- Email: [email protected]
Influence of molecular tilt angle on the SHG response of pyroelectric liquid crystal polymers
January 1998
Optical Materials 9 (1998) 220-225
ELSEVIER
Influence of molecular tilt angle on the SHG response of pyroelectric liquid crystal polymers Jonas &tegren a, Philippe Busson a, Mikael Lindgren b,c,*, David S. Hermann d, Per-Otto Arntzen ‘, Fredrik Sahl6n a, Mikael Troll&s a, Ulf W. Gedde a, Anders Hult a, Lechezar Komitov d, Sven T. Lagerwall d, Per Rudquist d, Bengt Stebler d a Department of Polymer Technology, Royal Institute of Technology, S-100 44 Stockholm, Sweden b IFM-Chemical Physics, Linkiiping University, S-581 83 Linkiiping, Sweden ’ National Defense Research Establishment, Division of Laser Systems, P.O. Box, S-581 I I Linkijping, Sweden d Physics Department, Chalmers Unioersity of Technology, S-412 96 Gb’teborg, Sweden
Abstract Second harmonic generation in novel pyroelectric liquid crystal polymers (PLCP) made from a series binary mixtures, was studied using 1100 nm as the fundamental wavelength. The PLCPs were prepared by photo-polymerization of binary mixtures of two monomers which exhibit a smectic C * phase, A2c (4”-{( I?)-( - )-2-[( IO-acrylo-yloxy)decyl]oxy}-3nitrophenyl 4-{4’-[(1l-acryloyloxy)-undecyloxy]phenyl}benzoate) and Alb (4”-((RI-( + )-2-octyloxy)-3”-nitrophenyl 4-(4’(( 1 1-acryloyloxy)undecyloxy)-phenyl)-benzoate). The highest d,, and d,, coefficients were found to be in the range 0.65-0.8 pm/V, and differed depending on the detailed preparation of the sample. All cases of polymers formed from the chiral smectic C * phase showed an SHG-signal with no external field present, indicating that polar order became fixed. The SHG-signal was found to increase with the tilt angle of the FLC molecules. 0 1998 Elsevier Science B.V.
1. Introduction Ferroelectric liquid crystals (FLC) are made up by chiral molecules having a pronounced elongated shape, giving rise to an intrinsically non-centrosymmetric system having high orientational order [1,2]. The FLCs have the possibility of forming domains possessing a spontaneous polarization, and in certain systems it is possible to switch between several states which make them useful in display technology
* Corresponding author. Tel.: +46-13-282485; fax: +46-13137568, e-mail: [email protected].
devices and other devices based on modification of the polarization state of the light [3]. The FLC molecules can be modified to contain additional functionality, such as, enhanced nonlinear optical activity [4,5] and/or ability for polymerization [6-S]. The ability to control both orientation and functionality of these novel materials could be interesting to exploit further in certain device applications based on the electrooptic function, such as devices based on the photorefractive effect, spatial light modulators, and integrated electrooptically active waveguides [9]. Here we give a brief summary of the nonlinear optical properties in terms of second harmonic gen-
00925-3467/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PII SO925-3467(97)00093-l
J. &tegren
et al. /Optical
eration of pyroelectric liquid crystal monomers (PLCP). The design and synthesis of the related materials have been reported on previously [6]. The samples discussed here were made from binary mixtures of two monomers (Fig. 1) which have the built-in property to polymerize, cross-link and also a lateral dipole to enhance the nonlinear optical properties. More details of the SHG-characteristics and electrooptical properties can be found in recent work [lO,l 11. Here it will be focused on how the molecu-
221
Materials 9 (1998) 220-225
lar tilt angle influences PLCP materials.
the SHG-response
of the
2. Methods The materials A2c and Alh (Fig. l>, were synthesized and prepared according to the procedure reported previously [6]. Conventional 4 pm shear cells (EHC) were filled with monomer materials, A2c and Alb, in different proportions, together with a pho-
a
Fig. 1. (a) Alb and Ale monomer molecules used for preparation and sample ( xyz) coordinate frames.
of PLCPs. (b) A scheme of a FLC or PLCP defining the laboratory
(XYZ)
222
J. htegren
et al. / Optical Materials 9 (1998) 220-225
toinitiator, Lucirin TPO. Proper alignment of the bookshelf geometry was obtained by varying the temperature while monitoring the cell structure in a polarization microscope. The ferroelectric switching properties as well as the SmC * tilt angle could be controlled and set prior to the polymerization by applying appropriate voltage on the shear cells. From the measured orientation of the optic axis the tilt angle could be examined after the polymerization had taken place. The frequency conversion from 1100 nm to 550 nm was examined using a modified Maker fringes set-up for second harmonic generation [ 121. The idler output of Sunlite OPO laser (Continuum), tuned to 1100 nm, was employed as pump beam. The details of the experimental set-up for sample positioning, data collection and analysis will appear elsewhere [lo]. By using two rotational degrees of freedom of the sample, 0 and 4 in Fig. lb, it was possible to investigate all kind of polarization combinations be-
tween the pump and SHG light for the specific symmetry of the FIX or PLCP film, i.e., the interaction of two waves, ordinary (0) and/or extraordinary (e), at the pump frequency, to yield o- or e-waves at the second harmonic frequency. 3. Results and discussion 3.1. Second harmonic generation
in the PLCPs
Representative data for p (fundamental) to p (SHG) polarized light and p to s polarized light obtained from a cell filled with the polymerized Alb sample, are shown in Fig. 2a,b, respectively. The plot of SHG intensity corresponding to the p to p conversion expresses a behavior very similar to the FLC monomer case regarding the orientation dependence [lo]. The magnitude of the measured SHG intensity is approximately the same for both the unwound FLC with an applied voltage and the PLCP
Fig. 2. SHG intensity as a function of the angles 8 and 4 of the PLCP based on the Alb monomer: (a) p (pump) to p (detected SHG) conversion, (b) p to s conversion. (c, d) Fits along with the measured SHG intensity of a PLCP made from 100% Alb, tilt angle 30”, using Eqs. (4a) and (4c) given in the text. The refractive indices were taken as no” = 1.47, n,” = 1.57, niW = 1.52, nz” = 1.62, nsaSb = 1.46, n;,zrs = 1.5, Apump= 1.1 pm, film thickness = 4 pm. The angles were approximated according to ezo = 0, and the o-dependent refractive index for the extraordinary wave was neglected (i.e., n,(0) = n,). Samples were measured approximately 5 days after the polymerization reaction.
J. &tegrm
et al. / Optical Materials 9 (1998) 220-225
film, and it appears evident that some of the spontaneous polarization has been ‘fixed’ in the PLCP by the polymer network. The maxima of the p to p conversion in the plot of the $-dependence (Fig. 2a) correspond to the orientations of the sample with the director perpendicular or parallel to the plane of incidence, and thus, it can also be related to the molecular tilt angle [10,13]. This will discussed more in Section 3.2. Using data collected with the x- or z-axes within the plane of incidence it is possible to calculate some of the elements in the d-tensor. The conversion efficiency of the second harmonic light can be written [14]: I 20 TM
2
_-
d&f~~l’ I sin*( Akl/2a)
e,“c,’ nin2w
o
Akl/2r
’
(1)
where the symbols of variables and constants have their usual meaning. Ignoring for the moment the complications which may arise from the biaxiality of the LC system with the sample in certain orientations, there are two parameters which can give rise to the orientation variation, namely the phase-mismatch A k = k, w - 2 k,,, = (4r/h)(n, w - nW), and the effective second order coefficient for frequency doubling, d,,. The phase-mismatch is important particularly near the so called phase match condition for samples thicker than the coherence length, and in the present configuration it is not likely to give a large contribution. The d,, coefficient expresses the coupling between the two interacting polarized waves and a specified polarization of the generated SHG light and generally it depends on the orientation of the sample and the incidence angle. Formally it can be written: deff= e^2,d”::e^,e^,,
(2)
where e^ are unit vectors describing the polarization state of the interacting waves and d” is a third rank tensor with components determined by the symmetry of the system [ 151. In the case of C, point group symmetry, which applies to the FLCs and PLCPs discussed here, the &ensor is given by: 0
0
0
4,
4,
0
0
d,, ’
d,,
0
4,
0
4,
0
0
43
0
d,, ,
,
(3)
223
where the assumption of Kleinman symmetry and the contracted notation, have been used. The relevant interactions possible to measure with the x- and z-axis (Fig. lb) in the plane of incidence are: x-axis in plane of incidence p-p:
d,,, = d,,(sin20,
cos 19~~+ co&,
sin e,,)
+ d,, sin*O, sin 02w p-s:
(4a)
d,, = - d,, sin 2 0,
(4b)
z-axis in plane of incidence p-p:
d,, = d,,(cos*O,
sin 02W + sin20,
cos e,,)
+ d,, sin26, sin 8, w, p-s:
(44
d,, = d,, sin 2 0,.
(44
In the analyses which follow it has been assumed that 020 = 0, [ 131. Thus, the third power dependence of the sin 6 factor for the d,,-component becomes small compared with the linear dependence for the d,, and d,, components, which means that this tensor element cannot be determined accurately for these canonical orientations. The experimental data was fitted to give the values 0.80 and 0.65 pm/V for the d,, and d,, components (Fig. 2c,d), where the absolute values were be obtained by a comparison of the SHG intensity produced in a 530 pm z-cut LiNbO, sample. The parameters used in the fits are given in the associated figure captions. For poled polymer films with axial symmetry (poling axis along z) there is a relation between two of the tensor components: d,, = id,, [16]. Taking the same relation for the polar orientation in our PLCP system (y-axis) the d,, coefficient could be estimated from the d,, and d,, coefficients to be ca. 2-2.5 pm/V [lo]. The d,, component is neglegible as judged from the very small s-polarized SHG intensity upon p-polarized fundamental with the x- and z-axis in the plane of incidence. 3.2. The influence from the molecular
tilt angle
The direction and magnitude of the tilt angle, that is, the angle of the long axis of the FLC molecules with respect to the smectic layers, is crucial for the understanding of the Sm C * phase and a fundamental property for FLC devices. The tilt angle of each
224
.I. htegren
et al. /Optical Materials 9 (1998) 220-225
sample was measured with a polarization microscope. For some samples it was found to deviate from the value set prior to the polymerization reaction. It seems reasonable to attribute such changes of the tilt angle to some kind of relaxation within the polymer system during the polymerization reaction. To find the origin of this effect further detailed studies of the polymerization reaction are necessary. The magnitude of the d,, and d,, coefficients of a series of samples of different composition were obtained by analyzing the SHG intensity vs. O-dependence with the plane of incidence coinciding with the xy and yz planes. The PLCP associated with the pure Alb monomer gave the largest SHG signal, however, values of the d coefficients were found to be approximately the same for a series of PLCPs formed from binary mixtures of the Alb and A2c monomers. For samples having the same or similar tilt angle we found no systematic differences depending on composition of the precursor mixture, however, the magnitudes were found to differ significantly due to variations in the molecular tilt angle. The d,, and d,, coefficients as a function of the tilt angle for a series of PLCP films are shown in Fig. 3a,b. This demonstrates the importance of a large molecular tilt angle in order to obtain a large SHG signal.
4. Summary
and coulusions
The results of second harmonic generation studies of various PLCPs made from binary mixtures have been summarized and showed useful for characterizing the nonlinear optical response following from the spontaneous polarization of these materials. The influence of the molecular tilt angle showed that it is important to design the precursor molecular systems to give a relatively large tilt angle in the polymerized systems. Thus, this consideration together with a large molecular hyperpolarizability are both necessary in order to enhance nonlinear, and presumably electrooptical, properties, of these and related systems.
Acknowledgements Financial support from the National Foundation for Strategic Research of Sweden, the National Board for Technical and Industrial Development (NUTEK), Defence Material Administration (FMV) and the Swedish Research Council for Engineering Sciences (TFR), is acknowledged.
References 111K.D. Singer, J.E. Sohn, S.J. Lalama, Appl. Phys. Lett. 49 (5) (1986) 248-250,
121R.B. Meyer, L. Liebert, L. Strzelecki,
I
10
30
20 Tilt angle [deg]
63
z
,
.
b)
0,2
.
B $!
I
.
0,l
??
.
4..
I
0
10
20
30
Tilt angle [deg]
Fig. 3. (a) d,, and (b) d,, coefficients as a function of the molecular tilt angle of PLCP films measured lo-11 days after polymerization.
P. Keller, J. Phys. (Paris) Lett. 36 (1975) L69-L71. [31 M. Schadt, Liq. Cryst. 14 (1993) 73-104. R. [41 K. Schmitt, R.-P. Herr, M. Schadt, J. Funfschilling, Buchecker, X.H. Chen, C. Benecke, Liq. Cryst. 14 (1993) 1735-1752. 151 D.M. Walba, M.B. Ros, N.A. Clark, R. Shao, K.M. Jonsson, M.G. Robinson, J.Y. Liu, D. Doroski, Mol. Cryst. Liq. Cryst. 198 (1991) 51-60. [61M. Troll&, C. Orrenius, F. Sahltn, U.W. Gedde, T. Norin, A. Hult, D. Hermann, P. Rudquist, L. Komitov, S.T. Lagerwall, J. Lindstriim, J. Am. Chem. Sot. 36 (1996) 8542-8548. t71 M. Troll&s, F. Sahlen, U.W. Gedde, A. Hult, D. Hermann, P. Rudquist, L. Komitov, S.T. Lagerwall, B. Stebler, J. Lindstrijm, 0. Rydlund, Macromolecules 29 (1996) 2590. w A. Hult, F. SahlBn, M. Troll&s, ST. Lagerwall, D. Hermann, L. Komitov, P. Rudquist, B. Stebler, Liq. Cryst. 20 (1996) 23.
J. btegren
[9] L.A.
et al. / Optical
Hornak (Ed.), Polymers for Lightwave and Integrated Optics - Technology and Applications, Dekker, New York, 1992. [lo] M. Lindgren, D.S. Hermann, J. Grtegren, P.-O. Arntzen, U.W. Gedde, A. Hult, L. Komitov, S.T. Lagerwall, P. Rudquist, B. Stebler, F. SahlCn, M. Troll&s, J. Opt. Sot. Am. B 101 (19981, in press. 1111 D.S. Hermann, Interaction of light with liquid crystals. Development of novel polymer nonlinear optical materials, Thesis, GGteborg (1997) ISBN: 91-7197-453-9.
Materials
9 (1998) 220-225
225
S.K. Kurtz, J. Appl. Phys. 41 (4) (1970) [121 J. Jerphagnon, 1667-1681. [131 B. Park, M. Lim, J.-H. Lee, J.-H. Kim, S.-D. Lee, Ferroelectrics 179 (1996) 23 l-240. [I41 P.N. Prasad, D.J. Williams, Wiley, New York, 1991. t151 V.G. Dmitriev, G.G. Gurzadyan, D.N. Nikogosyan, Handbook of Nonlinear Optical Crystals, 2nd ed., Springer, Berlin, 1997. K.D. Singer, M.G. Kuzyk, J.E. Sohn, J. Opt. Sot. Am. B 4 t161 (6) (1987) 968-976.
Optical Materials 9 (1998) 220-225
ELSEVIER
Influence of molecular tilt angle on the SHG response of pyroelectric liquid crystal polymers Jonas &tegren a, Philippe Busson a, Mikael Lindgren b,c,*, David S. Hermann d, Per-Otto Arntzen ‘, Fredrik Sahl6n a, Mikael Troll&s a, Ulf W. Gedde a, Anders Hult a, Lechezar Komitov d, Sven T. Lagerwall d, Per Rudquist d, Bengt Stebler d a Department of Polymer Technology, Royal Institute of Technology, S-100 44 Stockholm, Sweden b IFM-Chemical Physics, Linkiiping University, S-581 83 Linkiiping, Sweden ’ National Defense Research Establishment, Division of Laser Systems, P.O. Box, S-581 I I Linkijping, Sweden d Physics Department, Chalmers Unioersity of Technology, S-412 96 Gb’teborg, Sweden
Abstract Second harmonic generation in novel pyroelectric liquid crystal polymers (PLCP) made from a series binary mixtures, was studied using 1100 nm as the fundamental wavelength. The PLCPs were prepared by photo-polymerization of binary mixtures of two monomers which exhibit a smectic C * phase, A2c (4”-{( I?)-( - )-2-[( IO-acrylo-yloxy)decyl]oxy}-3nitrophenyl 4-{4’-[(1l-acryloyloxy)-undecyloxy]phenyl}benzoate) and Alb (4”-((RI-( + )-2-octyloxy)-3”-nitrophenyl 4-(4’(( 1 1-acryloyloxy)undecyloxy)-phenyl)-benzoate). The highest d,, and d,, coefficients were found to be in the range 0.65-0.8 pm/V, and differed depending on the detailed preparation of the sample. All cases of polymers formed from the chiral smectic C * phase showed an SHG-signal with no external field present, indicating that polar order became fixed. The SHG-signal was found to increase with the tilt angle of the FLC molecules. 0 1998 Elsevier Science B.V.
1. Introduction Ferroelectric liquid crystals (FLC) are made up by chiral molecules having a pronounced elongated shape, giving rise to an intrinsically non-centrosymmetric system having high orientational order [1,2]. The FLCs have the possibility of forming domains possessing a spontaneous polarization, and in certain systems it is possible to switch between several states which make them useful in display technology
* Corresponding author. Tel.: +46-13-282485; fax: +46-13137568, e-mail: [email protected].
devices and other devices based on modification of the polarization state of the light [3]. The FLC molecules can be modified to contain additional functionality, such as, enhanced nonlinear optical activity [4,5] and/or ability for polymerization [6-S]. The ability to control both orientation and functionality of these novel materials could be interesting to exploit further in certain device applications based on the electrooptic function, such as devices based on the photorefractive effect, spatial light modulators, and integrated electrooptically active waveguides [9]. Here we give a brief summary of the nonlinear optical properties in terms of second harmonic gen-
00925-3467/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved. PII SO925-3467(97)00093-l
J. &tegren
et al. /Optical
eration of pyroelectric liquid crystal monomers (PLCP). The design and synthesis of the related materials have been reported on previously [6]. The samples discussed here were made from binary mixtures of two monomers (Fig. 1) which have the built-in property to polymerize, cross-link and also a lateral dipole to enhance the nonlinear optical properties. More details of the SHG-characteristics and electrooptical properties can be found in recent work [lO,l 11. Here it will be focused on how the molecu-
221
Materials 9 (1998) 220-225
lar tilt angle influences PLCP materials.
the SHG-response
of the
2. Methods The materials A2c and Alh (Fig. l>, were synthesized and prepared according to the procedure reported previously [6]. Conventional 4 pm shear cells (EHC) were filled with monomer materials, A2c and Alb, in different proportions, together with a pho-
a
Fig. 1. (a) Alb and Ale monomer molecules used for preparation and sample ( xyz) coordinate frames.
of PLCPs. (b) A scheme of a FLC or PLCP defining the laboratory
(XYZ)
222
J. htegren
et al. / Optical Materials 9 (1998) 220-225
toinitiator, Lucirin TPO. Proper alignment of the bookshelf geometry was obtained by varying the temperature while monitoring the cell structure in a polarization microscope. The ferroelectric switching properties as well as the SmC * tilt angle could be controlled and set prior to the polymerization by applying appropriate voltage on the shear cells. From the measured orientation of the optic axis the tilt angle could be examined after the polymerization had taken place. The frequency conversion from 1100 nm to 550 nm was examined using a modified Maker fringes set-up for second harmonic generation [ 121. The idler output of Sunlite OPO laser (Continuum), tuned to 1100 nm, was employed as pump beam. The details of the experimental set-up for sample positioning, data collection and analysis will appear elsewhere [lo]. By using two rotational degrees of freedom of the sample, 0 and 4 in Fig. lb, it was possible to investigate all kind of polarization combinations be-
tween the pump and SHG light for the specific symmetry of the FIX or PLCP film, i.e., the interaction of two waves, ordinary (0) and/or extraordinary (e), at the pump frequency, to yield o- or e-waves at the second harmonic frequency. 3. Results and discussion 3.1. Second harmonic generation
in the PLCPs
Representative data for p (fundamental) to p (SHG) polarized light and p to s polarized light obtained from a cell filled with the polymerized Alb sample, are shown in Fig. 2a,b, respectively. The plot of SHG intensity corresponding to the p to p conversion expresses a behavior very similar to the FLC monomer case regarding the orientation dependence [lo]. The magnitude of the measured SHG intensity is approximately the same for both the unwound FLC with an applied voltage and the PLCP
Fig. 2. SHG intensity as a function of the angles 8 and 4 of the PLCP based on the Alb monomer: (a) p (pump) to p (detected SHG) conversion, (b) p to s conversion. (c, d) Fits along with the measured SHG intensity of a PLCP made from 100% Alb, tilt angle 30”, using Eqs. (4a) and (4c) given in the text. The refractive indices were taken as no” = 1.47, n,” = 1.57, niW = 1.52, nz” = 1.62, nsaSb = 1.46, n;,zrs = 1.5, Apump= 1.1 pm, film thickness = 4 pm. The angles were approximated according to ezo = 0, and the o-dependent refractive index for the extraordinary wave was neglected (i.e., n,(0) = n,). Samples were measured approximately 5 days after the polymerization reaction.
J. &tegrm
et al. / Optical Materials 9 (1998) 220-225
film, and it appears evident that some of the spontaneous polarization has been ‘fixed’ in the PLCP by the polymer network. The maxima of the p to p conversion in the plot of the $-dependence (Fig. 2a) correspond to the orientations of the sample with the director perpendicular or parallel to the plane of incidence, and thus, it can also be related to the molecular tilt angle [10,13]. This will discussed more in Section 3.2. Using data collected with the x- or z-axes within the plane of incidence it is possible to calculate some of the elements in the d-tensor. The conversion efficiency of the second harmonic light can be written [14]: I 20 TM
2
_-
d&f~~l’ I sin*( Akl/2a)
e,“c,’ nin2w
o
Akl/2r
’
(1)
where the symbols of variables and constants have their usual meaning. Ignoring for the moment the complications which may arise from the biaxiality of the LC system with the sample in certain orientations, there are two parameters which can give rise to the orientation variation, namely the phase-mismatch A k = k, w - 2 k,,, = (4r/h)(n, w - nW), and the effective second order coefficient for frequency doubling, d,,. The phase-mismatch is important particularly near the so called phase match condition for samples thicker than the coherence length, and in the present configuration it is not likely to give a large contribution. The d,, coefficient expresses the coupling between the two interacting polarized waves and a specified polarization of the generated SHG light and generally it depends on the orientation of the sample and the incidence angle. Formally it can be written: deff= e^2,d”::e^,e^,,
(2)
where e^ are unit vectors describing the polarization state of the interacting waves and d” is a third rank tensor with components determined by the symmetry of the system [ 151. In the case of C, point group symmetry, which applies to the FLCs and PLCPs discussed here, the &ensor is given by: 0
0
0
4,
4,
0
0
d,, ’
d,,
0
4,
0
4,
0
0
43
0
d,, ,
,
(3)
223
where the assumption of Kleinman symmetry and the contracted notation, have been used. The relevant interactions possible to measure with the x- and z-axis (Fig. lb) in the plane of incidence are: x-axis in plane of incidence p-p:
d,,, = d,,(sin20,
cos 19~~+ co&,
sin e,,)
+ d,, sin*O, sin 02w p-s:
(4a)
d,, = - d,, sin 2 0,
(4b)
z-axis in plane of incidence p-p:
d,, = d,,(cos*O,
sin 02W + sin20,
cos e,,)
+ d,, sin26, sin 8, w, p-s:
(44
d,, = d,, sin 2 0,.
(44
In the analyses which follow it has been assumed that 020 = 0, [ 131. Thus, the third power dependence of the sin 6 factor for the d,,-component becomes small compared with the linear dependence for the d,, and d,, components, which means that this tensor element cannot be determined accurately for these canonical orientations. The experimental data was fitted to give the values 0.80 and 0.65 pm/V for the d,, and d,, components (Fig. 2c,d), where the absolute values were be obtained by a comparison of the SHG intensity produced in a 530 pm z-cut LiNbO, sample. The parameters used in the fits are given in the associated figure captions. For poled polymer films with axial symmetry (poling axis along z) there is a relation between two of the tensor components: d,, = id,, [16]. Taking the same relation for the polar orientation in our PLCP system (y-axis) the d,, coefficient could be estimated from the d,, and d,, coefficients to be ca. 2-2.5 pm/V [lo]. The d,, component is neglegible as judged from the very small s-polarized SHG intensity upon p-polarized fundamental with the x- and z-axis in the plane of incidence. 3.2. The influence from the molecular
tilt angle
The direction and magnitude of the tilt angle, that is, the angle of the long axis of the FLC molecules with respect to the smectic layers, is crucial for the understanding of the Sm C * phase and a fundamental property for FLC devices. The tilt angle of each
224
.I. htegren
et al. /Optical Materials 9 (1998) 220-225
sample was measured with a polarization microscope. For some samples it was found to deviate from the value set prior to the polymerization reaction. It seems reasonable to attribute such changes of the tilt angle to some kind of relaxation within the polymer system during the polymerization reaction. To find the origin of this effect further detailed studies of the polymerization reaction are necessary. The magnitude of the d,, and d,, coefficients of a series of samples of different composition were obtained by analyzing the SHG intensity vs. O-dependence with the plane of incidence coinciding with the xy and yz planes. The PLCP associated with the pure Alb monomer gave the largest SHG signal, however, values of the d coefficients were found to be approximately the same for a series of PLCPs formed from binary mixtures of the Alb and A2c monomers. For samples having the same or similar tilt angle we found no systematic differences depending on composition of the precursor mixture, however, the magnitudes were found to differ significantly due to variations in the molecular tilt angle. The d,, and d,, coefficients as a function of the tilt angle for a series of PLCP films are shown in Fig. 3a,b. This demonstrates the importance of a large molecular tilt angle in order to obtain a large SHG signal.
4. Summary
and coulusions
The results of second harmonic generation studies of various PLCPs made from binary mixtures have been summarized and showed useful for characterizing the nonlinear optical response following from the spontaneous polarization of these materials. The influence of the molecular tilt angle showed that it is important to design the precursor molecular systems to give a relatively large tilt angle in the polymerized systems. Thus, this consideration together with a large molecular hyperpolarizability are both necessary in order to enhance nonlinear, and presumably electrooptical, properties, of these and related systems.
Acknowledgements Financial support from the National Foundation for Strategic Research of Sweden, the National Board for Technical and Industrial Development (NUTEK), Defence Material Administration (FMV) and the Swedish Research Council for Engineering Sciences (TFR), is acknowledged.
References 111K.D. Singer, J.E. Sohn, S.J. Lalama, Appl. Phys. Lett. 49 (5) (1986) 248-250,
121R.B. Meyer, L. Liebert, L. Strzelecki,
I
10
30
20 Tilt angle [deg]
63
z
,
.
b)
0,2
.
B $!
I
.
0,l
??
.
4..
I
0
10
20
30
Tilt angle [deg]
Fig. 3. (a) d,, and (b) d,, coefficients as a function of the molecular tilt angle of PLCP films measured lo-11 days after polymerization.
P. Keller, J. Phys. (Paris) Lett. 36 (1975) L69-L71. [31 M. Schadt, Liq. Cryst. 14 (1993) 73-104. R. [41 K. Schmitt, R.-P. Herr, M. Schadt, J. Funfschilling, Buchecker, X.H. Chen, C. Benecke, Liq. Cryst. 14 (1993) 1735-1752. 151 D.M. Walba, M.B. Ros, N.A. Clark, R. Shao, K.M. Jonsson, M.G. Robinson, J.Y. Liu, D. Doroski, Mol. Cryst. Liq. Cryst. 198 (1991) 51-60. [61M. Troll&, C. Orrenius, F. Sahltn, U.W. Gedde, T. Norin, A. Hult, D. Hermann, P. Rudquist, L. Komitov, S.T. Lagerwall, J. Lindstriim, J. Am. Chem. Sot. 36 (1996) 8542-8548. t71 M. Troll&s, F. Sahlen, U.W. Gedde, A. Hult, D. Hermann, P. Rudquist, L. Komitov, S.T. Lagerwall, B. Stebler, J. Lindstrijm, 0. Rydlund, Macromolecules 29 (1996) 2590. w A. Hult, F. SahlBn, M. Troll&s, ST. Lagerwall, D. Hermann, L. Komitov, P. Rudquist, B. Stebler, Liq. Cryst. 20 (1996) 23.
J. btegren
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et al. / Optical
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