes and presentations are easily achievable (segmented or dot-matrix cha- racters).

- A good contrast is generally obtained though there still remains some problems concerning the viewing angle.

- Small power consumption is related to the fact that they do not produce light and are generally high resistivity materials. Moreover, the required driving voltage levels are fairly compatible with integrated circuits and particularly with MOSFET integrated circuits, this technology being the cheapest nowadays.

- Transmissive or reflective modes of operation can be used depending on the particular application and some configurations allow building color displays.

F . E . T . N . (kIEMORY)

Some of the properties can be necessary or inaccep- table in peculiar applications, but the general feeling is that for most of the possible uses the price is the most sensitive criterion and in this respect liquid crystal displtiys are potentially highly competitive.

F?S=F;C.SeHS

1 . 2 PHYSICAL EFFECTS. - Let us now very briefly review the main physical effects used in liquid crystal displays (Fig. 2). Basically they are of two kinds : either related to ion motion within the structure (electrohydrodynamic effects [5,6a, 181) or to collective alignment of molecules in the electric field with their average director orientation at rest either in the direction of the field (anisotropy of the dielectric constant AE = E~~ - positive) or normal to the field (AE < 0). The medium being strongly bire- fringent (An = ne - no

-

Both arrangements are used in display cells and are

D.S.M. : DYNAMIC SCATTERING MODE F.E.: FIELD EFFECT

T.N. : TWISTED NEMATICS

F.C.S. : FOCAL CONIC STRUCTURE P. S . ^: PLANAR STRUCTURE

H.S. : HOMEOTROPIC STRUCTURE MAIN PHYSICAL EFFECTS FIG. 2. - Main physical phenomena involved..

transparent without external applied field. The driving voltage, in every case, changes the molecular arran- gement :

- Either by disturbing the texture by ion motion.

This gives a very strong diffusion of the incident light in nematics and the effect is known as dynamic scatte- ring mode (D. S. M.) [6a, 7, 81. If small amounts of a cholesteric liquid crystal are added, a memory of the disturbed state is seen [6b]. Generally molecules with AE < 0 are used.

- Or by changing the direction of molecules align- ments (with AE > 0 or AE < 0) either in a parallel oriented texture (field effect) or in an helicoidal texture : twisted nematics (T. N.) or cholesterics. In a parallel oriented texture, birefringence is induced by the electric field [36] ; cholesterics are arranged in an helicoidal texture with a pitch P along which the orientation of molecules changes gradually. In thin cholesteric layers two different kinds of textures can be found.

- The focal conic texture : the orientation of the helix axis is parallel to the plates, but generally random.

- The planar texture : the helix axes are oriented perpendicular to the glass surface.

The focal conic texture can scatter light and the planar texture is transparent. Texture changes from the planar structure to the focal conic structure (AE > 0) or even to the homeotropic structure can be obtained [37]. The texture change between planar structure and focal conic structure can be induced by ion motion (memory D. S. M.) and the reverse change by field effect (molecules with AE < 0) [38].

Among these physical effects some are preferred for the following reasons :

- They can be driven by low voltages : twisted

NEW APPLICATIONS OF LIQUID CRYSTALS

Comparison of typical electrooptic effects in L. C .

Type of D. S. M. F. E. T . N . P . S . e F . C . S . F . C . S . S H . S . Remarks effect

Property

- - - - - - -

Threshold voltage (V) 5 to 10 1 to 4 0.9 to 4 1391. 20 V/pm A. C. or D. C. drive

As (molecules) neg neg or pos POS POS POS

Current (PA cm-2) 10 1 1 1 1 D. C. drive

Response : time

<< on )> 10 to 20 ms 10 ms 5 ms 30 ms - For 1.15 V driving voltage

cc off ^)> 100 to 2Q0 ins 30 ms 200 rns I ms -- ^{100 ms} Depend on cell thickness

Slope at threshold poor good good good poor For multiplexing

f 45O f 20° f 45" f 45O f 45O A1

Viewing angle At--- _I 50 %

Memory Contrast

nematics for example because the,dielectric anisotropy ,SLOPE AT THRESHOLD of L. C. can be very large.

- They have a wide viewing angle : D. S . M. or -SPEED (RISE AND DELAY TIMES)

T. N. -VIEWING ANGLE

- They can be used for color displays F. E.

- They have an internal memory : D. S. M. using

a mixture of cholesterics and nematics, P. S. to F. C . S. -OUTPUT PADS transition.

- They can be multiplexed in their addressing. All the materials used are high resistivity materials and very low power consumption is required. The main features of the displays using these physical effects are summarized in the following table : table I.

1 . 3 ADDRESSING TECHNIQUES. - Contrast is a func- tion of the addressing technique, the electrooptical effect used and is generally greater than 20.

As previously mentioned an important feature of a display, in general, is its ability to be multiplexed.

This property has a direct incidence on the number of interconnecting pads for the display and on the complexity of the driving circuitry, two parameters that influence the cost per character displayed within a system. Particular care, mainly for large size displays, must be taken to use a cheap addressing technique. As far as liquid crystals are concerned several problems arise : figure 3.

- The slope of the electrooptic effect at threshold is generally poor and crosstalk between addressed and non addressed dots can be significant [40].

- The rise time, for driving pulse values compa- tible with integrated cricuits, is relatively long (10 ms range) compared with the decay time (100 ms range) ^A allowing few characters to be multiplexed.

- The viewing angle may depend on the fact that we use multiplexing or not (for a given driving voltage level multiplexing decreases the viewing angle).

- Lifetime has been found to be very sensitive to the shape of the driving voltage (D. C., A. C. low or high frequency). Reasonable values are now obtained

NBer O F O U T P U T PADS

0 PARALLEL

/

5 0 7 SEGMENTS

4 0 DISPLAYS

I : ; : ; : : : ; ! ; 3

0 5 10 NBer OF

CHARACTERS

PROBLEMS IN L.C. DISPLAYS ADDRESSING FIG. 3. - The addressing technique : the problems.

when avoiding D. C . drive (from 10 000 hours for D. S. M. to 50 000 hours expected for field effect).

- As mentioned, the number of output pads must be reduced to a minimum.

Figure 3 also shows the incidence of the addressing technique on the number of output pads as a function of the number of characters in the display. It is seen that this number quickly becomes prohibitive for parallel addressing, a particular situation to avoid when possible.

From these considerations several techniques have been proposed and are summarized figure 4. They use either a build in parallel addressing, a x - y addressing

C1-218 J. BOREL, G . LABRUNIE AND J. ROBERT ,PARALLEL ELECTRONIC ADDRESSING

(ELECTRON BEAM) ,X-Y ADDRESSING (L.C. MEMORY)

,PARALLEL ELECTRONIC ADDRESSING (I.C. MEMORY) P A R A L L E L OPTICAL ADDRESSING (RCJ

-1.C. MOUNTED ON L.C. CELL

ADDRESSING TECHNIQUES

FIG. 4. - Addressing techniques : solutions.

or an electronic addressing directly mounted on the display. This last technique combined with a x - y addressing of the liquid crystal cell is certainly the most economical.

Let us now describe in more details each of these techniques.

provided the electrical connection between the address- ing electron beam of the cathode ray tube and chro- mium dots acting as reflective electrodes. A mosaic of 3.1 cm x 3.1 cm was used and anode potentials of 20 to 25 kV were applied. The resolution of the display is approximately 150 to 175 lines over the 3.1 cm- square viewing area and contrast ratios of 7.511 were measured with + 450 viewing angle. More recently [42]

such a technique has been improved and the following results have been obtained :

- mosaic : 70 mm x 95 mm with conducting wires on 160 ym centers ;

- electrooptic effect : D. S. M. ;

- addressing conditions : T V standard, 625 lines, 25 images per second ;

- measured resolution : 300 x 400 dots ; - contrast ratio > 1011 ;

- applications : TV image projection or informa- tion display in high ambient light level.

An example of the operation of this display is given in figure 6.

The limitations of this technique are related to the fact that :

- vacuum is needed for the electron beam ; - flat displays cannot be built.

1 .3.1 Electron beam parallel addressing. - The elec- tron beam parallel addressing was first proposed by van Raalte in 1968 [41]. He used a liquid crystal cell (Fig. 5) sandwiched between a standard transparent electrode (reflective mode) and a mosaic face-plate.

The mosaic was a piece of thick glass in which were embedded 25 ym wires on 100 ym centers. This mosaic

MOSAIC

1 LIQUID CRYSTAL NICKEL DOTS

1

'I

CONDUCTIVE COATING VAN RAALTE 1968 ELECTRON BEAM PARALLEL ADDRESSING FIG. 5. - The parallel electronic addressing using an electron

beam.

FIG. 6. - 300 x 400 dots resolution E. B. addressed display.

I . 3 . 2 x - y-addressing. - The x - y electronic addressing allows a solution to these two problems but other limitations occur, as will be seen later. Generally, we use a strong non linearity in the optical response versus control voltage. An example of such a non- linearity is given in figure 7 corresponding to the relative light intensity versus the applied voltage for a D. S . M. operation. A high relative light intensity is obtained when applying half the voltage to each electrode of an addressed dot (or a similar value : one-third-select addressing scheme [43 to 471.. .). Two problems arise :

- We need a sharp threshold in the effect to avoid cross talk between adjacent dots.

- Cumulative effects on a non-addressed dot alter the contrast ratio.

NEW APPLICATIONS OF LIQUID CRYSTALS C1-219

RELATIVE TRANSMISSION The following results have been obtained :

THE X. Y. ELECTRONIC ADDRESSING FIG. 7. - The x - ^y electronic addressing.

'

0

The best situation is to use a pulsed a. c. addressing avoiding D. C. current degradation ; polarity symme- try [40, 50,651 gives a constant contrast ratio whatever the displayed picture may be. Depending on the nature of the electrooptical effect used, we obtain a more or less sharp threshold and cumulative effects always limit the contrast ratio.

f -

FIG. 8. - x - Y addressed display.

0 - Vo Vo VOLTAGE 2

Number

Electrooptical effect of colunms Contrast ratio Reference

- - ^-

D. S . M. 8 to 16 z 10 15 1 -401

T. N. 32 > 10 [@I

F. E. 50 > 10 (color) [40-511 C. N. transition 128 < 10 1 4 1

As an example we see, in figure 8, two kinds of x - y addressed displays built by the Thomson-CSF laboratory in Corbeville [51].

The first one is an 8 character multiplexed D. S. M. display driven by 15 V with a power-consump- tion of 10 mW (electronic circuitry -I- display).

The second one is a x - y F. E. matrix addressed display (12 x 18 characters of 35 dots). The supply voltage is 18 V and 2 imagesls can be displayed (size 60 mm x 60 mm).

Improvements are still possible in this field, but for larger displays, the main limitation is related to the ratio between rise time and memory time within the liquid crystal and, as mentioned, to the cross talk between dots. To increase the number of images/second erasing techniques can be used in certain configura- tions by utilizing dielectric relaxation versus fre- quency [52] or field effect erasing of D. S. M. [24], [471, [951.

1 .3 .3 Integrated circuit parallel addressing. - The I. C. parallel addressing was first proposed in

1971 [53] with a static MOSFET shift register for the parallel addressing of the L. C. display dots. Other configurations can be used for a larger,matrix needing less electronic circuitry [53-541. The liquid crystal is directly placed on top of the I. C. where metal dot eIect~o4es are evaporated using standard I. C . techno- logy. The layer of the liquid crystal is deposited on top of t h e j . C . and sandwiched between the evaporated metal dot electrodes, driven independently by the memory elements, and an upper nesa coated cover electrode. The addressing circuitry uses either static [53-541 or dynamic memorization [53]. In this technique each dot is driven in parallel by the elec- tronic circuitry, solving :

- the threshold problem (no voltage on unad- dressed dots),

- the speed problem (the electronic circuitry can be addressed in a time much shorter than the rise time of the liquid crystal).

On figure 9 we have shown two particular MOSFET circuits with very few (1 or 2) devices per dot. Both schemes use dynamic storage of the address in memory capacitance CM with either a D. C . (9b) or a D. C . or A. C. drive of the liquid crystal (9a). Grey scales can be obtained with analog dynamic storage of the driving signal.

C1-220 J. BOREL, G . LABRUNIE AND J. ROBERT

These displays are particularly attractive for

L.C. CELL a ) A.C. OR D.C. DRIVE

O - * ~ & L . C . CELL

b) D.C. DRIVE

CM MEMORY CAPACITANCE 'THE I.C. PARALLEL ADDRESSING

( WbIAMIC MEMORY l

FIG. 9. - The I. C. parallel addressing.

As an example, figure 10, gives a view of a 1 inch x 1 inch section of the Hughes liquid crystal display using the address circuitry of figure 9b. The main characteristics are :

- 100 x 100 dots (MOSFET circuits drive),

- 1 inch x 1 inch size,

- D. S. M. operation (D. S. M. mode).

The main limitations of this technique are :

- the yield problem of very large L. S. I. arrays, - the small size of the display.

FIG. 10. - I. C. parallel addressing of a 100 x ¹⁰⁰ display.

- watch displays (small size, very low power consumption),

- projection displays (reflective mode),

- good visibility under strong illumination.

1 .3.4 Optical parallel addressing. - An other parallel addressing techniques uses a sandwich of a photoconductor and a liquid crystal. Several mate- rials have been used for displaying the data : nema- tics [55, 56, 571 or cholesterics [12, 21, 36, 38, 58, 59, 60, 611. Illumination with various optical wavelengths has been done : such a cell structure built by Xerox [38]

is given in figure 11. The local conductivity change

FIG. 11. - Parallel optical addressing using a photoconductor.

of the photoconductor, due to imaging light, changes the voltage locally applied to the L. C. sandwich, causes an electrooptical effect (D. S. M., F. E. or F. C. S. to P. S.) change. One of the main problems is due to chemical and electrochemical stability of the cell ; the photoconductor and the liquid crystal generally operate under D. C. for good sensitivity. The photoconductor used must be chemically inert with regard to liquid crystal and alloys containing selenium have been proposed to solve this problem [38]. A. C . operation has also been reported [60]. The speed pro- perties do not depend on the photoconductor charac- teristics and are limited by the liquid crystal itself.

The main results obtained, using a focal conic structure to planar structure change in a mixture of dielectrically negative nematics and cholesterics, are given table IS.

TABLE I1

Characteristics of the Xerox display [38]

Resolution . . . 40 I/mm

Sensitivity . . . 25 ergs/cm2 (actinic) Storage time . . . 24 hours for 30 1/mm Contrast ratio . . . 711

Read-out efficiency . . _{10 %}

NEW APPLICATIONS OF HQUID CRYSTALS C1-221

loan. o f copy- d r w f o r in 1 its t h have im into be

ing a - d ma

r o g r a ~ h y 50. Xerox troduced

rnenze adv en i n c o r ] rcodern o f chine coa

Figure 12 is an image on storage panel projected from 5 output pads and act as a buffer memory with into a screen. each output connected to each segment of a 9 cha- This technique of parallel addressing is very attractive racter display (63 pads). D. C. drive was used to

for : simplify circuit technology, but A. C . drive is possible.

- projection, Such a technique can also be used to make x - y

- optical amplification, addressing on the L. C. display. This would be the

- optical frequency change (between incident and solution minimizing the overall number of inter- projected light), but requires an optical image or a connexions : 4 output pads and (7 + N ) = 16 internal light deflector for the source. connections (N being the number of characters). Let us point out that the number of output pads is inde- ent of the number of characters within the display.

reviously mentioned, price per digit is the most rtant parameter to compare several technologies isplays. This has to be done within acceptable limits of readability, power consumption, and ease to decode. We give in figure 14, as an example, the

PRICE PER DIGIT ($1

- 8 DIGITS L . C. DISPLAY c.475" ) 5 DIGITS L E D DISPLAY (.11") 15 . ^{. .} . ^{.I DlGlTS} LED MODULE ( 4")

10

( FEBRUARY 7 3 ) FIG. 12. - Example of optically addressed L. C. display.

5 1 .3.5 Integrated circuit on L. C. displays. - For 0 displays using a limited number of characters (say

10 to 12), the driving circuitry can be mounted directly on the glass plate carrying the electrodes pattern. Very few output pads (say 4 to 5) are needed to supply serial information to the integrated circuit.

Then this data is stored in the electronic circuit and COST COMPARISON O F

used to drive the appropriate character configuration. _LED'S

AND L C DISPLAYS

Due to the low cost of large scale integrated circuits

this seems to be a very attractive and economical FIG. 14. - Cost comparison of LEDS and L. C. displays.

solution. Such a display using D. S. M. has been proposed by Gerristma and Lorteye [62] and is

shown in figure 13. Three integrated circuits in ILL price per digit versus the number of units delivered technology receive power supply and input data for two kinds of LED's displays and a L. C. display built by two representative manufacturers. As can be seen, ,the price per digit is similar for LED's and L. C. cells and can decrease very significantly as a function of the number of units sold. The character size of L. C. cells can be varied easily without any significant increase in price. In figure 14 we give data for a liquid crystal cell with characters 0.475" in height. LED'S displays have generally (mainly for several characters) smaller sizes than 0.11". Let us mention that these prices are related either to a well established technology (LED's) or to an emerging technology (L. C.). It is believed that the tendency will be to decrease L. C. display prices by a greater amount

l.- than LED'S displays, the limiting factor for LED'S

B being the cost of the semiconductor material. This is just a matter of time. For large character displays FIG.. 13. - L. C. cells with I. C. drive circuitry. or panels L. C. are now the best solution.

Cl-222 J. BOREL, G. LABRUNIE AND J. ROBERT These economic considerations must be taken in

the light of the potential market. We give, in figure 15, some information on the needs in digitslyear for diffe- rent fields of application related to LED'S and L. C . displays. The most important needs are for clock radios, hand-held caIculators,. and electronic watches where both techniques are well suited. Other fields of application are instrumentation indicators, auto- motive, appliances.. . If we consider an asymptotic price of 1 $ per digit, this corresponds to a potential market ranging around one billion dollals a year in 1980. This is to be compared with the computer market at that time (non IBM market) evaluated to 40 billions dollars and the computer peripherals market evaluated to 15 billions dollars.

ELECTRONICS MAY 3 0 - 1974 FIELD

AUTOS CLOCK

RADIOS HAND - HELD CALCULATORS APPLIANCES

TOTAL

DIGITS / YEAR 36 MILLION SEVERAL HUNDRED

MILLION (1977) SATURATION MARKET

3 0 0 TO 4 0 0 MILLION 30 MILLION (1978) 5 0 0 TO 1000 MILLION

DIGITS / YEAR

ELECTRONICS MAY 22 1972 ELECTRONIC

WATCHES

POTENTIAL MARKET FOR LEOS AND L.C.

400 MILLION (1980)

FIG. 15. - Potential market for LEDS and L. C.

2. Advanced eleetrooptic devices using L. C. -

2.1 THE << TIME SELECTION ^>) MODE OF ADDRESSING. -

Liquid crystal cells are generally used for displays appli- cations. They present electrooptic characteristics sui- table for this kind of operation : contrast, rise, and decay times ... For particular applications where I. C . compatibility is not a major criterion, they have very different performances mainly connected with electric driving. Let us first define what we call time selection in figure 16 [49, 50, 651 : when a L. C . cell initially homeotropic is driven by a voltage V such as V 9 V,h (Vth = threshold voltage for electrooptic effect), the induced birefringence (for AE < 0) is related to pulse width t and a time constant T by an exponential relationship. The main parameters involved are defined in figure 16. This time constant T is related to the

INITIALLY HOMEOTROPIC STRUCTURE ( A & < ^{0 )}

A N ( t ) = AN00 EXP (B)

T I F AN(t) < ^{AN MAX}

WITH ANoo = THERMALLY INDUCED BIREFRINGENCE T *TIME CONSTANT

-

V =CELL APPLIED VOLTAGE t = DRIVING PULSE WIDTH

" TIME SELECTION" PRINCIPLE FIG. 16. - Time selection principle.

driving voltage and to physical parameters such as for T, (rise time), see figure 17 :

- AE the dielectric anisotropy, - y , a viscosity coefficient,

- E the applied field

A E . E ~

T; E -

4 n.3'1

LIGHT INTENSITY ( RELATIVE

t e * I O p m

I v . 2 0 0 VOLTS

-1;-

APPLIED VOLTAGE ( V 7 7 V t h ) 4 h Y 1

T m = A E . E 2 T d ' 4h. *d .1 A E . ^EthZ

RISE - STORAGE - DECAY TIMES IN HOMEOTROPIC CELLS (A& < ^{0 1}

BETWEEN CROSSED POLARIZERS

Fao. 17. - Rise-storage-decay times in homeotropic cells (As < 0).

NEW: APPLlCATIONS OF LIQUID CRYSTALS C1-223

and for T, (decay time) : These response time values (for the rise time) are

2 K 3 3 AE.E; very different from the one generally considered in

~^d -= L l . ^- displays. This means that, taking into account the

~ ~ 6 7 1 4W1 very different values of rise and storage plus decay times, complex displays can be addressed. In figure 17 - K , , an elastic constant in Frank's notation. we have represented the transmitted light through a

- L cell thickness. nematic liquid crystal cell between crossed polarizers.

- E,, = - vt, = field at threshold. With the typical values for cell thickness and driving L pulse characteristics used, a rise time t , of the electro- Let us give an order of magnitude of the parameters optical effect of 30 Ps is ~ e a s ~ r e d and decay times involved in these conditions. t, of 10 ms or more are seen. This shows that a ratio For M. B. B. A. where no = 1.545 and n, = 1.755 for of 3 ,

_

A = 6 328 A (ordinary and extraordinary indices) and t, 30 ps

L = 5 pm, V

-

ceable for transmitted light on the first line. This ratio is related

t ci10 to 30 ps to time constants T, and T, (Fig. 17). Driving a. c.

pulse is generally used to avoid ion migration and and storage time plus decay time ranges around 5 ms. increase cell lifetime. It can be shown that, in this case, the contrast ratio is independent of the addressed data configuration [49, 50, 651.

One of the main limitations is due to decay time.

In the previous example, a minimum image frequency of 100 Hz is expected. As shown, the decay time (time constant T,) in a homeotropic cell with AE < 0 depends only on cell and L. C . characteristics. To reduce, or to control, such a decay time, slightly diffe- rent cell thicknesses and L. C . materials must be taken. We use, in this case, relaxation of the parallel component of the dielectric constant versus fre- quency [52]. Starting from a homogeneous structure, a low frequency pulse (frequency lower than the critical frequency fc defined for A& = 0 ; fc

-

Cl-224 J. BOREL, G. LABRUNIE AND J. ROBERT Figure 18 the characteristics of the cell are :

- cell thickness : L = 10 pm,

- liquid crystal : A. B. A. B. N. mixture (initially in homeotropic structure),

- H. F. pulse amplitude : 100 V, - H. F. pulse frequency : 200 kHz, - M. F. pulse amplitude : 20 V,

- M. F. pulse frequency : 3 kHz,

- maximum pulse rate : 250 Hz.

Using this technique image frequencies higher than 1 kHz are expected.

2.2 EXAMPLES OF APPLICATIONS. - 2.2.1 A 128 x 128 digital data composer. - A 128 x 128 dots x - y addressed display using time selection has been built to demonstrate the feasability of large displays.

A homeotropic cell with M. B. B. A. is used to display a black and white image (Fig. 19). Typical addressing conditions are 1651 :

- Driving voltage on rows : 120 V (null mean value).

- Driving voltage columns : + 10 V (null mean value).

- Pulse width : 35 ps.

- Image frequency : 25 image+.

- No erasing.

The left part of figure 19 shows a black and white image on a TV monitor and the right part shows the same image displayed by the nematic L. C. cell. The difference in vertical sizes is due to a difference in the vertical sampling on the TV monitor and on the L. C . cell.

This figure shows the state of the art in flat panel displays or electrooptic interfaces using liquid crystal cells.

2.2'. 2 Analog imaging. - The quality of such an electrooptic interface can be increased easily using analog operation of the L. C. cell. As an example analog operation is represented in figure 20 where we have plotted the induced birefringence versus pulse width (a similar result is obtained if we use phase shift between x and y addressing pulses). Input signal quantification on 16 levels has been used to demons-

An (t) V = 150V

CELL THICKNESS = 16jm

ANALOG CHARACTERISTICS OF OPTICAL BIREFRINGENCE VERSUS INPUT PULSE WIDTH

FIG. 20. - Analog characteristics of L. C. cells.

trate feasability of analog addressing [66] and figure 21 is an example of an arrangement to obtain a real time Fourier transform of an analog signal. In this case the analog signal versus time (x direction) is quantified with 64 stripe cells (4 mm x 100 pm for example) in the pupil plane and the spectrum appears in the spec- tral plane where it can be analyzed or filtered. Crossed polarizers are used to change the phase signal into an amplitude signal and linearization can be obtained by electronic processing of the input signal. In the spectral plane, the observed spectrum corresponds to the convolution product of the input signal spectrum by the diffraction function of one column and the diffrac- tion function of the pupil.

u.

REAL TIME FOURIER TRANSFORM

FIG. 21. - Real time Fourier transform.

With an electronic buffer, memory signals can be analysed in a frequency range extending to around

[Celectronic sampling

2

I -

with image frequencies ranging from 10 to 100 Hz.

Signal processing can be done with such an electro- optical device as will be seen later.

2.2.3 Deflecting devices. - Refractive index changes can be used in image deflection [67-681. A very simple arrangement using a L. C. cell and two prisms has been done in figure 22 and uses a known principle [69, 70, 711. Total internal reflectioln is achieved depending on the refractive index of the liquid crystal cell compared to the prisms refractive index. We control the optical index of the L. C. medium by an a. c.

electric field applied between two transparent elec- trodes. With the correct direction of polarization of the input beam relative to the rubbing direction and with no applied field maximum, transmitted light is obtained for the Brewster angle. When molecules are tilted by an a. c. field in the incidence plane, total reflection is seen. Such a device presents the same dynamic characteristics as those given for the x - y addressed display :

NEW APPLICATIONS O F LIQUID CRYSTALS C1-225 Direction of input light Deflected beam

~ o l a r i z a t ion .

beam Indium oxiae

transparent electrodes

IMAGE DEFLECTION USING L. C. SANDWICHES FIG. 22. - Image deflection using L. C.

- access times from 80 ps to several ms (depending on L. C., L. C. thickness and operating voltage),

- rise time from 35 ps to several ms (same remark),

- optical contrast on both transmitted and deflected beams : 5011,

- overall optical efficiency (internal) near 97 %.

This device can be used in several applications :

- image deflection : either black and white as in figure 23 or color images,

- page addressing in optical memories : it is pos- sible either to switch between two different page composers or to address different pages of an optical memory using this technique.

FIG. 23. - Deflected image in the cell of figure 22.

The main advantages of L. C. cells used in electro- optic devices are the following :

- Rise and decay times of the electrooptic effect can be small (10 ps range) if 100 to 200 V a. c. pulses are used.

- Strong eIectrooptic effects are seen, even in thin L. C. cells.

- Very low light absorption is measured (< 1 %

in 10 pm thick cells).

- Large cells can be built easily and x - y addressing either in digital or analog operation can be used.

2.2.4 Optical information processing. - An example of a very general situation for optical signal processing is shown in figure 24. A distorted image is placed in the

F O ~ R I E R TRANSFORM I FOURIER TRANSFORM LENS I LENS 2

DISTORTED I L . C . I FILTERED I M A G F , 4 ^FILTER 4 IMJAGE

COMPUTER

f - l

EXAMPLE b~ OPTICAL IMAGE

PROCESSING USING ANALOG L.C. C E L L S FIG. 24. - Optical signal processing using analog L. C. cells.

object plane of lens 1 and in the image plane a filter (made of a L. C. cell) is placed and driven by an electronic buffer memory. This electronic buffer memory stores the data for the optical filter depending on the nature of defects in the image. The filtered spectrum of the distorted image is in the object plane of lens 2 and the filtered image is seen in the image plane of lens 2. A computer can be used to compute, in real time, various masks depending on the filtered image quality. This general scheme can be applied in various cases :

- The L. C. filter represents an optical filter for a distorted image coming from a scanning electron microscope, a transmission satellite ...

- The L. C. cell can be used to make a Fresnel lens with continuous variable focal length (autofocus rectifier).

The focal distance is given by the computer depend- ing on the quality of the filtered image measured by a photodetector : contrast ratio in particular. So we realize an automatic focusing :