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Efficient nonlinear optical ferroelectric liquid crystals for integrated optics devices
K. Schmitt, C. Benecke, M. Schadt, J. Fünfschilling, R. Herr, R. Buchecker
To cite this version:
K. Schmitt, C. Benecke, M. Schadt, J. Fünfschilling, R. Herr, et al.. Efficient nonlinear optical ferroelectric liquid crystals for integrated optics devices. Journal de Physique III, EDP Sciences, 1994, 4 (2), pp.387-400. �10.1051/jp3:1994121�. �jpa-00249110�
Classification Physic-s Abstracts
42.80 42.65 61.30
Efficient nonlinear optical ferroelectric liquid crystals for
integrated optics devices
K. Schmitt, C. Benecke, M. Schadt, J. Ffinfschilling, R. P. Herr and R. Buchecker F. Hoffmann-La Roche Inc., Dept. RLCR, 4002 Basel, Switzerland
(Received 9 July 1993, accepted J9 November 1993)
Abstract. The linear electro-optical effect of a novel ferroelectric liquid crystal (FLC) with
exceptionally large and stable second order nonlinear optical jnlo) response (d~~
= 5 pm/V) is
investigated. The electro-optical coefficients r~j are compared with the corresponding nlo coefficients dj,. It is shown that the electro-optical effect is mainly of electronic origin. This indicates that molecular motions of the nlo-FLC are negligible below its glass transition temperature. Moreover, waveguiding in planar short pitch bistable ferroelectric (SBF) configura-
tions comprising the new nlo-FLC material is demonstrated.
1. Introduction.
Recently we have reported novel short pitch ferroelecuic liquid crystals (FLCS) which have been designed for nonlinear optical (nlo) applications Ii- The new nlo FLCS are characterized by a nlo-active para-nitro-aniline group incorporated into the core of the FLC molecule such
that it is adjacent to the chiral center of the molecule. Due to the strong interaction of the nlo
chromophore with the chiral group their mutual orientations are correlated such that the nlo- active charge-transfer axis of the chromophore exhibits a large component parallel to the polar axis of the FLC. This design leads to an unusually pronounced second harmonic response (SHG) which renders the new compounds useful to generate coherent blue-green light.
Furthermore the new nlo FLCS are strongly birefringent and hence allow for phase matching.
Waveguiding in homeotropically as well as in planar aligned layers becomes possible.
In the following we focus on one particularly interesting member of this new class of nlo- FLC materials, namely on compound I (Tab. I). Apart from being ferroelectric and nlo-active, the short pitch compound I exhibits a glass transition above room temperture. This allows to
freeze in the nlo active, unwound helix configuration below T~ such that macroscopically
uniform polar layers of optical quality result. Despite the glass transition temperature being only lo °C above room temperature these layers are long term stable at room temperature and show no tendency for reorientation or crystallization over weeks. In this respect compound I behaves quite differently compared with poled amorphous nlo polymers where the polar order
decays within minutes under comparable conditions. The reason for this remarkable difference
388 JOURNAL DE PHYSIQUE III N° 2
Table I.- Structure,- phase ti"ansitions and spontaneous polarization of compound I
P~ is measured at loo °C.
N02 O
Compound C6Hj3 /
O
~-
O O O O C9Hj9
CH3 NH2
Phase transitions/°C C-Sc* Sc*-SA SA-1
80 132 186
Ps/(nC/cm2) 3 5 0 at 00 °C
in stability is a consequence of the electric field-induced unwound helix configuration, which is identical with the optically bistable states of a short pitch bistable ferroelectric (SBF)- configuration [2]. The relaxation of the metastable nlo-active SBF-configuration into a nlo- inactive helical configuration requires a collective motion of all molecules which is strongly suppressed in the glassy state.
Thus glassy short pitch nlo-FLC compounds represent a new class of interesting nlo- materials which combine the following important properties required for nonlinear optical
devices (ii thermodynamically stable noncentrosymmetric orientation of a high degree of
order, (iii waveguiding ability, (iii) phase matchability and (iv) transparency down to the blue- green spectral region.
In the following the electro-optical coefficients rjj of compound I are determined. These coefficients govem the performance of the new nlo-materials in electro-optical modulators or switches. Comparisons are made with the corresponding djj coefficients of the nlo tensor which reveal the electronic and motional contributions to the total electro-optical coefficients.
2. Experimental.
The synthesis of compound I will be published elsewhere. DSC spectra were determined with
a Perkin Elmer DSC7 instrument. The usual triangular voltage method was applied to
determine the spontaneous polarization of the FLC-samples i>emits temperature. Observations of liquid crystalline textures and smectic C* (St j tilt angle measurements were performed
with a polarizing microscope.
In reference I we determined the refractive indices and the dispersion of compound I from
waveguiding experiments in homeotropically oriented layers. Here we confirm these earlier
data by means of a modified Abbe refractometer (Tab. III. Within experimental error it
surprinsingly turnes out that compound is optically uniaxial. The refractive indices which are relevant to analyse the results below are summarized in table II.
The samples used for our electro-optical experiments are 8 ~Lm thick planar oriented layers (Fig. I). Cells with ITO coatings and polyimide alignment layers were filled by capillary
action at 180 °C. The samples were slowly cooled down to 130 °C, that is, just below the
S~/S/ transition. At this temperature an electric ac field of lo V/~Lm and 1.2 kHz was applied
Table II. -Refi.actii~e indices of compound I at various war,elengths.
A/nm 633 543 488 476 457
n, = n~ 1.554 1.575 1.595 1.610 1.621
n~ 1.682 1.702 1.724 1.735 1.743
v
, y
i
i
z i
i i
i
'
' Z
a) b)
Fig, I. Geometries applied in the experiments a) top view, j- -l'rubbing direction, (-) smectic
layers, b) side view, Fs
= spontaneous polarization,
H~ = tilt angle.
which induced a well aligned homogeneous SBF-configuration, Under the aligning field the samples were further cooled to 50 °C, During cooling the viscosity of the material increases, an effect which was compensated for by continuously decreasing the frequency of the electric field, At 50 °C the ac aligning field was removed and a dc field of lo V/~Lm was applied while the cell was quenched to room temperature, I-e- below T~ then the dc field was switched off.
After this alignment and poling treatment the configuration of the sample remains frozen in one of the two short pitch bistable (SBF) configurations, that is, in a field-aligned frozen quasi
bookshelf configuration [2], The apparent tilt angle of the director with respect to the substrate
rubbing direction was 27°.
The electro-optical experiments were performed at the wavelengths 633 nm, 543 nm (HeNe lasers) and at 488 nm, 476 nm, and 457 nm (Ar+ laser). An electric modulation voltage of 30 V~~~ and I kHz was applied to the cell electrodes. Phase modulation as well as amplitude
modulation techniques were used to determine the electro-optical coefficients and their
dispersion. The experimental set-up and the data analysis are described below.
3. Electro-optical experiments.
The linear electro-optical effect of a dielectric is defined by [3, 4]
_~~~EK Alfb~)"~
j
iJ '
390 JOURNAL DE PHYSIQUE III N° 2
where s~ is the optical perrnittivity tensor and EK is the applied electric field in K-direction.
The tensor rjjK is determined by the symmeuy of the FLC material, which is monoclinic and
belongs to the point group C2. In contracted notation the tensor has then the form
0 rj~ 0
0 ri~ 0
r,j =
° r~~ o
~41 0 r~~
0 r5~ 0
~61 ° ~63
where the 2
= ~y) axis is parallel to the C2 symmetry axis, the 3
= (z axis is parallel to the director, and the I
= ix ) axis is normal to z and y (Fig. I). In our experiment the modulating
electric field E is applied in y-direction (longitudinal eo-effect). E modulates the refractive index which is given by the equation of the index ellipsoid
l~~ + rj ~
~~ x~ + +
r E y2 ~
l ~
n
<
n( ~ ~ ~ ~2 ~ ~3>Ey z + 2 zxr~ ~E~ =
I jI
z
The coefficients rj~, r~i, and r~~ are determined below. While the electric field E~ changes the
lengths of the axes n~, n~, and n~ of the index ellipsoid directly via the coefficients rj~, r~i, and r~i without changing their direction, the last mixed term in ii rotates the ellipsoid
about the y-axis. As was shown by Yariv [3] this rotation is negligible if the coefficient is small and if n~ and n~ are sufficiently different, which is the case in our system (n~-
n~ = 0.15). Therefore the rotation of the index ellipsoid can be neglected in our analysis. For small perturbations (rE « I/n~) this leads to
Anj
=
(n), rj~,. E~) I
= 1, 2, 3. (2)
2
The electric field-induced index changes An, were determined in the usual way via phase- and amplitude modulation techniques. The first method allows to evaluate the rjj coefficients
individually. From amplitude modulation experiments follows the difference between the components. This difference is used to cross check the results obtained in the first experiment.
3.I PHASE MODULATION. The phase modulation experiments are performed in the
Michelson interferometer configuration [5] depicted in figure 2. The sample is oriented such that the 3-direction is parallel to Z. The light beam travelling in Y-direction transverses the
sample twice it is polarized either parallel to X or to Z. The beam along the second arm passes
a slowly rotating glass plate which continuously alters the optical path length of this arm. The two coherent beams are combined in front of the detector, thus giving rise to a well resolved
fringe pattem. The resulting intensity I of the two interfering beams of intensity Ii and I~ is given by
1
= Ii + Ii + 2 Q@
cos
A45 " n)r,~ 2LEy
,
(3)
A where
~~ ~~ ~~~' ~ ~~' ~ ~
and L is the thickness of the FLC layer at normal incidence. AL is the path difference between
j
~~m~~nenfx,y-Recorder 0eleclor
dc Componenl
~~~~r
~ Aperture
+~
Filler Lens
Beamspliffer
d Mirror
Mirror 2 Sample Polarizer
Phase plate
y~~
Mirror 3
Fig. 2. Schematic drawing of the Michelson interferometer used in the investigation.
the beams in the two interferometer arms including the phase plate. The experiments are
conducted such that the phase plate monotonously increases the optical path difference. From
applying a weak sinusoidal modulation voltage with amplitude Uo «V~ to the sample it follows for
A45
= ± ar in + I n
= 0, 1, 2..
~
~ ~
(4) 1=Ii +I~-2.,fi.I.n/.ij~,-.Uosin (wt).
From the definition
~'
" 'max 'm~n ~
~ /~ (~)
one obtaines from (3)
'eff " ( ) ''ll'
~ly '~j '~eff 16)
Uo where U~~~ =
-.
This finally leads to
/
j,,~ =
~ ~ ~~~~~
(7)
~ W 2 L AI n( U~j~
392 JOURNAL DE< PHYSIQUE III N° 2
I~~~ is detected via a lock-in amplifier as the ac component of the detector output, while the
intensity I is the dc component of the signal (Fig. 2). When recording both signals simultaneously on an x, y-recorder the recorder trace has the form of an ellipse whose main
axes are proportional to 2I~~~ and AI respectively. From determining U~~~ with a rrns-voltmeter the coefficients (rj~,( follow from equation (7). Figure 3 shows a typical recorder trace.
~~ ~~eff
) 'min 'max
z~
~
c 0
~~ 1J2
o
j~
E ~~eff
0 2 3 4
Intensity / V
Fig. 3. Typical x, y-recorder trace showing the ellipse obtained in the r~~-experiment ; 1
= 633 nm,
modulation frequency
= 1.2 kHz, sinusoidal modulation voltage U~~~ 30 V, light polarization fl z.
The three components rj~, r~~ and r~~ of the electro-optical tensor ijj are determined by applying appropriate input light polarizations. ri~ and r~~ are measured at normal incidence and
polarizations parallel to the I and 3 direction respectively. Component r~~ is not directly
accessible. However, it can be extracted from the angular dependence of r~~~(H') when the
sample is turned about the 3-axis and the light beam is polarized in the 1, 2-plane (Fig. I). It
can be shown that
r~~~(8')
= r~~ sin~ (8~) + r,~ cos~ (8') (8)
holds when optical uniaxiality (n~ = n~) is assumed.
3.2 AMPLITUDE MODULATION. Figure 4 schematically shows the equipment used for the
amplitude modulation experiments. The sample is positioned between crossed polarizers and
Polarizer Sample Analyzer Filler
i i
~ °~~~
i
.F45° -45°
Apcrlure Compensalor Aperlure Pholodiode
U
Fig. 4. Schematic drawing of the amplitude modulation apparatus.
the incoming laser light is polarized at + 45° with respect to the optical axis (3-direction) of the
sample~ The sample transmission in this geometry is [4]
The adjustable optical retardation A45 is induced by the compensator. Setting the compensator
such that A~b = I
at zero electric field and applying a modulation voltage U « V
~
leads to : 2
~ ~
2A,d.I~~~
~ ~~ ~~~ ~~' ~~~
ar U~~~. L lo ~~~~
The coefficient r* is the quantity directly determined in this type of experiment.
3.3 RESULTS. As discussed above the coefficients ri~ and r~~ can directly be calculated from equation (7); the overall precision of the measurements being about lo fb. The
coefficient r~~ is extracted from the angular dependence of i~j~( depicted in figure 5 and from equation (8). This leads to an error for r~~ of about 15 fb. Table III shows the coefficients r12, r22, r32, and r~~~ (H
=
45° ), as well as the corresponding reduced halfwave voltages
v~ = Al(n~ r~~~) determined at several wavelengths. A strong dispersion is observed which is due to the chromophore absorption of the nlo-FLC material. The maximum of the absorption
lies at 380 nm.
2
E(>
~
/~
m~
j X= 632nm
-=jr jsjn2@'+j~ j~~~28'
22 21
30 60 90
angle of incidence 8 /deg
Fig. 5. H-dependence of i~jj, Hand H'are defined in figures and 2 (H H,~).
For a crosscheck ri~ and r~~ were determined also with the amplitude modulation method. In this case the right hand side of equation (10) is directly measured, while the left hand side of (10) follows from the data of the phase modulation experiments. As shown in table IV both sets of data are in good agreement. Moreover, the agreement proves that rj~ and r~~ exhibit the
same sign. The weak chevron pattern which stabilizes SBF-configurations and which induces
394 JOURNAL DE PHYSIQUE III N° 2
Table III. -Electra-optical coefficients rj~, r~~, r~~~ (o =45°), r~~ and corresponding
reduced halJivave i,oltages u~ determined at various wavelengths.
A/nm r,~ /P) vj2/kv r~~ / f
vj2/kv
~ v©«/kv ~ v12/kv
632 0.58 = 0.05 300 ± 30 0.62 = 0.06 220 = 20 1.3 ± 0.1 136 = 15 3.8 ± 0.5 44 = 5
543 1.01 ± 0,1 137 ±15 1.05 = 0.1 107 ± 10 2,1 = 0,1 66 ± 6 6.4 = 0.8 21 ± 3
488 1.57 = 0.2 79 ± 8 1.39 = 0.2 70 ± 7 2.7 = 0.2 45 ± 5 7.2 = 1.5 17 ± 2
476 1.88±0.2 60=6 1.53±0.2 59±6 3.7±0.2 30=3 11.6=2 16±2
457 2.7 ±0.2 39=3 2.3 ±0.2 34=3 5.2±0.2 21=2 14.9=2 7±1.5
Table IV. -Comparison of r*
=
n(ri~ -n)r~~ determined in the amplitude modulation experiment and calculated from the phase modulation data.
r*/pm/V) r*/(pm/V)
(nm) (Amplitude (Phase
modulation) modulation)
633 0.79 0.77
543 1.27 1.23
small variations of the smectic director and the jefractive indices [2] does hardly affect the
electro-optical coefficients rjj. The slight alteration of the refractive indices due to averaging
over the periodic modulation of the director is negligible compared with the overall
experimental error.
4. Frequency doubling experiments.
The generation of second harmonic light in FLC-materials is governed by the tensor of the second order nlo coefficients djj defined by :
0 0 0 dj~ 0 di~
djj
= d~i d~~ d~3 0 d~5 0
° ° ° ~34 ° ~36
where dj4
= d~~ = d~~, d~i = di~, and d~~ = d~4 (assuming Kleinman degeneracy).
Earlier Ii we have determined these coefficients from the angular dependence of the SHG-
signal in homeotropically oriented nlo-FLC samples. In particular, type I phase matching (eeo experiment) has been demonstrated Iii. In these earlier experiments, the coefficient d~~ was directly determined in a d
=
2 ~Lm thick cell (d« coherence length) at normal
incidence (ooo experiment), while the other three components were extracted from the fit to the
phase matching curve [1, 61 determined in d
=
20 ~Lm thick cells. The coefficients that resulted from the generation of second harmonic light of a 1064 nm Nd : YAG laser are [Ii :
d~~ =
5 pm/V
,
dj4
= 0.49 pm/V
,
dj~
=
1.13 pm/V and d~~ = 1.46 pm/V
These coefficients are by far the largest ever observed in FLC materials. In particular
d~~ which is, thanks to the transverse nlo-molecular transition moment of compound I (Tab. I), accessible in planar waveguide configurations is large enough to be of practical interest for the
design of harmonic waveguide devices. The excellent performance is a consequence of our
design strategy of integrating an efficient (but compact) nlo-chromophore into the core of a FLC-structure adjacent to its chiral center (Tab. I). The interaction of the chiral group with the
chromophore forces the nlo-active axis to align parallel to the C2-axis of the SBF-FLC configuration.
5. Comparison of the electro-optical coeffcients with the nlo coefficients.
The elctronic contribution r)j to the total electro-optical coefficient rjj can be calculated from the corresponding nlo coefficient djj if the frequency dependence of the coefficients
I)jis taken into account [7, 8]. Assuming a two level model for the molecular hyperpolarizabili- ty the relation between r)j and djj follows from [7]
4 d)j~
r)j~
=
,
I I) (n~ n~)~
where
~~ ~ ~~~~~
f~ f/ f( (3 WI- w~). (WI- w'~)~. (wj-4w'~)
"~ ~~~ f(~ f~ f~ (3 wj 3
w
'~ WI w ~)~ WI ~
x djj§~ ~~ ~ (12) In equation (12) w' is the frequency of the fundamental wave in the SHG experiment, w is the
frequency of the light used in the electro-optical experiment, wo is the frequency which corresponds to the maximum of the first absorption band of the sample and 0 is the frequency of the modulating electric field applied to the sample. fare Lorentzian type local field factors of the form
fw (~~ ~ ~)
f0 ~(~~ ~ ~)
3 ~2 ~ ~
~
which are discussed in [9]. In our samples the dielectric constant e~ was measured at I kHz
e~ =3.5. In table V the coefficients r/j which follow from the above electro-optical
experiments are compared with the calculated values r)j that follow from the SHG experiments
and from equations (I I) and (12). The good agreement between the coefficients clearly shows that the origin of the electro-optical effect in our nlo-FLC material is basically electronic in
nature. This is further confirmed by the observed dispersion of the coefficients (Tab. V) which
agrees with the assumed two level model [7].
6. Waveguiding.
Waveguiding in planar unwound helix configurations is a prerequisite for the technical
applicability of the nlo-FLC materials in thin films. This is because in this geometry full use of the large d~~ or r~~ coefficients can be made for frequency doubling or electro-optical
modulation (Pockels effect). Moreover the ratio L/d in this configuration can be made very
large, where L is the path length of the guided wave and d is the electrode distance of the
waveguide modulator. This leads to low modulation or switching voltages.
Waveguiding experiments with oriented FLC-films require a cell design which provides for electro-optical switching (ITO coating), FLC-alignment treatment and at the same time
