DYE DOPED NEMATIC LIQUID CRYSTAL REORIENTATION IN A LINEAR POLARIZED LASER FIELD: THRESHOLD EFFECT

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1 DYE DOPED NEMATIC LIQUID CRYSTAL REORIENTATION IN A LINEAR POLARIZED LASER FIELD: THRESHOLD EFFECT NICOLETA ESEANU* 1, CORNELIA UNCHESELU 2, I. PALARIE 3, B. UMANSKI 4 1 Department of Physics, ''Politehnica'' University of Bucharest, 313 Splaiul Independentei, Bucharest 2 Department of Physics, Technical University of Civil Engineering of Bucharest, Ave. Lacul Tei, Bucharest 3 Faculty of Physics, University of Craiova,13 A. I. Cuza Str., Craiova 4 SSC RF NIOPIK (Organic Intermediate & Dyes Institute), B. Sadovaya 1/4, Moscow *nicoletaeseanu@ yahoo.com (Received July 15, 2003) Abstract. An experimental study of the molecular orientation induced by a polarized Ar + laser beam in a dye doped nematic liquid crystal (DDNLC) has been done. A sandwich type sample filled with 5CB+MR was aligned approximately planar by two glass surfaces, only one surface being rubbed. By measuring He-Ne probe beam intensity in time a threshold effect has been observed when modifying the Ar + laser power. This could be explained by using the percolation theory. Key words: dyes, liquid crystals, molecular orientation, parallel alignment, threshold effect. 1. Introduction The operation principle of the electro-optical liquid crystal devices (such as liquid crystal displays - LCD) combines bulk properties and surface interactions. Therefore controlling the alignment of the liquid crystal (LC) molecules is an attractive research field. It is well known that a laser beam passing through a transparent nematic liquid crystal (NLC) produces the reorientation of the nematic director. This effect is explained by the optical torque acting on NLC molecules due to their anisotropic optical polarizability [1]. Recently it was demonstrated that a small amount (less than 1%) of a dye dissolved in NLC can enhance the reorientation by almost two orders of magnitude and, in some cases, can even change its sign [2]. The explanation proposed by Janossy [3] is connected with the electronically excited metastable states appearing in the molecules of absorbing dye. The interaction of the excited dye molecule with the host LC molecules is different from the one of the dye molecule in the ground state. The Janossy model is suitable for the reorientational bulk effect. A surface driven reorientation effect as a result of the light action on the bulk of a light-sensitive NLC-azodye mixture was reported by Voloschenko and co-workers. The explanation was the adsorption of dye phototransformed molecules onto the aligning photopolymer surface [4].

2 Simoni and co-workers have pointed that the observed huge reorientational effect in a LC-dye mixture is due to light-induced modifications on the aligning surface (SINE - Surface-Induced Nonlinear Effect) [5]. Their investigations about the diffraction efficiency revealed the basic role of light-induced adsorption and desorption of the dye molecules onto the boundary surfaces [6, 7]. In this paper we report an experimental study of the dye aligning effect on the nematic director for various values of the exciting laser power and the presence of a threshold effect. A phenomenological explanation using some percolation theory notions is given. We have also investigated the differences in nematic director configurations obtained when the laser exciting beam radiation is impinging through the oriented surface or through the isotropic surface of the LC cell. 2. Materials and Experimental Device The experiments were realized using a standard sandwich glass cell filled with a mixture of NLC 4'-n-pentyl-4-cyanobiphenyl (5CB, clearing point T = 36 C) and azo-dye methyl red (MR) as a dopant (weight concentrations: 0.5%, 1% and 2%). Their structural formulas are presented in Fig.1. Fig.1 Pentyl-cyanobiphenyl (5CB) and methyl-red (MR). Fig.2 Absorption spectrum of methyl-red. Methyl red is a photosensitive dye [8]; its intrinsic photochemical processes are: rotation, trans-cis isomerization (for λ = 514 nm) and bleaching under the illumination in the absorption band (its absorption spectrum is shown in

3 Fig.2). The thickness of the cells was approximately 19 µm (obtained by Mylar spacers). The inner surfaces of the glasses were covered with a thin polymeric film, polyamide acid - PAC (the films of a solution of PAC in dimethylformamide were baked in an oven at 180 C for 90 minutes, then slowly cooled). This polymer has been chosen because it does not dissolve MR. One of the surfaces was rubbed in one direction provided a strong uniaxial anchoring of NLC molecules on the surface. This oriented surface (or passive surface, S pas ) imposed the planar orientation of the nematic director for the whole cell in the initial state. The other surface (control surface, S con ) is isotropic and in most of the experiments the exciting laser beam is impinging through it. The optical arrangement is shown in Fig. 3. The cell was placed normally to the exciting beam of Ar + laser (wavelength λ = nm; power P exc. was varied between 3 mw and 20 mw). The polarization of the excited beam was established by the polaryzer P, the electric vector being horizontal. The probe beam of He-Ne laser (wavelength λ = 633 nm; power P test = 1 mw) was focused by a lens L at the same plane with the Ar + beam. The two laser beams must be superposed in the same area of the NLC cell but the testing one must be smaller. The polarization of the He-Ne beam is 1000:1 for the vertical electric component. The rubbing direction was parallel with the electric vector of the probe beam. After having passed through the cell the testing beam was deviated by a beam-splitter (BS) and went through the analyzer A. The transmission direction of the analyzer was horizontal. The probe beam intensity measurement was performed by an optical fiber (OF) connected with an Ocean Optics spectrometer S 2000 (S) and a computer. The electric vector E test follows the nematic director through the whole cell (Mauguine limit [9] is fulfilled in our experiments). We have measured the intensity of the transmitted test light in the direction perpendicular to the initial nematic orientation as function of the irradiating time for different values of the exciting laser power.

4 Fig.3 Experimental set-up. III. Experimental results In most of our experiments the exciting laser radiation was normally incident through the isotropic polymer surface. The intensity of the transmitted test light in the direction mentioned above as function of the irradiating time for different values of the exciting laser power is presented for the cells containing 5CB doped with 1% MR and 5CB doped with 2% MR in Fig. 4 and Fig. 5, respectively. The trends of these dependencies are similar with that obtained by Voloschenko and co-workers [4] for 5CB doped with 1% MR when the exciting He - Cd laser power was 15 mw. We could have estimated the threshold exciting power: P th = 6 mw for 5CB doped with 1% MR and P th = 4 mw for 5CB doped with 2% MR. For the sample containing 5CB doped with 0.5% MR no reorientation effect was observed contrasting to some certain experiments [10], but according with other ones [11].

5 Fig.4 5CB doped with 1% MR.

6 Fig.5 5CB doped with 2% MR. We have evaluated the rotating angle θ of the nematic director as function of the irradiation time t for the cell containing 5CB doped with 2% MR by using the relation: sin 2 θ ( t ) r = r 2 I I ( t) γ I I sin tr dark 2 where I γ is the transmitted intensity when the analyzer is rotating with the angle γ (before irradiating the sample), I Dark is the dark signal and r, r are the Fresnel coefficients. The factor r r 2 dark is due to the light reflection on the beam-splitter. γ

7 Twist angle (degrees) Pexc20mW Pexc10mW Pexc5mW Pexc4mW Pexc3mW Irradiation time (s) Fig.6 Nematic twist angle versus irradiation time. Time-dependent rotating angle of the nematic director at different exciting laser power is presented in Fig. 6. We remark that at the end of irradiation (t = 1000 s) the rotating angle was 13 for P exc. = 4 mw and for P exc. = 20 mw. Our experimental results are similar with those reported in [10] for the same mixture (dye concentration 0.5%) but in different irradiating geometry, for different exciting wavelength (λ = 440 nm), different polymer substrates, and different measuring method of the twist angle. For the cell 5CB doped with 1% MR we have also investigated the intensity of the transmitted test light when the exciting laser radiation is normally incident through the oriented polymer surface. The results are shown in Fig. 7 for the same exciting power P exc. = 15 mw. When the exciting laser radiation enters the oriented (rubbed) polymer surface the intensity of transmitted testing light increases slowly as a function of the irradiation time and its values are smaller then in the case when the exciting laser beam enters the isotropic surface.

8 Fig.7 Exciting laser beam impinges through isotropic surface or through the rubbed surface. 4. Discussion The explanation of the existing threshold effect might be the following: the number of the excited dye molecules is dependent both on the exciting laser power by the probability of photon absorption and on the concentration of the dye molecules. An excited dye molecule produces, by its light-induced reorientation, a nematic local order in the bulk of NLC with the director perpendicular to the initial LC alignment. For the assumed spherical oriented zones the radius of alignment depends on the phototransformation of the dye molecules and on the interactions between these molecules and the host NLC molecules. When the exciting laser power is small the oriented zones can be treated as isolated defects in the bulk of NLC (see Fig. 8a). When the power increases (or if the dye concentration is increased) the number of these zones increases too and it appears a critical situation: the distribution of the oriented zones induces a reorientation of the whole NLC cell (see Fig. 8b). A quantitative model is possible using the methods of the percolation theory.

9 Fig.8a NLC orientation around isolated excited dye molecules. Fig.8b NLC orientation beyond the percolation threshold. The differences between experimental data when the exciting laser radiation is impinging through the isotropic surface (case 1) or through the oriented

10 surface (case 2) could be explained by the different nematic director configurations during the irradiation. In case 1 the reorientation of NLC molecules takes place in the NLC bulk from the isotropic surface to the oriented one. The result is a twisted configuration of the nematic director (TN twisted nematic) from the laser - induced orientation to the orientation imposed by the easy axis of the rubbed polymeric film. The two directions are orthogonal. The linearly polarized He-Ne probe beam having the electric vector E test parallel to the rubbing direction of the oriented surface is impinging through it one. This vector is rotated in the plane of the LC cell with an angle θ when passing through the reoriented NLC. The transmitted light intensity is increased if the rotation angle is bigger. In case 2 the number of the oriented dye molecules is increased in the vicinity of the oriented polymer surface and thus the easy axis is modified. The result is a new alignment of the bulk nematic director from this surface to the isotropic one. The configuration is still nematic but with an angular deviation from the initial alignment (DN deviated nematic). 5. Conclusions We have studied the aligning dye effect on the nematic director for various values of the exciting laser power and we have shown the presence of a threshold effect. We have estimated the values of the threshold exciting power: P th = 6 mw for 5CB doped with 1% MR and P th = 4 mw for 5CB doped with 2% MR. The light-induced aligning dye effect was demonstrated by the calculated rotating angle of the nematic director as a function of the laser irradiating time. The explanation was connected with the nematic local order in the NLC bulk produced by the excited dye molecules. The radius of the reorientation zone depends on the phototransformation of the dye molecules and on the interactions between these molecules and the host NLC molecules. Further investigations are necessary to identify the interactions responsible for these effects. We have also explained the differences in nematic director configurations, TN twisted nematic or DN deviated nematic, obtained when the laser-exciting beam is impinging through the isotropic surface or the oriented surface of the LC cell, respectively. Acknowledgements The authors acknowledge fruitful discussions with Professor Anca-Luiza Alexe-Ionescu and Professor Andrei Th. Ionescu.

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