Optical nonlinearities of C84 fullerenes

Chemical Physics Letters 432 (2006) 497–501 www.elsevier.com/locate/cplett

Optical nonlinearities of C84 fullerenes E. Xenogiannopoulou, P. Aloukos, S. Couris

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Institute of Chemical Engineering and High Temperature Chemical Processes (ICEHT), Foundation for Research and Technology-Hellas (FORTH), 26504 Patras, Greece Department of Physics, University of Patras, 26500 Patras, Greece Received 14 September 2006; in final form 22 October 2006 Available online 27 October 2006

Abstract The third-order nonlinear optical response of fullerene C84 (isomeric mixture as obtained from fullerenic soot) and its isomer C84–D2d (II) dissolved in toluene were studied using nanosecond and picosecond laser pulses employing the Z-scan and the Optical-Kerr-effect techniques. The nonlinear absorption and refraction parameters and the corresponding second hyperpolarizability c were determined, while the origins of the observed nonlinear optical response are discussed.  2006 Elsevier B.V. All rights reserved.

1. Introduction Although C84 is the third most abundant fullerene, after C60 and C70, its nonlinear optical properties have been less studied and only very few reports are available in the literature. As recent studies have shown, fullerene C84 has 24 possible isomers (obeying the isolated pentagon rule) with two of them, namely the C84–D2(IV) and C84–D2d(II) isomers, being the most stable and abundant ones in the C84 (isomeric mixture as isolated from fullerenic soot) with a ratio D2(IV):D2d(II) of approximately 2:1 [1–5]. Recently, in our group the nonlinear optical response of C84 isomeric mixture and C84–D2d(II) isomer has been accomplished under femtosecond laser excitation and a large enhancement of the response of the isomer C84–D2d(II) compared to that of the C84 isomeric mixture has been observed [6]. The motivation of the present work was to complete our knowledge on the nonlinear optical response of fullerene C84 (isomeric mixture) and its C84–D2d(II) isomer under nanosecond and picosecond laser excitation in the visible

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Corresponding author. Address: Institute of Chemical Engineering and High Temperature Chemical Processes (ICEHT), Foundation for Research and Technology-Hellas (FORTH), 26504 Patras, Greece. Fax: +30 2610 965223. E-mail address: [email protected] (S. Couris). 0009-2614/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2006.10.098

and the near infrared. In view of that, in the present study measurements were performed with: (i) 8 ns, 532 nm and (ii) 35 ps, 532 nm and 1064 nm laser pulses. Hereafter for simplicity, the fullerene C84 (isomeric mixture) and the C84–D2d(II) isomer will be referred to as C84 and C84– D2d respectively. 2. Experimental In order to investigate the nonlinear optical response of C84 and C84–D2d fullerenes, two different experimental approaches have been employed namely the Z-scan [7] and the Optical-Kerr-Effect [8,9] (OKE) techniques. The experimental setups are described in details elsewhere [10,11]. For the needs of the Z-scan experiments, a 532 nm, 8 ns Nd:YAG laser operating at 10 Hz was used, while in the OKE experiments, a 10 Hz, 35 ps Nd:YAG laser operating at 1064 and 532 nm has been employed. Z-scan technique allows for the simultaneous determination of the real and imaginary parts of the third-order susceptibility v(3), from transmission measurements, namely the closed and open aperture Z-scan recordings respectively, from which the nonlinear absorption parameter b and the nonlinear refraction parameter c 0 can be determined easily following the procedure described in details elsewhere [10].

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In OKE technique, the signal is related with the refractive index anisotropy Dn caused by a strong linearly polarized laser beam (i.e. pump beam), where the magnitude of the signal depends on the time delay between a pump and a probe beams. If the refractive index anisotropy is small, then the maximum of the OKE signal is directly proportional to the nonlinear refractive index n2, which is defined as: n2 = n0 + c 0 I = n0 + Dn, where n0 is the linear refractive index, c 0 is the nonlinear refractive index parameter and I is the incident laser intensity. For non resonant excitation, n2 is proportional to the modulus of v(3) which is determined by comparison with a reference material according to the standard procedure described elsewhere [9,12]. In this study, the reference material used was CS2 and the n2 and v(3) values used were those reported by Sheik-Bahae et al. [7] for picosecond excitation, i.e. n2 = 1.2 · 1011 esu and v(3) = 5.2 · 1013 esu. The second hyperpolarizability c has been calculated from the third-order susceptibility v(3) using relation c = v(3)/NL4, where N is the number of molecules per unit volume and L is the local field correction factor given by L ¼ ðn20 þ 2Þ=3, with n0 the refractive index of the solvent. The energy per pulse of the laser systems used was monitored with appropriate, previously calibrated, energy meters. The pulse-to-pulse fluctuations were measured to be less than 5% for the nanosecond and about 20% for the picosecond laser respectively. In addition, the beam diameters at the focal plane have been measured separately for each laser system, using a CCD camera and were determined to be 34 and 36 lm for the nanosecond and picosecond laser respectively. Fullerene C84 (purity 99.5%), as isolated from carbon soot, was purchased from Techno Carbo (France) and was mainly a mixture of the two most abundant isomers D2d(II) and D2(IV) with a ratio of 1:2. The C84–D2d(II) isomer, was produced with the arc discharge method [4] and was isolated from the fullerenic soot by multistage recycling high performance liquid chromatography (HPLC), resulting to high purity isomer, according to a published procedure [1]. All fullerenes were dissolved in toluene at

C84

0.20

0.02 mM

Absorption (a.u.)

C84 D2d 0.02 mM 0.15

0.10

0.05

0.00 300

400

500

600

700

800

900

1000 1100

Wavelength (nm) Fig. 1. Absorption spectra of C84 and C84–D2d solutions in toluene.

concentrations in the range of 104 to 105 M to avoid formation of aggregates as has been reported elsewhere [13]. In Fig. 1 some representative UV-VIS-NIR absorption spectra of a concentration of 2 · 105 M are shown. They were all similar to previously reported ones [13–15]. Absorption spectra of the prepared solutions were measured before and after each set of measurements to ensure that no photo-degradation or other undesirable changes have been occurred to the fullerenes studied. 3. Results and discussion The transient nonlinear optical response of fullerenes C84 and C84–D2d was investigated in the visible, at 532 nm, by means of the Z-scan technique using an 8 ns Nd:YAG laser. Open and closed aperture Z-scans of various concentration solutions at different incident laser intensities were recorded. In all cases, the solvent used, i.e. toluene, did not show any nonlinear optical response under the same experimental conditions. Therefore the observed nonlinear response of the solutions was attributed solely to the dissolved fullerenes. As a general observation, the Z-scans of both C84 and C84–D2d solutions exhibited weak nonlinear absorption and large nonlinear refraction, while the closed aperture Z-scans were characterized by a prefocal transmission maximum (i.e. peak) followed by a post-focal transmission minimum (i.e. valley), a behavior suggesting negative sign nonlinear refraction (i.e. defocusing) [7]. Moreover, the difference in transmission between the peak and the valley (denoted as DTpv) was found to increase linearly with the energy of the incident radiation, as shown in Figs. 2a,b for different concentration toluene solutions of C84 and C84–D2d respectively. From the corresponding open and divided Z-scan recordings and following the procedure described elsewhere [e.g. Ref. [10], the nonlinear absorption and refraction parameters b and c 0 were determined respectively. Then, the real and imaginary parts of the third-order susceptibility v(3) were calculated. The variation of the Re v(3) of the studied solutions versus concentration are shown in Fig. 3, where the solid line represents the linear fit of the experimental data. From the slopes of the straight lines, the real parts of the second hyperpolarizability, Re c, were determined to be (84 ± 10) · 1030 esu and (260 ± 60) · 1030 esu for C84 and C84–D2d respectively. Similarly, from the imaginary part of the third-order susceptibility, the corresponding imaginary parts of the second hyperpolarizability, Im c, were determined to be (8.6 ± 0.3) · 1030 esu and <12 · 1030 esu for C84 and C84–D2d respectively. So, both fullerenes were found to exhibit refractive part significantly larger than their absorptive one in agreement with previous reports for other fullerenes studied under nanosecond excitation [11]. Finally, the second hyperpolarizability c of fullerenes C84 and C84–D2d were determined to be (85 ± 10) · 1030 esu and (260 ± 60) · 1030 esu respectively. Under visible nanosecond excitation it is well known [10] that the response of the lower fullerenes C60 and C70

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participation of the triplet state to the observed nonlinearity can be ruled out, since neither significant triplet–triplet absorption has been observed nor a sizeable intersystem crossing quantum yield has been found. In fact, an almost zero (<0.01) intersystem crossing quantum yield has been measured and a lifetime of about 35 ps has been reported [13,19] for the lowest singlet state S1 of C84 together with the lack of any experimental evidence [13] of fluorescence from this state to the S0, implying that the S1 state can only relax to the ground state S0 through radiationless processes (e.g. internal conversion). As a result, the transient response of the fullerenes studied here should result from ground and first excited singlet state contributions while contributions from higher excited singlet states are also possible. Next, the nonlinear optical response of C84 and C84– D2d(II) fullerenes was investigated under 35 ps laser excitation at both 532 and 1064 nm using OKE technique. The dependence of the OKE signal upon the incident pump intensity at 1064 nm is presented in Figs. 4a,b for different concentration solutions of both fullerenes. The straight lines, corresponding to linear fits of the

Fig. 2. Variation of the parameter DTpv on the incident laser energy for: (a) C84 and (b) C84–D2d toluene solutions, under 8 ns, 532 nm laser excitation.

0 -20

(3)

Re χ (x 10

- 13

esu)

-40

C84 C84-D2d

-60 -80 -100 -120 -140 -160 -180 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 Concentration (mM)

Fig. 3. Dependence of the Re v(3) of some C84 and C84–D2d solutions in toluene upon concentration, under 8 ns, 532 nm laser excitation.

is described successfully in terms of reverse saturable absorption, a process which depends on the ground and excited singlet and triplet states absorption cross sections, their lifetimes and the intersystem crossing efficiency. To the best of our knowledge, there are very few reports about the photophysical parameters of C84 (isomeric mixture) [3,16,17] and the C84–D2d isomer [18]. Considering the photophysical parameters reported in these studies, any

Fig. 4. Intensity dependence of the OKE signal of: (a) C84–D2d and (b) C84 toluene solutions, together with the corresponding signals of toluene and CS2 (for comparison purposes) under 35 ps, 1064 nm laser excitation.

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experimental data sets, were all found exhibiting a slope of about two. The OKE signals of CS2 and toluene were also measured under the same experimental conditions and are also presented in these figures for comparison purposes. As a general trend, all solutions’ OKE signals were found smaller than the corresponding signals of toluene, indicating an opposite [11,20] sign of the Re v(3) of the solute (i.e. the fullerene) compared to that of toluene, the latter being known to exhibit positive sign Re v(3) at the wavelengths of interest (as most of the organic solvents). The v(3) of the solutions studied, considered as dilute solutions of non-interacting particles and assuming a pair wise additive model [20], is given by relation:  2 ð3Þ v ¼ Imvð3Þ þ L4 N fullerene ci toluene fullerene  2 1=2 ð3Þ 4 r þ Revtoluene þ L N fullerene cfullerene

ð1Þ

where the superscripts r and i denote the real and imaginary part respectively, N is the fullerene number density and L is the local field correction factor. The v(3) of toluene using CS2 as reference was measured in the present study and was found to be 1.42 · 1013 esu. Then, the v(3) values of the different concentration solutions of C84 and C84–D2d were determined. As a result, the second hyperpolarizability of C84 and D2d at 1064 nm were determined to be (0.34 ± 0.07) · 1030 esu and (2.94 ± 0.84) · 1030 esu respectively. The isomer C84–D2d fullerene exhibited an almost one-order of magnitude larger second hyperpolarizability than that of the C84 fullerene. Similar OKE measurements were performed under 35 ps, 532 nm laser excitation. As an example of these measurements, the OKE signals of a 0.11 mM and a 0.027 mM C84 and C84–D2d toluene solutions respectively, together with the corresponding OKE signal of CS2 are presented in Fig. 5. As shown, both solutions exhibited similar

CS2

OKE signal (a.u.)

C84

10

0

0.11 mM

C84 D2d 0.027 mM

-1

10

-2

10

9

10

10

10 2

Ipump (W/cm ) Fig. 5. Intensity dependence of the OKE signals of a 0.11 mM C84 and a 0.027 mM C84–D2d toluene solutions and CS2, under 35 ps, 532 nm laser excitation.

OKE signals although their concentrations were different by a factor of four, indicating that the nonlinearity of the isomer was about four times larger than that of C84 fullerene. The values of the effective second hyperpolarizability of C84 and C84–D2d at 532 nm were then determined to be (1.15 ± 0.60) · 1030 esu and (5.5 ± 2.1) · 1030 esu respectively. Using DFWM technique and employing 70 ps laser pulses at 532 nm Huang et al. [21] have reported a hyperpolarizability value of (1.2 ± 0.3) · 1030 esu for C84, which is in good agreement with the value determined in this work. Taking into account the fact that the most stable and abundant isomers in the C84 fraction of fullerenic soot are the C84–D2(IV) and C84–D2d(II) ones, with a ratio D2:D2d of 2:1, the hyperpolarizability values of D2d and C84 determined here, indicate that the nonlinear optical response of the D2d isomer should be significantly larger than that of the D2 isomer although the later has not been measured in the present study. However, considering the molecular symmetry of the D2 isomer which is lower than that of D2d, its molecular volume which is relatively larger than that of D2d and their photophysical parameters which are not very different, the opposite behavior was expected, at least qualitatively, for the nonlinear response of the two isomers. A similar trend, i.e. opposite to what is expected, has been also found in our previous investigation [6] using 100 fs laser pulses at 800 nm. However, in this latter case, the larger nonlinear response of the D2d isomer at 800 nm could be understood considering the stronger absorption exhibited by this isomer at 800 nm. On the contrary, in the present case, at 1064 nm, such explanation does not hold since both fullerenes exhibit similar absorption as can be seen from the absorption spectra shown in Fig. 1 (and also in Fig. 1 of Ref. [6], where equal concentration solutions of C84, C84–D2d and C84–D2 exhibited similar absorption as well). For excitation at 532 nm, the resonance enhancement can be more efficient for the D2d isomer compared to the D2 one, due to the strong absorption nearby the excitation wavelength (i.e. the absorption peak at 400 and the unresolved shoulder at 472 nm). In that case, for the D2d isomer, a larger nonlinear response can be understood in principle for excitation at 532 nm. In any case, the situation with multiphoton induced resonance enhancements can be more complicated, as it has been reported in Ref. [22], where the frequency dependence of the second hyperpolarizability of several fullerenes and among them the D2d and D2 isomers has been studied by the SOSCNDO/S CI semiempirical technique. According to this study, two- and three-photon resonances are expected to be active for the D2 isomer, while one- and three-photon resonances can be active in the case of the C2d isomer, therefore increasing the possibilities of resonance enhancements to occur. Besides, in this study, a particular behavior of the D2d isomer has been reported. More experimental studies at different wavelengths could possibly help to clarify these issues.

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4. Conclusions In summary, a systematic experimental investigation of the optical nonlinearities of C84 and the isomer C84– D2d(II) has been performed under ps and ns excitation. Measurements performed at 35 ps, 532 and 1064 nm, have shown that the D2d isomer has significantly larger nonlinear response than the C84 mixture and the D2 isomer. Furthermore, measurements performed using ns excitation revealed significant transient response for both fullerenes, arising only from contributions of the singlet states, while triplet states’ contributions are negligible. Acknowledgements This work has been partially supported by the EU HPRN-CT-2002-00177 project ‘WONDERFULL’. E.X. acknowledges financial support from GSRT through a PENED 2001 project (No. 395 ‘EPOPTIC’). P.A. acknowledges financial support from the Greek Ministry of Education through an ‘HRAKLEITOS’ grant. All authors wish to thank Prof. H. Shinohara and Dr N. Tagmatarchis for making the isomer C84–D2d(II) available and Prof. E. Koudoumas for helpful discussions. References [1] T.J.S. Dennis, T. Kai, T. Tomiyama, H. Shinohara, Chem. Commun. 5 (1998) 619. [2] G. Sun, M. Kertesz, J. Phys. Chem. A 105 (2001) 5212.

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