Application of antiferroelectric liquid crystals with high tilt

Application of Antiferroelectric

Liquid Crystals with High Tilt

Koen D’havé

Promotor: prof. dr. ir. H. Pauwels

Proefschrift ingediend tot het behalen van de graad van

Doctor in de Toegepaste Wetenschappen: Elektrotechniek

Vakgroep Elektronica en Informatiesystemen

Voorzitter: prof. dr. ir. J. Van Campenhout

Faculteit Toegepaste Wetenschappen

Academiejaar: 2001-2002

i

Acknowledgement

ii

Table of contents

1 Introduction 1

1.1 The current display market . . . . . . . . . . . . . . . . . . . . . . . . . 1

1.2 Liquid crystal displays . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.2.1 The liquid crystal phase . . . . . . . . . . . . . . . . . . . . . . 3

1.2.2 Liquid crystal displays; layer by layer . . . . . . . . . . 6

1.3 Quantification of the image quality . . . . . . . . . . . . . . . . . 10

1.4 An overview of this work . . . . . . . . . . . . . . . . . . . . . . . . . . 12

2 Ferroelectric and antiferroelectric liquid crystals 15

2.1 SmC and SmC* phases . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.1.1 Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

2.1.2 Dielectric tensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

2.1.3 Optical properties . . . . . . . . . . . . . . . . . . . . . . . . . . 21

2.1.4 Ferroelectric liquid crystal displays . . . . . . . . . . . 22

2.2 The SmCa and SmCa* phases . . . . . . . . . . . . . . . . . . . . . . . 28

2.2.1 Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

2.2.2 Dielectric tensor . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

2.2.3 Optical properties . . . . . . . . . . . . . . . . . . . . . . . . . . 33

2.2.4 Antiferroelectric liquid crystal displays . . . . . . . 36

2.3 The first goal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

3 Alignment of AFLCs 39

3.1 Prototype cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

3.2 Optimization of the buffing parameters . . . . . . . . . . . . . . 42

3.3 Compensating the rubbing directions . . . . . . . . . . . . . . . 45

3.4 Obliquely evaporated SiOx . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

3.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

iv

4 Orthoconic antiferroelectric liquid crystals 53

4.1 Uniaxial anticlinic conditions . . . . . . . . . . . . . . . . . . . . . . . 53

4.2 Isotropic anticlinic conditions . . . . . . . . . . . . . . . . . . . . . . 56

4.3 A solution for the dark state problem of an AFLCD . . . 56

4.4 Orthoconic antiferroelectric liquid crystals . . . . . . . . . . . 58

4.5 Phase modulation by means of OAFLCs . . . . . . . . . . . . . 61

4.5.1 Polarisation switches . . . . . . . . . . . . . . . . . . . . . . . 61

4.5.2 An alternative construction for an OAFLC

display . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

4.5.3 Ternary phase modulation . . . . . . . . . . . . . . . . . . 64

4.6 The pretransitional effect in AFLCDs . . . . . . . . . . . . . . . . 70

4.7 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

4.8 The second goal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

5 Reflective AFLCDs 75

5.1 Normally bright mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

5.2 Normally dark mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

5.2.1 A λ/4 film between liquid crystal layer

and mirror . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

5.2.2 A λ/4 film between polariser and

liquid crystal layer . . . . . . . . . . . . . . . . . . . . . . . . . 80

5.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

5.4 The third goal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82

6 Light scattering polymer dispersions of OAFLC 83

6.1 Polymer Dispersed Liquid Crystals . . . . . . . . . . . . . . . . . 83

6.2 OAFLC in a polymer matrix . . . . . . . . . . . . . . . . . . . . . . . . 85

6.3 The influence of the material parameters . . . . . . . . . . . . . 86

6.3.1 Extinction in a light scattering medium . . . . . . . . 86

6.3.2 Anomalous diffraction . . . . . . . . . . . . . . . . . . . . . . 87

6.3.3 Influence of the tilt angle . . . . . . . . . . . . . . . . . . . . 91

6.3.4 Influence of the birefringence . . . . . . . . . . . . . . . . 93

6.3.5 Viewing angle dependency of the transparent

state . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93

6.3.6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96

7 Conclusions 97

7.1 Achievements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

7.2 Some remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98

v

A Data sheets 99

A.1 CS4001 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99

A.2 W107 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

A.3 W107a . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102

A.4 W107b . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

A.5 W123 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

A.6 W124 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

A.7 W129 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

Bibliography 113

vi

List of figures

5

1.1 A schematic comparison between the nematic (N), the cholesteric

(N*) and the smectic A (SmA) phases. . . . . . . . . . . . . . . . . . . . . . . . .

6

1.2 A schematic cross section of the pixels of a typical liquid crystal

display. The purpose and the materials used for the different

layers are further explained in the text. . . . . . . . . . . . . . . . . . . . . . . .

10

1.3 A schematic comparison between (a) a passive matrix and (b) an

active matrix. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

13

1.4 The wavelength dependency of the transmission of a birefringent

layer between crossed polarisers. The optical thickness of the layer

was optimised for a wavelength of 550 nm. The best choice is k=0,

corresponding to a optical thickness of λ/2. That plot shows least

curvature and hence has the lowest wavelength dependency. The

curve with k=1 corresponds to an optical thickness of 3λ/2. . . . . .

16

2.1 A schematic illustration of the difference between (a) the SmA and

(b) the SmC phases. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

16

2.2 A representation of a full pitch length of the helix in the SmC*

phase. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.3 A review of the phenomenological angles. . . . . . . . . . . . . . . . . . . . . 18

2.4 The direction of the optic axes for a ferroelectric liquid crystal.. . . 23

23

2.5 The bookshelf geometry of an SSFLCD and its electro-optic appli￾cation between crossed polarisers. . . . . . . . . . . . . . . . . . . . . . . . . . . .

25

2.6 The chevron profile in an SSFLCD and the stable states in which

the material can be switched. In order to keep a constant layer spa￾cing at the surfaces, the layers have to tilt when the layer thickness

decreases due to the increasing tilt angle of the material. If this

phenomenon occurs in opposite directions at both surfaces a che￾vron with a sharp tip is created. For the situation represented here,

without surface pretilt, both chevron directions are equally proba￾ble and optically identical. If both directions are present in one

cell, they give rise to the characteristic zig-zag or lightning defects.

27

2.7 The operation of the Twisted Smectic Mode in the SmC* phase.

This mode is similar to the twisted nematic mode. The electro￾optic behaviour is ‘normally bright’.. . . . . . . . . . . . . . . . . . . . . . . . . .

viii

28

2.8 The operating principle of the V-shaped switching mode in the

SmC* phase. Note that the choice of the directions of polariser and

analyser differ from the TS-mode. The electro-optic behaviour is

also reversed. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

29

2.9 A schematic illustration of the difference between (a) synclinic and

(b) anticlinic behaviour. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

30

2.10 A representation of a full pitch length of the helix in the SmCa* pha￾se. The helix represented here is an idealized structure. To be more

precise the director in adjacent layers of the unit cell does not make

a phase angle difference of exactly 180°. . . . . . . . . . . . . . . . . . . . . . .

32

2.11 The position of the glide mirror plane and the symmetric deforma￾tion with respect to the ideal anticlinic structure. When the symme￾tric deformation reaches 90°, one obtains a synclinic state. At that

moment the angle ψ describes the phase angle of the SmC phase.

This description will allow us to determine the direction of the

principle axes and their matching refractive indices in a more sim￾plified manner. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

2.12 The direction of the optic axes for an antiferroelectric liquid crystal. 35

2.13 The working principle of an AFLCD. . . . . . . . . . . . . . . . . . . . . . . . . . 37

41

3.1 An adapted buffing machine which allows for easy control over the

buffing directions on the substrates. . . . . . . . . . . . . . . . . . . . . . . . . . .

43

3.2 An image of the alignment for (left) parallel and (right) anti-parallel

assembly. The rubbing directions as well as the directions of

polariser and analyser are indicated in the top right corner of each

picture.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

43

3.3 The alignment (left) obtained for a cell of which only one substrate

received an optimal buffing treatment. The right part of that picture

shows the deformation of the structure when applying even small

electric fields. The other picture (right) shows the structure after

prolonged addressing of such a cell.. . . . . . . . . . . . . . . . . . . . . . . . . .

45

3.4 The straightening of the smectic layers by an electric field. The che￾vrons, which are created due to shrinkage of the layers during cool

down, endure a straightening torque. Instead of creating a books￾helf structure, a defect structure in the plane of the cell, which is cal￾led the striped texture, is obtained. Between crossed polariser one

sees a grid of alternating bright and dark lines. . . . . . . . . . . . . . . . .

47

3.5 Rubbing and assembly of the substrates for compensation

experiments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

48

3.6 Structures obtained for (a) a too small, (b) an almost ideal and (c) a

too large angle between the buffing directions. When the directions

on both substrates are switched we obtain a structure as shown

in (d). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

50

3.7 Typical texture for hybrid cells for which the evaporation angle is

(a) lower than 70°, (b) between 70° and 83° and (c) higher than 83°

with respect to the substrate normal. . . . . . . . . . . . . . . . . . . . . . . . . .

ix

55

4.1 The necessary tilt angle as a function of the symmetric deformation

with respect to the ideal anticlinic structure in order to obtain a

uniaxial phase. The parameters used are: , and

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

57

4.2 Approximation of the striped texture by an alternating bookshelf

structure. The slow axis for a normal AFLC (a) alternates, for an

AFLC with 45° tilt (b) the optic axis is perpendicular to the layer

normal and can therefore not alternate, there is no slow axis in that

case. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

58

4.3 This sequence shows the switching from the anticlinic state to the

synclinic state for the orthoconic antiferroelectric material W107.

Note that the defect structure is not visible in the dark anticlinic sta￾te. For the bright state the defect structure is of minor importance.

59

4.4 The switching process for a “non-aligned” OAFLC sample between

crossed polarisers. The layer normal is oriented parallel to the

substrates. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

60

4.5 The rotation of an OAFLC sample between crossed polarisers.

From the structure outside the pixel area we can conclude that the

structure inside the pixel can not be a perfect alignment of the ma￾terial. Nevertheless the transmission through the pixel is practical￾ly zero for all angles. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

61

4.6 A summary of the switchable polarisations for an OAFLC with an

optical thickness equal to a half wavelength. . . . . . . . . . . . . . . . . . .

63

4.7 A summary of the switchable polarisation states for an OAFLC

with an optical thickness equal to a half wavelength. . . . . . . . . . . .

65

4.8 The relation between refractive and diffractive optical elements.

Material with a thickness equal to a multiple of the wavelength and

thus causes a relative phase shift of a multiple of 2π may be remo￾ved. The example given here is the principle of non-mechanical

beam steering.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

66

4.9 Discretisation of the phase profile for pixelated devices. One can ei￾ther modulate the thickness or the refractive index in order to mo￾dulate the relative phase shifts between the pixels. . . . . . . . . . . . . .

67

4.10 Ternary phase modulation in an orthoconic antiferroelectric liquid

crystal. Note that if one rotates the polarisation of the incident light

by 90°, the outer states will be switched. . . . . . . . . . . . . . . . . . . . . . .

69

4.11 A transmissive 1D SLM with 256 electrodes and uncut flexible

interconnect. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

70

4.12 Two SLMs each mounted on a PCB with the necessary driving elec￾tronics (designed by T. Matuszczyk). . . . . . . . . . . . . . . . . . . . . . . . . .

4.13 The electroclinic effect in the SmA* and the SmCa* phases. . . . . . . 71

72

4.14 Director profile obtained after exceeding the Fréedericksz

threshold.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

76

5.1 A schematic representation of the optical geometry for the normal￾ly bright mode of a reflective AFLCD. The anticlinic state is in this

case the bright state.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

n1 = 1.5 n2 = 1.51

n3 = 1.7

x

78

5.2 A schematic representation of the optical geometry for a normally

dark mode of a reflective AFLCD wherein the quarter wave film is

placed between the liquid crystal layer and the mirror. The

switched synclinic states are now the bright states. . . . . . . . . . . . . .

79

5.3 Visualisation of the solutions for an optimal bright state. The dotted

line represents the locus of the solutions corresponding to the first

and second maxima. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

80

5.4 A schematic representation of the optical geometry for a normally

dark mode of a reflective AFLCD wherein the quarter wave film is

placed between the polariser and the liquid crystal layer. The syn￾clinic states are the bright states. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

6.1 The principle behind PDLC devices.. . . . . . . . . . . . . . . . . . . . . . . . . . 84

86

6.2 The principle behind PDOAFLC devices. The layers are straighte￾ned out by applying an elctric field. . . . . . . . . . . . . . . . . . . . . . . . . . .

88

6.3 The geometry assumed for the calculation of the total scattering

cross section. The fields are decomposed along the directions of the

eigenpolarisations. Therefore we do not need to take the orienta￾tion of the droplets themselves into account. . . . . . . . . . . . . . . . . . .

91

6.4 The influence of the tilt angle on the relative scattering cross section

of the droplets PDOAFLC in the anticlinic state. The parameters

which are used here are: , and . The

polarisation is taken to be along the axis corresponding to . For

the polarisation perpendicular to it, , thus according to

the axis belonging to , the sensitivity of the tilt angle is found to

be even lower. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

92

6.5 The influence of the index matching on the relative scattering cross

section of droplets PDOAFLC. The parameters used here are:

and . For the perpendicular polarisation the

curves for transform into those for and vice

versa. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

93

6.6 The influence of the birefringence on the scattering properties. For

all curves an equally good index matching is assumed. This implies

that the curves for both perpendicular polarisations are almost

equal. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

94

6.7 The viewing angle dependency of the transparent state for a nemat￾ic PDLC. The parameters which are used here are: ,

and . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

94

6.8 The viewing angle dependency of the transparent state for a

PDOAFLC. The parameters which are used here are: ,

and . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

95

6.9 The viewing angle dependency of the transparent state of a

PDOAFLC with low optical anisotropy. The parameters which are

used are: , and . . . . . . . . . . . . . .

100

A.1 Spontaneous polarisation of W107, measured with capacitance

bridge method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

100

A.2 Apparent tilt angle of W107, measured with polarizing microscope

and rotation stage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

n1 = 1.52 n3 = 1.7 nm = 1.61

n3

α0 = 90°

n1

n1 = 1.52 n3 = 1.7

nm = 1.59 nm = 1.63

ne = 1.7

no = 1.52 nm = 1.521

neff = 1.61

n2 = 1.52 nm = 1.611

neff = 1.56 n2 = 1.52 nm = 1.561

xi

A.3 Helical pitch of W107, measured through selective reflection.. . . . 101

102

A.4 Spontaneous polarisation of W107a, measured with capacitance

bridge method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

102

A.5 Apparent tilt angle of W107a, measured with polarizing micro￾scope and rotation stage.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A.6 Helical pitch of W107a, measured through selective reflection.. . . 103

A.7 Threshold field of W107a, measured with a square wave of 10 Hz. 103

104

A.8 Spontaneous polarisation of W107b, measured with capacitance

bridge method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

104

A.9 Apparent tilt angle of W107b, measured with polarizing micro￾scope and rotation stage.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A.10 Threshold field of W107b, measured with a square wave of 10 Hz. 105

106

A.11 Spontaneous polarisation of W123, measured with capacitance

bridge method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

106

A.12 Apparent tilt angle of W123, measured with polarizing microscope

and rotation stage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A.13 Threshold field of W123, measured with a square wave of 10 Hz. 107

108

A.14 Spontaneous polarisation of W124, measured with capacitance

bridge method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

108

A.15 Apparent tilt angle of W124, measured with polarizing microscope

and rotation stage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A.16 Threshold field of W124, measured with a square wave of 10 Hz. 109

110

A.17 Spontaneous polarisation of W129, measured with capacitance

bridge method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

110

A.18 Apparent tilt angle of W129, measured with polarizing microscope

and rotation stage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

A.19 Threshold field of W129, measured with a square wave of 10 Hz. 111