Liquid-crystal photonic applications
Jeroen Beeckman
Kristiaan Neyts
Pieter J. M. Vanbrabant
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Optical Engineering 50(8), 081202 (August 2011)
Liquid-crystal photonic applications
Jeroen Beeckman
Kristiaan Neyts
Pieter J. M. Vanbrabant
Ghent University
Department of Electronics & Infor mation Systems
Sint-Pietersnieuwstraat 41
Ghent, East-Flanders B-9000, Belgium
E-mail: Jeroen.Beeckman@elis.ugent.be
Abstract. Liquid crystals are nowadays widely used in all types of dis-
play applications. However their unique electro-optic properties also make
them a suitable material for nondisplay applications. We will focus on the
use of liquid crystals in different photonic components: optical filters and
switches, beam-steering devices, spatial light modulators, integrated de-
vices based on optical waveguiding, lasers, and optical nonlinear compo-
nents. Both the basic operating pr inciples as well as the recent state-of-the
art are discussed.
C
2011 Society of Photo-Optical Instrumentation Engineers (SPIE)
.
[DOI: 10.1117/1.3565046]
Subject terms: liquid crystals; photonic applications; review; liquid-crystal lasers;
spatial light modulators; tunable lenses; nematicons; optical nonlinearity.
Paper 100913SSR received Nov. 5, 2010; revised manuscript received Jan. 11,
2011; accepted for publication Jan. 13, 2011; published online Jun. 14, 2011.
1 Introduction
Liquid crystals (LCs) are organic materials that are liquid
but that show a certain degree of ordering (positional and/or
orientational). With this definition, many materials can be
classified as liquid crystals, but the majority of liquid crystals
that are used in photonic applications are of the thermotropic
type. Thermotropic means that the liquid-crystal phase
exists within a certain temperature interval (in contrast to
lyotropic materials for which the material is liquid-crystal
within a certain concentration range). Various types of
thermotropic liquid-crystal materials exist, and many differ-
ent mesophases have been discovered in the last decades:
nematic, smectic A, smectic C, columnar, blue phases, and
many many more. The diversity of liquid-crystal materials is
huge, but this diversity is even overshadowed by the number
of applications in which liquid crystals are used nowadays.
The majority of applications are related to information-
display applications. Liquid crystals have conquered the
major market share i n different display application areas:
television screens, laptop screens, screens in mobile phones,
etc. Only in projection displays does a tough competitor
exist, namely microelectromechanical systems or licenced
by Texas Instruments: Digital light processing. Organic light
emitting diodes (OLEDs) are the obvious next generation
technology that could overtake the LC domination, but today
OLED displays have not penetrated the market and only the
future will tell if they will. In this review article, we will
focus on nondisplay applications, but because we cannot go
too broad, we restrict ourselves to photonic applications in
which the light is actively manipulated by the LC. In this re-
view article, a certain external influence (surface anchoring,
electric fields, optical fields) is used to (re)orient the LC in
a certain way. In turn, the LC then changes the light that is
propagating through it. Of course, as in every review article,
it is impossible to list all the fascinating new scientific results
or breakthroughs. Therefore, the authors apologize for not
including some novel exciting phenomena or applications
such as light-induced switching of LC polymers
1–3
or
novel retardation structures such as broadband cholesteric
filters.
4, 5
0091-3286/2011/$25.00
C
2011 SPIE
Numerous books and articles exist that deal with physical
properties of LCs
6–8
and with display applications.
9–11
A
very nice review on the different types of LC materials that
exist for photonic applications is written by de Bougrenet
de la Tocnaye.
12
In this review article, we will focus on the
diversity of applications of LCs with special attention to the
different configurations and engineering techniques that are
applied.
2 Classification and Properties of Liquid Crystals
2.1
Liquid Crystals: A Fourth Phase of Matter
The liquid crystalline phase can be formed by different types
of organic molecules. Anisotropy in either shape or solubility
of the molecules is required. A balanced interaction between
the molecules is the key to obtain a liquid crystalline be-
havior. In thermotropic liquid crystals, this interaction is in
competition with the thermal motion of the molecules and
the liquid-crystal phase is only stable within a certain tem-
perature interval. Thermotropic liquid crystals usually have
a high degree of anisotropy in the shape of the molecules (or
mesogens). The most common type consists of rod-shaped
molecules. The organic molecules typically have a long chain
structure containing a side chain, multiple aromatic rings,
and a terminal group on the other side.
13
As an illustration,
Fig. 1 shows a 4-pentyl-4’-cyanobiphenyl molecule, which
constitutes the well-known liquid crystal 5CB, which has
been widely used in mixtures for display applications.
The 5CB molecule depicted in Fig. 1 has mirror plane
symmetry and is therefore called achiral. The structure of
chiral molecules is not planar but has different functional
groups above and below the plane. Depending on the ar-
rangement of the groups with respect to each other, two
configurations (enantiomers) are possible. Enantiomers typi-
cally have identical chemical and physical properties, except
for their optical activity.
2.2
Nematic and Smectic Phases
The ordering of thermotropic liquid-crystal molecules de-
pends both on their chemical structure and temperature. For
rodlike molecules, the ordering can be further classified as ei-
ther nematic or smectic. The transitions from one mesophase
to the other are reversible and occur at well-defined temper-
atures. The nematic phase of achiral liquid crystals can be
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Beeckman, Neyts, and Vanbrabant: Liquid-crystal photonic applications
Fig. 1 Chemical structure of a 5CB molecule (4-pentyl-4’-
cyanobiphenyl).
observed at temperatures below the clearing temperature for
transition to the isotropic phase. In this case, the long-range
orientational order dominates over thermal fluctuations and
the molecules tend to align their long axes in a common di-
rection (denoted by the director L),asshowninFig.2(a).
However, no long-range positional order of the molecules
with respect to each other is present. The chiral nematic phase
is very similar as for achiral mesogens, but the asymmetry
of the molecules causes a gradual rotation of the director,
which induces a spontaneous and continuous twist around
the normal of the preferred molecular directions [Fig. 2(b) ].
The distance over which the director has rotated over an an-
gle 2π is called the pitch P
0
of the chiral nematic phase. If
the temperature is decreased below the nematic range, then
certain liquid crystals arrange in smectic layers. Depending
on the molecular interaction energy, the smectic mesophase
is typically further divided into the smectic A and smectic
C phases. The molecules in the smectic A phase can freely
rotate around the director, and the end group can point either
up or down. In the smectic C phase, the molecules are ar-
ranged in smectic planes similar as in the smectic A phase.
There is a higher order because the molecules are not able to
rotate freely around their long axis and they are tilted with
respect to the layer normal as indicated in Fig. 2(c). The end
group of the molecules can still be oriented up or down.
2.3
Properties of the Nematic Phase
The mix of properties on the microscopic scale has lead
to the excellent macroscopic electro-optic properties of liq-
uid crystals, combining low viscosity (as in liquids) with
anisotropy (typical for crystals). The nematic phase is most
commonly used in applications because it is the easiest
mesophase in terms of technology and in terms of under-
standing its behavior. The molecules are free to rotate around
their long axis in the homogeneous nematic phase, yield-
ing a f ull rotational symmetry. The symmetry elements of
any physical property of a crystal must include the symme-
try elements of the point group of the crystal (Neumann’s
principle
14
). This implies that homogeneous nematic liq-
Fig. 2 Director configuration i n (a) achiral nematic, (b) chiral nematic,
and (c) smectic liquid-crystals.
uid crystals have uniaxial macroscopic properties. Nematic
liquid crystals exist with wide temperature range, different
dielectric anisotropy (−10 <ε<20), different birefrin-
gence (0.07 <n < 0.5) and high stability (mainly due to
the development of fluorinated compounds).
15
A certain orientation of the liquid-crystal layer in the re-
laxed state can be realized by using an appropriate align-
ment layer at the interfaces of the liquid crystal with the
surrounding media. Even patterned alignment of the liq-
uid crystal is possible by using photoaligned surfaces,
16, 17
(e.g., for use in polarization gratings
18
or micropolarizers.
19
)
The electrical properties of the nematic phase can be de-
scribed by the 3×3 dielectric tensor, which takes the fol-
lowing form: ε
ij
= ε
⊥
δ
ij
+ εL
i
L
j
(with i, j = x, y, z and
ε = ε
− ε
⊥
). The orientation of the liquid crystal can be
changed because of its dielectric anisotropy ε by apply-
ing an external electric field
E. The corresponding energy
density f
electric
can be calculated as:
20
f
electric
=−
1
2
[ε
⊥
|E|
2
+ ε(L · E)
2
]. (1)
On the basis of this free energy, one can define a torque that
acts on the liquid-crystal molecules due to the electric field.
This torque is equal to
¯
= ε
0
ε
¯
L.
¯
E
¯
L×
¯
E
. The torque
on the molecules is zero for an angle of 0 or 90 deg between
the electric-field direction and the director axis. For an angle
of 45 deg, the torque is maximal and, for a positive ε,the
electric-field tries to align the director along the electric-field
direction. Liquid-crystal molecules tend to reach an equi-
librium state for which the free energy is minimized. If the
applied voltage exceeds the Freedericksz threshold for elastic
deformation, then the liquid-crystal tends to align parallel (as
in case ε
>ε
⊥
) or perpendicularly (as in case ε
<ε
⊥
)tothe
electric field. Because of the optical anisotropy n, changing
the orientation of the liquid-crystal also modifies the phase
retardation for polarized light induced by the material. Such
electrically controlled birefringence has been applied in the
vast majority of liquid-crystal devices to date.
2.4
Polymerization of Liquid Crystals
In several applications, the liquid-crystal is stabilized by a
polymer network by adding reactive compounds to the mix-
ture. After exposure by UV light, the mixture with a pho-
toinitiator is polymerized. In the polymerization reaction,
the compounds are linked together and an anisotropic net-
work is formed inside the liquid-crystal mixture, as shown in
Fig. 3(a). The orientation of the liquid-crystal can still be
changed electrically, but higher voltages are required be-
cause of the strong interaction between the liquid-crystal and
Fig. 3 Director configuration in (a) polymer stabilized and (b) polymer
dispersed liquid-crystals.
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Beeckman, Neyts, and Vanbrabant: Liquid-crystal photonic applications
the polymer network. When the electric field is removed,
the liquid-crystal rapidly reaches the relaxed state because
of the “memory effect” induced by the polymer network.
When high polymer concentrations are used in the mixture,
the liquid-crystal and polymer can become phase separated
after polymerization. In this case, micrometer-sized droplets
of liquid-crystal are embedded in a polymer matrix as shown
in Fig. 3(b). Such polymer dispersed liquid-crystals (PDLCs)
have been applied in switchable scattering devices. When no
electric field is present, the liquid-crystal droplets are ran-
domly oriented and a PDLC slab will scatter light because of
the variations in refractive index. However, the liquid-crystal
droplets can align in a common direction when an electric
field is applied. Depending on the configuration, the system
can act as a homogeneous transparant layer in case the re-
fractive index of the polymer matches either the ordinary or
extraordinary refractive index of the liquid-crystal.
2.5
Blue Phases
It has been observed that the helical structure of the chiral
nematic phase [see Fig. 2(b)] of sufficiently low pitch can-
not deform continuously on transition to the isotropic phase
without the creation of defects.
21
As a result, an interme-
diate phase with a three-dimensional lattice of double-twist
cilinders (in which the director rotates in a helical fashion
about any axis perpendicular to a normal) occurs as shown in
Fig. 4. Because the lattice constant is on the order of a few
hundred nanometers, the structure can behave as a Bragg re-
flector in the visible part of the spectrum. In the original ob-
servation of this intermediate phase by Reinitzer, the material
appeared as a blue substance because of Bragg reflection.
22
Hence, this interim phase was called the “blue phase”. Blue
phases are optically isotropic and have a high optical non-
linearity (Kerr effect, see Sec. 7.1), Thus, applying a suf-
ficiently strong electric field induces birefringence in the
liquid-crystal. Blue phases typically exist over a very narrow
temperature range (∼1 K), which restricted their attractive-
ness for applications for a long time. However, blue phases
stabilized over a wide temperature range (260–326 K in
Ref. 23) have been demonstrated more recently, making them
eligible to become the liquid-crystal phase of the future.
3 Filters and Switches
In display applications, the LC is used to modulate the light
transmission through the different pixels. Crossed polarizers
are used, and the liquid-crystal layer acts as a variable retarder
to modulate the polarization of light passing through. In ver-
Fig. 4 Three-dimensional lattice of double helix cilinders in the blue
phase.
tically aligned or in-plane-switching displays, a bright pixel
is obtained by rotating the incoming polarization over 90 deg.
This rotation is done by using the liquid-crystal layer as a
half wave plate and thus engineering the optical retardation.
In Twisted Nematic (TN) (or Super Twisted Nematic (STN))
displays, the situation is a bit more complicated because the
retarder axis rotates. In order to make an optical filter or
optical switch, one can use the same physical principles as
the ones used in display applications. Here, we will focus
on wavelength tunable filters, switches for optical signals in
telecommunication, and tunable liquid-crystal lenses.
3.1
Wavelength Tunable Filters
In standard three-color imaging, one actually takes three im-
ages for three different colors or wavelength ranges. A color
CCD, for example, often consists of RGB pixels in a Bayer
arrangement, with double the amount of green pixels, com-
pared to red and blue. In hyperspectral imaging, the aim is
to obtain images of a certain object for much more different
wavelengths and each image should be related to only a small
wavelength range. The Lyot–Ohman tunable filter
24
was pro-
posed in 1938, and today it is still the reference for hyperspec-
tral imaging. These filters are commercially available, such
as the VariSpec from Cambridge Research & Instrumenta-
tion (CRI). The principle of such a filter is best explained
by considering Fig. 5. The device consists of a number of
parallel polarizers with planar nematic liquid-crystal cells in
between. The thickness of the liquid-crystal cells doubles
with respect to the previous one: d
i+1
= 2.d
i
= 2
i
d
1
.The
transmission of two polarizers in combination with one LC
cell gives rise to the following transmission:
T =
1
2
cos
i
2
2
=
1
2
cos
πnd
i
λ
2
. (2)
By creating a stack of N steps, we obtain a transmission
T =
1
2
N
i=1
cos
2
i−1
1
2
2
=
1
2
sin(2
N −1
1
)
2
N
sin(
1
/2)
2
. (3)
Fig. 5 Lyot–Ohman filter, which consists of a number of planarly
oriented LC cells in-between parallel polarizers.
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Beeckman, Neyts, and Vanbrabant: Liquid-crystal photonic applications
This results in a transmission spectrum with a central
peak at a certain wavelength and a nearly zero transmission
for other wavelengths. The width of t he transmission peak
decreases with increasing N , and also, the unwanted trans-
mission at other wavelengths is suppressed. By changing
the voltage applied to the different LC cells, the transmitted
wavelength can be tuned. The thickest LC cell determines the
speed of the device; thus, in practical devices, this thickness
is minimized by combining the LC cell with a fixed retarder
made, e.g., of quartz.
Even in the last few years there has been quite some work
on Lyot-Ohman type devices because of the wide range of
applications. Effort has been devoted to, e.g., sensing the
different polarization properties of the incoming light,
25, 26
polarization insensitivity,
27
or wider spectral ranges.
28–31
The main drawback of these Lyot–Ohman filters is the
high loss in transmission because of the series of polariz-
ers. In any case, 50% of the light is lost because of the first
polarizer. Additionally, practical polarizers only have a trans-
mission of about 30–40% for unpolarized light.
Another class of tunable filters is based on the use of
Holographic polymer dispersed liquid crystals. The liquid
crystal is actually a mix of polymerizable material (typically,
UV curing glue) and nematic LC. Interference of two UV
laser beams, leading to a sinusoidal UV intensity, gives rise
to polymer-rich regions and liquid-crystal rich regions in
the cell in a periodic way. Writing a grating that is along
the substrates leads to switchable diffractive gratings.
32
A
grating with a period perpendicular to the substrates leads
to a wavelength dependent filter with typically a reflection
for a rather narrow wavelength band.
33, 34
These gratings can
be switched off by applying a voltage, providing that the
refractive index of the polymer is matched with either n
⊥
or n
. The switching speed of these gratings are typically
<1ms.
3.2
Optical Switches in Free Space
Optical switches that switch light from one input fiber to a
certain output fiber is not a simple task. Light in the fiber is
caught in a high refractive index region and a set of high-
precision lenses is necessary to couple the light coming out
of one fiber into an output fiber with low losses. On top of
that, it is necessary to perform switching with an electro-
optic element with a certain speed and a minimum of cross
talk. A very simple design for an N ×N switch (i.e. with
N inputs and N outputs) is shown in Fig. 6. Light from
two input fibers goes to a matrix of 2 ×2 LC shutters by
means of 50/50 splitters. The LC shutter either transmits or
blocks the light. The light that passes through the shutter is
then coupled into an output fiber. Again, a 50/50 splitter is
necessary. In this way, an N ×N switch can be fabricated with
any arbitrary value of N , but in practice, this geometry is not
usable because the losses are too high. First, for an N×N
switch, only a factor 1/N
2
of the light is coupled into the
desired output. Additionally, 50% of the light is lost because
the switch is usually polarization dependent.
A number of free-space switches have been presented in
literature based on liquid crystals. There is nice, but out-
dated review by d’Alessandro and Asquini.
35
Different de-
signs have been presented in order to reduce losses and cross
talk and avoid the polarization dependency.
36–38
However,
most of the promising free-space LC switches are based on
Fig. 6 Simple 2×2 optical fiber switch based on LC shutters.
a different principle, which is shown in Fig. 7. Light from an
input fiber is projected onto a spatial light modulator (SLM)
by means of a lens. The SLM changes the phase of the beam
in a pixelated way after relection and in the Fourier plane of
the lens, a certain intensity profile can be created by loading
the correct phase pattern onto the SLM. By loading differ-
ent phase patterns onto the SLM, the light can be coupled
into one (or more) of the output fibers. These switches are
actually based on holographic projection, and a number of
devices have been demonstrated.
39, 40
SLMs and their appli-
cations are discussed in more detail in Sec 4.
3.3
Liquid-Crystal Lenses
Lenses with an electrically tunable focal length and/or a
repositionable focus are of broad interest for a number of
applications. Because there are no moving parts in such
a lens, they can be more reliable and able to withstand
mechanical shock. Such tunable lenses can be applied in
adaptive binocular glasses, autofocus cameras, LED light-
steering applications,
41
3-D confocal microscopy,
42
etc. A
wide number of approaches have been studied to realize
tunable lenses, and some of them are illustrated in Fig. 8.
Figure 8(a) shows a configuration with a LC layer with
nonuniform thickness. By using a liquid-crystal with n
⊥
<
n
glass
< n
, it possible to tune the lens, going from con-
cave to convex.
43
The biggest problem with LC is the
fact that such lenses only work with one linear polariza-
tion component of the light so that a polarizer is necessary.
Fig. 7 Optical switch in freespace based on holographic projection.
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