Photoresponsive Liquid-Crystalline Polymers - ACS Publications

Chapter 28

Photoresponsive Liquid-Crystalline Polymers Holographic Storage, Digital Storage, and Holographic Optical Components

Downloaded by CORNELL UNIV on September 1, 2016 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1994-0579.ch028

Klaus Anderle and Joachim H. Wendorff Fachbereich Physikalische Chemie und Wissenschaftliches Zentrum für Materialwissenschaft, Philipps-Universität Marburg, D-35032 Marburg, Germany

A storage process is described which is based on light-induced trans-cis-trans-isomerization reactions of azobenzene chromophores attached to a polymer backbone as side groups in liquid crystalline polymers. The chromophores are able to rotate in the glassy state if subjected to linearly polarized light: the azobenzene units approch a saturation orientation which is perpendular to the polarization direction of the light. The contribution discusses the molecular mechanism of this process as well as possible applications. In the past decade optical recording and the development of suitable recording media have become a subject of extensive scientific and industrial interest (1). High optical sensitivity, an enhanced storage density and short switching as well as access times are as important as, in certain applications, reversibility and high signal to noise ratio after many write-erase cycles. The contribution describes the principles of a storage process based on the liquid crystalline state of organic polymeric materials (2-6). Such materials are capable of forming anisotropic glasses, which can be obtained as thin films. B y suitable means one is able to align the optical axis of the uniaxial 0097-6156/94/0579-0357$08.00/0 © 1994 American Chemical Society

Ito et al.; Polymeric Materials for Microelectronic Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


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POLYMERIC MATERIALS FOR MICROELECTRONIC APPLICATIONS

system within the film along a given direction or parallel to the film normal. A storage process is made possible by incorporating into the polymer a suitable dye - such as azobenzene characterized by the fact that it is able to undergo a lightinduced isomerization process even in the solid glassy state. This in turn leads to a reorientation of the optical axis within the film and thus to strong modifications of the optical properties. The information written-in in this way can be erased either by heating to temperatures above the glass transition temperature or by light. The paper describes the physical processes involved in the storage process and the capability of such materials to store holograms. Of particular interest is that the novel type of storage material is able to store information not only on amplitude and phase (scalar properties) of the light but also on the state of polarization. It may thus be used for polarization holography. M a t e r i a l s Used and Preparation the L i q u i d C r y s t a l l i n e Polymer Recording Films

of (LCP)-based

The compounds used are predominantly nematic side chain polymers with acrylate or polyester-type chain backbone (Figure 1). The polymers contain azobenzene-units in different concentrations. This allows to tailor the absorption and optical and dielectric properties more or less at will, depending on the choice of the copolymer. The synthesis has been the topic of many previous publications (2-6). To prepare the films used for storage the liquid crystalline polymers were routinely annealed above their glass transition temperature in a vacuum oven to remove residual solvent. Prefabricated liquid crystalline display (LCD)-cells of various sizes and thicknesses were filled at temperatures well above the clearing temperature by capillary action. The cells consist usually of two quartz substrates which were covered by spin coating with a thin polyamide layer. This layer was subsequently structured by a buffing technique. Defect free and large size (as large as 25 cm2 ) homogeneous monodomain films of high optical quality can be achieved by annealing the cells at temperatures close to but still below the clearing temperature. Figure 2 displays the birefringence of such a sample as a function of the temperature within the nematic and isostropic phase.


28.

ANDERLE & WENDORFF

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Photoresponsive Liquid-Crystalline Polymers 359

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Figure 1 Chemical structure of some of the polymers used for optical storage. The transition temperatures (in ° C ) are also given in the usual notation with the symbols: g: glass transition; s :smectic transition; η nematic transition; i isotropic melt

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360


Basic Mechanism Reorientation by

Controlling the Storage Photoselection (4)

Process:


The modulation of the optical properties of monodomains of liquid crystalline polymers is achieved by the trans-cis isomerization which is caused by the interaction of light with the covalently attached azobenzene groups of the polymer. This type of reaction is of particular interest in a liquid crystalline state: the cis isomer is bent and nonmesogenic and hence cannot form a liquid crystalline phase. An effective storage process results if linearly polarized light is used and if the illumination is done in such a way that the liquid crystalline state is not destabilized (2-6). Typically intensities of the order of mW/cm^ are used. In particular the illumination can be performed within the liquid crystalline glassy state i.e. using solid films. The experimental finding is that by using polarized light one is able to reorient the director significantly. This is apparent from the polar diagram of extinction, as displayed in Figure 3. The saturation value of the reorientational angle amounts to 90 deg with respect to the polarization direction of the light. This result is independent of the relative orientation of the polarization direction with respect to the director. This modulation of the direction of the nematic director is directly reflected in the variation of the birefringence: the birefringence is reduced and the sign of the birefringence is reversed due to the induced director reorientation. Grating

Experiments

(3)

The simplest hologram one can think of is obtained by superimposing two planar waves. This leads to a characteristic periodic intensity modulation in the plane of the recording film and thus to an optical grating in the material. Its characteristic features which can be read out by a planar wave (periodicity of the grating and the amplitude of modulation of the optical properties) allow to obtain valuable information of the storage capability of the recording material (diffraction efficiency, resolution, etc, see below). A second type of a simple hologram is obtained by the superposition of a planar and a spherical wave which gives rise to a Fresnel type interference pattern. The hologram stored in a recording film can be considered as an optical element manufactured by holography (HOE: holographic


ANDERLE & WENDORFF

Photoresponsive Liquid-Crystalline Polymers 361 ο


270

b

270

180

Figure 3 Polar diagram of the extinction as obtained for a nematic monodomain for a side chain liquid crystalline polymer a ) before and b) after irradiation with polarized light Ito et al.; Polymeric Materials for Microelectronic Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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optical component). It can be used as a Fresnel lens to enlarge the size of an object or to decrease ist size (5). In order to test the capability of the liquid crystalline polymers with respect to the storage of holograms we performed grating experiments. The result is that a very high diffraction efficiency which can be achieved combined with an excellent resolving power which makes this type of liquid crystalline polymer an attractive novel recording medium (Figure 4). Actually we were able to store real holograms reversibly in such materials and we were also able to prepare holographic optical components, such as a Fresnel lens, using azobenzene containing liquid crystalline side chain polymers. Polarization

Holography

(6)

The characteristic feature of holographic recording is the transformation of phase information into intensity using the interference of the beam carrying the information (object beam) with a reference beam. The reason is that optical storage media are not able to record phase information directly. Optical recording media have still another disadvantage. The recorded pattern does not contain information on the state of polarization of the incident light. So if the surface of the object to be recorded has the property of changing the polarization of the reflected light one will not be able to store this information in the hologram. Sometimes this kind of information may be crucial. Polarization holography is the answer. It has become apparent that the material described here is to a certain extent able to store information on the polarization of the light: it tends to change the direction of its main optical axis which approachs a saturation value of 90° relative to the polarization direction (Figure 5). So in principle this material may be capable of storing polarization holograms. To test this capability we performed grating experiments (6). The result is that the material is able to store to a certain extent information related to polarization and that one is able to retrieve such information using an unpolarized beam. The pattern obtained are, however, quite complex. Figure 6 gives an example for the angle of maximum diffraction efficiency as a function of the irradiation time for a nonparallel polarization of the intersecting beams. We are currently performing experiments to try to develop an understanding of the basic


Photoresponsive Liquid-Crystalline Polymers

ANDERLE & WENDORFF

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Figure 5 Dependence of the orientation of the director after irradiation with polarized light on the original relative orientation of the polarization of the light and the nematic director


36

364

POLYMERIC MATERIALS FOR MICROELECTRONIC APPLICATIONS 90-

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Figure 6 Angle of maximum diffraction efficiency as a function of the irradiation time, nonparallel polarization of intersecting beams features in relation to the processes happening during storage in the liquid crystalline polymers. Literature

Cited

1.) Hartmann, M; Jakobs, B.J.Α.; .Braat, J.J.M. Philips Techn. Rev. (1985),42, 37; Gravesteijn,D.J.; van der Poel,C.J.; Scholte, L.O.; van Uitjen, C.M.J. Philips. Techn. Rev. (1987) 44, 250 2) Eich,M; J.H.Wendorff,J.H. Makromol. Chem. Rapid Commun. (1986) 8,467 3. ) Eich. M; Wendorff,J.H. J.Opt. Soc. Am. B, 7, 1428 (199o) 4. ) Birenheide, R.; Wendorff J.H. Proc.SPIE-Int. Soc. Opt. Eng. (1990), 1213 5. ) Anderle, K.; Birenheide,R.; Wendorff,J.H. Liq. Cryst. (1991) 5,691 6. ) Anderle,K; Wendorff,J.H. Mol. Cryst. Liq. Cryst, in press; Birenheide, R.PhD-Thesis, Darmstadt (1991) RECEIVED September 13, 1994 Ito et al.; Polymeric Materials for Microelectronic Applications ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

Photoresponsive Liquid-Crystalline Polymers - ACS Publications

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