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Magnetic field effects on the optical properties of an azo-dye doped liquid crystal
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 306 (2006) 103–107 www.elsevier.com/locate/jmmm
Magnetic field effects on the optical properties of an azo-dye doped liquid crystal Cornelia Motoca, Gabriela Iacobescub, a
Department of Physics, University ‘‘Politehnica’’, Splaiul Independentei 313, Bucharest, 060032, Romania b Faculty of Physics, University of Craiova, A. I. Cuza 13, Craiova, 200585, Romania Received 2 September 2005; received in revised form 14 February 2006 Available online 23 March 2006
Abstract Homeotropically aligned nematic liquid crystals doped with azo-dye were subjected both to a linear polarized light of a He–Ne laser and to a magnetic field perpendicular to the incident light beam. We found that the emerging light was elliptically polarized when using magnetic field strengths higher than the threshold value for the magnetic Freedericksz transition. The light transmission, the rotatory power (induced by azo-dye) and the ellipticity varied quasiperiodically when increasing magnetic field strength. The number and positions of maxima and minima depend on the cell thicknesses. Changes in the phase difference between the emergent ordinary and extraordinary rays were computed from the experimental data and the magnetic field dependence of the birefringence was determined. r 2006 Elsevier B.V. All rights reserved. PACS: 42.70.Df; 78.20.Ls; 78.20.Ek; 61.30.Gd; 42.25.Dd Keywords: Liquid crystals; Azo-dyes; Rotatory power; Birefringence
1. Introduction Dye doped liquid crystal guest–host systems were intensively studied in the last years, due to their potential applications to optoelectronic devices and information processing [1–7]. When the dye contains the azo- group, the photoinduced reversible conformational transformations are very promising for reversible optical memories or photochemical switching operations. There are also many studies which investigate the phenomena occurring in such systems [8–13]. Most of these studies are focused on molecular reorientation produced by electrical or optical fields. Still, less works were dedicated to the study of the magnetic field effects on nematic liquid crystal (NLC) doped with azo- dyes [14,15]. These effects could be important to elucidate the type of the interaction between the nematic host and the guest dye.
Corresponding author. Tel.: +40 4251 593599; fax: +40 4251 413977.
E-mail address: [email protected] (G. Iacobescu). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.02.230
Unlike the electric field which is a vectorial one and induces a linear optical anisotropy, the magnetic field is an axial field and, consequently, induces a circular anisotropy [16]. Therefore, the magnetic fields acting upon such guest–host systems could produce interesting optical effects. In this paper we investigate the magneto-optical effects in nematic liquid crystals doped with azo-dyes, when subjecting them to laser irradiation. The studies were performed with the magnetic field applied normally with respect to the optical beam propagation direction (Voigt configuration). The laser beam falls at normal incidence on the homeotropically aligned liquid crystal cells. The paper is organized as follows. First, the experimental set-up and procedures are described. Second, experimental results referring to magnetic field effects on light transmissions, induced rotatory powers and ellipticities are shown and discussed. Finally, the plots showing the phase retardation variation as a function of the magnetic field strengths, for two different cell thicknesses, are computed from the experimental data. The maximum birefringence of the cell was determined using the extrapolation method.
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2. Experimental Liquid crystal cells with 50 and 180 mm thicknesses were obtained using Mylar spacers with uniform thickness. The cells were filled by capillarity with a mixture of nematic liquid crystal (Merck MLC-6601) and a small amount (1.8% by wt) of Methyl Orange (MO) as azo-dye. The host NLC has the clearing point at 77 1C and the refractive indices ne ¼ 1:5498, no ¼ 1:4735, Dn ¼ 0:0763 (determined for l ¼ 589:3 nm at 20 1C). Before filling the cells, the glass substrates were coated with DMOAP surfactant, in order to obtain a homeotropic alignment of the liquid crystal (the long molecular axis normal with respect to the glass substrates). The experimental set-up is shown in Fig. 1. A He–Ne laser beam (632.8 nm, 1 mW) falls at normal incidence on a liquid crystal cell (LC) placed in the middle of the electromagnet (E). The magnetic field direction was set to be normal with respect to the light propagation direction (Voigt configuration). At the exit, the plane of polarization was determined by rotating a Glan–Thomson polarizer (P) to obtain the extinction of the transmitted light. The intensity of the transmitted light was recorded by means of a photomultiplier (Ph), connected at a multimeter (M). Changes in the magnetic field strength were achieved by a DC power supply (PS) which allowed both current adjustment and change of polarity. The current through the electromagnet was slowly increased and for each fixed value of the magnetic field the angle of rotation corresponding to maximum extinction was measured. The intensity of the transmitted light, registered by the photomultiplier, was recorded before and after rotating the polarizer. All measurements were performed at room temperature and the experimental procedure was identically applied for both cells with two different thicknesses.
the guest azo-dye has also rodlike shape. It aligns in a parallel direction with respect to the host molecules favoring the nematic order, unlike the bent shaped cis isomer which tends to disturb the nematic phase. The torques induced by the trans/cis azo-dye’s isomers on the nematic molecules were intensively studied in the last years [10]. When a magnetic field is applied on guest–host mixtures, the molecules of the NLC and azo-dye receive a magnetic torque from the magnetic field and the molecular orientation will change. The NLC molecules have the tendency to orient with their long molecular axis along the magnetic field direction. The initial homeotropic alignment of the mixture molecules is disturbed (tends to change into a parallel one), except for two very thin layers in contact with the glass substrates. The equilibrium molecular orientation of the entire system depends on the balance between the magnetic, azo-dye and glass surface induced torques. If a polarized monochromatic light beam propagates through such anisotropic medium an interference of the ordinary and extraordinary rays in the emergent light could be observed [16]. The plots showing changes in the light transmission as a function of applied magnetic field, for two cells with different thicknesses, are given in Fig. 2. In all figures from below the lines are drawn to guide eyes. The homeotropic cells exhibit dark states for magnetic field strength below the threshold value for the magnetic Freedericksz transition, since the incident linearly polarized light behaves like an ordinary ray and experience no phase retardation [17]. Above the threshold value, the NLC directors are reoriented by the magnetic field and phase retardation between the ordinary and extraordinary rays takes place. It may be seen from Fig. 2 that the light transmission varies quasi-periodically when the magnetic field strength is increased above the threshold value. These changes are quite similar to those appearing in nematic
3. Results and discussion Nematic liquid crystals consist of rodlike molecules which, on the average, line up parallel to a preferential direction named molecular director. The trans isomer of
Fig. 1. Experimental set-up.
Fig. 2. The intensities of transmitted light (He–Ne, l ¼ 632:8 nm) as a function of the magnetic field strength, for fixed polarization angle.
ARTICLE IN PRESS C. Motoc, G. Iacobescu / Journal of Magnetism and Magnetic Materials 306 (2006) 103–107
liquid crystals subjected to electric field exceeding the critical value for electrical Freedericksz transition. This effect, reported by Assouline et al. [18], was termed ‘‘electrically controlled birefringence’’ [19]. It was investigated in several nematic liquid crystals due to its possible applications in optoelectronics [17]. A voltage-dependent transmission technique was proposed to measure the birefringence of liquid crystals [20,21]. As known, in nematic liquid crystals the phenomena induced by magnetic fields are similar to those induced by electric fields. Therefore, in order to fit our experimental data we used the following equation giving the transmitted light intensity as a function of magnetic field [16] d I ¼ I 0 sin2 ½2FðBÞsin2 , 2
(1)
where I0 is the incident beam intensity, F(B) is the angle between the polarization axis of the monochromatic light wave and the molecular director of the liquid crystal, and d is the phase difference between the extraordinary and ordinary rays in the outgoing light. Eq. (1) is valid under the assumption that the absorption difference for the extraordinary and ordinary rays is negligible. For pure NLC, which displays no optical activity (rotatory power), the angle F from Eq. (1) is not controlled by the external fields [22]. Optical activity may be induced in NLC by adding small amounts of cholesteric compounds; in this case F depends quasiperiodically on the external field strengths [23]. For NLC containing a small amount of azo-dye we have found that the angle F(B) varied with magnetic field as shown in Fig. 3 (the positive values correspond to clockwise rotation). This shows that optical activity is induced in nematic MLC-6601 by the azo-dye. When the magnetic field lines are normal with respect to the light propagation direction, there are two possible Faraday configurations: longitudinal and transversal. In
Fig. 3. Magnetic field dependences of the rotation angles, for two different cell thicknesses.
105
the particular case of normal incidence of the light beam on the LC cell, the longitudinal and transversal Faraday configurations are equivalent and any rotation of the polarization plane appears for either s- or p- polarization in the isotropic media. From Fig. 3 it may be seen that, above the threshold value of the magnetic field, the rotation of the polarization plane of the light beam passing through the LC anisotropic medium has a periodical variation. The linear dependence on the magnetic field may be neglected. In this case we did not notice a linear Faraday effect. The peculiar periodical dependence of the rotation angle may be ascribed to the gyroscopic-like movement of the molecular LC director imposed by the magnetic field and azo-dye induced torques acting on the system. These torques give rise to 3D reorientation of the LC director which could be defined by the azimuthal angle F(B) (rotation angle) and the polar angle y(B) [24]. When the alignment of molecules is changed, the phase velocities of the light are modulated. Then, the value of the phase difference, d, is controlled by the applied magnetic field. The phase difference d, for the geometry of our experiment, is given by d¼
2pdDnðBÞ 2 sin yðBÞ, l
(2)
where d is the LC layer thickness, Dn ¼ ne no is the birefringence of the LC, l is the wavelength of the light, and y(B) is the angle between the optic axis of the LC and the light propagation direction (polar angle) [21]. The dependence of the tilt angle y on the magnetic field strength is similar to that for the homeotrop-planar Freedericksz transition in nematic liquid crystals [22]. Another important parameter is the ellipticity of the emerging light, e. The ellipticity is given by e ¼ B=A (where A, B are the half axes of the ellipse). It p may be determined ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi experimentally using the formula e ¼ I min =I max , where Imin is the minimum value of the intensity recorded by the photomultiplier (after rotating the polarizer) and Imax is the maximum value of the intensity. As it may be seen from Fig. 4, the ellipticities of the emergent light depend also periodically on the applied magnetic field, for both cells. The positions of the maxima and minima strongly depend on the cell thickness, above the threshold magnetic field. When the polarizer is removed, the LC cells act as phase retardation plates: half or quarter wave plates for the magnetic field values corresponding to minima or maxima of the ellipticity curves, respectively. Thus, the linearly polarized monochromatic light may be reversibly changed into any desired polarization state; these polarization changes are controlled by the magnetic field. The thicker LC cells used as phase retardation plates are suitable for lower magnetic field strengths, as the magnetic field threshold is lower. When examining the minima or maxima of light intensity it results that the phase shift changes when increasing the magnetic field strength above the threshold
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Fig. 4. The ellipticities of transmitted light beams (He–Ne, l ¼ 632:8 nm) through LC cells with two different thickness, as a function of magnetic field strength.
value. This is an indication for the existence of a fielddependent birefringence. From each maximum to the adjacent minimum in the transmitted light intensity plot (Fig. 2), the phase change is p. As mentioned by Khoo and Wu [17], when applying high magnetic fields the transmitted light intensity reaches a saturation level. This limiting value is never reached because it is hardly to reorient the layers in the proximity of the glass substrates. Following the method explained in detail in Refs. [17,21,22] we have calculated the phase retardation, d, as a function of the magnetic field, for the cells with 50 mm and 180 mm thickness (Fig. 5). The threshold fields for magnetic Freedericksz transition could be obtained from the linear extrapolation near the threshold regions; we have found Bth ¼ 2880 Gs and Bth ¼ 1039 Gs for 50 mm and 180 mm cell thickness, respectively. The phase difference as a function of Bth/B for each cell is shown inn Fig. 6. Linear dependences d ¼ dðBth =BÞ were found when Bth/Bo0.75 for 50 mm cell thickness and when Bth/ Bo0.45 for 180 mm cell thickness. The total phase differences, dmax, were obtained from the plots’ extrapolation at Bth/B ¼ 0 (or B-N). The values for dmax deduced by this method are: dmax ¼ 11.4p (for d ¼ 50 mm) and dmax ¼ 41:54p (for d ¼ 180 mm). From Eq. (2), we have calculated the maximum birefringence for each cell, under the assumption that at very high magnetic fields (B-N), sin2y-1. The computed maximum birefringence values are: Dnmax ¼ 0:0721 (for d ¼ 50 mm) and Dnmax ¼ 0:073 (for d ¼ 180 mm). The dependence of the birefringence on the applied magnetic field is shown in Fig. 7. The birefringence abruptly increases when the magnetic field exceeds the Freedericksz transition threshold. The increasing rate is lower for high magnetic fields, as a consequence of the balance of surface torques and external magnetic field
Fig. 5. Phase retardation dependence on the magnetic field for two different cells.
Fig. 6. Phase difference as a function of the ratio Bth/B.
induced torques. When B-N, the birefringence reached its maximum value. 4. Conclusions Linear polarized light from a He–Ne laser passing through azo-dye doped NLC cells becomes elliptically polarized when the cells are subjected to magnetic field. The transmitted light intensity, the rotatory power and ellipticity exhibit a quasiperiodically behaviour when using magnetic field strength above the threshold value for magnetic Freedericksz transition. The magnitude and positions of maxima and minima depend on cell thickness (60, 180 mm). The LC cells act like retardation plates and provide any degree of phase shift from 0 to 2p. It may be suggested that the rotatory polarization arises as a result of disturbing the nematic order by azo-dyes, rather than a result of the Faraday effect. Its periodical
ARTICLE IN PRESS C. Motoc, G. Iacobescu / Journal of Magnetism and Magnetic Materials 306 (2006) 103–107
Fig. 7. Magnetic field dependence of birefringence for azo-dye doped NLC, for two different cells.
variation could be a consequence of the magnetic field and azo-dye induced torques balance. The birefringence of the mixture was computed from the transmitted light intensity data. The birefringence showed a strong variation just above the threshold magnetic field and a slower one for higher field values. Within the range of high fields the birefringence reached a saturation value which was estimated by using the phase retardation plots. References [1] I. Janossy, Phys. Rev. E 49 (4) (1994) 2957. [2] L. Marrucci, D. Paparo, Phys. Rev. E 56 (2) (1997) 1765. [3] R. Muenster, M. Jarasch, X. Zhuang, Y.R. Shen, Phys. Rev. Lett. 78 (1) (1997) 42.
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[4] V.I. Kopp, B. Fan, H.K.M. Vithana, A.Z. Genack, Opt. Lett. 23 (1998) 1707. [5] H. Ono, Y. Harato, J. Appl. Phys. 85 (2) (1999) 676. [6] V.I. Kopp, Z.-Q. Zhang, A.Z. Genack, Prog. Quantum Electron. 27 (2003) 369. [7] S.M. Morris, A.D. Ford, M.N. Pivnenko, H.J. Coles, J. Opt. A: Pure Appl. Opt. 7 (2005) 215. [8] R.A. Hill, S. Dreher, A. Knoesen, D.R. Yankelevich, Appl. Phys. Lett. 66 (17) (1995) 2156. [9] T.V. Galstyan, B. Saad, M.M. Denariez-Roberge, J. Chem. Phys. 107 (22) (1997) 9319. [10] I. Janossy, L. Szabados, Phys. Rev. E 58 (4) (1998) 4598. [11] B. Saad, T.V. Galstyan, M.M. Denariez-Roberge, M. Dumont, Opt. Commun. 151 (1998) 235. [12] E. Ouskova, Y. Reznikov, S.V. Shiyanovskii, L. Su, J.L. West, O.V. Kuksenok, O. Francescangeli, F. Simoni, Rev. E 64 (2001) 051709. [13] M. Becchi, I. Janossy, D. S. Shankar Rao, D. Statman, Electronic-Liq. Cryst. Commun. http://www.e-lc.org/docs/ 2004_01_22_04_39_35 [14] E.F. Carr, Mol. Cryst. Liq. Cryst. 3 (1–2) (1986) 1. [15] W. Jian, L. Yu, C. Yonglie, L. Zhaoxi, X. Ying, L. Ziyang, S. Longpei, Z. Jianying, J. Mat. Sci.: Mater. Electron. 12 (10) (2001) 597. [16] M. Born, E. Wolf, Principles of Optics, Pergamon Press, London, 1965. [17] I.-C. Khoo, S.-T. Wu, Optics and Nonlinear Optics of Liquid Crystals, World Scientific, Singapore, 1993, p. 113. [18] D. Assouline, M. Hareng, E. Leiba, Electron. Lett. 7 (1971) 7699. [19] R.A. Soref, M.J. Rafuse, J. Appl. Phys. 43 (1972) 2029. [20] W. Haase, D. Potzsch, Mol. Cryst. Liq. Cryst. 38 (1977) 77. [21] S.-T. Wu, U. Efron, L.D. Hess, Appl. Opt. 23 (21) (1984) 3911. [22] M. Ohtsu, T. Akahane, T. Tako, Jap. J. Appl. Phys. 13 (4) (1974) 621. [23] V.V. Belyayev, Krystallogr. 5 (24) (1979) 1092. [24] S.-W. Suh, Z. Zhuang, J.S. Patel, SID 29/1 (1998) 997.
Journal of Magnetism and Magnetic Materials 306 (2006) 103–107 www.elsevier.com/locate/jmmm
Magnetic field effects on the optical properties of an azo-dye doped liquid crystal Cornelia Motoca, Gabriela Iacobescub, a
Department of Physics, University ‘‘Politehnica’’, Splaiul Independentei 313, Bucharest, 060032, Romania b Faculty of Physics, University of Craiova, A. I. Cuza 13, Craiova, 200585, Romania Received 2 September 2005; received in revised form 14 February 2006 Available online 23 March 2006
Abstract Homeotropically aligned nematic liquid crystals doped with azo-dye were subjected both to a linear polarized light of a He–Ne laser and to a magnetic field perpendicular to the incident light beam. We found that the emerging light was elliptically polarized when using magnetic field strengths higher than the threshold value for the magnetic Freedericksz transition. The light transmission, the rotatory power (induced by azo-dye) and the ellipticity varied quasiperiodically when increasing magnetic field strength. The number and positions of maxima and minima depend on the cell thicknesses. Changes in the phase difference between the emergent ordinary and extraordinary rays were computed from the experimental data and the magnetic field dependence of the birefringence was determined. r 2006 Elsevier B.V. All rights reserved. PACS: 42.70.Df; 78.20.Ls; 78.20.Ek; 61.30.Gd; 42.25.Dd Keywords: Liquid crystals; Azo-dyes; Rotatory power; Birefringence
1. Introduction Dye doped liquid crystal guest–host systems were intensively studied in the last years, due to their potential applications to optoelectronic devices and information processing [1–7]. When the dye contains the azo- group, the photoinduced reversible conformational transformations are very promising for reversible optical memories or photochemical switching operations. There are also many studies which investigate the phenomena occurring in such systems [8–13]. Most of these studies are focused on molecular reorientation produced by electrical or optical fields. Still, less works were dedicated to the study of the magnetic field effects on nematic liquid crystal (NLC) doped with azo- dyes [14,15]. These effects could be important to elucidate the type of the interaction between the nematic host and the guest dye.
Corresponding author. Tel.: +40 4251 593599; fax: +40 4251 413977.
E-mail address: [email protected] (G. Iacobescu). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.02.230
Unlike the electric field which is a vectorial one and induces a linear optical anisotropy, the magnetic field is an axial field and, consequently, induces a circular anisotropy [16]. Therefore, the magnetic fields acting upon such guest–host systems could produce interesting optical effects. In this paper we investigate the magneto-optical effects in nematic liquid crystals doped with azo-dyes, when subjecting them to laser irradiation. The studies were performed with the magnetic field applied normally with respect to the optical beam propagation direction (Voigt configuration). The laser beam falls at normal incidence on the homeotropically aligned liquid crystal cells. The paper is organized as follows. First, the experimental set-up and procedures are described. Second, experimental results referring to magnetic field effects on light transmissions, induced rotatory powers and ellipticities are shown and discussed. Finally, the plots showing the phase retardation variation as a function of the magnetic field strengths, for two different cell thicknesses, are computed from the experimental data. The maximum birefringence of the cell was determined using the extrapolation method.
ARTICLE IN PRESS 104
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2. Experimental Liquid crystal cells with 50 and 180 mm thicknesses were obtained using Mylar spacers with uniform thickness. The cells were filled by capillarity with a mixture of nematic liquid crystal (Merck MLC-6601) and a small amount (1.8% by wt) of Methyl Orange (MO) as azo-dye. The host NLC has the clearing point at 77 1C and the refractive indices ne ¼ 1:5498, no ¼ 1:4735, Dn ¼ 0:0763 (determined for l ¼ 589:3 nm at 20 1C). Before filling the cells, the glass substrates were coated with DMOAP surfactant, in order to obtain a homeotropic alignment of the liquid crystal (the long molecular axis normal with respect to the glass substrates). The experimental set-up is shown in Fig. 1. A He–Ne laser beam (632.8 nm, 1 mW) falls at normal incidence on a liquid crystal cell (LC) placed in the middle of the electromagnet (E). The magnetic field direction was set to be normal with respect to the light propagation direction (Voigt configuration). At the exit, the plane of polarization was determined by rotating a Glan–Thomson polarizer (P) to obtain the extinction of the transmitted light. The intensity of the transmitted light was recorded by means of a photomultiplier (Ph), connected at a multimeter (M). Changes in the magnetic field strength were achieved by a DC power supply (PS) which allowed both current adjustment and change of polarity. The current through the electromagnet was slowly increased and for each fixed value of the magnetic field the angle of rotation corresponding to maximum extinction was measured. The intensity of the transmitted light, registered by the photomultiplier, was recorded before and after rotating the polarizer. All measurements were performed at room temperature and the experimental procedure was identically applied for both cells with two different thicknesses.
the guest azo-dye has also rodlike shape. It aligns in a parallel direction with respect to the host molecules favoring the nematic order, unlike the bent shaped cis isomer which tends to disturb the nematic phase. The torques induced by the trans/cis azo-dye’s isomers on the nematic molecules were intensively studied in the last years [10]. When a magnetic field is applied on guest–host mixtures, the molecules of the NLC and azo-dye receive a magnetic torque from the magnetic field and the molecular orientation will change. The NLC molecules have the tendency to orient with their long molecular axis along the magnetic field direction. The initial homeotropic alignment of the mixture molecules is disturbed (tends to change into a parallel one), except for two very thin layers in contact with the glass substrates. The equilibrium molecular orientation of the entire system depends on the balance between the magnetic, azo-dye and glass surface induced torques. If a polarized monochromatic light beam propagates through such anisotropic medium an interference of the ordinary and extraordinary rays in the emergent light could be observed [16]. The plots showing changes in the light transmission as a function of applied magnetic field, for two cells with different thicknesses, are given in Fig. 2. In all figures from below the lines are drawn to guide eyes. The homeotropic cells exhibit dark states for magnetic field strength below the threshold value for the magnetic Freedericksz transition, since the incident linearly polarized light behaves like an ordinary ray and experience no phase retardation [17]. Above the threshold value, the NLC directors are reoriented by the magnetic field and phase retardation between the ordinary and extraordinary rays takes place. It may be seen from Fig. 2 that the light transmission varies quasi-periodically when the magnetic field strength is increased above the threshold value. These changes are quite similar to those appearing in nematic
3. Results and discussion Nematic liquid crystals consist of rodlike molecules which, on the average, line up parallel to a preferential direction named molecular director. The trans isomer of
Fig. 1. Experimental set-up.
Fig. 2. The intensities of transmitted light (He–Ne, l ¼ 632:8 nm) as a function of the magnetic field strength, for fixed polarization angle.
ARTICLE IN PRESS C. Motoc, G. Iacobescu / Journal of Magnetism and Magnetic Materials 306 (2006) 103–107
liquid crystals subjected to electric field exceeding the critical value for electrical Freedericksz transition. This effect, reported by Assouline et al. [18], was termed ‘‘electrically controlled birefringence’’ [19]. It was investigated in several nematic liquid crystals due to its possible applications in optoelectronics [17]. A voltage-dependent transmission technique was proposed to measure the birefringence of liquid crystals [20,21]. As known, in nematic liquid crystals the phenomena induced by magnetic fields are similar to those induced by electric fields. Therefore, in order to fit our experimental data we used the following equation giving the transmitted light intensity as a function of magnetic field [16] d I ¼ I 0 sin2 ½2FðBÞsin2 , 2
(1)
where I0 is the incident beam intensity, F(B) is the angle between the polarization axis of the monochromatic light wave and the molecular director of the liquid crystal, and d is the phase difference between the extraordinary and ordinary rays in the outgoing light. Eq. (1) is valid under the assumption that the absorption difference for the extraordinary and ordinary rays is negligible. For pure NLC, which displays no optical activity (rotatory power), the angle F from Eq. (1) is not controlled by the external fields [22]. Optical activity may be induced in NLC by adding small amounts of cholesteric compounds; in this case F depends quasiperiodically on the external field strengths [23]. For NLC containing a small amount of azo-dye we have found that the angle F(B) varied with magnetic field as shown in Fig. 3 (the positive values correspond to clockwise rotation). This shows that optical activity is induced in nematic MLC-6601 by the azo-dye. When the magnetic field lines are normal with respect to the light propagation direction, there are two possible Faraday configurations: longitudinal and transversal. In
Fig. 3. Magnetic field dependences of the rotation angles, for two different cell thicknesses.
105
the particular case of normal incidence of the light beam on the LC cell, the longitudinal and transversal Faraday configurations are equivalent and any rotation of the polarization plane appears for either s- or p- polarization in the isotropic media. From Fig. 3 it may be seen that, above the threshold value of the magnetic field, the rotation of the polarization plane of the light beam passing through the LC anisotropic medium has a periodical variation. The linear dependence on the magnetic field may be neglected. In this case we did not notice a linear Faraday effect. The peculiar periodical dependence of the rotation angle may be ascribed to the gyroscopic-like movement of the molecular LC director imposed by the magnetic field and azo-dye induced torques acting on the system. These torques give rise to 3D reorientation of the LC director which could be defined by the azimuthal angle F(B) (rotation angle) and the polar angle y(B) [24]. When the alignment of molecules is changed, the phase velocities of the light are modulated. Then, the value of the phase difference, d, is controlled by the applied magnetic field. The phase difference d, for the geometry of our experiment, is given by d¼
2pdDnðBÞ 2 sin yðBÞ, l
(2)
where d is the LC layer thickness, Dn ¼ ne no is the birefringence of the LC, l is the wavelength of the light, and y(B) is the angle between the optic axis of the LC and the light propagation direction (polar angle) [21]. The dependence of the tilt angle y on the magnetic field strength is similar to that for the homeotrop-planar Freedericksz transition in nematic liquid crystals [22]. Another important parameter is the ellipticity of the emerging light, e. The ellipticity is given by e ¼ B=A (where A, B are the half axes of the ellipse). It p may be determined ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ffi experimentally using the formula e ¼ I min =I max , where Imin is the minimum value of the intensity recorded by the photomultiplier (after rotating the polarizer) and Imax is the maximum value of the intensity. As it may be seen from Fig. 4, the ellipticities of the emergent light depend also periodically on the applied magnetic field, for both cells. The positions of the maxima and minima strongly depend on the cell thickness, above the threshold magnetic field. When the polarizer is removed, the LC cells act as phase retardation plates: half or quarter wave plates for the magnetic field values corresponding to minima or maxima of the ellipticity curves, respectively. Thus, the linearly polarized monochromatic light may be reversibly changed into any desired polarization state; these polarization changes are controlled by the magnetic field. The thicker LC cells used as phase retardation plates are suitable for lower magnetic field strengths, as the magnetic field threshold is lower. When examining the minima or maxima of light intensity it results that the phase shift changes when increasing the magnetic field strength above the threshold
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Fig. 4. The ellipticities of transmitted light beams (He–Ne, l ¼ 632:8 nm) through LC cells with two different thickness, as a function of magnetic field strength.
value. This is an indication for the existence of a fielddependent birefringence. From each maximum to the adjacent minimum in the transmitted light intensity plot (Fig. 2), the phase change is p. As mentioned by Khoo and Wu [17], when applying high magnetic fields the transmitted light intensity reaches a saturation level. This limiting value is never reached because it is hardly to reorient the layers in the proximity of the glass substrates. Following the method explained in detail in Refs. [17,21,22] we have calculated the phase retardation, d, as a function of the magnetic field, for the cells with 50 mm and 180 mm thickness (Fig. 5). The threshold fields for magnetic Freedericksz transition could be obtained from the linear extrapolation near the threshold regions; we have found Bth ¼ 2880 Gs and Bth ¼ 1039 Gs for 50 mm and 180 mm cell thickness, respectively. The phase difference as a function of Bth/B for each cell is shown inn Fig. 6. Linear dependences d ¼ dðBth =BÞ were found when Bth/Bo0.75 for 50 mm cell thickness and when Bth/ Bo0.45 for 180 mm cell thickness. The total phase differences, dmax, were obtained from the plots’ extrapolation at Bth/B ¼ 0 (or B-N). The values for dmax deduced by this method are: dmax ¼ 11.4p (for d ¼ 50 mm) and dmax ¼ 41:54p (for d ¼ 180 mm). From Eq. (2), we have calculated the maximum birefringence for each cell, under the assumption that at very high magnetic fields (B-N), sin2y-1. The computed maximum birefringence values are: Dnmax ¼ 0:0721 (for d ¼ 50 mm) and Dnmax ¼ 0:073 (for d ¼ 180 mm). The dependence of the birefringence on the applied magnetic field is shown in Fig. 7. The birefringence abruptly increases when the magnetic field exceeds the Freedericksz transition threshold. The increasing rate is lower for high magnetic fields, as a consequence of the balance of surface torques and external magnetic field
Fig. 5. Phase retardation dependence on the magnetic field for two different cells.
Fig. 6. Phase difference as a function of the ratio Bth/B.
induced torques. When B-N, the birefringence reached its maximum value. 4. Conclusions Linear polarized light from a He–Ne laser passing through azo-dye doped NLC cells becomes elliptically polarized when the cells are subjected to magnetic field. The transmitted light intensity, the rotatory power and ellipticity exhibit a quasiperiodically behaviour when using magnetic field strength above the threshold value for magnetic Freedericksz transition. The magnitude and positions of maxima and minima depend on cell thickness (60, 180 mm). The LC cells act like retardation plates and provide any degree of phase shift from 0 to 2p. It may be suggested that the rotatory polarization arises as a result of disturbing the nematic order by azo-dyes, rather than a result of the Faraday effect. Its periodical
ARTICLE IN PRESS C. Motoc, G. Iacobescu / Journal of Magnetism and Magnetic Materials 306 (2006) 103–107
Fig. 7. Magnetic field dependence of birefringence for azo-dye doped NLC, for two different cells.
variation could be a consequence of the magnetic field and azo-dye induced torques balance. The birefringence of the mixture was computed from the transmitted light intensity data. The birefringence showed a strong variation just above the threshold magnetic field and a slower one for higher field values. Within the range of high fields the birefringence reached a saturation value which was estimated by using the phase retardation plots. References [1] I. Janossy, Phys. Rev. E 49 (4) (1994) 2957. [2] L. Marrucci, D. Paparo, Phys. Rev. E 56 (2) (1997) 1765. [3] R. Muenster, M. Jarasch, X. Zhuang, Y.R. Shen, Phys. Rev. Lett. 78 (1) (1997) 42.
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