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Investigation of dielectric properties and diffraction efficiency enhancements caused by photothermal effect in DR9 dye-doped nematic liquid crystal
Optics Communications 284 (2011) 4924–4928
Contents lists available at ScienceDirect
Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m
Investigation of dielectric properties and diffraction efficiency enhancements caused by photothermal effect in DR9 dye-doped nematic liquid crystal Oğuz Köysal a,⁎, Mustafa Okutan b, Muharrem Gökçen a a b
Department of Physics, Faculty of Arts and Sciences, Düzce University, Düzce, 81620, Turkey Department of Physics, Yıldız Technical University, Davutpaşa, İstanbul, 34210, Turkey
a r t i c l e
i n f o
Article history: Received 28 February 2011 Accepted 23 June 2011 Available online 7 July 2011 Keywords: Dye Liquid crystal Photothermal effect Holography Diffraction efficiency Dielectric anisotropy
a b s t r a c t In this work, we studied dielectric properties and laser-induced refractive index changes originating from photothermal effects of liquid crystal material doped with Disperse Red 9 (DR9) dye. Dye concentration is arranged to be between percentages changing from 0.2 wt.% to 1 wt.% in E63 nematic liquid crystal. Nonlinear optical properties such as diffraction efficiency (η) and refractive index modulation (Δn) were investigated by diffraction grating measurements. It was found the diffraction efficiency of pure E63 nematic liquid crystal is 1%. As the doping amount of DR9 dye in nematic LC is increased, diffraction efficiency took higher values and the maximum diffraction efficiency of 10% was gained with E63 doped with 0.8 wt.%DR9 dye. Moreover, dielectric permittivity and dielectric anisotropy values of the samples were investigated in the frequency range of 100 Hz–10 MHz by using dielectric spectroscopy technique. It was observed that dielectric constant values of the liquid crystal material are strongly affected by doping with dye. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent years, liquid crystal materials gained much attention owing to their wide range of possible application and structural features in electronics and optics. Especially, rod-like (nematic) liquid crystals have been used to modulate refractive index by molecular reorientation with an external field or under illumination. Nematic liquid crystals have highly anisotropic compounds. Dielectric anisotropy (Δε) is one of the most important physical properties of the LC compounds. This property enables that LC molecules are easily reoriented in low electrical and optical field. Dielectric anisotropy is expressed as Δε = ε// − ε⊥ where ε// and ε⊥ are the parallel and perpendicular components of the electric permittivity, respectively [1]. Dielectric properties of LC are largely influenced by doping agent. Favorable works have been performed about the mechanism of the doped LC systems [2]. Actually, dye doped LC composite materials have been studied because of numerous properties and their utility in wide variety of applications such as electrically controllable switching, displays, optical shutters and holographic data storage [3, 4]. In this research, LC material was doped with dye at various concentrations. In order to investigate the holographic data storage capacity, we focused on dye doped materials such that the change in the refractive index of the liquid crystal is caused by the mechanism of photo-thermal effect [5, 6]. In these mechanisms, with the absorption ⁎ Corresponding author at: Department of Physics, Faculty of Arts and Sciences, Düzce University, Düzce, 81620, Turkey. Fax: + 90 380 5412403. E-mail address: [email protected] (O. Köysal). 0030-4018/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2011.06.046
of light by the dye molecules doped into liquid crystal, the increase in temperature causes some changes in the degree of molecular order parameters of the liquid crystal. In liquid crystals, adding a small amount of dye into LC would create some influence on electro-optic properties and hence improve the device performance. Dielectric Spectroscopy (DS) is the most preferred technique to understand molecular mechanisms and full electro-optical characterization for acquiring and optimizing device parameters such as dielectric anisotropy and permittivity as a function of frequency at various bias voltage values [7]. In this study, we have investigated the influence of doping DR9 dye into E63 nematic liquid crystal on holographic diffraction efficiency and dielectric behaviors. Relationship between photothermal effect and diffraction efficiency of the pure and dye doped liquid crystals were investigated depending on applied DC voltage with using two wave mixing experiment. Dielectric characters of the samples were investigated by impedance spectroscopy technique. 2. Experimental The liquid crystal used in this experiment is E63 which was obtained from Merck. LC material E63 exhibits high positive dielectric anisotropy (Δε N 0). Doping material DR9 dye was purchased from Aldrich. Dye concentration was acquired at percentages changing from 0.2 wt.% to 1 wt.% in E63. The mixtures were stirred in isotropic phase at ~ 80 °C for ~ 1 h to make the constituents uniformly mixed and then were filled into a 9 μm gap LC cell in the isotropic phase by capillary technique. LC cell was made up of two conductive glass plates (ITO) with planar alignment.
O. Köysal et al. / Optics Communications 284 (2011) 4924–4928
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Diffraction efficiency performance was tested by using two-beam coupling (2BC) method and its experimental set-up was also used in our previous work [8]. Fig. 1 shows the absorbencies of pure E63 LC and E63 LC doped with DR9 dye at various concentrations in the visible spectrum. This configuration is almost appropriate for efficient absorbance to happen at the characteristic wavelength of He–Cd laser around 441 nm. He–Cd laser was used as a pumping source and this source was split into two components having approximately equal power by a beam splitter. Polarization of laser is arranged to be parallel to preliminary orientation of LC molecules. Pumping beams, having ~ 20 mW power, were intersected on the sample with, 2θ = ~ 3 o that makes grating constant Λ to be 24 μm and since Λ 2 NN λd, diffraction is considered to be in the Raman–Nath regime. He–Ne laser, which has 1.7 mW power and 632 nm wavelength, was used as probe beam and also it was parallel to the director of LC molecules. Computer controlled Hewlett-Packard 4194 impedance/gain-phase analyzer was used in dielectric measurements which are performed at room temperature. 3. Result and discussion Grating diffraction experiments are the basis for performance evaluation of holographic applications. Therefore, the character of the systems was investigated in terms of the diffraction signals depending on applied DC voltage. Normalized diffraction signals and diffraction efficiency on the applied dc voltage were investigated for pure E63 and E63 doped with DR9 dye at various concentrations ranging from 0.2 wt.% to 1 wt.%. Favorable concentration 0.8 wt.% was chosen for characterization of our system. The applied DC voltage dependence of diffraction signal is shown in Fig. 2. In our system, diffraction signal enhances with the increasing doping concentration whereas threshold voltage of the reorientation shifts to higher values as the concentration of DR9 doping material is decreased (Fig. 3). Threshold voltage (Vth) values were found as 2.50, 2.55, 2.78, 2.90 and 3.30 V for the samples 0.8 wt.% DR9, 0.6 wt.% DR9, 0.4 wt.% DR9, 0.2 wt.% DR9 and pure E63, respectively. Fig. 4 shows the normalized diffraction efficiency of DR9 doping samples with different concentrations amounts. Diffraction efficiency η is defined by percentage of transmitted laser light intensity over the incoming light intensity in this type of experimental setup and given by the following equation;
η=
I1 × 100% I0
Fig. 1. An absorbance spectrum of pure E63 LC and it's doped with DR9 dye at various concentrations in the visible spectrum.
Fig. 2. Dependency of first order diffraction signals on the applied dc voltage for pure E63 and its doped forms with different concentrations of DR9 dye.
where I1 is the first order probe diffraction and I0 is the incident power of probe beam. Efficiency is increased as the percentage of the doping material DR9 rises up to 0.8 wt.%. For the constructed system, diffraction efficiency is 10% under optimum circumstances. As can be seen in Fig. 5, 1 wt.% concentration of DR9 added to the nematic liquid crystal causes screening effect and transmitted laser intensity decreases slowly after that value. That is why diffraction efficiency dropped at 8.4%, for this reason, DR9 concentration is optimized at 0.8 wt.% in our study. According to mechanism of light induced photo-thermal effect, refractive index modulation Δn is calculated using the following formula
η=
πΔnd 2 ; λ
where d is cell thickness and λ is wavelength of pumping laser beam. The refractive index change Δn was found as 1.5 × 10 − 3 and 4.9 × 10 − 3 for pure E63 and 0.8 wt.% DR9 doped materials, respectively. It was observed that the doping agent causes positive effect such that it increases the refractive index modulation almost three times compared to that of pure E63 material.
Fig. 3. Threshold voltage (Vth) values vs. dye concentrations for investigated samples. Solid line meant only as a guide to the eye.
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Fig. 4. Normalized diffraction efficiency of the investigated samples.
Molecular orientation of pure LC and its forms doped with dye molecules determines the electro optical behavior of composite systems. Permittivity is the most important parameter which describes how the electric field is affected by a dielectric medium in the materials. The frequency-dependent complex permittivity can written as [9,10]
00
ε = ε ′ −iε ; where ε' and ε" are the real (dielectric dispersion) and imaginary parts (dielectric loss) of complex dielectric constant and i is the root square of “–1”. Fig. 6 shows the frequency dependence of dielectric constants of the samples. It is seen that both parts of dielectric constant strongly depends on frequency. For pure E63, the real part of the dielectric constant is weak at frequency values ranging from 100 Hz to 1 MHz, while it significantly increases for dye doped material in the same frequency range (Fig. 6(a)). This increase in dielectric dispersion comes from the volume effect of DR9 dye molecules in LC composite system. For pure and doped samples, the values of real part of the dielectric constant ε' decrease with the increasing frequency up to 1 MHz. After this frequency, the ε' takes same values from 1 MHz to 10 MHz for both samples. Therefore, it is suggested that dielectric polarization of pure and doped samples no longer responds to electric field in the high frequency range. In Fig. 6(b) shows the frequency dependence of dielectric loss ε" of the samples. It is seen that
Fig. 5. First order diffraction efficiency vs. various percentage of DR9 dye.
Fig. 6. Frequency dependence of (a) real dielectric constant ε', (b) imaginary parts ε" for pure E63 and E63/0.8 wt.% DR9 samples.
imaginary dielectric constant takes higher values in the low frequency region for pure and doped samples. This effect can occur because of the impurity ions in dye and pure LC. The values of ε" increase with the frequency up to about ~2 MHz, where dielectric loss takes maximum value, and then decrease after that point for E63 and dye doped forms. This frequency value is named as relaxation frequency. Fig. 7 shows DC bias voltage dependence of the real part of dielectric constant of pure E63 and dye-doped E63 at various frequencies. It is clearly seen that the change in dielectric constant is relevant to molecular reorientation of LC molecules which is triggered by external applied voltage. Molecular axis is perpendicular to the field when there is no voltage applied (at bias 0 V) and molecules orient parallel to the electric field when the electric field is strong enough (at bias 10 V). Dielectric anisotropy is expressed as Δε = ε// − ε⊥ where ε// and ε⊥ are parallel and perpendicular components of the real dielectric permittivity, respectively. The values of ε' is named as perpendicular (ε⊥) when the applied bias voltage is zero and as parallel (ε//) when the applied bias voltage is maximum. Liquid crystal materials have either positive (p-type) Δε where ε// component is greater than ε⊥, or negative (n-type) Δε where ε⊥, component is greater than ε// [11]. Dielectric anisotropy values of the pure E63 LC and E63/0.8 wt.% DR9 samples are calculated from Fig. 7 and given in Table 1. Doping DR9 dye into pure E63 decreased the dielectric anisotropy in the large frequency range such that the decrement in dielectric anisotropy in lower frequencies is bigger than that in higher frequencies. Samples dielectrically behave as positive
O. Köysal et al. / Optics Communications 284 (2011) 4924–4928
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Fig. 7. Dependence of reel part of dielectric constant on DC bias voltage at various frequency; a) 100 Hz, b) 1 kHz, c) 10 kHz, d) 100 kHz, e) 1 MHz, f) 10 MHz.
structure (p-type) until 1 MHz, whereas they behave like dielectrically n-type after ~ 1 MHz such that dielectric anisotropy does not take positive values for the frequency values up to ~ 10 MHz. 4. Conclusion In this study, we characterized the diffraction efficiency and dielectric behavior of pure and dye doped LC materials. Diffraction efficiency η and refractive index change Δn of the investigated
samples were calculated by using two wave-mixing experiment. It is seen that favorable concentration for characterization of our system is 0.8 wt.%. The results indicated that holographic diffraction efficiency enhances with percentage of doping DR9 dye due to the photothermal effect. Maximum diffraction efficiency of 10% was gained with E63/0.8 wt.% DR9. Threshold voltage of the reorientation of LC molecules shifts to higher values with the increasing concentration of DR9 doping material. This effect comes from ionic charge due to the exiting dye molecules under laser illumination in
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Table 1 The variation of dielectric anisotropy of pure E63 and E63/0.8 wt.% DR9 samples as a function of frequency. Δε
100 Hz
1 kHz
10 kHz
100 kHz
1 MHz
10 MHz
E63 E63/0.8%DR9
13.21 12.24
12.84 12.51
11.94 11.75
9.91 9.44
− 1.10 − 1.062
0.0068 0.0084
structure (p-type) until 1 MHz. After this frequency point, dielectric anisotropy takes negative values up to 10 MHz with the applied bias voltage. In conclusion, electro-optic measurements and performance analysis of DR9 dye doped LC structure will pave the way for the new studies by providing a vision about their usage in device design.
References LC-dye hybrid structure. It is clarified that increasing dye concentration creates more ionic charges which create strong dipole moment in LC-dye hybrid structure. As a result, the threshold voltage shifts to lower values since LC molecules orient easily with external applied bias voltage because of the dipole-dipole interaction. Furthermore, dielectric characterization of the samples has been investigated by using dielectric spectroscopy technique. The results indicated that both parts of dielectric constant strongly depend on frequency. For the dye doped material, real part of the dielectric constant increases from 100 Hz to 1 MHz and dielectric loss is slightly higher than that of pure LC material. Another result which was obtained is that the dielectric anisotropy is in strong correlation with molecular orientation. Both samples dielectrically behave as positive
[1] M. Okutan, S.E. San, O. Köysal, Dyes and Pigments 65 (2005) 169. [2] F. Simoni, O. Francescangeli, Journal of Physics. Condensed Matter 11 (1999) 439. [3] H. Ono, T. Kawamura, N.M. Frias, K. Kitamura, N. Kawatsuki, H. Norisada, Advanced Materials 12 (2000) 143. [4] P. Nolan, M. Tillin, D. Coates, Liquid Crystals 14 (1993) 339. [5] H. Ono, Y. Harato, The Journal of the Optical Society of America B 16 (1999) 2195. [6] H.J. Hoffman, The Journal of the Optical Society of America B 3 (1986) 253. [7] A.M. Biradar, D. Kilian, S. Wrobel, W. Haase, Liquid Crystals 27 (2000) 225. [8] F. Yakuphanoglu, M. Okutan, O. Koysal, S.M. Ahn, S.R. Keum, Dyes and Pigments 76 (3) (2008) 721. [9] A.K. Jonscher, Dielectric relaxation in solids, Chelsea Dielectrics Press, London, 1983. [10] O. Köysal, M. Okutan, M. Durmuş, F. Yakuphanoğlu, S.E. San, V. Ahsen, Synthetic Metals 156 (2006) 58. [11] H. Kawamoto, Proceedings of the IEEE 90 (2002) 460.
Contents lists available at ScienceDirect
Optics Communications j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / o p t c o m
Investigation of dielectric properties and diffraction efficiency enhancements caused by photothermal effect in DR9 dye-doped nematic liquid crystal Oğuz Köysal a,⁎, Mustafa Okutan b, Muharrem Gökçen a a b
Department of Physics, Faculty of Arts and Sciences, Düzce University, Düzce, 81620, Turkey Department of Physics, Yıldız Technical University, Davutpaşa, İstanbul, 34210, Turkey
a r t i c l e
i n f o
Article history: Received 28 February 2011 Accepted 23 June 2011 Available online 7 July 2011 Keywords: Dye Liquid crystal Photothermal effect Holography Diffraction efficiency Dielectric anisotropy
a b s t r a c t In this work, we studied dielectric properties and laser-induced refractive index changes originating from photothermal effects of liquid crystal material doped with Disperse Red 9 (DR9) dye. Dye concentration is arranged to be between percentages changing from 0.2 wt.% to 1 wt.% in E63 nematic liquid crystal. Nonlinear optical properties such as diffraction efficiency (η) and refractive index modulation (Δn) were investigated by diffraction grating measurements. It was found the diffraction efficiency of pure E63 nematic liquid crystal is 1%. As the doping amount of DR9 dye in nematic LC is increased, diffraction efficiency took higher values and the maximum diffraction efficiency of 10% was gained with E63 doped with 0.8 wt.%DR9 dye. Moreover, dielectric permittivity and dielectric anisotropy values of the samples were investigated in the frequency range of 100 Hz–10 MHz by using dielectric spectroscopy technique. It was observed that dielectric constant values of the liquid crystal material are strongly affected by doping with dye. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent years, liquid crystal materials gained much attention owing to their wide range of possible application and structural features in electronics and optics. Especially, rod-like (nematic) liquid crystals have been used to modulate refractive index by molecular reorientation with an external field or under illumination. Nematic liquid crystals have highly anisotropic compounds. Dielectric anisotropy (Δε) is one of the most important physical properties of the LC compounds. This property enables that LC molecules are easily reoriented in low electrical and optical field. Dielectric anisotropy is expressed as Δε = ε// − ε⊥ where ε// and ε⊥ are the parallel and perpendicular components of the electric permittivity, respectively [1]. Dielectric properties of LC are largely influenced by doping agent. Favorable works have been performed about the mechanism of the doped LC systems [2]. Actually, dye doped LC composite materials have been studied because of numerous properties and their utility in wide variety of applications such as electrically controllable switching, displays, optical shutters and holographic data storage [3, 4]. In this research, LC material was doped with dye at various concentrations. In order to investigate the holographic data storage capacity, we focused on dye doped materials such that the change in the refractive index of the liquid crystal is caused by the mechanism of photo-thermal effect [5, 6]. In these mechanisms, with the absorption ⁎ Corresponding author at: Department of Physics, Faculty of Arts and Sciences, Düzce University, Düzce, 81620, Turkey. Fax: + 90 380 5412403. E-mail address: [email protected] (O. Köysal). 0030-4018/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2011.06.046
of light by the dye molecules doped into liquid crystal, the increase in temperature causes some changes in the degree of molecular order parameters of the liquid crystal. In liquid crystals, adding a small amount of dye into LC would create some influence on electro-optic properties and hence improve the device performance. Dielectric Spectroscopy (DS) is the most preferred technique to understand molecular mechanisms and full electro-optical characterization for acquiring and optimizing device parameters such as dielectric anisotropy and permittivity as a function of frequency at various bias voltage values [7]. In this study, we have investigated the influence of doping DR9 dye into E63 nematic liquid crystal on holographic diffraction efficiency and dielectric behaviors. Relationship between photothermal effect and diffraction efficiency of the pure and dye doped liquid crystals were investigated depending on applied DC voltage with using two wave mixing experiment. Dielectric characters of the samples were investigated by impedance spectroscopy technique. 2. Experimental The liquid crystal used in this experiment is E63 which was obtained from Merck. LC material E63 exhibits high positive dielectric anisotropy (Δε N 0). Doping material DR9 dye was purchased from Aldrich. Dye concentration was acquired at percentages changing from 0.2 wt.% to 1 wt.% in E63. The mixtures were stirred in isotropic phase at ~ 80 °C for ~ 1 h to make the constituents uniformly mixed and then were filled into a 9 μm gap LC cell in the isotropic phase by capillary technique. LC cell was made up of two conductive glass plates (ITO) with planar alignment.
O. Köysal et al. / Optics Communications 284 (2011) 4924–4928
4925
Diffraction efficiency performance was tested by using two-beam coupling (2BC) method and its experimental set-up was also used in our previous work [8]. Fig. 1 shows the absorbencies of pure E63 LC and E63 LC doped with DR9 dye at various concentrations in the visible spectrum. This configuration is almost appropriate for efficient absorbance to happen at the characteristic wavelength of He–Cd laser around 441 nm. He–Cd laser was used as a pumping source and this source was split into two components having approximately equal power by a beam splitter. Polarization of laser is arranged to be parallel to preliminary orientation of LC molecules. Pumping beams, having ~ 20 mW power, were intersected on the sample with, 2θ = ~ 3 o that makes grating constant Λ to be 24 μm and since Λ 2 NN λd, diffraction is considered to be in the Raman–Nath regime. He–Ne laser, which has 1.7 mW power and 632 nm wavelength, was used as probe beam and also it was parallel to the director of LC molecules. Computer controlled Hewlett-Packard 4194 impedance/gain-phase analyzer was used in dielectric measurements which are performed at room temperature. 3. Result and discussion Grating diffraction experiments are the basis for performance evaluation of holographic applications. Therefore, the character of the systems was investigated in terms of the diffraction signals depending on applied DC voltage. Normalized diffraction signals and diffraction efficiency on the applied dc voltage were investigated for pure E63 and E63 doped with DR9 dye at various concentrations ranging from 0.2 wt.% to 1 wt.%. Favorable concentration 0.8 wt.% was chosen for characterization of our system. The applied DC voltage dependence of diffraction signal is shown in Fig. 2. In our system, diffraction signal enhances with the increasing doping concentration whereas threshold voltage of the reorientation shifts to higher values as the concentration of DR9 doping material is decreased (Fig. 3). Threshold voltage (Vth) values were found as 2.50, 2.55, 2.78, 2.90 and 3.30 V for the samples 0.8 wt.% DR9, 0.6 wt.% DR9, 0.4 wt.% DR9, 0.2 wt.% DR9 and pure E63, respectively. Fig. 4 shows the normalized diffraction efficiency of DR9 doping samples with different concentrations amounts. Diffraction efficiency η is defined by percentage of transmitted laser light intensity over the incoming light intensity in this type of experimental setup and given by the following equation;
η=
I1 × 100% I0
Fig. 1. An absorbance spectrum of pure E63 LC and it's doped with DR9 dye at various concentrations in the visible spectrum.
Fig. 2. Dependency of first order diffraction signals on the applied dc voltage for pure E63 and its doped forms with different concentrations of DR9 dye.
where I1 is the first order probe diffraction and I0 is the incident power of probe beam. Efficiency is increased as the percentage of the doping material DR9 rises up to 0.8 wt.%. For the constructed system, diffraction efficiency is 10% under optimum circumstances. As can be seen in Fig. 5, 1 wt.% concentration of DR9 added to the nematic liquid crystal causes screening effect and transmitted laser intensity decreases slowly after that value. That is why diffraction efficiency dropped at 8.4%, for this reason, DR9 concentration is optimized at 0.8 wt.% in our study. According to mechanism of light induced photo-thermal effect, refractive index modulation Δn is calculated using the following formula
η=
πΔnd 2 ; λ
where d is cell thickness and λ is wavelength of pumping laser beam. The refractive index change Δn was found as 1.5 × 10 − 3 and 4.9 × 10 − 3 for pure E63 and 0.8 wt.% DR9 doped materials, respectively. It was observed that the doping agent causes positive effect such that it increases the refractive index modulation almost three times compared to that of pure E63 material.
Fig. 3. Threshold voltage (Vth) values vs. dye concentrations for investigated samples. Solid line meant only as a guide to the eye.
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Fig. 4. Normalized diffraction efficiency of the investigated samples.
Molecular orientation of pure LC and its forms doped with dye molecules determines the electro optical behavior of composite systems. Permittivity is the most important parameter which describes how the electric field is affected by a dielectric medium in the materials. The frequency-dependent complex permittivity can written as [9,10]
00
ε = ε ′ −iε ; where ε' and ε" are the real (dielectric dispersion) and imaginary parts (dielectric loss) of complex dielectric constant and i is the root square of “–1”. Fig. 6 shows the frequency dependence of dielectric constants of the samples. It is seen that both parts of dielectric constant strongly depends on frequency. For pure E63, the real part of the dielectric constant is weak at frequency values ranging from 100 Hz to 1 MHz, while it significantly increases for dye doped material in the same frequency range (Fig. 6(a)). This increase in dielectric dispersion comes from the volume effect of DR9 dye molecules in LC composite system. For pure and doped samples, the values of real part of the dielectric constant ε' decrease with the increasing frequency up to 1 MHz. After this frequency, the ε' takes same values from 1 MHz to 10 MHz for both samples. Therefore, it is suggested that dielectric polarization of pure and doped samples no longer responds to electric field in the high frequency range. In Fig. 6(b) shows the frequency dependence of dielectric loss ε" of the samples. It is seen that
Fig. 5. First order diffraction efficiency vs. various percentage of DR9 dye.
Fig. 6. Frequency dependence of (a) real dielectric constant ε', (b) imaginary parts ε" for pure E63 and E63/0.8 wt.% DR9 samples.
imaginary dielectric constant takes higher values in the low frequency region for pure and doped samples. This effect can occur because of the impurity ions in dye and pure LC. The values of ε" increase with the frequency up to about ~2 MHz, where dielectric loss takes maximum value, and then decrease after that point for E63 and dye doped forms. This frequency value is named as relaxation frequency. Fig. 7 shows DC bias voltage dependence of the real part of dielectric constant of pure E63 and dye-doped E63 at various frequencies. It is clearly seen that the change in dielectric constant is relevant to molecular reorientation of LC molecules which is triggered by external applied voltage. Molecular axis is perpendicular to the field when there is no voltage applied (at bias 0 V) and molecules orient parallel to the electric field when the electric field is strong enough (at bias 10 V). Dielectric anisotropy is expressed as Δε = ε// − ε⊥ where ε// and ε⊥ are parallel and perpendicular components of the real dielectric permittivity, respectively. The values of ε' is named as perpendicular (ε⊥) when the applied bias voltage is zero and as parallel (ε//) when the applied bias voltage is maximum. Liquid crystal materials have either positive (p-type) Δε where ε// component is greater than ε⊥, or negative (n-type) Δε where ε⊥, component is greater than ε// [11]. Dielectric anisotropy values of the pure E63 LC and E63/0.8 wt.% DR9 samples are calculated from Fig. 7 and given in Table 1. Doping DR9 dye into pure E63 decreased the dielectric anisotropy in the large frequency range such that the decrement in dielectric anisotropy in lower frequencies is bigger than that in higher frequencies. Samples dielectrically behave as positive
O. Köysal et al. / Optics Communications 284 (2011) 4924–4928
4927
Fig. 7. Dependence of reel part of dielectric constant on DC bias voltage at various frequency; a) 100 Hz, b) 1 kHz, c) 10 kHz, d) 100 kHz, e) 1 MHz, f) 10 MHz.
structure (p-type) until 1 MHz, whereas they behave like dielectrically n-type after ~ 1 MHz such that dielectric anisotropy does not take positive values for the frequency values up to ~ 10 MHz. 4. Conclusion In this study, we characterized the diffraction efficiency and dielectric behavior of pure and dye doped LC materials. Diffraction efficiency η and refractive index change Δn of the investigated
samples were calculated by using two wave-mixing experiment. It is seen that favorable concentration for characterization of our system is 0.8 wt.%. The results indicated that holographic diffraction efficiency enhances with percentage of doping DR9 dye due to the photothermal effect. Maximum diffraction efficiency of 10% was gained with E63/0.8 wt.% DR9. Threshold voltage of the reorientation of LC molecules shifts to higher values with the increasing concentration of DR9 doping material. This effect comes from ionic charge due to the exiting dye molecules under laser illumination in
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O. Köysal et al. / Optics Communications 284 (2011) 4924–4928
Table 1 The variation of dielectric anisotropy of pure E63 and E63/0.8 wt.% DR9 samples as a function of frequency. Δε
100 Hz
1 kHz
10 kHz
100 kHz
1 MHz
10 MHz
E63 E63/0.8%DR9
13.21 12.24
12.84 12.51
11.94 11.75
9.91 9.44
− 1.10 − 1.062
0.0068 0.0084
structure (p-type) until 1 MHz. After this frequency point, dielectric anisotropy takes negative values up to 10 MHz with the applied bias voltage. In conclusion, electro-optic measurements and performance analysis of DR9 dye doped LC structure will pave the way for the new studies by providing a vision about their usage in device design.
References LC-dye hybrid structure. It is clarified that increasing dye concentration creates more ionic charges which create strong dipole moment in LC-dye hybrid structure. As a result, the threshold voltage shifts to lower values since LC molecules orient easily with external applied bias voltage because of the dipole-dipole interaction. Furthermore, dielectric characterization of the samples has been investigated by using dielectric spectroscopy technique. The results indicated that both parts of dielectric constant strongly depend on frequency. For the dye doped material, real part of the dielectric constant increases from 100 Hz to 1 MHz and dielectric loss is slightly higher than that of pure LC material. Another result which was obtained is that the dielectric anisotropy is in strong correlation with molecular orientation. Both samples dielectrically behave as positive
[1] M. Okutan, S.E. San, O. Köysal, Dyes and Pigments 65 (2005) 169. [2] F. Simoni, O. Francescangeli, Journal of Physics. Condensed Matter 11 (1999) 439. [3] H. Ono, T. Kawamura, N.M. Frias, K. Kitamura, N. Kawatsuki, H. Norisada, Advanced Materials 12 (2000) 143. [4] P. Nolan, M. Tillin, D. Coates, Liquid Crystals 14 (1993) 339. [5] H. Ono, Y. Harato, The Journal of the Optical Society of America B 16 (1999) 2195. [6] H.J. Hoffman, The Journal of the Optical Society of America B 3 (1986) 253. [7] A.M. Biradar, D. Kilian, S. Wrobel, W. Haase, Liquid Crystals 27 (2000) 225. [8] F. Yakuphanoglu, M. Okutan, O. Koysal, S.M. Ahn, S.R. Keum, Dyes and Pigments 76 (3) (2008) 721. [9] A.K. Jonscher, Dielectric relaxation in solids, Chelsea Dielectrics Press, London, 1983. [10] O. Köysal, M. Okutan, M. Durmuş, F. Yakuphanoğlu, S.E. San, V. Ahsen, Synthetic Metals 156 (2006) 58. [11] H. Kawamoto, Proceedings of the IEEE 90 (2002) 460.