Photorefractive Mesogenic Composites

$Photorefractive Mesogenic Composites$ Research News Photorefractive Mesogenic Composites By Hiroshi Ono,* Tomomi Kawamura, Nazarene Mocam Frias, Keiko Kitamura, Nobuhiro Kawatsuki, and Hideki Norisada Mesogenic composites, consisting of low-molar-mass liquid crystals, polymers, and photoconductive sensitizers, constitute novel organic materials possessing high-performance photorefractivity. Polymeric materials in the photorefractive mesogenic composites play a very important role in terms of improving the resolution, stabilizing the homeotropic alignment, and func- tionalizing the materials. 1. Introduction terial. Electric fields as large as 50±100 V/mm are used to pole these materials and several kilovolts of DC field are Photorefractive materials have been investigated exten- applied to the popular photorefractive polymer composites sively because of the possibility of applications in optical because the film thickness is about 50±150 mm. The high signal processing, dynamic holography, and phase conjuga- electric field causes serious problems, such as dielectric tion.[1] Photorefractive gratings appear in materials that ex- breakdown and phase separation, in these photorefractive hibit both an electric-field-dependent refractive index polymer composites. change and an optically induced charge distribution. The In 1994, Khoo et al. found that low-molar-mass nematic refractive-index modulation is out of phase with the optical liquid crystals (L-LCs) show high-performance photorefrac- interference pattern, and this phase shift can induce an tivity under low driving voltage (<1 V/mm) by giving photo- energy exchange between two coherent beams, which leads conductivity to the L-LCs.[5] This was accomplished by dop- to a variety of useful applications. Photorefractive materi- ing the L-LC 4¢-(n-pentyl)-4-cyanobiphenyl (5CB) with als are by far the most efficient nonlinear optical media for fullerene (C60). The photorefractivity in the L-LCs is mainly wave mixing and phase conjugation with low intensity re- based on orientational birefringence and the effects are quirements. In addition, holograms can be recorded and named ªorientational photorefractive effectsº.[5,6] The per- erased in these media. formance was improved by doping with electron donor and Studies on photorefractive materials have been extended acceptor molecules.[7] However, the resolution of photore- to organic materials, since Sutter et al. first demonstrated fractive LCs is very low and good photorefractive perfor- the photorefractive effect in an organic crystal in 1990.[2] In mance is only observed for large fringe spacing (>10 mm), the next year, the photorefractive effect was observed in a which is not suitable for high-resolution applications.[5±9] polymer system by Ducharme et al.[3] and photorefractive polymer composites have drawn much attention due to their advantages, which include low cost, ease of fabrica- 2. Improved Resolution and Gain Coefficient of tion, and the ability to fabricate complex structures.[4] Some Photorefractive LCs of the most promising results have been obtained for the compositional guest±host approach of doping photocon- Recently, it was found that the resolution of the photore- ductive polymers with nonlinear optical chromophores. fractive mesogenic material was improved by combining High-performance photorefractivity in such polymer com- the L-LCs with a polymer.[10±13] We performed a study on posites permits the use of a larger applied electric field, the photorefractivity by using a L-LC mixture consisting of which breaks the inversion symmetry to achieve a Peckels 4-cyanobiphenyl (E7), a polymer containing mesogenic effect, and also enhances the photoconductivity of the ma- side-groups (P1, P2), and photoconductive sensitizer [C60, 2,4,7-trinitro-9-fruorene (TNF)]. Both P1 and P2 have ± mesogenic groups in the side-chain. L-LC (E7) can dissolve [*] Dr. H. Ono, Dr. T. Kawamura, Dr. N. M. Frias, Dr. K. Kitamura these polymers without phase separation and the resulting Department of Electrical Engineering Nagaoka University of Technology polymer-dissolved L-LC composites show the mesophase 1603-1 Kamitomioka, Nagaoka 940-2188 (Japan) at the appropriate compositions. Dr. N. Kawatsuki, Dr. H. Norisada The non-local origin of the photorefractive effect mani- Department of Applied Chemistry Himeji Institute of Technology fests itself in a phase shift (f) between the recording index 2167 Shosha, Himeji 671-2201 (Japan) and the light fringe pattern that created it, enabling two- Adv. Mater. 2000, 12, No. 2 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2000 0935-9648/00/0201-0143 $ 17.50+.50/0 143 Research News energy transfer in the two-beam coupling (2BC) experi- ments. A continuous-wave TEM00-mode (TEM = transmis- sion electron spectroscopy) He±Ne laser (633 nm) beam with p-polarization was split into two beams (beam 1 and beam 2) and made to overlap in the photorefractive mesogenic composites. After the two beams transmit the photorefractive mesogenic composites, I1 and I2, which corre- spond to the transmitted light intensities, become 1 1 z I I 0 (2) 1 1 1 z 1 eGd 1 z I I 0 (3) 2 2 1 ze Gd where x is the input intensity ratio and unit in our experi- mental conditions. G is the gain coefficient and is given by 1 G ln g ln z 1 g (4) d where I g 2 (5) I 0 2 Figures 1a and 1b show typical 2BC signals for photorefractive mesogenic materials without and with polymers, respectively. The intensity of beam 1 was monitored while beam coupling. Diffraction on a photorefractive grating is treated differently if the grating is ªthinº or ªthickº. The two limiting cases can be characterized by the following pa- rameter: 2pld Q (1) nL2 where l is the wavelength, n the refractive index, L the grating spacing, and d the thickness of the grating. Accord- Fig. 1. The typical asymmetric energy transfer in photorefractive mesogenic ing to Kogelnik's coupled-wave model, the most rigorous composites. The composition by weight (P1/E7) was a) 0:100 and b) 10:90. Fullerene C60 was (0.05 wt.-%) used as photoconductive sensitizer for the treatments suggest that Q values of 10 are required to pro- two composites. The fringe spacing was 2.6 mm and the applied DC field duce a true volume hologram (Bragg regime). When the Q was 0.18 V/mm. No signals were detected in the case of E7 without P1, while large 2BC signals appeared upon addition of P1. These results indicate that value is over 10, the coupled-wave theory is valid and the the resolution of the photorefractive mesogenic material was improved by photorefractive nature can be characterized by asymmetric adding the polymer. 144 Ó WILEY-VCH Verlag GmbH, D-69469 Weinheim, 2000 0935-9648/00/0201-0144 $ 17.50+.50/0 Adv. Mater. 2000, 12, No. 2 Research News beam 2 was opened at t = 0 s, and the same experiment was repeated for beam 2. The fringe spacing was set to be 2.6 mm and the Bragg condition for diffraction on thick gratings was achieved. No signals were detected in the case of the E7/C60 without P1 (L-LC), as shown in Figure 1a. In contrast, a large beam coupling signal appeared by adding P1 to E7/C60, as shown in Figure 1b. This means that the resolution of the photorefractive LC was improved by the addition of the polymer. In our materials system, the polymers have a dual function. One is to improve the resolution of the photorefractive mesogenic materials as mentioned above, and another is to achieve a stable homeotropic alignment. We observed that the mesogenic groups in the Fig. 2. Absorption spectra of P2/E7/TNF films. The concentration ratio by photorefractive mesogenic composite aligned homeotropi- weight (P2/E7) was 20:80 and the TNF concentration was a) 0.64, b) 1.00, cally by applying a DC field and this homeotropic align- and c) 2.00 wt.-%. A frequency doubled YAG laser (532 nm) was used as the hologram writing beam and He±Ne laser (633 nm) was used as the probe. ment was stable under no electric field by a polymer-stabilizing effect (see Scheme 1). concentration. Under the above-mentioned 2BC experi- mental conditions by means of He±Ne laser (633 nm), high net gain coefficients were also achieved. In addition, P2/ E7/TNF films show high-performance holographic recording properties. A linearly polarized frequency doubled YAG laser (532 nm) was used as the writing beam. The writing beam is divided into two equal-power beams and the resulting beams are incident to the sample cell and overlap at the P2/E7/TNF films. The fringe spacing was set to be 2.8 mm. A linearly polarized 1 mW He±Ne laser is used to probe the grating generation. Volume holographic gratings were recorded and a Bragg diffraction spot was observed in the far field. Figure 3 shows the typical holographic recording properties of P2/E7/TNF films. The diffraction efficiency increases with increasing TNF concentration and the maximum diffraction efficiency in the present experiment reached a value of ~40 %. The sensitivity was improved by increasing the TNF concentration. The grating was generated by irradiating at a low power (~1 mW/cm2) of writing beams, while maintaining a high diffraction efficiency of around 15 %. Scheme 1. 4. Conclusions 3. High-Sensitivity and Efficient Holographic Photorefractive mesogenic composites represent an Materials emerging class of new photorefractive and holographic recording materials. The photorefractive gratings are pro- One of the merits of the photorefractive polymer-dis- duced under a low applied DC field (<1 V/mm) due to their solved liquid-crystal composite (PDLCC) is that highly large birefringence and easy reorientation of the mesogen. functionalized polymers can be used. A functionalized The polymeric materials in the photorefractive mesogenic copolymer, P2, consists of mesogenic 4-cyanobenzoate and composites play a very important role in improving the N-carbazoyl side-groups.

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