All-optical switching modulation in poly(vinyl carbazole)--based azobenzene blends.

INTRODUCTION

In recent years, azobenzene-containing polymer systems have received increasing attention because of their unique and unexpected properties, which allow various applications triggered by light (1-5). One of the attractive phenomena is photoinduced alignment of the azobenzene moieties on irradiation of linearly polarized light. The photoinduced anisotropy of azobenzene dyes has been studied quite a lot for promising applications in optical data storage and all-optical switching (5-12). As for optical data storage, polymer systems containing azobenzene dyes have been reported by many researchers, e.g. surface relief gratings and photoinduced birefringence resulted from the photoinduced anisotropy of azobenzene in these systems (6, 7, 13-15). As for all-optical switching, various polymer liquid crystal (LC) blends containing azobenzene dyes have been studied; in these blends, phase transition of LCs induced by the photoinduced anisotropy of azobenzene is the key to optical switching effect (16-19). Few composite or blend systems containing only a host polymer and azobenzene dyes have been reported for the all-optical switching effect. In addition, traditional Kerr-type optical switching usually requires high driving power (e.g. 1 MW/[cm.sup.2]), thus limiting its applications. In order to make practical optical switching devices, it is necessary to reduce the optical power requirements and achieve long-term stability. In the present paper, we report a simple method to make long-term stable all-optical switching devices based on the blends consisting of poly(vinly carbazole) (PVK) and the donor-acceptor type azobenzene dyes 4'-(2-hydroxyethyl)ethylamino-2-chloro-4-nitroazobezene (Disperse Red 13, DR 13) and 4'-bis-hydroxyethylamino-4-nitroazobenzene (Disperse Red 19, DR 19). The all-optical switching effects of these blends were studied by using the 514 nm pump beam and 632.8 nm probe beam. In addition, the effects of the modulation frequency of the chopper and the pump beam intensity on the optical switching of the blends were investigated experimentally as well. Long-term stability of the devices, of over one year, was accomplished by carefully eliminating impurities, residual solvents, and air pockets in the material. The optical switching devices made herein can be operated with a relatively low driving power (12.5 mW ~ 30 mW). And the response speed of about 2.5 ms can be achieved at the modulation frequency of 200 Hz.

EXPERIMENTAL

Sample Preparation

Poly(vinyl carbazole) ([M.sub.w] = 1,100,000), 4'-(2-hydroxyethyl)ethylamino-2-chloro-4-nitroazobenzene (Disperse Red 13, DR 13) and 4'-bis-hydroxyethylamino-4-nitroazobenzene (Disperse Red 19, DR 19) were purchased from Aldrich Chemical Company.

The samples were prepared as follows:

Five grams (5 g) of PVK was dissolved in 600 mL of toluene and cyclohexanone (4:1 by volume) in a beaker through heating and stirring at 50[degrees]C for 4 h. The beaker was covered with aluminum foil to prevent any significant evaporation of solvents. A clear solution was obtained. To this clear solution, DR 13 or DR 19 (equivalent to 20% of PVK by weight) was added. Then the mixture was heated and stirred for 5-6 h until a clear, red or orange solution was obtained. Afterwards, the solution was filtered (5.0 [micro]m, DuPont Teflon) into another beaker. Later, the solution was heated on a hotplate to evaporate some of the solvents. After the solution volume was reduced to about 30 mL, the remaining solvents were removed by heating the solution on a piece of stainless steel. The stainless steel plate was placed on a hotplate and heated to 60[degrees]C ~ 70[degrees]C. The solution was pipetted onto the stainless steel plate drop by drop and stirred with a stainless steel spatula until all of the solvents appe ared to have evaporated. The soft polymeric material was then rolled into a ball and cut into small pieces for film preparation.

Prior to film preparation, the above-mentioned material was placed in the vacuum oven and dried at 6500 for four days Just before use. The drying was found to be necessary for preventing formation of bubbles in the film due to residual solvent. Then the dried material was placed on a glass slide, four Teflon spacers (50 [micro]m thick) were placed on the four corners of the glass slide. The glass slide was placed on the hotplate, until the material softens. Afterwards, the second piece glass slide is used to sandwich the material In between the two glass slides. At the same time, pressure is applied to the glass slides. Finally, the device was allowed to cool to room temperature in air.

Measurement

The all-optical switching effects of the blends were measured by using the experimental setup as shown in Fig. 1. An argon laser beam (514 nm) with a diameter of 0.5 mm was used as the pump beam. The sample was placed between two orthogonal polarizers (P2 and P3) in the path of a He-Ne laser beam (632.8 rim, probe beam, 1.16 mW. with a diameter of 0.3 mm). The angle between the polarization direction of polarizers P1 and P2 is 45[degrees] The optical change of the samples induced by pump beam was monitored by detecting intensity of the probe beam transmitting through polarizer P3. The intensity was automatically recorded with a digital oscilloscope.

RESULTS AND DISCUSSION

Figure 2 shows the transmission spectra of the blends (PVK-DR 13 and PVK-DR 19). Figures 3 and 4 show the intensity of the transmitted probe beam as a function of time at different modulation frequencies and different pump beam intensities for the blend systems PVK-DR 13 and PVK-DR 19, respectively. In these figures, at the peak points of the curves, the pump beam was turned off; while at the valley points of the curves, the pump beam was turned on. When the pump beam was blocked, the transmitted intensity of the probe beam was quite low; this is because the angle between polarizers P2 and P3 is 90[degrees]. While when the pump beam was unblocked, the probe beam could be detected. This is because the polarization of the probe beam is changed while passing through the sample due to the photoinduced isomerization of azobenzene molecules resulted from the irradiation of the pump beam. It is well known that azobenzene groups can exist in two configurations: the cis form and the more stable trans form. When expose d to light of a certain wavelength, the stable trans form can be photoisomerized to the cis form. Cis-trans isomerization can occur thermally or/and photochemically. By the repetition of the trans-cis-trans isomerizations, photoinduced anisotropy will occur. in our blend systems, the change of the polarization of the probe beam results from the photoinduced anisotropy.

From Fig. 3, we can see that as the pump beam is turned on, the intensity of the probe beam increases rapidly; while as the pump beam is turned off, the intensity of the probe beam decreases quickly. This indicates the all-optical switching effect has quick response. As the modulation frequency increases (from

Fig. 3 (I) to (VI)), the switching response becomes quicker, but the modulation depth (all-optical switching effect) becomes less. This is because at higher modulation frequency, there might not be sufficient time for the oriented azoberizene molecules to relax to their fully random structure; hence the modulation depth becomes less. Under a given modulation frequency, the modulation depth becomes greater as the pump beam intensity is increased. This is because at higher pump beam intensities, the photoinduced anisotropy of the blends is stronger, which in turn produces greater modulation depth. From Fig. 3, it also can be seen that under different modulation frequencies and pump beam intensities, there is no obvious crosstalk for the device.

In Fig. 4, it can be seen that at 20 Hz modulation frequency, the all-optical switching effect of PVK-DR 19 blend is quite obvious. While as the modulation frequency increases, the optical switching effect deteriorates. At the modulation frequency of 110 Hz, there is serious crosstalk. Compared with PVK-DR 13 blend system, the PVK-DR 19 blend has much slower optical switching response and higher crosstalk. The azobenzene dyes, DR 13 and DR 19, used in our blend systems all contain the electron-donor and electron-acceptor groups at the both ends of the azobenzene moiety. DR 13 contains the electron-acceptor group-[NO.sub.2] at the 4-position and -CI at the 2-position of azobenzene; and it contains the electron-donor groups ([H.sub.5][C.sub.2])([HOCH.sub.2][CH.sub.2])N- at the 4'-position of azabenzene. While DR 19 contains the electron-acceptor group -[NO.sub.2] at the 4-position of azabenzene and the electrondonor group ([HOCH.sub.2]C)([HOCH.sub.2][CH.sub.2])N- at the 4'-position of azobenzene. The strength o f electron-accepting ability of the substituents on DR 13 is stronger than that of DR 19 (20). And the rate of cis-trans isomerization of azoberizene dyes increases with the increase in the donor-acceptor strength (17). This explains why PVK-DR 13 blend displays better optical switching response than PVK-DR 19 blend.

CONCLUSIONS

In this article, we have prepared PVK-based azobenzene blends. The blends' photoinduced reversible and repeatable change in the transmittance of the probe beam is attributed to the photoinduced anisotropy of the azobenzene dyes. PVK-DR 13 blend's all-optical switching effect is much better than that of PVK-DR 19 due to the stronger electron donor-acceptor strength of DR 13. As the modulation frequency increases, the switching response becomes quicker, but the modulation depth becomes less. At a given modulation frequency. the modulation depth becomes greater as the pump beam intensity is Increased. The long-term stability was accomplished by carefully eliminating impurities, residual solvents, and air pockets in the materials. The optical switching devices made herein can be operated with a relatively low driving power (11.8 mW ~ 23 mW). And the response speed of about 2.5 ms can be achieved at the modulation frequency of 200 Hz.

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

ACKNOWLEDGMENTS

This work is part of Project 50173007 and Project 59803003 supported by National Natural Science Foundation of China and the project supported by Guangdong Provincial Government of China (Grant No.: A1060201 and 980505). The authors also would like to thank the financial supports by the Visiting Scholar Foundation of Key Lab. in University of China.

REFERENCES

(1.) A. Shlshido. O. Tsutsumi, A. Kanazawa, T. Shiono, and T. Tamal. J. Plugs. Chem. B, 101, 2806 (1997).

(2.) W. M. Gibbons, P. J. Shannon, Sh. T. Sun, and B. J. Swetlin, Nature, 351, 49 (1991).

(3.) D. Eaton, Science, 253, 281 (1991).

(4.) T. Ikeda and O. Tsutsumi, Science, 268, 1873(1995).

(5.) S. Bian. J. A. He, L. Li, J. Kumar, and S. K. Tripathy, Adv. Mater., 12, 1202 (2000).

(6.) R. Rochon, E. Batalla, and A. Natansobn, Appl. Plugs. Lett., 66, 136 (1995).

(7.) D. Y. Kim, S. K. Tripathy, L. Li, and J. Kumar, Appl. Plugs. Lett., 66, 1166 (1995).

(8.) M. S. Ho, C. Barrett, J. Paterson, M. Esteghanlatian. A. Natansobn, and P. Rochon, Macromoiecules, 29, 4613 (1996).

(9.) X. Wang, S. Balasubramanian, L. Li, X. Jiang, D. J. Sandman, M. F. Rubner, J. Kumar, and S. K. Tripathy, Macromol. Chem. Rapid Commun., 18, 451(1997).

(10.) T. S. Lee, D. Y. Kim, X. L. Jiang, L. Li, J. Kumar, and S. K. Tripathy, J. Polym. Sci. Part A. 36, 283(1998).

(11.) S. Hvilsted, F. Andruzzi, and P. S. Ramanujam, Opt. Lett., 17, 1234 (1992).

(12.) S. Hvilsted, F. Andruzzi, C. Kulinna, H. W. Siesler, and P. S. Ramanujam, Macromolecules, 28, 2172 (1995).

(13.) C. Egami, Y. Kawata, Y. Aoshima, and S. Alasfar, Jpn. J. Appl. Phys., 39, 1558 (2000).

(14.) U. Pletsch, P. Rochon, and A. Natansohn, Adv. Mater., 12, 1129 (2000).

(15.) D. Y. Kim, L. Li, X. L. Jiang, V. Shivshankar, J. Kumar, and S. K. Tripathy, Macromolecules, 28, 8835 (1995).

(16.) H. K. Lee, K. Doi, A. Kanazawa, T. Shiono, T. Ikeda, T. Fujisawa. M. Aizawa, and B. Lee, Polymer, 41, 1757 (2000).

(17.) H. K. Lee, A. Kanazawa. T. Shiono, T. Ikeda, T. Fujisawa, M. Aizawa, and B. Lee, Chem. Mater., 10, 1402 (1998).

(18.) H. K. Lee, A. Kanazawa, T. Shiono, T. Ikeda, T. Fujisawa, M. Aizawa, and B. Lee, J. AppL Plugs., 86, 5927 (1999).

(19.) A. Shishido, O. Tsutsumi, A. Kanazawa, T. Shiono, and T. Tamai, J. Am. Chem. Soc., 119, 7791 (1997).

(20.) T. W. Graham Solomons, Organic Chemistry, John Wiley & Sons, New York (1988).

* SHUIZHU WU (1, 2), SHENGLAN YAO (1), WEILONG SHE (2), KUN YAN (2), and JIANQING ZHAO (1)

* Corresponding author. E-mail: [e-mail]

(1.) Department of Polymer Science & Engineering South China University of Technology Guangzhou 510640, China

(2.) State Key Laboratory of Optoelectronic Materials and Technologies Zhongshan University Guangzhou 510275, China