- Email: [email protected]
Electro-optical modulators in polymer-dispersed liquid crystal complexes
ARTICLE IN PRESS Optics and Lasers in Engineering 48 (2010) 856–858
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Electro-optical modulators in polymer-dispersed liquid crystal complexes R. W˛eg"owski a, S.J. K"osowicz a, A. Majchrowski a, K. Ozga b, I.V. Kityk c,n, S. Calus c, M. Chmiel d a
Institute of Applied Physics, Military University of Technology, Gen. Sylwestra Kaliskiego 2, 00-908 Warsaw, Poland Chair of Public Health, Czestochowa Technological University, Armii Krajowej Av. 36B, 42-200 Czestochowa, Poland Electrical Engineering Department, Czestochowa Technological University, Armii Krajowej Av. 17/19, 42-200 Czestochowa, Poland d Central School of the State Fire Services, Sabinowska Av. 62, 42-200 Czestochowa, Poland b c
a r t i c l e in f o
a b s t r a c t
Article history: Received 18 November 2009 Received in revised form 13 April 2010 Accepted 13 April 2010 Available online 13 May 2010
The influence of bicolor optical treatment on the behaviour of the linear and quadratic electro-optical effects in the LCBO NC incorporated into the liquid crystalline matrix is studied. Simultaneously their dependence on frequency is analyzed. With increasing power density up to 250–270 MW/cm2 there is a substantial increase of the electro-optical coefficient at 1150 nm up to 1.75 pm/V and 1.53 pm/V for 5% and 3% LCBO nanocrystallites, respectively. Further enhancement of the photo-inducing treatment leads to a saturation of the process. The behaviour of the maximal quadratic electro-optics coefficient R2233 demonstrates principally different photo-inducing dependences. For both LCBO contents we observe an occurrence of the maximum in the electro-optical coefficients situated at 120 and 180 MW/cm2 powers for the fundamental laser beams. However, contrary to the Pockels effect the enhanced content of the LCBO NC leads to a decrease of the observed effect. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Electro-optical modulators Liquid crystalline optical cells Modulators Polymer-dispersed devices Liquid crystal systems
1. Introduction The recent progress in the electro-optical modulators is caused by a search of principally new media which present the organic chromophore embedded into the liquid crystalline matrices [1]. The switching characteristics of multilayer electro-optical structures contain quartz substrates, transparent conducting layers and an oriented nematic liquid crystal (NLC) film doped with photosensitive charge-transfer complexes based on electrooptically active organic monomer or polymer molecules and fullerenes. Organic–inorganic hybrid materials (SIAZO-VP2) for the modulators are prepared usually by the sol–gel processable monomer derived from 3-isocyanatopropyltriethoxysilane with hydroxy-functionalized nonlinear optical (NLO) chromophores. The NLO susceptibilities strongly depended on conditions such as poling temperature, poling time and heating rate. These modulators are used in a wide spectral range, even up to 10.6 mm [2]. Our previous studies have shown that the nanocrystallites incorporated into the polymer matrices may enhance the electro-optical and nonlinear optical properties [3]. Additional treatment by bicolor coherent laser beams may cause an enhancement of the corresponding susceptibilities [4]. Among such materials there is exceptional interest in polymer-dispersed liquid crystals (PDLC) that are heterogeneous composites in which micrometer liquid crystal droplets are embedded in a solid
polymer matrix [5]. Those droplets are usually spherical; however droplets of another shape can be also obtained. They are very interesting due to the curvilinear geometry of confined liquid crystal and the pronounced effect of interactions with the surface of polymer cavity, and thus size-confined effects [6]. Depending on the properties of components and a preparation process PDLC exhibit different morphologies, i.e. concentration, size, shape and the director field inside droplets of liquid crystal droplets, which in turns affect optical properties of the composite. An orientation of nematic droplets’ optical axes is usually random. An application of bias electric field results in reorientation of nematic liquid crystal aligning droplets’ optical axes in one direction. This is the reason for electro-optical effects being both linear (Pockels effect described by third rank polar tensor) and quadratic (Kerr effect described by fourth rank tensor). On the other hand the reorientation of liquid crystal can also align non-spherical guest molecules or nanocrystallites. Additional treatment by external laser beams originating from the same laser (fundamental and its second harmonic generation) may be of particular interest. In the present work we study the influence of bicolor optical treatment on the behaviour of the linear and quadratic electro-optical effects in such prepared samples. Simultaneously their dependence on frequency is analyzed.
2. Materials and methods n
Corresponding author. E-mail address: [email protected] (I.V. Kityk).
0143-8166/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2010.04.004
Nanocrystallites of La2CaB10O19 (LCBO) were prepared from LCBO single crystals by acoustic milling of them using the focused
ARTICLE IN PRESS R. W˛eg!owski et al. / Optics and Lasers in Engineering 48 (2010) 856–858
frequency by a lock-in amplifier. The method allows us to determine the birefringence with a precision equal to about 6 10 6.
3. Results and discussion Our studies have shown that the enhancement of the bicolor treatment by 1064 nm fundamental 10 ns laser beams and its second harmonic generation lead to the increase of the linear electro-optical effect as shown in Fig. 1. The maximal effect is observed for the r233 tensor components. With increasing power density up to 250–270 MW/cm2 one can see a substantial increase of the electro-optical coefficient at 1150 nm up to 1.75 pm/V and 1.53 pm/V for 5% and 3% LCBO nanocrystallites, respectively. The further enhancement of the photo-inducing treatment leads to a saturation of the process. The obtained electro-optical coefficients are sufficient to use them as electro-optical laser modulators. The appeared photo-induced electro-optical anisotropy remained on the level about 70% with respect to the maximally achieved electro-optical Pockels coefficient during several months at ambient temperature. The behaviour of the maximal quadratic electro-optical coefficient R2233 demonstrates principally different photo-inducing dependences (see Fig. 2). For both LCBO contents we observe an occurrence of the maximum in the electro-optical coefficients situated at 120 and 180 MW/cm2 powers for the fundamental laser beams. However, contrary to the Pockels effect the enhanced content of the LCBO NC leads to a decrease of the effect observed. The observed maximum may be a consequence of a competition between the formation of the optically oriented polarized states and their reorientation due to photo-thermal processes. The relaxation of the quadratic electro-optical coefficients is larger than linear electro-optical coefficients and achieves 45% with respect to the starting parameters (see Fig. 3). Comparing these two dependences one can conclude that the mechanism of all-optical treatment is principally different for the linear electro-optical coefficient and the quadratic electro-optical effect. As a confirmation of this fact one can see the typical dependence of both electro-optical effects for two different NC contents versus the voltage frequencies. One can see that for the Kerr effect the observed dependences have maximum at 900 Hz and the Pockels coefficients have maximum at 170 Hz. So the
2.0 3 % NC 5 % NC
1.5
r233 [pm/V]
acoustical field with power about 130 W and frequency 4.2 kHz. LCBO melts incongruently near 1050 1C [7], so it had to be crystallized from high temperature solution, to lower the temperature of crystallization below the temperature of the peritectic transition. Calcium tetraborate (CaB4O7) was used as the solvent. The growth was carried out in a two-zone resistance furnace under conditions of low-temperature gradients by means of top seeded solution growth (TSSG) technique. The synthesis of the starting melt was carried out at a temperature 100 1C above the melting point. Possible pre-crystallization of unwanted LaB3O6 phase was avoided by mixing the melt with Pt stirrer for several hours and then fast cooling to obtain uniform glass. In this way the homogeneous composition of the starting melt was achieved and crystallization of wanted LCBO phase was allowed. LCBO seed, after immersing in the melt, was rotated at 30 rpm without pulling. The temperature was lowered at a rate of 0.02 1C/h. LCBO single crystals grew under low-temperature gradients in the volume of the melt. As-grown LCBO single crystals were confined with natural crystallographic faces and were of good optical quality. The PDLC samples were prepared using photo polymerizationinduced phase separation method (PPIPS). The initial homogeneous mixture of NOA-65 (optical glue from Norland Optical Adhesives) and nematic liquid crystal composition W-765 or W-819 was prepared by vigorous mixing. The liquid crystal content was equal to 25% by weight. Both nematic mixtures were designed for PDLC applications. The ordinary refractive index was equal to 1.52 while the extraordinary one was remarkably larger. Then glass spacers 18 mm thick and LCBO nanocrystallites were added (the latter up to 5% by weight) and the mixture was carefully mixed. The drop of the obtained system was placed onto glass plate 0.7 mm thick coated with conductive indium-tin oxide (ITO) layer. After a few minutes of degassing the mixture, the drop was covered by another ITO glass plate and the sample was weighed to confirm uniform thickness equal to the spacers’ thickness. Finally the samples were irradiated by UV flux. The prepolymer was cured and the solubility of the liquid crystal dramatically decreased. As a result droplets of liquid crystal emerged in the solidified polymer. The mean diameter of the droplets was inversely proportional to the intensity of the UV power density. For investigations we chose the nanocomposites containing 3% and 5% of the LCBO NC. The PDLC film was confined between two ITO covered glass substrates (transparent electrodes) biased from a source supplying combined direct and alternative voltage. The electro-optical properties were measured using an illumination of 1150 nm He–Ne CW 10 mW laser. We performed experiments corresponding to measurements of different experimental geometries. We established that maximal values of the Pockels effect have the tensor components r233 ðoÞ, where index 3 corresponds to the direction perpendicular to ITO surfaces. The measurements were performed using the AC dynamics Senarmont method described in Ref. [8]. For higher precision of the laser measurements we made them at 20 different values of the polarization directions intra the xy plane. To distinguish a contribution of quadratic and linear contribution to electrooptical effect we have carried out the measurements with varying polarity of dc electric field pulses. The spot diameter of the laser beam was equal to about 900 mm. General determination of the birefringence induced by low frequency ac field was done by the Senarmont method. This method uses phase shifting plate l/4 situated in diagonal position (i.e., 451 with respect to crossed polarizer and analyzer directions). The sinusoidal ac electric field had a frequency of about 1 kHz with maximal amplitude of up to 25 V. On varying the amplitude of the ac voltage, we measured the voltage corresponding to the doubling of the modulated
857
1.0
0.5
0.0 0
100
200
300
400
2
I [MW/cm ] Fig. 1. Dependence of the linear electro-optical coefficients for two different contents of the LCBO NC versus the power density of the laser-induced field.
ARTICLE IN PRESS 858
R. W˛eg!owski et al. / Optics and Lasers in Engineering 48 (2010) 856–858
polymer dispersed LC cells [9–12]. At the same time additional potential exists in the use of crystallites possessing good optical quality, i.e. low number of defect states [13] and rare earth doping [14].
45 40 35
R2233 [m2/V2]
30 4. Conclusions
25 20 15 10 5 0
100
200
300
400
2
Ip [MW/cm ] Fig. 2. Quadratic electro-optical coefficient features versus the applied photoinducing optical power density for the two types of samples. The indication are the same as in the Fig. 1.
Pockels coefficient Kerr coefficient
EOE [arb.un.]
10
During investigations of bicolor laser treatment of the LCBO NC incorporated into the LC matrices, we established that with enhancement of bicolor treatment by 1064 nm fundamental 10 ns laser beams and its second harmonic generation the maximal effect is observed for the r233 tensor components. On increasing power density up to 250–270 MW/cm2 there is a substantial increase of the electro-optical coefficient at 1150 nm up to 1.75 pm/V and 1.53 pm/V for 5% and 3% LCBO nanocrystallites, respectively. Further enhancement of the photo-inducing treatment leads to a saturation of the process. The behaviour of the maximal quadratic electro-optical coefficient R2233 demonstrates principally different photo-inducing dependences. For both LCBO contents we observe an occurrence of the maximum in the electro-optical coefficients situated at 120 and 180 MW/cm2 powers for the fundamental laser beams. However, contrary to the Pockels effect the enhanced content of the LCBO NC leads to a decrease of the observed effect. Moreover, the observed Kerr effect dependences have a maximum at 900 Hz and the Pockels coefficients have a maximum at 170 Hz. So the relaxation mechanism is different for these types of effects.
References
1
0.1 0
200
400
600
800
1000
1200
1400
f [Hz] Fig. 3. Frequency dependence of the relative Kerr (rings) and Pockels constant (squares) for the samples containing 3% of the LCBO NC. Data were normalized to the maximum value found in each series.
relaxation mechanism is different for these types of effects. This allows one to propose the electro-optical modulators with varied relaxation times. It is important that the value of the effects observed are comparable with the ones obtained in the other
[1] Vasilyev PYa, Kamanina NV. Tech Phys Lett 2007;33:8. [2] Brugioni S, Meucci R. Opt Commun 2003;216(4):453. [3] Majchrowski A, Kityk IV, Lukasiewicz T, Mefleh A, Benet S. Opt Mater 2000;15:51. [4] Kos´cien´ E, Sanetra J, Gondek E, Jarosz B, Kityk IV, Ebothe J, et al. Opt Commun 2004;242:401. [5] Weg"owski R, K"osowicz SJ, Majchrowski A, Tkaczyk S, Reshak AH, Pisarek J, et al. Mater Lett 2010;64:1176. [6] Kamanina NV, Reshak AH, Vasilyev PYa, Vangonen AI, Studeonov VI, Usanov YuE, et al. Physica E 2009;41(3):391. [7] Wu Yicheng, Fu Peizhen, Zheng Feng, Wan Songming, Guan Xiangguo. Opt Mater 2003;23:373. [8] Kityk IV, Makowska-Janusik M, Fontana MD, Aillerie M. Cryst Res Technol 2001;36:577. [9] Iwamochi H, Yoshizawa A. Appl Phys Express 2008;1:111801. [10] Nishioka M, Kikuchi K, Yoshioka K. Polymer 1975;16(11):791. [11] Kikuchi H. Structure and bonding. In: Takashi Kato, editor. Liquid crystalline functional assemblies and their supramolecular structures, 128. Heidelberg: Springer; 2008. p. 99. [12] Hori M, Toki M. J Sol–Gel Sci Technol 2000;19:349. [13] Dinakaran S, Verma Sunil, Das SJerome, Kar S, Bartwal KS, Gupta PK. Physica B: Condens Matter 2010;405:1809. [14] Brenier A, Kityk IV, Majchrowski A. Opt Commun 2002;203:125.
Contents lists available at ScienceDirect
Optics and Lasers in Engineering journal homepage: www.elsevier.com/locate/optlaseng
Electro-optical modulators in polymer-dispersed liquid crystal complexes R. W˛eg"owski a, S.J. K"osowicz a, A. Majchrowski a, K. Ozga b, I.V. Kityk c,n, S. Calus c, M. Chmiel d a
Institute of Applied Physics, Military University of Technology, Gen. Sylwestra Kaliskiego 2, 00-908 Warsaw, Poland Chair of Public Health, Czestochowa Technological University, Armii Krajowej Av. 36B, 42-200 Czestochowa, Poland Electrical Engineering Department, Czestochowa Technological University, Armii Krajowej Av. 17/19, 42-200 Czestochowa, Poland d Central School of the State Fire Services, Sabinowska Av. 62, 42-200 Czestochowa, Poland b c
a r t i c l e in f o
a b s t r a c t
Article history: Received 18 November 2009 Received in revised form 13 April 2010 Accepted 13 April 2010 Available online 13 May 2010
The influence of bicolor optical treatment on the behaviour of the linear and quadratic electro-optical effects in the LCBO NC incorporated into the liquid crystalline matrix is studied. Simultaneously their dependence on frequency is analyzed. With increasing power density up to 250–270 MW/cm2 there is a substantial increase of the electro-optical coefficient at 1150 nm up to 1.75 pm/V and 1.53 pm/V for 5% and 3% LCBO nanocrystallites, respectively. Further enhancement of the photo-inducing treatment leads to a saturation of the process. The behaviour of the maximal quadratic electro-optics coefficient R2233 demonstrates principally different photo-inducing dependences. For both LCBO contents we observe an occurrence of the maximum in the electro-optical coefficients situated at 120 and 180 MW/cm2 powers for the fundamental laser beams. However, contrary to the Pockels effect the enhanced content of the LCBO NC leads to a decrease of the observed effect. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Electro-optical modulators Liquid crystalline optical cells Modulators Polymer-dispersed devices Liquid crystal systems
1. Introduction The recent progress in the electro-optical modulators is caused by a search of principally new media which present the organic chromophore embedded into the liquid crystalline matrices [1]. The switching characteristics of multilayer electro-optical structures contain quartz substrates, transparent conducting layers and an oriented nematic liquid crystal (NLC) film doped with photosensitive charge-transfer complexes based on electrooptically active organic monomer or polymer molecules and fullerenes. Organic–inorganic hybrid materials (SIAZO-VP2) for the modulators are prepared usually by the sol–gel processable monomer derived from 3-isocyanatopropyltriethoxysilane with hydroxy-functionalized nonlinear optical (NLO) chromophores. The NLO susceptibilities strongly depended on conditions such as poling temperature, poling time and heating rate. These modulators are used in a wide spectral range, even up to 10.6 mm [2]. Our previous studies have shown that the nanocrystallites incorporated into the polymer matrices may enhance the electro-optical and nonlinear optical properties [3]. Additional treatment by bicolor coherent laser beams may cause an enhancement of the corresponding susceptibilities [4]. Among such materials there is exceptional interest in polymer-dispersed liquid crystals (PDLC) that are heterogeneous composites in which micrometer liquid crystal droplets are embedded in a solid
polymer matrix [5]. Those droplets are usually spherical; however droplets of another shape can be also obtained. They are very interesting due to the curvilinear geometry of confined liquid crystal and the pronounced effect of interactions with the surface of polymer cavity, and thus size-confined effects [6]. Depending on the properties of components and a preparation process PDLC exhibit different morphologies, i.e. concentration, size, shape and the director field inside droplets of liquid crystal droplets, which in turns affect optical properties of the composite. An orientation of nematic droplets’ optical axes is usually random. An application of bias electric field results in reorientation of nematic liquid crystal aligning droplets’ optical axes in one direction. This is the reason for electro-optical effects being both linear (Pockels effect described by third rank polar tensor) and quadratic (Kerr effect described by fourth rank tensor). On the other hand the reorientation of liquid crystal can also align non-spherical guest molecules or nanocrystallites. Additional treatment by external laser beams originating from the same laser (fundamental and its second harmonic generation) may be of particular interest. In the present work we study the influence of bicolor optical treatment on the behaviour of the linear and quadratic electro-optical effects in such prepared samples. Simultaneously their dependence on frequency is analyzed.
2. Materials and methods n
Corresponding author. E-mail address: [email protected] (I.V. Kityk).
0143-8166/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlaseng.2010.04.004
Nanocrystallites of La2CaB10O19 (LCBO) were prepared from LCBO single crystals by acoustic milling of them using the focused
ARTICLE IN PRESS R. W˛eg!owski et al. / Optics and Lasers in Engineering 48 (2010) 856–858
frequency by a lock-in amplifier. The method allows us to determine the birefringence with a precision equal to about 6 10 6.
3. Results and discussion Our studies have shown that the enhancement of the bicolor treatment by 1064 nm fundamental 10 ns laser beams and its second harmonic generation lead to the increase of the linear electro-optical effect as shown in Fig. 1. The maximal effect is observed for the r233 tensor components. With increasing power density up to 250–270 MW/cm2 one can see a substantial increase of the electro-optical coefficient at 1150 nm up to 1.75 pm/V and 1.53 pm/V for 5% and 3% LCBO nanocrystallites, respectively. The further enhancement of the photo-inducing treatment leads to a saturation of the process. The obtained electro-optical coefficients are sufficient to use them as electro-optical laser modulators. The appeared photo-induced electro-optical anisotropy remained on the level about 70% with respect to the maximally achieved electro-optical Pockels coefficient during several months at ambient temperature. The behaviour of the maximal quadratic electro-optical coefficient R2233 demonstrates principally different photo-inducing dependences (see Fig. 2). For both LCBO contents we observe an occurrence of the maximum in the electro-optical coefficients situated at 120 and 180 MW/cm2 powers for the fundamental laser beams. However, contrary to the Pockels effect the enhanced content of the LCBO NC leads to a decrease of the effect observed. The observed maximum may be a consequence of a competition between the formation of the optically oriented polarized states and their reorientation due to photo-thermal processes. The relaxation of the quadratic electro-optical coefficients is larger than linear electro-optical coefficients and achieves 45% with respect to the starting parameters (see Fig. 3). Comparing these two dependences one can conclude that the mechanism of all-optical treatment is principally different for the linear electro-optical coefficient and the quadratic electro-optical effect. As a confirmation of this fact one can see the typical dependence of both electro-optical effects for two different NC contents versus the voltage frequencies. One can see that for the Kerr effect the observed dependences have maximum at 900 Hz and the Pockels coefficients have maximum at 170 Hz. So the
2.0 3 % NC 5 % NC
1.5
r233 [pm/V]
acoustical field with power about 130 W and frequency 4.2 kHz. LCBO melts incongruently near 1050 1C [7], so it had to be crystallized from high temperature solution, to lower the temperature of crystallization below the temperature of the peritectic transition. Calcium tetraborate (CaB4O7) was used as the solvent. The growth was carried out in a two-zone resistance furnace under conditions of low-temperature gradients by means of top seeded solution growth (TSSG) technique. The synthesis of the starting melt was carried out at a temperature 100 1C above the melting point. Possible pre-crystallization of unwanted LaB3O6 phase was avoided by mixing the melt with Pt stirrer for several hours and then fast cooling to obtain uniform glass. In this way the homogeneous composition of the starting melt was achieved and crystallization of wanted LCBO phase was allowed. LCBO seed, after immersing in the melt, was rotated at 30 rpm without pulling. The temperature was lowered at a rate of 0.02 1C/h. LCBO single crystals grew under low-temperature gradients in the volume of the melt. As-grown LCBO single crystals were confined with natural crystallographic faces and were of good optical quality. The PDLC samples were prepared using photo polymerizationinduced phase separation method (PPIPS). The initial homogeneous mixture of NOA-65 (optical glue from Norland Optical Adhesives) and nematic liquid crystal composition W-765 or W-819 was prepared by vigorous mixing. The liquid crystal content was equal to 25% by weight. Both nematic mixtures were designed for PDLC applications. The ordinary refractive index was equal to 1.52 while the extraordinary one was remarkably larger. Then glass spacers 18 mm thick and LCBO nanocrystallites were added (the latter up to 5% by weight) and the mixture was carefully mixed. The drop of the obtained system was placed onto glass plate 0.7 mm thick coated with conductive indium-tin oxide (ITO) layer. After a few minutes of degassing the mixture, the drop was covered by another ITO glass plate and the sample was weighed to confirm uniform thickness equal to the spacers’ thickness. Finally the samples were irradiated by UV flux. The prepolymer was cured and the solubility of the liquid crystal dramatically decreased. As a result droplets of liquid crystal emerged in the solidified polymer. The mean diameter of the droplets was inversely proportional to the intensity of the UV power density. For investigations we chose the nanocomposites containing 3% and 5% of the LCBO NC. The PDLC film was confined between two ITO covered glass substrates (transparent electrodes) biased from a source supplying combined direct and alternative voltage. The electro-optical properties were measured using an illumination of 1150 nm He–Ne CW 10 mW laser. We performed experiments corresponding to measurements of different experimental geometries. We established that maximal values of the Pockels effect have the tensor components r233 ðoÞ, where index 3 corresponds to the direction perpendicular to ITO surfaces. The measurements were performed using the AC dynamics Senarmont method described in Ref. [8]. For higher precision of the laser measurements we made them at 20 different values of the polarization directions intra the xy plane. To distinguish a contribution of quadratic and linear contribution to electrooptical effect we have carried out the measurements with varying polarity of dc electric field pulses. The spot diameter of the laser beam was equal to about 900 mm. General determination of the birefringence induced by low frequency ac field was done by the Senarmont method. This method uses phase shifting plate l/4 situated in diagonal position (i.e., 451 with respect to crossed polarizer and analyzer directions). The sinusoidal ac electric field had a frequency of about 1 kHz with maximal amplitude of up to 25 V. On varying the amplitude of the ac voltage, we measured the voltage corresponding to the doubling of the modulated
857
1.0
0.5
0.0 0
100
200
300
400
2
I [MW/cm ] Fig. 1. Dependence of the linear electro-optical coefficients for two different contents of the LCBO NC versus the power density of the laser-induced field.
ARTICLE IN PRESS 858
R. W˛eg!owski et al. / Optics and Lasers in Engineering 48 (2010) 856–858
polymer dispersed LC cells [9–12]. At the same time additional potential exists in the use of crystallites possessing good optical quality, i.e. low number of defect states [13] and rare earth doping [14].
45 40 35
R2233 [m2/V2]
30 4. Conclusions
25 20 15 10 5 0
100
200
300
400
2
Ip [MW/cm ] Fig. 2. Quadratic electro-optical coefficient features versus the applied photoinducing optical power density for the two types of samples. The indication are the same as in the Fig. 1.
Pockels coefficient Kerr coefficient
EOE [arb.un.]
10
During investigations of bicolor laser treatment of the LCBO NC incorporated into the LC matrices, we established that with enhancement of bicolor treatment by 1064 nm fundamental 10 ns laser beams and its second harmonic generation the maximal effect is observed for the r233 tensor components. On increasing power density up to 250–270 MW/cm2 there is a substantial increase of the electro-optical coefficient at 1150 nm up to 1.75 pm/V and 1.53 pm/V for 5% and 3% LCBO nanocrystallites, respectively. Further enhancement of the photo-inducing treatment leads to a saturation of the process. The behaviour of the maximal quadratic electro-optical coefficient R2233 demonstrates principally different photo-inducing dependences. For both LCBO contents we observe an occurrence of the maximum in the electro-optical coefficients situated at 120 and 180 MW/cm2 powers for the fundamental laser beams. However, contrary to the Pockels effect the enhanced content of the LCBO NC leads to a decrease of the observed effect. Moreover, the observed Kerr effect dependences have a maximum at 900 Hz and the Pockels coefficients have a maximum at 170 Hz. So the relaxation mechanism is different for these types of effects.
References
1
0.1 0
200
400
600
800
1000
1200
1400
f [Hz] Fig. 3. Frequency dependence of the relative Kerr (rings) and Pockels constant (squares) for the samples containing 3% of the LCBO NC. Data were normalized to the maximum value found in each series.
relaxation mechanism is different for these types of effects. This allows one to propose the electro-optical modulators with varied relaxation times. It is important that the value of the effects observed are comparable with the ones obtained in the other
[1] Vasilyev PYa, Kamanina NV. Tech Phys Lett 2007;33:8. [2] Brugioni S, Meucci R. Opt Commun 2003;216(4):453. [3] Majchrowski A, Kityk IV, Lukasiewicz T, Mefleh A, Benet S. Opt Mater 2000;15:51. [4] Kos´cien´ E, Sanetra J, Gondek E, Jarosz B, Kityk IV, Ebothe J, et al. Opt Commun 2004;242:401. [5] Weg"owski R, K"osowicz SJ, Majchrowski A, Tkaczyk S, Reshak AH, Pisarek J, et al. Mater Lett 2010;64:1176. [6] Kamanina NV, Reshak AH, Vasilyev PYa, Vangonen AI, Studeonov VI, Usanov YuE, et al. Physica E 2009;41(3):391. [7] Wu Yicheng, Fu Peizhen, Zheng Feng, Wan Songming, Guan Xiangguo. Opt Mater 2003;23:373. [8] Kityk IV, Makowska-Janusik M, Fontana MD, Aillerie M. Cryst Res Technol 2001;36:577. [9] Iwamochi H, Yoshizawa A. Appl Phys Express 2008;1:111801. [10] Nishioka M, Kikuchi K, Yoshioka K. Polymer 1975;16(11):791. [11] Kikuchi H. Structure and bonding. In: Takashi Kato, editor. Liquid crystalline functional assemblies and their supramolecular structures, 128. Heidelberg: Springer; 2008. p. 99. [12] Hori M, Toki M. J Sol–Gel Sci Technol 2000;19:349. [13] Dinakaran S, Verma Sunil, Das SJerome, Kar S, Bartwal KS, Gupta PK. Physica B: Condens Matter 2010;405:1809. [14] Brenier A, Kityk IV, Majchrowski A. Opt Commun 2002;203:125.