Download n - UCL Department of Electronic and Electrical Engineering

Liquid Crystal Devices
Dr. Sally E. Day
[email protected]
1
Abstract:
Liquid Crystals Displays (LCD) – very common, low power, light-weight displays,
as well as larger area flat panel displays for monitors and TV applications.
Liquid Crystals have a remarkable electro-optic coefficient, a large birefringence
is switched with a very low voltage.
Newer displays require complex structures with careful control of small features
in the liquid crystal. This makes them of interest in other applications
besides displays.
This tutorial will cover the physical properties essential for the operation of liquid
crystal devices including displays and non-display applications.
2
Contents:
1. Structure-property relationships in liquid crystals
a.
b.
c.
d.
Phases of liquid crystals
Order parameter in liquid crystals
Anisotropy in a liquid: Dielectric, optical and viscoelastic properties
Molecular structure and influence on the physical properties
2. Optical properties of liquid crystals
a. Birefringence
b. Polarisation of light
c. Control of polarisation
3. Structure of liquid crystal devices (LCD)
a. Alignment
b. Basic construction of LCDs
4. Optical properties of display and other devices
a. Twisted nematic, In-plane switching, Vertically aligned nematic
b. Holograms and Beam steering
c. Micro and nano-structures and liquid crystals
3
Structure property relationships
Phases of liquid crystals
• Liquid crystal materials are made of organic
molecules.
C2H5
CN
• But to understand the phase behaviour these can be
considered as rods.
4
Structure property relationships
Phases of liquid crystals
• Liquid crystals are liquids, but have some additional order
associated with them, which is crystalline like.
• The simplest is the nematic phase:- the rods align in a particular
direction, but have no positional order.
• Nematic liquid crystals are ‘milky’ looking liquids
5
Structure property relationships
Phases of liquid crystals
• Smectic phases have the additional order of layers, but they are
not precise layers, but ‘density waves’
• In addition to layering, there may be some other order, e.g.
tilting within the layer.
• Smectic liquid crystals tend to be ‘wax’ like substances
6
Structure property relationships
Phases of liquid crystals
• Other smectic phases have additional order within the layers
• This order may be in the form of hexagonal packing
• The phases can be identified by the patterns that form and can
be seen using a polarising microscope, or by X-ray scattering.
• The order between the molecules can also be seen by NMR
• Some of the polarising microscope images can be seen at
http://reynolds.ph.man.ac.uk/~dierking/gallery/gallery1.html
7
Structure property relationships
Phases of liquid crystals –
Discotic Liquid crystals
• Disc shaped molecules are
the basic building blocks,
and the order can be in in
terms of the orientation
(nematic discotics) or in the
form of columns.
Nematic discotic
8
Structure property relationships
Phases of liquid crystals –
Discotic Liquid crystals
• Disc shaped molecules are
the basic building blocks,
and the order can be in in
terms of the orientation
(nematic discotics) or in the
form of columns.
Nematic discotic
Columnar phase
• The columns can then pack
together to form a two
dimensional crystalline array.
• The columnar structure could
be useful for 1-D conductors
and semi-conductors and
other properties along the
columns.
Hexagonal Columnar phase
9
Structure property relationships
Phases of liquid crystals
Thermotropic
liquid crystals
phase forms as a function of temperature
Lyotropic
liquid crystals
Phase forms as a function of concentration
in a solvent
10
Structure property relationships
Phases of liquid crystals –
Lyotropic liquid crystals.
• Lyotropic phases occur for molecules dissolved in a solution
• Different phases occur with concentration
• Often the solvent is water and the molecules have an hydrophilic end and
an hydrophobic end (e.g. detergents with polar (hydrophilic) and non-polar
(hydrophobic) end groups).
• The lyotropic liquid crystals form many different phases, as with the
thermotropic liquid crystals, but depending on concentration as well as
temperature
Hexagonal phase
11
Liquid crystal templates
• Lyotropic liquid crystal structures can be converted to
solid structures using the sol-gel process to give
silicates with the same structure as the liquid crystal
phase.
• Other methods can be used to form metal nanoparticles
12
Structure property relationships
Phases of liquid crystals –
Lyotropic liquid crystals.
• The lamellar phases are found in cell membranes
• This allows a liquid environment to exist, so transporting material around, but
with a layer which controls the transport of material across the layer
• An example of the phase transition is from the lamellar liquid crystal phase to
a gel phase, sometimes an undesirable transition.
• This transition occurs at different temperatures and pressures depending on
the environment that the organism lives in and what is required
water
Lamellar phase
water
gel phase
13
Chemical structure of liquid crystal molecules
C2H5
CN
• Cyano biphenyl, shown above was the first stable liquid crystal developed at
Hull University Chemistry Dept. – enabled the LCD industry to develop.
• Generally the rod shaped molecules can have the following structure:
X
Y
n
n=1,2,3
X,Y CmH2m+1; CmH2m+1-O; CN etc
F
F
F
Aromatic
CN
Aliphatic
CH2
CH2
CH
CH
CH
CH
CO
O
N
N
O
N
O
Hetrocyclic
N
14
Chemical structure of liquid crystal molecules
C2H5
CN
• Cyano biphenyl, shown above was the first stable liquid crystal developed at
Hull University Chemistry Dept. – enabled the LCD industry to develop.
• Generally the rod shaped molecules can have the following structure:
X
Y
n
n=1,2,3
X,Y CmH2m+1; CmH2m+1-O; CN etc
F
F
F
Aromatic
CN
Aliphatic
CH2
CH2
CH
CH
CH
CH
CO
N
N
O
N
O
Hetrocyclic
N
15
Chemical structure of liquid crystal molecules
C2H5
CN
• Cyano biphenyl, shown above was the first stable liquid crystal developed at
Hull University Chemistry Dept. – enabled the LCD industry to develop.
• Generally the rod shaped molecules can have the following structure:
X
Y
n
n=1,2,3
X,Y CmH2m+1; CmH2m+1-O; CN etc
F
F
F
Aromatic
CN
Aliphatic
CH2
CH2
CH
CH
CH
CH
CO
N
N
O
N
O
Hetrocyclic
N
16
Chemical structure of liquid crystal molecules
C2H5
CN
• Cyano biphenyl, shown above was the first stable liquid crystal developed at
Hull University Chemistry Dept. – enabled the LCD industry to develop.
• Generally the rod shaped molecules can have the following structure:
X
Y
n
n=1,2,3
X,Y CmH2m+1; CmH2m+1-O; CN etc
F
F
F
Aromatic
CN
Aliphatic
CH2
CH2
CH
CH
CH
CH
CO
N
N
O
N
O
Hetrocyclic
N
17
Chemical structure of liquid crystal molecules
• Chirality is an important property of some of the molecules:
– A chiral molecule cannot be superimposed on its mirror image. The
carbon centre of the molecules below is the chiral centre. The
enantiomers are identical except for the way in which they are arranged
in space.
– Solutions or mixtures containing chiral molecules will rotate the plane of
polarisation of light travelling through: Optical activity.
– A racemic mixture has equal amounts of each enantiomer.
– Synthesis of chiral compounds must be carried out carefully to make sure
that a racemic mixture is not obtained.
H13C6
C6H13
H
H
OH
H3C
HO
CH3
18
Chiral liquid crystals
• The chiral nematic (Cholesteric) liquid crystal phase is
a nematic phase, but the average direction of the
molecules rotates through the material.
19
Chemical structure of liquid crystal molecules
•
The different chemical groups affect the physical properties in
many ways, some important effects are as follows
–
–
–
–
–
–
Phase transition temperatures
Dielectric properties
Optical properties
Visco-elastic properties
Ferroelectric, flexoelectric coefficients
Chirality
• These physical properties in turn affect the performance of
the displays and other devices that contain liquid crystals
20
Order parameter
• The order parameter is the degree to which
the individual molecules align with the
average direction.
• It is defined in terms of the angle that the
molecules make with n, the vector
describing the average direction
• An important property of this vector is that
n=-n
• The order parameter (S) is typically S ≈ 0.65
for a liquid crystal; for a perfectly ordered
crystal S = 1 and for an isotropic liquid S = 0
• If the temperature is increased in a
thermotropic liquid crystal, the molecules
become more disordered and so the order
parameter will reduce.
n – the director
q
S
1
3 cos2 q  1
2
21
Anisotropy in a liquid
• The order in the liquid allows the material to have
different properties in different directions
• In a liquid domains will form. Alignment methods will
have to be used to obtain a uniform structure
• Scattering of light at the domain boundaries give the
bulk a ‘milky’ appearance
22
Elastic properties
• The molecules in the liquid crystal have a preferred orientation
(the director) and as a result if there is a distortion in the
structure then there is an elastic energy associated with the
distortion
• The elastic energy is anisotropic and is described by three
elastic constants, k11, k22, k33.
Splay, k11
Bend, k33
Twist, k22
23
Dielectric properties
• The electric permittivity of the liquid crystal is
anisotropic
• The permittivity is concerned with the polarisability of
the material and the response of a material to an
electric field.
• D=eoerE
24
Dielectric properties
• The electric permittivity of the liquid crystal is
anisotropic
• The permittivity is concerned with the polarisability of
the material and the response of a material to an
electric field.
• D=eoerE
The field will induce dipoles in
the material, which will create a
field inside. P the polarisation.
E
+
+
+
-
+
-
-
P
+
+
+
-
-
D = eoE+P║ = eoe║E
25
Dielectric properties
• The electric permittivity of the liquid crystal is
anisotropic
• The permittivity is concerned with the polarisability of
the material and the response of a material to an
electric field.
E
+
-
P
+
-
+
-
+
-
+
-
+
+
-
If the field direction changes
then the size of the dipoles will
be different in an anisotropic
material
D = eoE+P┴ = eoe┴E
26
Measurement of permittivity
• The permittivity is measured by making a capacitor filled with
liquid crystal and measuring the capacitance.
• The two values are measured by orienting the liquid crystal in
two directions
• The anisotropy in the liquid crystal has values in the range from
-10 ≤ De ≥ 40 in mixtures
(where De = e║-e┴)
Guard ring to
avoid the
effect of
fringing fields.
A
Capacitance
meter
d
C
e oe  A
d
27
Permittivity or dielectric constant, from
capacitance measurements
Permittivity, dielectric constants.
Reduced Temperature T/ TNI
(TNI is the nematic to isotropic transition temperature)
28
Dielectric anisotropy and electric fields in a liquid
crystal.
• When an electric field is applied the energy can be minimised by
reorientation of the liquid crystal, because it is a liquid.
• the stored energy of a parallel plate capacitor is:
1 Q 2 1 Q 2d
W

2 C 2 e oe r A
• So W is minimised by making the dielectric constant as large as possible.
• Note: this is not the effect of a dipole and does not depend on the polarity
(sign) of the field
• A liquid crystal responds to the average (r.m.s) value of the electric field.
29
Dielectric anisotropy and electric fields in a liquid
crystal.
E
E
With positive
dielectric
anisotropy the
director will
line up with the
electric field
With negative
dielectric
anisotropy the
director will
line up
perpendicular
to the electric
field
De = e║-e┴ > 0
De = e║-e┴ < 0
30
Influence of chemical structure on permittivity
• Conjugation will increase the polarisability
• Dipolar groups will increase the dipoles, either el or et depending
on the position in the molecule
X
Y
n
n=1,2,3
X,Y CmH2m+1; CmH2m+1-O; CN etc
F
F
F
Aromatic
CN
Aliphatic
CH2
CH2
CH
CH
CH
CH
CO
O
N
N
O
N
O
Hetrocyclic
N
31
Influence of chemical structure on permittivity
• Conjugation will increase the polarisability
• Dipolar groups will increase the dipoles, either el or et depending
on the position in the molecule
X
Y
n
n=1,2,3
X,Y CmH2m+1; CmH2m+1-O; CN etc
F
F
F
Aromatic
CN
Aliphatic
CH2
CH2
CH
CH
CH
CH
CO
O
N
N
O
N
O
Hetrocyclic
N
32
Permittivity as a function of frequency
• As the frequency of the electric field is increased the permittivity
will change.
• At optical frequencies the dielectric anisotropy will be positive
and can be related to the birefringence as follows:
•
n║2 = e║
•
Dn = n║ - n┴, the birefringence
and
n┴2 = e┴
• The refractive index of the bulk depends on
– the polarisability of the molecules
– the order parameter
as the temperature is increased the birefringence will reduce.
33
Measurement of refractive indices
• The optical refractive indices can be obtained from Abbé
refractometer measurements.
• An aligned sample of liquid crystal is put onto a prism
• The critical angle, qc, at which total internal reflection occurs is
measured
• By changing the polarisation of the light observed both
refractive indices can be measured.
qc
qc
34
Refractive indices, from Abbé
refractometer measurements
Refractive indices of liquid crystal
Reduced Temperature T/ TNI
(TNI is the nematic to isotropic transition temperature)
35
Refractive indices of liquid crystal
• The refractive index depends on the polarisability of
the molecules and also depends on the order
parameter
• Polarisability varies with chemical group, increasing
with increased conjugation
• The birefringence is always positive, because there is
no influence due to purely dipoles.
increasing conjugation
CH
CH
CH
CH
CH2
CH2
36
Refractive indices of liquid crystal
• The birefringence is critical to the optical properties of the liquid
crystal and underlies many of the applications of liquid crystals.
• By reorienting the liquid crystal the effective birefringence will
change and so the optical properties will change
37
Birefringence
• Optically anisotropic materials have different optical properties depending on
the polarisation of the light travelling through the material.
• This is described by different refractive indices in the material.
• For a uniaxial material such as liquid crystals there are two values for the
refractive index.
The refractive indices can be
described by an optical indicatrix.
Shown in the figure.
ne
nx
q
 cos2 q
n x q   
 n2
 o

1
2
sin q  2

ne2 
no
38
Optical Indicatrix
ne
no
39
Optical Indicatrix
ne
q
no
40
Optical Indicatrix
ne
q
no
41
Optical Indicatrix
ne
nx
q
no
42
Optical Indicatrix
ne
nx
q
no
 cos2 q sin 2 q 

n x q   

2
 n2
ne 
 o
1
2
43
Optical Indicatrix
The angle of incidence
of the light may change
ne
nx
q
no
 cos2 q sin 2 q 

n x q   

2
 n2
ne 
 o
1
2
44
Optical Indicatrix
ne
nx
q
no
 cos2 q sin 2 q 

n x q   

2
 n2
ne 
 o
1
2
45
Optical Indicatrix
The orientation of
the optical
indicatrix may
change – this
occurs when liquid
crystals switch
ne
nx
q
no
 cos2 q sin 2 q 

n x q   

2
 n2
ne 
 o
1
2
46
Polarised light
l0
Light is a transverse electromagnetic wave,
the electric field, the magnetic field and
the direction of propagation are all at right
angles to each other.
The wave is time varying
frequency given by n,
speed given by c= nlo in a vacuum
in a medium of refractive index n, the
wavelength is changed by llo/n.
x

E

H
y
A full analysis of polarised light must include
both the electric and magnetic components of
the light; this is particularly necessary when
considering reflected components
In transmissive optical systems the reflected
light does not have to be considered in such
detail and the light is considered only in terms
of the electric components.
47
z
Polarised light
Light can have two orthogonal states or
polarisations. The waves can be written as
follows:
If  = 0 or 2 or an integral multiple
then the light is plane polarised
E x  ˆiE0 x e j (t kz )
E y  ˆjE0 y e j (t kz  )
with
k
2n
l
The direction of propagation is taken to be along z
These are waves travelling in the z-direction with a
relative phase between them of .
E  (ˆiE0 x  ˆjE0 y )e j(t48 kz)
Circularly polarised light
If the waves have equal amplitudes E0
and the relative phase is -/2 + 2m (where m
= 0, 1, 2, …..) then circularly polarised light
is obtained. The components are then
E x  E0ˆie j (t kz )
E y  E0ˆje j (t kz  2)  E0ˆj je j (t kz )

i.e. E  E0 ˆie j (t kz )  ˆj je j (t kz )

The intensity of the light is E.E = E02, a constant, but the direction of E is
time varying and is rotating with angular frequency of  = 2f. The light
is described as circularly polarised – it can be right or left circularly
polarised.
49
Elliptical polarisation
In a general case E0x  E0y and  has any value,
the light is elliptically polarised. The electric
vector E then rotates as a function of time and
the amplitude varies as well. Linear and circular
polarisations are special cases.
time
y
Ex component

E at t  0

E (t )
x
Ey component
50
Optical phase through a birefringent layer
• As the two polarisations pass through the crystal the relative
phase between them will change, because the values of
k=2n/l will be different.
• If a crystal has a thickness of d then one polarisation will have
phase relative to z=0 of =2nod/l, and the other of =2nxd/l.
• The phase difference is D=2nx-no)d/l
E x  E0 x ˆie j (t k x z )
E y  E0 y ˆje
j (t  k y z  D )
51
Viewing angle
• When light is incident from different directions, the effective
refractive index can change – so the change in polarisation will
be different for the different directions of the light.
• As a result the grey levels are not reproduced accurately and
colour distortion occurs, at the extreme reverse contrast can
occur.
n e, n o
n x, n o
52
Viewing angle – calculation of reflected light
- bright state
Reflected Intensity - Bright
Total Twist, =45
Dn.d/l=0.29
Polarizer orientation, qpol=-15
Quarter-wave layer orientation, qqwp=-60
Reflected Intensity - Bright
53
Viewing angle – calculation of reflected light
- dark state
Reflected Intensity - Dark
Total Twist, =45
Dn.d/l=0.29
Polarizer orientation, qpol=-15
Quarter-wave layer orientation, qqwp=-60
Reflected Intensity - Dark
54
Viewing angle – calculation of reflected light
- contrast ratio
Contrast Ratio (Bright / Dark) contour plot
(clipped to ~ 20:1) Max:66
Contrast Ratio (Bright / Dark) area plot
(clipped to ~ 20:1) Max:66
Viewing angle plot covers polar angles from 0 to 60.
55
Nematic continuum theory
• The energy density F is expressed as a function of
–
–
–
–
The director n
The elastic contants k11, k22, k33.
The pitch, P
The electric field E and dielectric anisotropy De.


2
2
2
2

2

1






F
k
div
n

k
n
.
curl
n


k
n

curl
n

e
D
e
n
.
E

11
22
33
o
2
P


Splay, k11
Twist, k22
Bend, k33
56
Numerical modelling of the director (n)
distribution
57
1
0.5
z [ mm]
Numerical modelling of
the director distribution
0
0.5
1
1.5
Stable Vertical State
1
0.5
Stable Hybrid Aligned State
0
x [ mm]
0.5
1
1.5
58
Threshold voltage for switching
Permittivity, measured from the
capacitance
The competition between the elastic and electric energies, results in a
threshold voltage for switching
k
slope 33
k11
k11 2  e o DeVc2
Voltage (V)
59
Alignment
• The liquid crystal director must be fixed somewhere, and the best place is at
the surface of a substrate
• The director must be fixed so that the elastic forces restore the original
structure after the electric field has been removed.
• The director can be fixed parallel to the surface or perpendicular, or at an
angle.
• Alignment is achieved by rubbing a polymer surface, or by treating the
surface with a surfactant.
Energetically unfavourable –
because of the distortion in
the director field
Energetically favourable – the
director field is undistorted
The alkyl chains of the surfactant
interact with the long chains of
the liquid crystal to give
alignment perpendicular to the
60
substrate
Structure of an LCD
The polarisers are needed to modulate
the light intensity, when the liquid crystal
changes the polarisation state of the
light
The glass is used to give structure to
the display – spacers are used to
maintain the cell gap (typically 5mm)
The ITO (Indium Tin Oxide) is a
transparent conductor, to allow a
voltage to be applied across the liquid
crystal layer
•
http://www.hylcd.com/images/color.gif
An alignment layer is needed to fix the
liquid crystal at the surface, so that the
un-switched structure is obtained with
the voltage is removed.
Colour filters are used to give Red,
Green and Blue pixels for colour
displays.
61
Twisted nematic (TN) LCD
A very common type of display uses an alignment
on the two glass substrates at 90° to each other,
so that there is a 90° twist in the director in going
through the cell. When a voltage is applied the
director tilts to align with the electric field in the
cell.
Figures taken from http://sharp-world.com/sc/library/lcd_e/s2_4_3e.htm
If the electrodes are patterned
into lines, then addressing a row
and a column selects a
particular pixel, defined by the
intersection of the electrodes. 62
Multiplexing
Multiplexing is implemented by applying a select voltage along a row and the data voltages to the columns, as shown in the
diagram below. The voltage levels as a function of time are shown and the rms voltage is also given. The liquid crystal
responds to the rms voltage.
Row select voltages
S
A
Normally white display i.e. white in the
un-switched state
and dark
in the switched state.
B
t
The rms select and non-select voltages are given below
Data voltages
D
S  D 2  N  1D
Vs 
Vns 
2
S  D 2  N  1D
N
2
N
r.m.s voltage on A
Resultant voltage on A
row voltage – data voltage
frame time = no. of rows (N) × row select time
S+D
D
t
t
r.m.s voltage on B
S-D
D
Resultant voltage on B
t
t
The multiplexing limit is:
Vs

Vns
N 1
N 1
63
Active Matrix Twisted Nematic Displays (AMTN)
•
•
•
•
•
•
•
•
Direct multiplexing only gives about 30 to 200 lines, with limited grey level and
response time.
An alternative is needed for computer displays or TV/video displays.
A non-linear element or a switch is required at each pixel so that row and column
addressing can be used.
A suitable non-linear device is the transistor.
A display may contain millions of pixels and therefore transistors
Only very few can be defective before the display appearance is degraded.
Displays are generally very large in comparison to normal silicon transistors
Often the displays are back lit so the display must be transparent.
http://sharp-world.com/sc/library/lcd_e/s2_4_3e.htm
64
Thin Film Transistors (TFTs) for LCDs
http://www.xbitlabs.com/articles/other/display/response-2_3.html
•
•
•
•
The material used to make the TFT is usually amorphous Silicon.
This is easy to deposit over large areas of glass with its low processing temperature
A storage capacitor is also needed so that the charge can be maintained on the LCD
pixel after it has been addressed
The change in dielectric constant in the LC layer can alter the voltage for a fixed
charge on the pixel capacitor.
65
Materials for TFTs
Amorphous Silicon
+ easy to deposit over large areas and at low temperature - compatible with glass
substrates
- But it has relatively low mobility
- Larger areas are required to overcome the low mobility, so that the pixel can be
addressed in the line address time
- The transistor must be shielded so that no photoconduction occurs
- Large shielding reduces the light throughput
- Large transistors mean that the leakage current can be a problem
Poly-silicon
- Amorphous Silicon can be re-crystallised by local heating to form poly-crystalline
Silicon
+ The higher mobility means that transistors are fast enough to be used for control and
signal processing as well as the pixel switches
+ Inclusion of some electronics onto the glass means fewer connections to the glass,
improving performance and simplifying fabrication.
+ Smaller transistors mean lower leakage currents and so the pixels hold the charge
better
66
Liquid Crystal on Silicon (LCOS)
• Although Silicon is not transparent, reflective displays can be made with a
Silicon backplane.
• High resolution can be obtained – with 10mm pitch pixels and HDTV
resolution (1M+ pixels)
• Complex processing can be carried out in the backplane.
• Planarisation of the wafer must be carried out so that the pixels are optically
flat.
• Electrical connections are made from the circuit through the planarisation
layer to the metal pixel electrode, which can also act as mirror
• The liquid crystal layer is added above the mirrors, with an ITO ground plane.
• Reflective operation means that the light travels twice through the liquid
crystal layer so it can be made thinner – resulting in faster switching
• The very small displays can be used in projector displays and for diffraction 67
LC modes for Active Matrix Liquid Crystal Displays
(AMTN)
• Conventionally the 90° twisted nematic liquid crystal is used but:
Twisted nematics have a problem with the viewing angle
characteristics and the dark state.
• Other LC modes to improve the properties are
-
Vertically Aligned Nematic (VAN)
-
In-Plane Switching
• Compensation films, which are birefringent films, can be added to the
front surface of the device to compensate for the off-axis birefringence
variations.
68
In-plane switching (IPS) LCDs
• Switching is in the plane of the cell.
• The optical properties are then less angle dependant and it is
easier to compensate for the changes off-axis.
• High anisotropy of the dielectric permittivity is possible because
positive materials are used, hence fast switching can be
obtained
• Patterned electrodes are needed, which require relatively high
resolution lithography.
69
In-plane switching (IPS) LCDs
LC
• Alternate voltages are applied to
the electrodes
• The modelled results show the
director distribution around the
end of an IPS electrode structure
70
Vertically Aligned Nematic
• Using a negative dielectric anisotropy and alignment
perpendicular to the substrates, switching can from a uniform
dark (off) state to a switched state which can be average to give
a more uniform viewing angle.
• Slits or other shaped holes in the electrodes provide shaped
electric fields so that the tilt direction of the switching is
controlled
71
Vertically Aligned Nematic
•
•
•
•
•
Numerical modelling of the director pattern and the optical properties show the
switching of a VAN cell with a cross shaped hole in the electrode.
Four domains result, giving very uniform viewing angle properties
The off-axis birefringence of the uniform dark state is easily compensated with a
birefringent film with a negative birefringence, giving a very good dark state, hence a
good contrast ratio.
Materials with a high negative dielectric anisotropy are more difficult to obtain and so
switching times can be slower than positive materials
The birefringence may be lower for these materials and so thicker layers are needed,
again making the switching slower
72
Cholesteric liquid crystals
• Essentially the nematic phase, but with a chiral group which gives a twist to
the structure.
• The twist is often temperature dependant, but only if there is a SmA phase
below the nematic and then the pitch increases in going closer the the SmA
phase, i.e. as the temperature decreases
• The pitch of the twist can match the wavelength of light, l, in which case
selective reflection occurs across a band when l = nP, with n, the refractive
index, going from ne to no. (Sometimes called Bragg scattering)
• Only the circular polarisation matching the twist sense is selectively reflected
• In droplet form, encapsulated into a polymer the cholesteric can be used as a
thermometer
• In an aligned form it can be used as a circular polariser, or as the reflector for
a laser system.
73
Liquid crystals for laser systems and photonics
•
•
•
•
•
•
•
•
•
Laser dye molecules can be added to the cholesteric liquid crystal and then optically
pumped and the liquid crystal structure forms the laser cavity.
Changing the temperature slightly would then tune the wavelength of the laser
Some cholesteric systems have been shown to have electrically tuned pitch, which
allows electrical tuning of the laser wavelength
The cavity can be made very small, which has advantages in laser operation
Improvements will be to synthesis liquid crystal laser dye materials, which would
allow very high concentrations of the dye and to find a method of electrically pumping
the laser
Blue phases are a three dimensional twisted structure, which is very interesting as a
3D distributed Bragg system.
Recent developments using dimers have allowed blue phases to exist over a wide
temperature range.
These have potential as self assembled photonic band gap structures
A defect formed in the 3D structure, or a line of defects give localised transmission of
light in the band gap and control of photon movement through the structure.
74
Diffractive displays
• Conventional LC displays rely on transmission/reflection or absorption of the
light
• This is inefficient since light is absorbed rather than redirected
• Diffraction offers a way of steering the light into the region where it is
required
• Requires coherent illumination (i.e. laser light) and very high resolution
structures (sub-wavelength) in the diffracting medium
• The Fourier transform of the image is obtained and this is displayed on a
Spatial Light Modulator (SLM) – or display!
• Diffraction from the Fourier transform reconstructs the original image
• Often image compression relies on taking the transform, so the processing
could be simplified, so reducing the data rates required to the display
• Only LCOS systems provide the resolution required
• Both binary Ferroelectric liquid crystal and nematic LC can be used.
• Intensity modulation of the diffraction pattern can be used, but phase
modulation is more efficient
75
Diffractive Display demonstrated at Cambridge
University
Using Binary FLC LCOS
Using analogue nematic LCOS
76
Beam steering using LCOS
• A grating can be used to direct light into different
regions
• This can be used in telecom systems to reconfigure
light from an array of optical fibre outputs into an array
of optical fibre inputs
Modelling of a subset of a 2D array of LC pixels
to produce a programmable phase grating
77
Holograms in LC devices
• As well as providing a 2D image by diffraction, at a
higher resolution a hologram can be generated, which
is a 3D ‘image’.
• The phase front of the light reflected from an object is
reconstructed and this then appears like the object
itself.
• This has been demonstrated by Qinetiq, but is not in
production
78
LCOS-based 3D Holographic Displays
3D holographic CAD Workstation Concept
Demonstration System
Stanley, M, Conway, P B, Coomber, S, Jones, C J, Scattergood, D C, Slinger C W, Bannister, B W, Brown, C V, Crossland W A, and Travis A R L,
"A novel electro-optic modulator system for the production of dynamic images from giga-pixel computer generated holograms",
Practical Holography XIV, Proceedings of SPIE Vol 3956, San Jose, 24 January 2000, pages 13 to 22
79
Bistable displays
•
•
•
•
For portable displays the energy consumption is critical
Bistability allows the display to be addressed and then it can be left in that state.
The power can be removed once the display has been addressed, giving very low
power operation for displays which do not have rapidly varying information
Applications are for e-books, supermarket shelf labels, some mobile phone
applications
80
Post Aligned Bistable Device (PABN)
3-D Modelling Window
[1], [2]
•
Device under development at Hewlett-Packard.
•
Periodic array of microscopic square posts.
•
Two stable states of operation, planar and tilted.
2.6 – 3.0 µm
Top: Homeotropic
Sides: Periodic
0.6 µm
1.2 µm
Bottom and Post: Planar Degenerate
[1] Kitson S. Geisow A., “Bistable Alignment of Nematic Liquid Crystals Around Microscopic Posts”, Mol.Cryst. Liq.
Cryst., vol.412, pp.153-161, 2004
[2] Kitson S. Geisow A., ”Controllable Alignment of Liquid Crystals Around Microscopic Posts: Stabilisation of Multiple
States”, Appl. Phys. Lett., vol. 80, no. 19, pp. 3635-3637, May 2002
81
The Two Stable States
• The background colour
indicates the tilt of the
director, showing the
two stable states.
• Switching is via the
flexo-electric effect and
involves defect
movement.
• Adaptive meshing is
used to concentrate
calculation where
needed e.g. at the
defects.
Planar
Tilted
82
Doping liquid crystals
• Adding anisotropic dyes to liquid crystals has been done for a long time for
simple displays.
• The alignment in the liquid crystal aligns the dye molecules which can then
be switched
• Absorption occurs only along one direction of the dye and so the light
absorption can be switched
• Other additives are being considered – carbon nano-tubes, very small
particles of ferroelectric crystal, biological molecules
• This could be to enhance the anisotropy of the liquid crystal properties or to
control the orientation of the dopant
• Control of the position of defects in the liquid crystal can be used to move
particles in the liquid crystal and so could be used to position very small
particles
A Pair of Point Defects
http://www.lci.kent.edu/polmicpic.html 83
Colloidal crystals in liquid crystal
“Two-Dimensional Nematic Colloidal
Crystals Self-Assembled by
Topological Defects”
Muševič Igor, Škarabot Miha, Tkalec
Uroš, Miha Ravnik, Slobodan Žumer,
Science (vol.313 p.954)
84
Summary
• Liquid crystals give electrically switchable anisotropic properties
• The organic molecules can be synthesized to have many different properties
and mixture formulation allows greater flexibility and control
• Optical anisotropy can be switched so as to modulate the polarisation of
light, or the phase of light passing through the layer
• Displays are the most common application, but there are many different
configurations depending on the application requirements
• High resolution structures in the liquid crystal allow them to be used as
diffraction or holographic displays
• Future applications in photonics may be based on the use of particles or
dopants in the liquid crystal
85
Switching Between the Stable States
via defect formation along post.
• The Flexoelectric
effect responsible
for switching
between the stable
states.
Post
Top view
Post
Stable planar State
Post
Post
Side view
Stable tilted State
Intermediate, defect state.
86