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Third-order nonlinear optical properties of undoped polyaniline solutions and films probed at 532 nm Glauco S. Maciel, Arandi G. Bezerra, Jr., Nikifor Rakov, Cid B. de Arau´jo, and Anderson S. L. Gomes Departamento de Fı´sica, Universidade Federal de Pernambuco, 50670-901 Recife, Pernambuco, Brazil
Walter M. de Azevedo Departamento de Quı´mica Fundamental, Universidade Federal de Pernambuco, 50670-901 Recife, Pernambuco, Brazil Received August 14, 2000; revised manuscript received Febraury 6, 2001 The third-order nonlinear optical properties of polyaniline (PANI) solutions and films were investigated at 532 nm by use of Z-scan, power limiting, and optical Kerr gate techniques. The polymers studied were the undoped partially oxidized (emeraldine base) and fully reduced (leucoemeraldine base) forms of PANI. Our results demonstrate that the leucoemeraldine base is more suitable for use in devices such as all-optical switches and optical power limiters operating at 532 nm. The worse performance of the emeraldine base is due to the presence of defects inside the bandgap of the polymer. © 2001 Optical Society of America OCIS codes: 190.4710; 160.5470.
1. INTRODUCTION Organic molecular structures represented by conjugated -electron polymer chains play a key role in the realm of optoelectronics.1,2 Among the conjugated polymers, polyaniline (PANI) has received special attention because it exhibits a controlled reversibility of its electrical conductivity.3 Upon protonation, it is possible to control the electrical properties of the polymer by changing it from an insulating (base) form to a conducting (salt) form. Furthermore, the redox property of PANI has been exploited in the development of a great number of novel devices such as, for example, biotransistors.4 In addition to a wide range of desirable electrical, electrochemical, and optical properties, PANI also exhibits excellent environmental and thermal stability, being easily cast into films, gels, and fibers.5 While research on the magnetic properties of PANI and its derivatives has drawn attention only recently,6 the linear and nonlinear optical properties of PANI are active areas of investigation [see for example Refs. 7 and 8]. Indeed, the third-order optical susceptibility, ( 3 ) , of PANI has been studied by use of third-harmonic generation,9 a Z-scan technique,10 and degenerate fourwave mixing.10–12 Most of the research has concentrated on the undoped partially oxidized form of PANI, the emeraldine base (EB), and its protonated analog, the emeraldine salt. So far, not much attention has been paid to the capabilities of the undoped, fully reduced form of PANI, the leucoemeraldine base (LEB), as a nonlinear optical material. This investigation can also clarify, for example, how defects, such as those present in the EB form, modify the nonlinear optical properties of the polymer. To achieve a better understanding, we performed our investigation at 532 nm. At this wavelength the excitation 0740-3224/2001/081099-05$15.00
is quasi resonant with respect to the absorption band corresponding to defects (excitons) in the EB form of PANI. We were able to analyze separately the real and the imaginary parts of the third-order nonlinearity of PANI, and we observed that LEB is more suitable than EB for application in devices such as all-optical switches and optical power limiters operating at 532 nm.
2. EXPERIMENT PANI was prepared by chemical oxidative polymerization of aniline with ammonium peroxydisulfate as oxidant in an aqueous solution in the presence of HCl. The emeraldine hydrochloric precipitate was filtered and washed with 1M HCl and acetonitrile to remove low-molecularweight and monomeric material. After that, the samples were dried under dynamic vacuum at room temperature and stored. The EB form was obtained from the emeraldine salt by treating the precipitate with 1M ammonium hydroxide for 10 h before the samples were washed with deionized water and dried under dynamic vacuum. Stock solutions were prepared by dissolving 0.259 g of PANI in 200 ml of dimethyl sulfoxide (DMSO), and dilution of this solution was made when necessary. The LEB form was achieved by treating the polymer solution with hydrazine. Films for PANI were prepared using a semiinterpenetrating polymer network (SIPN) approach.13 Poly(vinyl-alcohol) coacetate (PVA) and glutaraldehyde (GLUT) are used in the SIPN to improve the mechanical strength and chemical stability without compromising the optical properties of PANI. The thickness of the films used for the optical measurements was ⬃0.4 mm. To probe the contribution of defects to the nonlinear response of the polymer, we used a quasi-resonant excita© 2001 Optical Society of America
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tion approach. Three different experimental techniques were employed: one-color Z-scan,14 optical power limiting,15 and optical Kerr gate16 (OKG). The light source for the Z-scan and OKG experiments was the second harmonic of a cw-pumped, Q-switched, and modelocked Nd:YAG laser that delivered pulses of 70 ps FWHM at 532 nm in bursts of ⬃20 pulses at a 10-Hz repetition rate. By means of a pulse picker, a single pulse was selected from the pulse train. The maximum energy per pulse used was ⬃1 J (intensity, ⬃6 GW/cm2 ). The PANI solutions were contained in 1-mm-optical-pathlength quartz cuvettes. The light source for the optical power limiting experiments was the second harmonic (532 nm) of a Q-switched Nd:YAG laser emitting 10-ns pulses at a 5-Hz repetition rate (a few millijoules per pulse). The laser was focused in the sample with a 10-cm focal-length lens providing intensities up to ⬃100 GW/ cm2 . The incident and the transmitted pulse energies were simultaneously measured with two identical largearea photodetectors, and the signal was monitored pulse by pulse in a fast oscilloscope. The sample solutions were contained in 1-cm-long quartz cuvettes.
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photon absorption and saturated absorption were not observed, indicating that the  of PANI is smaller than the detection limit of the system (0.1 cm/GW). We also performed measurements on samples of pure DMSO and PVA GLUT film, and the results showed a negligible contribution for n 2 (⬍10⫺15 cm2 /W). We can analyze our results by keeping in mind that the macroscopic and the microscopic (molecular) nonlinear coefficients are related to each other.10 Using the sumover-states approach,19 it is possible to describe, in a first approximation, the real part of the off-resonant thirdorder nonlinear microscopic polarizability as
␥ 共 ⫺ ; ,⫺ , 兲 ⬀ ␣
冋
2 ⌬ ge
共 ge ⫺ 2 兲共 ge ⫺ 兲 2
⫹
⫺
M ee ⬘ 共 ge ⬘ ⫺ 2 兲共 ge ⫺ 兲 2 M ge
共 ge ⫺ 兲共 ge ⫹ 兲
册
,
(1)
3. RESULTS AND DISCUSSION Optical linear absorption of PANI in DMSO solutions is shown in Fig. 1 for the LEB and EB forms. The band in the UV region at ⬃330 nm corresponds to – * bandgap electronic transitions associated with the -electron extended molecular orbitals on the backbone of the polymer. The peak at ⬃270 nm is due to a residual amount of hydrazine. The band centered at ⬃610 nm in the EB form can be ascribed to a localized molecular exciton formation associated with transitions from the benzene highest occupied molecular orbital ( B ) to the quinone lowest unoccupied molecular orbital ( Q ). Note that after treatment with hydrazine the absorption band at ⬃610 nm was eliminated, confirming that the polymer was fully reduced to the LEB form. In the first set of nonlinear experiments we used the sensitive Z-scan technique to probe the optical response of PANI. By measurement of the beam intensity transmitted through a circular aperture in front of a detector placed in the far-field region, the Z-scan method yields the sign and magnitude of the nonlinear index of refraction, n 2 , which is proportional to the real part of ( 3 ) . The nonlinear absorption coefficient, , which is proportional to the imaginary part of ( 3 ) , is measured by collection of all the light intensity transmitted through the sample. A typical Z-scan trace obtained for the samples studied here is shown in Fig. 2, and it is characteristic of a selfdefocusing nonlinearity.14 The solid line shown in Fig. 2 represents the best-fit curve using the equations provided in Ref. 14, and the results found for n 2 are shown in Table 1. We note that the present values found for n 2 , by use of only one pulse, are 1–2 orders of magnitude smaller than those obtained using a Q-switched pulse train.17 This is because a pulse train’s accumulative effects and thermal loading also contribute to the nonlinearity, as discussed in Ref. 18. Furthermore, the results found here for LEB were two times larger than the results found for EB in both solutions and films. Nonlinear effects such as two-
Fig. 1. Linear absorption spectra of PANI in DMSO solutions. The forms of PANI studied in this work were the EB (solid curve) and the LEB (dashed curve). PANI concentration, 0.1 g/l.
Fig. 2. Typical Z-scan trace showing self-defocusing for the emeraldine base in DMSO solution. The Z-scan experiment was performed with 70-ps single pulses at 532 nm. The pulse peak intensity was ⬃5 GW/cm2 . Sample thickness, 1 mm; PANI concentration, 0.1 g/l.
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Table 1. Nonlinear Index of Refraction (n 2 ), Linear ( ␣ 0 ), and Nonlinear (  ) Absorption Coefficients for PANIa DMSO Solutions PANI EB Z-scan setup n 2 (⫻10⫺14 cm2 /W) a 0 (cm⫺1) n 2 I/2␣ 0 b 2  /n 2 c Optical power limiting setup  (cm/GW) ␣ 0 (cm⫺1)
PVA-GLUT SIPN Films
PANI LEB
⫺2 1.5 0.6 ⬍0.5
⫺4 1 1.9 ⬍0.3
⫺0.002 0.3
0.002 0.05
PANI EB
PANI LEB
⫺20 12 0.8 ⬍0.1
⫺40 10 1.9 ⬍0.1
— —
— —
a In DMSO solutions and cast in PVA-GLUT SIPN films at 532 nm, the Z-scan experiments were performed with 70-ps single pulses. determined in an optical limiting setup with a pulsed laser delivering pulses of 10 ns at 5 Hz. b I ⫽ 5 GW/cm2 . c It was assumed that  ⬍ 0.1 cm/GW.
where ␣ is the linear polarizability due to the electrons, M ij and ij are the electronic transition moment and the transition frequency, respectively, from level i to level j, ⌬ ge is the difference between the dipole moments in states e and g, and is the optical frequency. The level g stands for the ground state, level e is the first onephoton optically allowed excited state, and e ⬘ is an excited state accessible from e. The first term in the brackets, representing the dipolar term, is zero for PANI. The second term, involving two-photon transitions g → e → e ⬘ → e → g, can also be disregarded, as we did not observe any nonlinear absorption for the laser intensities used. The last term in the brackets is related to one-photon transitions g → e → g → e → g. Note that the thirdorder polarizability is proportional to the linear polarizability in relation (1). Therefore, we should expect the third-order polarizability of compounds containing both benzene and quinone rings, such as EB, to be larger than that of compounds containing only benzene rings, such as LEB.20 However, this was not observed experimentally. Our explanation for the larger value of n 2 obtained for LEB is the following: We note that self-defocusing was observed for both samples studied, implying negative values of ␥ and n 2 .14,16 For LEB, the level e is located at ⬃3.7 eV and ge ⬎ . Thus the negative value of n 2 indicates that the last term inside the brackets in relation (1) is dominant. For EB the presence of another optically accessible absorption band related to the defect states at ⬃2 eV should be taken into account, and relation (1) has yet to be modified. Hence, in a first approximation, ␥ for the EB form is written as
冋
␥ 共 ⫺ ; ,⫺ , 兲 ⬀ ␣ ⫺ ⫺
2 M ge
共 ge ⫺ 兲共 ge ⫹ 兲 2 M gd
共 gd ⫺ 兲共 gd ⫹ 兲
册
,
(2)
where the second term in the brackets is related to the transitions g → d → g → d → g and level d stands for the defect states. It is important to note that the second term in relation (2) has a negative denominator because gd ⬍ , and therefore it represents a positive contribu-
Values of  were
tion for ␥ and n 2 . For LEB the term that is due to defect states is null. We therefore conclude that, even though the first term in the brackets of relation (2) dominates for both forms of PANI studied here, the contribution from the second term cannot be neglected for the EB form. This would imply less negative values of ␥ and n 2 for EB than for LEB. This is an important result, which implies that the coupling between the ground state and the bandgap states ( – * transition) is stronger than that between the ground state and the defect states ( B – Q transition). We emphasize that this conclusion is in agreement with Libert and co-workers, who performed intermediate neglect of differential overlap/configuration interaction scheme theoretical calculations of the electronic and optical properties of oligoanilines.21 They predicted that the oscillator strength of B – Q transitions would be smaller than that for – * transitions in EB. To complete the characterization of the third-order nonlinearity of PANI, it is important to determine the nonlinear absorption coefficient also. We already know that it must be small, less than 0.1 cm/GW, since it was not detected in our Z-scan setup. Therefore we used an excitation source with a higher pulse intensity: the second harmonic of a Q-switched Nd:YAG laser delivering pulses of 10 ns at a 5-Hz repetition rate. We used the opticalpower-limiting setup described in Section 2, and the results found are shown in Fig. 3. Here transmittance is defined as the ratio between the transmitted intensity and the incident intensity. Note that the transmittance of EB shown in Fig. 3(a) increases with the incident laser intensity. This behavior indicates that saturated absorption is occurring as a result of the quasi-resonance of the laser energy with defect states. The transmittance for LEB shown in Fig. 3(b), on the other hand, exhibits nonlinear absorption. The values of  were determined for propagation of the light beam through the medium by assuming the absorption coefficient of the material to be given by ␣ (I) ⫽ ␣ 0 ⫹  I.14,15 In this case the transmittance can be written as T(I) ⫽ T 0 /(1 ⫹ I  L eff), where T 0 is the linear transmittance given by Beer’s law, I is the incident peak intensity, and L eff is given by 关 1 ⫺ exp(⫺␣0L)兴/␣0 ; L is the sample thickness. The values of  were estimated
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from the best fit of T(I) with the experimental data (shown in Fig. 3 as solid curves). A negative  value for EB and a positive  value for LEB were obtained (see Table 1), as should be expected for the case of saturated absorption and two-photon absorption, respectively.
Note that the quasi-resonance condition between the excitation photon energy and the defect state energies in EB enhances one-photon processes, which compete with twophoton transitions. Thus, for high intensity levels of excitation at 532 nm, we found that the negative contribution to  due to the saturation channel is larger than the positive contribution to  due to the two-photon absorption channel in EB. The LEB form of PANI, on the other hand, does not exhibit defect states, and therefore resonant one-photon processes are not relevant. This explains the opposite signs of  obtained for the two forms of PANI studied here. To evaluate the performance of PANI for all-optical switching, we recall that a good nonlinear optical material must present reasonable figures of merit, i.e., n 2 I/2␣ 0 ⬎ 1 and 2  /n 2 ⬍ 1.16 These quantities were calculated for both forms of PANI studied, and, from the results shown in Table 1, we see that LEB is the most suitable for this application. We also studied the response time of the third-order nonlinearity of our PANI films operating a picosecond OKG at 532 nm. The laser beam with 70-ps single pulses was split into two beams with different intensities. The electric field of the strong (pump) beam was set at 45° with respect to the electric field of the weak (probe) input beam. When the pulses of the two beams overlap spatially and temporally at the sample position, the probe beam polarization rotates owing to the birefringence induced in the sample by the pump beam. Then a fraction of the probe beam passes through a polarizer crossed to the input probe beam polarization. A slow detector connected to a boxcar integrator was used to record the probe signal as a function of the time delay between the pump and the probe beams. Figure 4 shows the observed OKG signal obtained for the LEB film. The signal is symmetrical; i.e., the material response time was shorter than the pulse coherence time (⬃50 ps). This corroborates the hypothesis of an offresonant electronic nonlinearity.
Fig. 3. Transmittance as a function of incident peak intensity for the forms of PANI studied here: (a) EB and (b) LEB. The excitation beam is a pulsed laser (⬃10 ns) emitting at 532 nm. Sample thickness, 1 cm. PANI concentration, 0.01 g/l.
4. CONCLUSIONS The real and the imaginary parts of the third-order nonlinearity of solutions containing the undoped partially oxidized form of polyaniline, the emeraldine base (EB), and the reduced form of EB, the leucoemeraldine base (LEB), were investigated at 532 nm. The contribution of electronic defects for the nonlinear response of the polymer as well as the contribution of the – * bandgap electronic transitions were evaluated. The present results suggest that further studies of PANI derivatives are worthwhile in the search for materials with better nonlinear performance.
ACKNOWLEDGMENTS
Fig. 4. Optical Kerr gate signal as a function of delay time for a LEB film.
This research work was financially supported by the Brazilian agencies Conselho Nacional de Desenvolvimento Cientı´fico e Tecnolo´gico, Fundac¸˜ao de Amparo a` Cieˆncia e Tecnologia de Pernambuco, and Programa de Apoio a Nu´cleos de Exceleˆncia/Ministe´io de Cieˆncia e Tecnologia. Glauco S. Maciel’s fax number is 55-81-32710359. His e-mail address is
[email protected].
Maciel et al.
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