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2008, 112, 9561–9564 Published on Web 07/23/2008
Nonlinear Optical Transmission Properties of C60 Dyads Consisting of a Light-Harvesting Diphenylaminofluorene Antenna Hendry I. Elim,† Sea-Ho Jeon,‡ Sarika Verma,‡ Wei Ji,*,† Loon-Seng Tan,§ Augustine Urbas,§ and Long Y. Chiang*,‡ Department of Physics, National UniVersity of Singapore, 2 Science DriVe 3, Singapore 117542, Department of Chemistry, UniVersity of MassachusettssLowell, Lowell, Massachusetts 01854, and Materials & Manufacturing Directorate, Air Force Research Laboratory, AFRL/RX, Wright-Patterson Air Force Base, Dayton, Ohio 45433 ReceiVed: June 08, 2008; ReVised Manuscript ReceiVed: July 06, 2008
Highly enhanced nonlinear absorption cross section values of C60(>DPAF-C2M), C60(>DPAF-C9), and C60(>DPAF-C10) dyads were detected up to 5400, 9700, and 14000 GM, respectively, in the 2.0 ps region in toluene at the concentration of 1.5 × 10-3 M. They were correlated to a trend showing higher efficiency in light transmittance attenuation down to 39-46% for the dyads C60(>DPAF-C10) and C60(>DPAF-C9) with the increase of irradiance intensity up only to 140 GW/cm2. The phenomena were attributed to additional enhancement on the excited-state absorption of 1C60*(>DPAF-Cn) in the subpicosecond to picosecond region over the two-photon absorption of C60(>DPAF-Cn) in the femtosecond region. Its accumulative 2.0 ps absorption cross sections were estimated to be 8900 GM for 1C60*(>DPAF-C9), roughly one order of magnitude higher than its intrinsic femtosecond 2PA cross sections. The approach of using one or multiple photoactive chromophores covalently bound on a C60 cage for enhancing photoinduced intramolecular energy- or electron-transfer events1–3 becomes a significant tactic in the design of nanomaterials for the applications of photovoltaics4 and nonlinear photonics.5–8 It is interesting to note that these two photoprocesses may coexist in the same molecular system upon photoexcitation in a solvent-dependent manner.2,3 Among them, intramolecular energy-transfer processes were reported to be more favorable in nonpolar solvents, such as toluene and CS2. Coupling of this process with the intersystem crossing characteristics of an excited singlet C60 cage facilitates the population of the excited triplet C60 state. Utilization of the absorption of both the transient excited singlet and triplet fullerenyl states was proposed to be one of prerequisites for the enhancement of nonlinear photophysical properties of the materials.9,10 Meanwhile, covalent attachment of one or several chromophore antenna on C60 to increase the two-photon absorption capability in the visible and near-infrared region gives remedy to the low absorption of the C60 cage at these wavelengths. The approach was applied in the extensive study of methano[60]fullerene (C60>)-9,9dialkyldiphenylaminofluorene (DPAF-Cn) conjugates, where DPAF-Cn arms serve as the light-harvesting antenna.5–7 With the covalent linkage of sterically hindered 9,9-di(3,5,5-trimethylhexyl)-2-diphenylaminofluorene (DPAF-C9) antenna in a periconjugation position with a C60 cage, the resulting dyad C60(>DPAF-C9), as shown in Figure 1, exhibited large con* To whom correspondence should be addressed. Phone: +1 (978)-9343663. Fax: +1 (978)-934-3013. E-mail:
[email protected].. † National University of Singapore. ‡ University of MassachusettssLowell. § Wright-Patterson Air Force Base.
10.1021/jp8050356 CCC: $40.75
Figure 1. Molecular structure of C60(>DPAF-Cn) using the diphenylaminofluorene ring system as a photoactive antenna moiety for the light-harvesting effect.
centration-dependent two-photon absorption (2PA) cross section (σ2) values in the femtosecond (fs) region in CS2 upon photoexcitation at 780 nm. The value reaches 290, 1140, and 2530 GM at the concentrations of 1.0 × 10-2, 1.86 × 10-3, and 1.0 × 10-4 M, respectively, using 150 fs laser pulses with the power intensity of 163 GW/cm2.8 In this study, we carried out the time-resolved 2PA cross section and nonlinear light-transmission measurements for a 2.0 ps duration using low laser intensity of 48 GW/cm2 to provide confirmation of the large nonlinear photonic behavior of C60(>DPAF-C9)-related nanomaterials in the short picosecond region. Application of laser irradiation at a much lower power intensity than 163 GW/cm2 is expected to minimize the potential thermal scattering effect during the measurement. In addition, even though CS2 gave the highest solubility of C60(>DPAFCn) materials when applied in previous measurements, certain 2008 American Chemical Society
9562 J. Phys. Chem. B, Vol. 112, No. 32, 2008
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Figure 3. Light transmittance of C60(>DPAF-Cn) in toluene showing 85-89% transmittance at 780 nm and the concentration of 1.5 × 10-3 M used for NLO measurements.
Figure 2. Energy diagram of C60> and DPAF-C9 moieties of the C60(>DPAF-C9) dyad showing the path of the ultrafast intramolecular energy-transfer process.6
two-photon absorption of CS211 itself is problematic for the calculation and calibration of the material’s 2PA cross section values. To avoid this complication, we use toluene as the solvent for the current measurements at a concentration of 1.5 × 10-3 M. Intramolecular energy-transfer processes constitute the major photophysical events in enhancing the nonlinear photonic properties of C60(>DPAF-Cn) nanomaterials. It is plausible since the lowest excited singlet energies of C60(>1DPAF*-C2) and 1C *(>DPAF-C ) were estimated to be 2.74 (452 nm) and 1.74 60 2 (714 nm) eV, respectively, in toluene, based on steady-state fluorescence measurements.2a The formation of an excited fullerenyl triplet state was evident from the detection of the triplet-triplet absorption band of 3C60*(>DPAF-C9) at ∼730 nm in nanosecond transient absorption measurements after 532 nm pulse laser irradiation on the DPAF-C9 moiety.6 That implied clearly the occurrence of efficient intramolecular energy-transfer events going from the photoexcited C60(>1DPAF*-C9) transient state to the 1C60*(>DPAF-C9) transient state in toluene prior to the process of intersystem crossing of the latter to 3C60*(>DPAFC9). The kinetic rate of the former event was found to be ultrafast at DPAFC9) by 780 nm irradiation with energy that is roughly half of DPAF-C9’s HOMO-LUMO energy gap should facilitate both two-photon absorption of the DPAF-C9 moiety and excitedstate absorption of the fullerene cage moiety in a similar wavelength range, as shown in the energy diagram of Figure 2. As the two-photon absorption and excited-state absorption of both DPAF-C9 and C60> moieties9 occur in a nearly concurrent event, large enhancement of the overall nonlinear optical response capability of C60(>DPAF-Cn) nanomaterials
should be achievable in the ultrashort fs-ps time scale. The phenomena follow the proposed concept of the combination of 2PA at the ground state and reverse saturable absorption (RSA) at the excited states of organic complex chromophores for the potential reduction and attenuation of light transmittance.10 Accordingly, extension of the transient absorption and optical transmittance measurements at a longer time scale than 130 fs into the region of several picoseconds should allow us to follow such early, stepwise photophysical events of C60(>DPAF-Cn) dyads and correlate the measured absorption cross sections and optical transmittance reduction efficiency to these events among energy bands of the material. Experimentally, a recently reported synthetic procedure7 with modifications was used for the preparation of the dyads C60(>DPAF-C2M), C60(>DPAF-C9), and C60(>DPAF-C10), as shown in Figure 1. Owing to the concern of potential two-photon absorption arising from CS2, a solution of all materials in toluene was prepared for the nonlinear optical (NLO) studies. At the concentration of 1.5 × 10-3 M used in NLO measurements, a solution of C60(>DPAF-Cn) showed a good linear (or lowfluence) optical transmittance (T) of 89% for C60(>DPAF-C9) and C60(>DPAF-C10) at 780 nm (Figure 3), whereas it was ∼85% for C60(>DPAF-C2M). A weak absorption band centered at 698 nm for S0-Sn transitions of the C60> cage is visible for all compounds. A minor variation of the transmittance profile among these three samples at 780 nm may be attributed to the slightly higher molecular coalescence tendency of C60(>DPAFC2M) than that of hindered C60(>DPAF-C9) due to its lower solubility in toluene. Molecular aggregation is expected to result in broadening of the transmission edge at long wavelengths. Simultaneous two-photon absorption cross sections (σ2) and nonlinear light transmittance measurements of C60(>DPAF-Cn) samples in toluene were carried out with 2.0 ps Z-scans and irradiance-dependent transmission counting at the wavelength of 780 nm. To reduce potential accumulative thermal scattering effects,12 we employed laser pulses with a light intensity of 48 GW/cm2. Laser pulses were generated by a mode-locked Ti: Sapphire laser (Quantronix, IMRA), which seeded a Ti:Sapphire regenerative amplifier (Quantronix, Titan) and focused onto a 1-mm-thick quartz cuvette containing a solution of C60(>DPAFCn) with a minimum beam waist of ∼12 µm. Incident and transmitted laser intensities were monitored as the cuvette was moved (or Z-scanned) along the propagation direction of the laser pulses. The data sets were normalized to the linear transmittance and sample inhomogeneities for all Z-scans by the correction of the background transmittance, T(|Z| . Zo). The normalized transmittance ∆T(Z) was expressed as T(Z)/T(|Z| . Zo). Accordingly, the change in the normalized transmittance
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J. Phys. Chem. B, Vol. 112, No. 32, 2008 9563
TABLE 1: Two-Photon Absorption Cross Sections (σ2) of C60(>DPAF-C9), C60(>DPAF-C10), and C60(>DPAF-C2M) Measured Using Laser Pulses Working at 780 nm with a ∼2.0 ps Duration and a Light Intensity of I ) 48 GW/cm2 chromophore
[C] (M)
β (cm/GW)
σ2 (×10-48 cm4 sec photon-1 molecule-1)
C60(>DPAF-C9) C60(>DPAF-C10) C(>DPAF-C2M)
1.5 × 10-3 1.5 × 10-3 1.6 × 10-3
0.34 0.48 0.20
97 (9700 GM) 140 (14000 GM) 54 (5400 GM)
is indicative of the nonlinear (or light-fluence-dependent) part in the sample’s absorption. Total absorption was described by the change in the absorption coefficient ∆R ) βI, where β and I are the absorption coefficient and the light intensity, respectively. The absorption coefficient could be extracted from the best fitting between the Z-scan theory13 and the data. The 2PA cross section value was then calculated from the coefficient by the formula σ2 ) βpω/N, where pω is the photon energy and N is the number of the molecules. As a result, the 2PA cross section values of three C60(>DPAF-Cn) compounds measured are summarized in Table 1. It is interesting to observe a higher nonlinear absorption cross section value of 140 × 10-48 cm4 sec photon-1 molecule-1 (or 14000 GM) for C60(>DPAF-C10) at the concentration of 1.5 × 10-3 M than that, 97 × 10-48 cm4 sec photon-1 molecule-1 (or 9700 GM), for C60(>DPAF-C9). A lower value of 54 × 10-48 cm4 sec photon-1 molecule-1 (or 5400 GM) for C60(>DPAFC2M) was detected, perhaps owing to its higher particle aggregation tendency at 1.6 × 10-3 M. On the basis of the energy diagram given in Figure 2, a 2.0 ps pulse duration is longer than 130 fs required for the intramolecular energy transfer from the photoexcited 1(DPAF)*-Cn antenna moiety to the C60> cage of C60(>DPAF-Cn). Completion of this energy-transfer event at the early time scale leads to the formation of an excited 1C *(>DPAF-C ) state. Therefore, the measured σ values at 60 n 2 2.0 ps should cover both the two-photon absorption of the DPAF-Cn moiety in the femtosecond region and the excited singlet-state absorption (S1-Sn) of the 1(C60>)* cage moiety in the subsequent subpicoseconds to picoseconds. The previously measured absorption cross section value8 of C60(>DPAF-C9) of 8.0 × 10-48 cm4 sec photon-1 molecule-1 (or 800 GM, adjusted for the absorption contribution of CS2)11 in the 160 fs region at a similar concentration of 1.9 × 10-3 M in CS2 represents mainly the two-photon absorption contribution of the DPAF-C9 moiety forming the transient C60(>1DPAF*-C9) state. Accordingly, this value can be applied as the reference for the correlation and differentiation of the excited-state absorption contribution of 1(C60>)*. The argument is valid due to the low linear C60> cage absorption at 780 nm as compared with that of the DPAF-C9 moiety. By taking the σ2 values of C60(>DPAF-C9) obtained in either the 160 fs or 2.0 ps time scale for comparison, we were able to derive a large picosecond σ2 contribution of the excited-state absorption in 1C60*(>DPAF-C9) samples as 89 × 10-48 cm4 sec photon-1 molecule-1 (or 8900 GM), corresponding to the singlet-singlet (S1-Sn) transitions of 1(C60>)*. It is roughly one order of magnitude higher than intrinsic femtosecond 2PA cross sections of C60(>DPAF-C9). Contribution of the laserinduced thermal scattering effect at the 2.0 ps region might be minimized by taking the reported fast mode of the stimulated diffractive light scattering,14 as developed within a 10 ps duration on a slightly longer time scale, rather than the current measurement into consideration as the reference. In addition, the
Figure 4. Open-aperture Z-scan profiles of C60(>DPAF-C9), C60(>DPAFC10), and C60(>DPAF-C2M) in toluene. The measurements were carried out with a 2.0 ps laser duration at 780 nm.
Figure 5. Nonlinear light transmittance of C60(>DPAF-C9), C60(>DPAFC10), and C60(>DPAF-C2M) in toluene (1.5 × 10-3 M) measured as a function of irradiance with 2.0 ps laser pulses operated at 780 nm.
occurrence of transient conversion from the 1C60*(>DPAF-C9) state to the corresponding 3C60*(>DPAF-C9) state via intersystem crossing was reported to be effective at a much longer time scale of ∼1.4 ns.6 Therefore, the absorption contribution of the 3C60*(>DPAF-C9) state can be excluded in this measurement. These highly enhanced fs-ps absorptions may be correlated to the following nonlinear light transmittance attenuation measurements. Open-aperture Z-scans carried out under the irradiance of 48 GW/cm2 were taken on the samples of C60(>DPAF-C9), C60(>DPAF-C10), and C60(>DPAF-C2M) in toluene at the concentration of 1.5-1.6 × 10-3 M, with the profile plots shown in Figure 4. These Z-scans displayed positive signs for absorptive nonlinearities with the decrease of light transmittance in the order of C60(>DPAF-C10) < C60(>DPAF-C9) < C60(>DPAFC2M) in solution. This molecular trend persists in the following irradiance-dependent NLO measurements. Nonlinear optical transmittance attenuation properties of C60(>DPAF-Cn) were investigated by irradiance-dependent transmission measurements at the wavelength of 780 nm using the same setup as that used in 2PA cross section measurements conducted by 2.0 ps laser pulses, except the sample was fixed at the focus area when the incident pulse intensity was varied. Technical details of the measurements were described elsewhere.15 Nonlinear light transmittance intensity results for 1.0mm-thick solutions of C60(>DPAF-C9), C60(>DPAF-C10), and C60(>DPAF-C2M) in toluene at a concentration of 1.5 × 10-3 M are illustrated in Figure 5. All of the samples showed a linear transmission (T ) ∼89%) with an input intensity of up to 27 GW/cm2 or a fluence of 4.05 mJ/cm2. When the incident intensity was increased above 27 GW/cm2, the transmittance
9564 J. Phys. Chem. B, Vol. 112, No. 32, 2008 (%) began to deviate from the linear transmission line and decrease, indicating the initiation of nonlinearity and the limiting effect. The transmitted fluence further departed from the linear line upon the increase in the incident fluence. A systematic trend showing higher efficiency with reducing light transmittance down to 39, 46, and 55% for the dyads C60(>DPAF-C10), C60(>DPAF-C9), and C60(>DPAF-C2M), respectively, was observed with the increase of irradiance intensity up to 140 GW/ cm2 (Figure 5). Improvement in lowering the transmittance can be correlated to the higher solubility of the former two dyads, consistent with the positive contribution of C60(>DPAF-C10) and C60(>DPAF-C9) to a larger transient absorption, concluded by Z-scans in Figure 4. When these data were compared with the nonlinear light transmittance results of C60(>DPAF-C9) in a similar concentration range conducted with 160 fs laser pulses at the wavelength of 780 nm, its ability to reduce light transmittance down to only ∼75% at an irradiance intensity of 140 GW/cm2 was much less efficient than that of 46% in the 2.0 ps region obtained by the current measurement. To reach the same light transmittance reduction level of 46% in the femtosecond region, an irradiance intensity of ∼750 GW/cm2 was required.8 Significant enhancement of the observed light transmittance attenuation behavior in the 2.0 ps region may be attributed predominantly to the largely increased excited-state absorption of the lowest excited singlet state 1(C60>)* moiety and, to a lesser extent, to the twophoton absorption processes of the DPAF-C9 moiety. Accumulative absorption cross sections of the former transient state are larger than that of the latter process by more than 10-fold. Population of the transient 1C60*(>DPAF-C9) state was highly increased by ultrafast intramolecular energy-transfer processes from the two-photon-pumped excited transient C60(>1DPAF*C9) state. In conclusion, we observed highly enhanced absorption cross section values of C60(>DPAF-C2M), C60(>DPAF-C9), and C60(>DPAF-C10) dyads up to 5400, 9700, and 14000 GM, respectively, in the 2.0 ps region in toluene at the concentration of 1.5 × 10-3 M. They were correlated to a trend showing higher efficiency in the attenuation of light transmittance down to 39-46% for the dyads C60(>DPAF-C10) and C60(>DPAF-C9) with the increase of irradiance intensity up only to 140 GW/ cm2. This efficiency is largely improved from that of ∼750 GW/ cm2 required to reach the same light transmittance reduction level in the 160 fs region. The phenomena were attributed to additional enhancement in the excited-state absorption of 1C *(>DPAF-C ) and 1C *(>DPAF-C ) in the subpicosecond 60 10 60 9 topicosecondregionoverthetwo-photonabsorptionofC60(>DPAFCn) in the femtosecond region. Its accumulative 2.0 ps absorptioncrosssectionswereestimatedtobe8900GMfor 1C60*(>DPAFC9), roughly one order of magnitude higher than its intrinsic femtosecond 2PA cross sections. Ultrafast population of the 1(C >)* transient state via intramolecular energy transfer from 60 C60(>1DPAF*-Cn) within 130 fs, upon two-photon excitation of the DPAF-Cn moiety, was thought to be the reason for the largely increased nonlinear light transmittance attenuation. Acknowledgment. We thank the Air Force Office of Scientific Research for funding under the Contract Number FA955005-1-0154.
Letters Supporting Information Available: General synthetic procedures, 13C NMR spectra of (a) C60(>DPAF-C2) and (b) C60(>DPAF-C9) (Figure S1) and the positive ion FAB mass spectrum(directprobe)of(a)C60(>DPAF-C9)and(b)C60(>DPAFC9)2 (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) For recent reviews, see: (a) Martin, N.; Sanchez, L.; Illescas, B.; Perez, I. Chem. ReV. 1998, 98, 2527–2548. (b) Prato, M.; Maggini, M. Acc. Chem. Res. 1998, 31, 519–530. (c) Diederich, F.; Gomez-Lopez, M. Chem. Soc. ReV. 1999, 28, 263–277. (d) Guldi, D. M.; Prato, M. Acc. Chem. Res. 2000, 33, 695–703. (e) Guldi, D. M. Chem. Soc. ReV. 2002, 31, 22–36. (f) El-Khouly, M. E.; Ito, O.; Smith, P. M.; D’Souza, F. J. Photochem. Photobiol., C 2004, 5, 79–104. (2) (a) Luo, H.; Fujitsuka, M.; Ito, O.; Padmawar, P.; Chiang, L. Y. J. Phys. Chem. B 2003, 107, 9312–9318. (b) El-Khouly, M. E.; Padmawar, P.; Araki, Y.; Verma, S.; Chiang, L. Y.; Ito, O. J. Phys. Chem. A 2006, 110, 884–891. (3) El-Khouly, M. E.; Anandakathir, R.; Ito, O.; Chiang, L. Y. J. Phys. Chem. A 2007, 111, 6938–6944. (4) (a) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science 1992, 258, 1474–1476. (b) Kraabel, B.; McBranch, D.; Sariciftci, N. S.; Moses, D.; Heeger, A. J. Phys. ReV. B 1994, 50, 18543–18552. (c) Halls, J. J. M.; Pichler, K.; Friend, R. H.; Moratti, S. C.; Holmes, A. B. Appl. Phys. Lett. 1996, 68, 3120–3122. (d) Brabec, C. J.; Sariciftci, N. S.; Hummelen, J. C. AdV. Funct. Mater. 2001, 11, 15–26. (e) Li, G.; Shrotriya, V.; Yao, Y.; Huang, J.; Yang, Y. J. Mater. Chem. 2007, 17, 3126–3140. (f) Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T.-Q.; Dante, M.; Heeger, A. J. Science 2007, 317, 222–225. (5) (a) Chiang, L. Y.; Padmawar, P. A.; Canteewala, T.; Tan, L.-S.; He, C. S.; Kanna, R.; Vaia, R.; Lin, T.-C.; Zheng, Q.; Prasad, P. N. Chem. Commun. 2002, 1854–1855. (b) Padmawar, P. A.; Canteenwala, T.; Verma, S.; Tan, L.-S.; Chiang, L. Y. J. Macromol. Sci., Pure Appl. Chem. 2004, 41, 1387–1400. (c) Padmawar, P. A.; Canteenwala, T.; Verma, S.; Tan, L.-S.; He, G. S.; Prasad, P. N.; Chiang, L. Y. Synth. Met. 2005, 154, 185– 188. (6) Padmawar, P. A.; Rogers, J. O.; He, G. S.; Chiang, L. Y.; Canteenwala, T.; Tan, L.-S.; Zheng, Q.; Lu, C.; Slagle, J. E.; Danilov, E.; McLean, D. G.; Fleitz, P. A.; Prasad, P. N. Chem. Mater. 2006, 18, 4065– 4074. (7) Padmawar, P. A.; Canteenwala, T.; Tan, L.-S.; Chiang, L. Y. J. Mater. Chem. 2006, 16, 1366–1378. (8) Elim, H. I.; Anandakathir, R.; Jakubiak, R.; Chiang, L. Y.; Ji, W.; Tan, L. S. J. Mater. Chem. 2007, 17, 1826–1838. (9) (a) Tutt, L. W.; Kost, A. Nature 1992, 356, 225–226. (b) McLean, D. G.; Sutherland, R. L.; Brant, M. C.; Brandelik, D. M. Opt. Lett. 1993, 18, 858–860. (c) Riggs, J. E.; Sun, Y.-P. J. Chem. Phys. 2000, 112, 4221– 4230. (d) Sun, N.; Guo, Z.-X.; Dai, L.; Zhu, D.; Wang, Y.; Song, Y. Chem. Phys. Lett. 2002, 356, 175–180. (10) (a) Lin, T. C.; Chung, S. J.; Kim, K. S.; Wang, X.; He, G. S.; Swiatkiewicz, J.; Pudavan, H. E.; Prasad, P. N. AdV. Polym. Sci. 2003, 161, 157–193. (b) He, G. S.; Bhawalkar, J. D.; Zhao, C. F.; Prasad, P. N. Appl. Phys. Lett. 1995, 67, 2433–2435. (c) Ehrlich, J. E.; Wu, X. L.; Lee, L.-Y.; Hu, Z.-Y.; Roeckel, H.; Marder, S. R.; Perry, J. W. Opt. Lett. 1997, 22, 1843–1845. (d) Silly, M. G.; Porres, L.; Mongin, O.; Chollet, P.-A.; Blanchard-Desce, M. Chem. Phys. Lett. 2003, 379, 74–80. (11) (a) Ganeev, R. A.; Ryasnyansky, A. I.; Ishizawa, N.; Baba, M.; Suzuki, M.; Turu, M.; Sakakibara, S.; Kuroda, H. Opt. Commun. 2004, 231, 431–436. (b) Liu, Z.-B.; Liu, Y.-L.; Zhang, B.; Zhou, W.-Y.; Tian, J.-G.; Zang, W.-P.; Zhang, C.-P. J. Opt. Soc. Am. B 2007, 24, 1101–1104. (12) (a) Kost, A.; Tutt, L. W.; Klein, M. B.; Dougherty, T. K.; Elias, W. E. Opt. Lett. 1993, 18, 334–336. (b) Justus, L. B.; Kafafi, Z. H.; Huston, A. L. Opt. Lett. 1993, 18, 1603–1605. (13) Sheik-Bahae, M.; Said, A. A.; Wei, T.; Hagan, D. J.; Van Stryland, E. W. IEEE J. Quantum Electron. 1990, 26, 760–769. (14) (a) Scarlet, R. I. Phys. ReV. A 1972, 2281–2291. (b) Guo, S.; Busch, D. G.; Ho, W. Surf. Sci. 1995, 344, L1252-L1258. (15) (a) Elim, H. I.; Ouyang, J. Y.; He, J.; Goh, S. H.; Tang, S. H.; Ji, W. Chem. Phys. Lett. 2003, 369, 281. (b) Elim, H. I.; Ji, W.; Meng, G. C.; Ouyang, J. Y.; Goh, S. H. J. Nonlinear Opt. Phys. 2003, 12, 175–186.
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