Nonlinear-optical properties of a poly[vinyl alcohol ... - OSA Publishing

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OPTICS LETTERS / Vol. 20, No. 6 / March 15, 1995

Nonlinear-optical properties of a poly(vinyl alcohol)– polyaniline interpenetrating polymer network ´ D. V. Petrov,* A. S. L. Gomes, and Cid B. de Araujo Departamento de F´ısica, Universidade Federal de Pernambuco, 50670-910 Recife, PE, Brazil

J. M. de Souza, W. M. de Azevedo, J. V. de Melo, and F. B. Diniz Departamento de Qu´ımica Fundamental, Universidade Federal de Pernambuco, 50670-910 Recife, PE, Brazil Received November 14, 1994 The nonlinear-optical properties of a semi-interpenetrating polymer network of poly(vinyl alcohol) glutaraldehyde – polyaniline were studied. Large ($10212 cm2yW) and fast (,50 ps) refractive optical nonlinearites were observed. The potential of this novel material for photonic applications is evaluated.

Polymers have been among the most recently studied candidates for all-optical devices because of their large nonlinear susceptibilities, allied to fast response time. Several compounds, including polyacetylene, polyaniline, and polythiophene, have been studied in the past few years.1 In particular, a significant effort has been put into understanding the properties of polyaniline (PANI), which exhibits excellent environmental stability. Besides having controlled electrical conductivity, low density in comparison with metallic materials, and stability, PANI is easy to prepare from commercially available chemicals. However, because of the limited solubility of PANI, several methods have been developed to control the polymerization process,2 and recently a dispersion polymerization method to prepare soluble nanometersized polymer particles was proposed.3 The method is based on steric stabilization mechanics, in which the conducting polymer is prepared in an aqueous medium in the presence of a water-soluble polymer such as poly(vinyl alcohol) coacetate (PVA). Because of the partial solubility of PANI in organic solvents and the steric stabilization properties of PVA, an easy and fast method has been developed4 to produce a stable semi-interpenetrating polymer network (IPN), with glutaraldehyde as a cross-linking agent. The resulting IPN obtained by this technique combines the interesting properties of PANI, such as redox recyclability and doping reversibility, with the superior mechanical strength and low cost of the PVA network. The details of the synthesis and characterization of the samples used in the present work have been described in Ref. 4. We prepared PANI, in the emeraldine form, by oxidative polymerization of aniline, using (NH4 )2 S2 O8 at a temperature between 25 and 28 ±C. We prepared the IPN by dissolving emeraldine-base polyaniline and PVA in a mutually compatible solvent, dimethyl sulfoxide, which was chosen as the carrier solvent because of the large degree of solubility of PANI, PVA, and glutaraldehyde (GLUT). The dissolved polymers were mixed in a beaker and stirred for 5 min, after which GLUT, at 0146-9592/95/060554-03$6.00/0

concentrations of 3% and 6%, and a small amount of sulfuric acid were added. To obtain thin films, we poured the solution into a Petri dish and left it to react for 24 h at room temperature. The last step consisted of drying the sample under vacuum at 50 ±C for ,24 h. We examined the morphology of the PVAyGLUT–PANI samples by scanning electron microscopy, which revealed phase separation and domains with a mean diameter of 1 mm. Also, the morphology of the domains changed from rodlike to spherical shapes as the cross-linking density increased, in agreement with theoretical predictions for IPN’s.5 The absorption spectra of the films were obtained from 200 to 900 nm and from 2.5 to 25 mm. Light scattering by the domains was negligible because of the small concentration of the domains. Curve (a) of Fig. 1 shows that the PVAyGLUT network is transparent from 300 to 900 nm, and the band near 230 nm is evidence of ethylenic linkage conjugated with an aldehyde group. Curve (b) corresponds to the IPN soon after it was synthesized. The bands centered at 347, 420, and 820 nm are characteristics of doped PANI, and, after treatment with NH4 OH, only two absorption-band characteristics of the polyemeraldine base remain, as shown in curve (c). The

Fig. 1. (a) Optical absorption spectra of the PVAyGLUT network, (b) PVAyGLUT – PANI treated with acid, (c) PVAyGLUT – PANI treated with a base solution.  1995 Optical Society of America

March 15, 1995 / Vol. 20, No. 6 / OPTICS LETTERS

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samples A and B, whereas the nonlinear absorption of sample C is negligible. Furthermore, the negative signs of n2 and b (as for the polythiophene films8 ) give evidence of the electronic origin of the nonlinearity in this material. The time-delayed degenerate four-wave mixing setup used is standard.7 The laser beam is split

Fig. 2. Linear absorption coefficient of the three samples studied, as a function of the optical wavelength.

main features of the infrared spectra, not shown here, are two bands at 1007 and 1136 cm21 , assigned to the C—O and C —O—C groups, and a third band °° at 1720 cm21 , assigned to the C ° ° O group of nonconjugated aldehydes whose intensity is dependent on the aldehyde concentration. Because the ratio PANIyPVA is small, we do not observe the infrared bands of PANI. The samples used (A, B, and C) are thin films with thicknesses of 750, 350, and 250 mm, respectively. They correspond to the PVAyGLUT network doped with polyaniline at slightly different oxidation states: sample A (less oxidized), sample B (more oxidized), and sample C (undoped PVAyGLUT). Their absorption coefficients a in the visible range are shown in Fig. 2. For the more oxidized sample, a increases because the near-infrared band shifts to the visible part of the spectrum. Two nonlinear techniques were used: the Z-scan technique6 and time-delayed degenerate forward four-wave mixing.7 The pump source was the second harmonic of a cw-pumped Q-switched and modelocked Nd:YAG laser, delivering pulses of 70 ps (FWHM) at 532 nm, in pulse trains containing ,20 pulses at a 5-Hz repetition rate. The low repetition rate was used to avoid thermal effects. For the Z-scan experiments a 10-cm focal-length lens was used, and the intensity at the focal spot was determined by calibration with CS2 , whose nonlinear parameters are known.6,7 We used the small-aperture Z scan (corresponding to S ­ 0.1) to determine n2 , the nonlinear refractive index, and Fig. 3 shows the normalized transmittance obtained. The Z ordinate is measured along the beam direction, and Z , 0 corresponds to locations of the sample between the focusing lens and its focal plane. The signal profile with a peak followed by a valley indicates a self-defocusing nonlinearity sn2 , 0d. The calculated values of n2 are given in Table 1, and for the calculation the equations given in Ref. 6 were used with the intensities indicated in the caption of Fig. 3. The results obtained with the open aperture sS ­ 1d are shown in Fig. 4. The calculated transmittances are depicted by the curves in the figure, and the nonlinear absorption coefficients deduced, b, are given in Table 1. The negative sign of b indicates that saturation of the absorption dominates the behavior of

Fig. 3. Normalized transmittance of the Z-scan measurements with the small aperture sS ­ 0.1d at 532 nm. The open squares and solid curves correspond to the experimental and theoretical results, respectively. The peak irradiance was 0.16 GWycm2 for sample A and 0.18 GWycm2 for samples B and C. The measured 1ye2 diameter of the beam was 40 mm. Table 1. Parameters of the Studied PVAyGLUT – PANI Samples Sample A B C

Thickness a n2 b scm21 d scm2yW 3 10212 d scmyGW d smmd 750 350 250

35 56 4

26.4 22.9 22.0

2170 283 ,210

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OPTICS LETTERS / Vol. 20, No. 6 / March 15, 1995

ample, for all-optical switching, we calculated the figp ure of merit W ­ n2 Iy 2 al,10 where l and I are the laser wavelength and peak intensity, respectively p (the factor 2 arises from a temporal average, assuming Gaussian pulses). For the intensities used we obtained 0.1 , W , 1.2.11 These results together with the fast response time provide a favorable indication of the usefulness of PVAyGLUT-PANI for photonic applications. This research was supported by the Brazilian Agencies Conselho Nacional de Desenvolvimento Cient´ıfico e Tecnol´ogico and Financiadora de Estudos e Projetos. *Permanent address, The Institute of Semiconductor Physics, 630090 Novosibirsk, Russia.

References

Fig. 4. Normalized transmittance with the open aperture sS ­ 1d at 532 nm. The solid curves represent the theoretical results obtained with the b values indicated in Table 1. The peak irradiance was 0.16 GWycm2 for sample A and 0.18 GWycm2 for samples B and C.

into two, which after they are properly delayed are recombined, forming a small crossing angle (,2±). Self-diffracted beams are generated when the two beams temporally and spatially overlap. The response time of the nonlinearity can be inferred by adjustment of the temporal delay and recording of the intensity dependence of the self-diffracted signal. No asymmetry was observed in the time-delayed degenerate four-wave mixing recorded trace, indicating a response time smaller than the laser coherence time (50 ps).9 Care was taken to ensure that no thermal effect was giving rise to a temporal artifact. The nonlinear parameters obtained compare favorably with those of other organic materials,8 and their values may be controlled by an appropriate choice of the polyaniline doping fraction and oxidation state. To assess the suitability of this material, for ex-

1. See, for example the proceedings of the International Conference on the Science and Technology of Synthetic Metals, G¨oteberg, Sweden, 1992 [Synth. Met. 55 – 57 (1993)]; proceedings of the International Conference on the Science and Technology of Synthetic Metals, ¨ Tubingen, Germany, 1990 [Synth. Met. 41– 43 (1991)]. 2. Y. Wei, W. W. Folke, G. E. When, A. Ray, and A. G. MacDiarmid, J. Phys. Chem. 53, 495 (1989); J. Yue and A. J. Epstein, J. Am. Chem. Soc. 112, 2800 (1990); Y. Cao, P. Smith, and A. J. Heeger, Synth. Met. 32, 263 (1989). 3. C. De Armitt and S. P. Arms, J. Colloid Interface Sci. 150, 134 (1992); J. Stepskal, P. Kratochvil, and W. Radhakrishnan, Synth. Met. 61, 225 (1993); J. Stejskal, P. Kratochcil, N. Gospodinova, L. Teklemezyan, and P. Mokrena, Polymer 33, 4857 (1992). 4. J. M. de Souza, W. M. de Azevedo, J. V. de Melo, and F. B. Diniz, presented at the Second Ibero-American Polymer Symposium, Gramado, Brazil, 1994. 5. L. H. Sperling, Polymer Blends: Processing, Morphology and Properties (Plenum, New York, 1984), Vol. 2. 6. M. Sheik-Bahae, A. A. Said, T. H. Wei, D. J. Hagan, and E. W. Van Stryland, IEEE J. Quantum Electron. 26, 760 (1990). 7. Y. R. Shen, The Principles of Nonlinear Optics (Wiley, New York, 1984); M. D. Levenson, Introduction to Nonlinear Laser Spectroscopy (Academic, New York, 1982). 8. R. Dorsinville, L. Yang, R. R. Alfano, R. Zamboni, R. Danieli, G. Ruani, and C. Taliani, Opt. Lett. 14, 1321 (1989); L. Yang, R. Dorsinville, Q. Z. Wang, P. X. Ye, R. R. Alfano, R. Zamboni, and C. Taliani, Opt. Lett. 17, 323 (1992); S. Molyneux, A. K. Kar, B. S. Wherrett, T. L. Axon, and D. Bloom, Opt. Lett. 18, 2093 (1993). 9. We started experiments with these samples by using a dye laser delivering pulses of ,200-fs duration. Preliminary results with a Kerr gate show a subpicosecond response time for the nonlinearity. A similar response time for pure polyaniline was reported by K. S. Wong, S. G. Han, and Z. V. Vardeny, J. Appl. Phys. 70, 1896 (1991). 10. G. I. Stegeman and E. M. Wright, J. Opt. Quantum Electron. 22, 95 (1990). 11. The figure of merit W is more appropriate to characterize Kerr materials. In the present case, because of the saturated absorption, it gives only an indication of the materials’ performance.