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Third-order nonlinear optical properties of a π-conjugated biradical molecule investigated by third-harmonic generation spectroscopy
Thin Solid Films 519 (2010) 1028–1030
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Third-order nonlinear optical properties of a π-conjugated biradical molecule investigated by third-harmonic generation spectroscopy Hideo Kishida a,⁎, Kenichi Hibino a, Arao Nakamura a, Daisuke Kato b, Jiro Abe b a b
Department of Applied Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603 Japan Department of Chemistry, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara 252-5258 Japan
a r t i c l e
i n f o
Available online 20 August 2010 Keywords: Optical nonlinearity Third-harmonic generation Biradical
a b s t r a c t Third-order optical nonlinearity of a biradical molecule with open-shell π-electron system, 1,4-bis- (4,5diphenylimidazole-2-ylidene)-cyclohexa-2,5-diene (BDPI-2Y) was investigated by the third-harmonic generation method. The figure of merit for third-order optical nonlinearity in BDPI-2Y is comparable to those for conjugated polymers, which have elongated closed π-conjugated systems. This indicates that the biradical nature of BDPI-2Y enhances third-order optical nonlinearity. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Absorption measurements
Third-order optical nonlinearity of π-conjugated systems has long been investigated owing to their potential application in photonics and nanotechnology. Most of such studies are devoted to closed-shell systems [1–3]. However, the third-order optical nonlinearity of openshell systems is of considerable interest because some theoretical studies on singlet biradical molecules suggest that such systems can have enhanced optical nonlinearity [4–8]. Recently, exceptionally large two-photon absorption (TPA) cross sections of singlet biradical hydrocarbons in solution were reported by the open-aperture Z-scan method with a femtosecond laser pulse [9]. The open-shell system is an attractive target from the viewpoint of the exploration of third-order nonlinear optical materials, although only a few experimental studies on the open-shell systems exist. The biradical character of 1,4-bis- (4,5-diphenylimidazole-2-ylidene)cyclohexa-2,5-diene (BDPI-2Y) (Fig. 1) [10] was investigated by the theoretical calculation, and the experimental evidence for the contribution from a biradical state was presented by the observation of the radical dimerization reaction for a BDPI-2Y derivative, tF-BDPI2Y, where four hydrogen atoms at the central phenylene ring are substituted with four fluorine atoms of BDPI-2Y [11–13]. Nakano et al. reported the large second hyperpolarizabilities of a simplified analogue of BDPI-2Y by the CCSD(T)/6-31G level of the theory; here, CCSD (T) is the coupled cluster method including single and double excitation operators and a perturbative treatment of triple excitations [6]. In this study, we clarify by third-harmonic generation (THG) spectroscopy that BDPI-2Y shows enhanced optical nonlinearity compared with typical closed-shell conjugated polymers.
A thin film was fabricated onto synthesized quartz substrates by thermal evaporation and a high-quality optical film was obtained. The thickness was 94 nm when measured by a stylus profilometer. Fig. 2 (solid line) shows the absorption spectrum of the thin film. The absorption peak energy is 2.05 eV (605 nm), which is nearly the same as for benzene solution [10]. This absorption band is related to delocalized π-electrons, as evidenced by the observation of a red shift in the absorption peak energy for the polymer consisting of BDPI-2Y [14], in which the π-conjugated systems are elongated. The maximum absorption coefficient in thin films reaches 2.2 × 105 cm− 1, which is large in comparison with fully π-conjugated materials such as polythiophenes. It is thus desirable to compare BDPI-2Y with various conjugated polymers as is shown in Fig. 3. In poly(3-hexylthiophenes), which are typical and widely used conjugated polymers, the absorption coefficient varies from 0.6 × 105 cm− 1 to 1.4 × 105 cm− 1 depending on the head-to-tail coupling ratio r [15]. Even for the most regioregular P3HT (r = 0.985), the absorption coefficient is smaller than for BDPI-2Y. The large absorption coefficient of BDPI-2Y results from full elongation of the π-electrons over the molecule.
⁎ Corresponding author. E-mail address: [email protected] (H. Kishida). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.08.037
3. Nonlinear optical measurements An evaluation of the third-order optical nonlinearity was performed by THG spectroscopy, which gives the magnitude of third-order nonlinear susceptibility, |χ(3)|. Since |χ(3)| values are dependent on the photon energy of the incident laser due to resonant effects, a comparative study among various materials must be performed under the same resonant condition. In this study, |χ(3)| values are evaluated under the three-photon resonant condition, where they show the maximum values. The measuring range is from
H. Kishida et al. / Thin Solid Films 519 (2010) 1028–1030
1029
Fig. 1. Biradical and quinoidal forms of BDPI-2Y.
0.51 − 1.03 eV excitation photon energy. Excitation laser light was generated by a system consisting of Spitfire Pro (Spectra-Physics) and TOPAS (Light Conversion), with a pulse width of 100 fs and a repetition rate of 1 kHz. Measurement of |χ(3)| was performed by the standard Maker fringe method [16], with a SiO2 reference sample [17–19]. To avoid effects of THG in ambient air, the sample was mounted and measured in a vacuum cell. Fig. 2 (dots) shows the THG spectrum. A clear resonant enhancement is observed at 0.65-eV excitation photon energy. This resonant energy is just one-third of the absorption peak energy. Therefore, we can easily assign this enhancement to a three-photon resonance. The value of |χ(3)| reaches a maximum of 3.0 × 10− 11 esu. 4. Discussion Fig. 3 shows the maximum |χ(3)| values versus the maximum absorption coefficient (α) for various conjugated materials. The |χ(3)| value for BDPI-2Y is larger than for P3HTs, but smaller than for artificially designed nonlinear optical materials such as chargetransfer (CT) conjugated polymers like PAE and PThTz [20–22], and highly regioregular head-to-head type polythiophenes (HH-P3 (C ≡ CR)Th) [23,24]. In comparing materials, we should consider the difference in density of the π-electrons. |χ(3)| and α are both linearly proportional to the density of the molecules and π-electrons [25]. Therefore, the ratio of |χ(3)| to α, |χ(3)|/α, in which the difference in density is cancelled, gives a good measure of optical nonlinearity inherent to each π-electron system. Indeed, this quantity is called as a “figure of merit” for optical nonlinearity. In Fig. 3, |χ(3)|/α is the slope from the origin to the sample point. |χ(3)|/α for BDPI-2Y is 1.37 × 10− 16 cm esu, which is larger than for regiorandom P3HT (0.28 − 0.87 × 10− 16 cm esu; r = 0 − 0.80) but smaller than for P3HT (1.97 × 10− 16 cm esu). Considering that the π-conjugated system is smaller in BDPI-2Y than in conjugated polymers, |χ(3)|/α for BDPI-2Y is quite large. Thus, some enhancement effect characteristic of open-shell electronic systems is evident. Theoretical studies suggested an enhancement effect in optical
nonlinearity in singlet biradical states [6], where the charge distribution is favorable to third-order nonlinear optical processes. Thus, the theoretical expectations and experimental results are compatible, although the theoretical study considers static nonlinearity, namely, the off-resonant condition where the photon energy ħω = 0. The enhancement due to the biradical character under the resonant condition is still theoretically unclear. The THG peak is broader than the absorption peak in the higherenergy side. This broadening in the THG spectrum may be ascribed to the three-photon resonance to higher-lying excited states, because some resonant shoulder structure is often observed in the higherenergy side of the THG peak. In nonlinear optical measurements, higher-lying states are often sensitively observed because nonlinear optical processes include not only the transition from the ground state but also transitions between excited states. In contrast, the absorption process includes only the transition from the ground state. The transition dipole moment between the higher-lying state and the other excited states, which is included in the THG process, can be large, while that between the higher-lying state and the ground state is relatively small. As a result, THG measurements can be sensitive to higher-lying excited states, while they are not clearly observed in the absorption spectra. Decisive assignment requires further and different nonlinear optical measurements such as electroabsorption and twophoton absorption spectroscopy. 5. Summary We have measured the third-order optical nonlinearity of thin films of a π-conjugated singlet biradical molecule, BDPI-2Y, by THG spectroscopy. The three-photon resonant |χ(3)| value reaches 3.0× 10−11 esu, which is comparable to that of typical π-conjugated materials like poly (3-hexylthiophenes). Its figure of merit, |χ(3)|/α, in which the difference in density of π-electrons is cancelled, is also large in comparison with
Photon Energy (eV) 1
2
3
4
3
3 2 2 1
α (105 cm-1)
|χ(3)| (10-11 esu)
0 4
1
0
0
0.5
1.0
0
Excitation Photon Energy (eV) Fig. 2. Absorption coefficient α (solid line, upper and right axes) and |χ(3)| (dots, lower and left axes) spectra of BDPI-2Y.
Fig. 3. Max |χ(3)| versus peak values of absorption coefficients α for various conjugated systems.
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H. Kishida et al. / Thin Solid Films 519 (2010) 1028–1030
polythiophenes. This strongly suggests that the open-shell electronic states and singlet biradical character enhance third-order optical nonlinearity. Acknowledgement This work was supported by a Grant-in-Aid for Science Research in a Priority Area, “Super-Hierarchical Structures”, from the Ministry of Education, Culture, Sports, Science and Technology, Japan. References [1] C. Sauteret, J.–P. Hermann, R. Frey, F. Pradère, J. Ducing, R.H. Baughman, R.R. Chance, Phys. Rev. Lett. 36 (1976) 956. [2] H.S. Nalwa, Adv. Mater. 5 (1993) 341. [3] F. Meyers, S.R. Marder, B.M. Pierce, J.L. Brédas, J. Am. Chem. Soc. 116 (1994) 10703. [4] M. Nakano, T. Nitta, K. Yamaguchi, B. Champagne, E. Botek, J. Phys. Chem. A 108 (2004) 4105. [5] M. Nakano, R. Kishi, T. Nitta, T. Kubo, K. Nakasuji, K. Kamada, K. Ohta, B. Champagne, E. Botek, K. Yamaguchi, J. Phys. Chem. A 109 (2005) 885. [6] M. Nakano, R. Kishi, N. Nakagawa, S. Ohta, H. Takahashi, S. Furukawa, K. Kamada, K. Ohta, B. Champagne, E. Botek, S. Yamada, K. Yamaguchi, J. Phys. Chem. A 110 (2006) 4238. [7] S. Ohta, M. Nakano, T. Kubo, K. Kamada, K. Ohta, R. Kishi, N. Nakagawa, B. Champagne, E. Botek, A. Takebe, S. Umezaki, M. Nate, H. Takahashi, S. Furukawa, Y. Morita, K. Nakasuji, K. Yamaguchi, J. Phys. Chem. A 111 (2007) 3633.
[8] M. Nakano, R. Kishi, S. Ohta, H. Takahashi, T. Kubo, K. Kamada, K. Ohta, E. Botek, B. Champagne, Phys. Rev. Lett. 99 (2007) 033001. [9] K. Kamada, K. Ohta, T. Kubo, A. Shimizu, Y. Morita, K. Nakasuji, R. Kishi, S. Ohta, S. Furukawa, H. Takahashi, M. Nakano, Angew. Chem. Int. Ed. 46 (2007) 3544. [10] U. Mayer, H. Baumgärtel, H. Zimmermann, Angew. Chem. Int. Ed. 5 (1966) 311. [11] A. Kikuchi, F. Iwahori, J. Abe, J. Am. Chem. Soc. 126 (2004) 6526. [12] A. Kikuchi, H. Ito, J. Abe, J. Phys. Chem. B 109 (2005) 19448. [13] A. Kikuchi, J. Abe, Chem. Lett. 34 (2005) 1552. [14] D. Kato, K. Inoue, J. Akimitsu, J. Abe, Chem. Lett. 37 (2008) 694. [15] H. Kishida, K. Hirota, T. Wakabayashi, H. Okamoto, H. Kokubo, T. Yamamoto, Appl. Phys. Lett. 87 (2005) 121902. [16] F. Kajzar, J. Messier, C. Rosilio, J. Appl. Phys. 60 (1986) 3040. [17] R.C. Miller, Appl. Phys. Lett. 5 (1964) 17. [18] C.G.B. Garrett, F.N.H. Robinson, IEEE J. Quantum Electron. 2 (1966) 328. [19] B. Buchalter, G.R. Meredith, Appl. Opt. 21 (1982) 3221. [20] T. Yamamoto, H. Kokubo, M. Kobashi, Y. Sakai, Chem. Mater. 16 (2004) 4616. [21] T. Morikita, I. Yamaguchi, T. Yamamoto, Adv. Mater. 13 (2001) 1862. [22] H. Kishida, K. Hirota, H. Okamoto, H. Kokubo, T. Yamamoto, Appl. Phys. Lett. 92 (2008) 033309. [23] T. Sato, H. Kishida, A. Nakamura, T. Fukuda, T. Yamamoto, Synth. Met. 157 (2007) 318. [24] T. Yamamoto, T. Sato, T. Iijima, M. Abe, H. Fukumoto, T. Koizumi, M. Usui, Y. Nakamura, T. Yagi, H. Tajima, T. Okada, S. Sasaki, H. Kishida, A. Nakamura, T. Fukuda, A. Emoto, H. Ushijima, C. Kurosaki, H. Hirota, Bull. Chem. Soc. Jpn 82 (2009) 896. [25] P.N. Butcher, D. Cotter, The elements of Nonlinear Optics, Cambridge University Press, Cambridge, 1990.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Third-order nonlinear optical properties of a π-conjugated biradical molecule investigated by third-harmonic generation spectroscopy Hideo Kishida a,⁎, Kenichi Hibino a, Arao Nakamura a, Daisuke Kato b, Jiro Abe b a b
Department of Applied Physics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603 Japan Department of Chemistry, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara 252-5258 Japan
a r t i c l e
i n f o
Available online 20 August 2010 Keywords: Optical nonlinearity Third-harmonic generation Biradical
a b s t r a c t Third-order optical nonlinearity of a biradical molecule with open-shell π-electron system, 1,4-bis- (4,5diphenylimidazole-2-ylidene)-cyclohexa-2,5-diene (BDPI-2Y) was investigated by the third-harmonic generation method. The figure of merit for third-order optical nonlinearity in BDPI-2Y is comparable to those for conjugated polymers, which have elongated closed π-conjugated systems. This indicates that the biradical nature of BDPI-2Y enhances third-order optical nonlinearity. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Absorption measurements
Third-order optical nonlinearity of π-conjugated systems has long been investigated owing to their potential application in photonics and nanotechnology. Most of such studies are devoted to closed-shell systems [1–3]. However, the third-order optical nonlinearity of openshell systems is of considerable interest because some theoretical studies on singlet biradical molecules suggest that such systems can have enhanced optical nonlinearity [4–8]. Recently, exceptionally large two-photon absorption (TPA) cross sections of singlet biradical hydrocarbons in solution were reported by the open-aperture Z-scan method with a femtosecond laser pulse [9]. The open-shell system is an attractive target from the viewpoint of the exploration of third-order nonlinear optical materials, although only a few experimental studies on the open-shell systems exist. The biradical character of 1,4-bis- (4,5-diphenylimidazole-2-ylidene)cyclohexa-2,5-diene (BDPI-2Y) (Fig. 1) [10] was investigated by the theoretical calculation, and the experimental evidence for the contribution from a biradical state was presented by the observation of the radical dimerization reaction for a BDPI-2Y derivative, tF-BDPI2Y, where four hydrogen atoms at the central phenylene ring are substituted with four fluorine atoms of BDPI-2Y [11–13]. Nakano et al. reported the large second hyperpolarizabilities of a simplified analogue of BDPI-2Y by the CCSD(T)/6-31G level of the theory; here, CCSD (T) is the coupled cluster method including single and double excitation operators and a perturbative treatment of triple excitations [6]. In this study, we clarify by third-harmonic generation (THG) spectroscopy that BDPI-2Y shows enhanced optical nonlinearity compared with typical closed-shell conjugated polymers.
A thin film was fabricated onto synthesized quartz substrates by thermal evaporation and a high-quality optical film was obtained. The thickness was 94 nm when measured by a stylus profilometer. Fig. 2 (solid line) shows the absorption spectrum of the thin film. The absorption peak energy is 2.05 eV (605 nm), which is nearly the same as for benzene solution [10]. This absorption band is related to delocalized π-electrons, as evidenced by the observation of a red shift in the absorption peak energy for the polymer consisting of BDPI-2Y [14], in which the π-conjugated systems are elongated. The maximum absorption coefficient in thin films reaches 2.2 × 105 cm− 1, which is large in comparison with fully π-conjugated materials such as polythiophenes. It is thus desirable to compare BDPI-2Y with various conjugated polymers as is shown in Fig. 3. In poly(3-hexylthiophenes), which are typical and widely used conjugated polymers, the absorption coefficient varies from 0.6 × 105 cm− 1 to 1.4 × 105 cm− 1 depending on the head-to-tail coupling ratio r [15]. Even for the most regioregular P3HT (r = 0.985), the absorption coefficient is smaller than for BDPI-2Y. The large absorption coefficient of BDPI-2Y results from full elongation of the π-electrons over the molecule.
⁎ Corresponding author. E-mail address: [email protected] (H. Kishida). 0040-6090/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2010.08.037
3. Nonlinear optical measurements An evaluation of the third-order optical nonlinearity was performed by THG spectroscopy, which gives the magnitude of third-order nonlinear susceptibility, |χ(3)|. Since |χ(3)| values are dependent on the photon energy of the incident laser due to resonant effects, a comparative study among various materials must be performed under the same resonant condition. In this study, |χ(3)| values are evaluated under the three-photon resonant condition, where they show the maximum values. The measuring range is from
H. Kishida et al. / Thin Solid Films 519 (2010) 1028–1030
1029
Fig. 1. Biradical and quinoidal forms of BDPI-2Y.
0.51 − 1.03 eV excitation photon energy. Excitation laser light was generated by a system consisting of Spitfire Pro (Spectra-Physics) and TOPAS (Light Conversion), with a pulse width of 100 fs and a repetition rate of 1 kHz. Measurement of |χ(3)| was performed by the standard Maker fringe method [16], with a SiO2 reference sample [17–19]. To avoid effects of THG in ambient air, the sample was mounted and measured in a vacuum cell. Fig. 2 (dots) shows the THG spectrum. A clear resonant enhancement is observed at 0.65-eV excitation photon energy. This resonant energy is just one-third of the absorption peak energy. Therefore, we can easily assign this enhancement to a three-photon resonance. The value of |χ(3)| reaches a maximum of 3.0 × 10− 11 esu. 4. Discussion Fig. 3 shows the maximum |χ(3)| values versus the maximum absorption coefficient (α) for various conjugated materials. The |χ(3)| value for BDPI-2Y is larger than for P3HTs, but smaller than for artificially designed nonlinear optical materials such as chargetransfer (CT) conjugated polymers like PAE and PThTz [20–22], and highly regioregular head-to-head type polythiophenes (HH-P3 (C ≡ CR)Th) [23,24]. In comparing materials, we should consider the difference in density of the π-electrons. |χ(3)| and α are both linearly proportional to the density of the molecules and π-electrons [25]. Therefore, the ratio of |χ(3)| to α, |χ(3)|/α, in which the difference in density is cancelled, gives a good measure of optical nonlinearity inherent to each π-electron system. Indeed, this quantity is called as a “figure of merit” for optical nonlinearity. In Fig. 3, |χ(3)|/α is the slope from the origin to the sample point. |χ(3)|/α for BDPI-2Y is 1.37 × 10− 16 cm esu, which is larger than for regiorandom P3HT (0.28 − 0.87 × 10− 16 cm esu; r = 0 − 0.80) but smaller than for P3HT (1.97 × 10− 16 cm esu). Considering that the π-conjugated system is smaller in BDPI-2Y than in conjugated polymers, |χ(3)|/α for BDPI-2Y is quite large. Thus, some enhancement effect characteristic of open-shell electronic systems is evident. Theoretical studies suggested an enhancement effect in optical
nonlinearity in singlet biradical states [6], where the charge distribution is favorable to third-order nonlinear optical processes. Thus, the theoretical expectations and experimental results are compatible, although the theoretical study considers static nonlinearity, namely, the off-resonant condition where the photon energy ħω = 0. The enhancement due to the biradical character under the resonant condition is still theoretically unclear. The THG peak is broader than the absorption peak in the higherenergy side. This broadening in the THG spectrum may be ascribed to the three-photon resonance to higher-lying excited states, because some resonant shoulder structure is often observed in the higherenergy side of the THG peak. In nonlinear optical measurements, higher-lying states are often sensitively observed because nonlinear optical processes include not only the transition from the ground state but also transitions between excited states. In contrast, the absorption process includes only the transition from the ground state. The transition dipole moment between the higher-lying state and the other excited states, which is included in the THG process, can be large, while that between the higher-lying state and the ground state is relatively small. As a result, THG measurements can be sensitive to higher-lying excited states, while they are not clearly observed in the absorption spectra. Decisive assignment requires further and different nonlinear optical measurements such as electroabsorption and twophoton absorption spectroscopy. 5. Summary We have measured the third-order optical nonlinearity of thin films of a π-conjugated singlet biradical molecule, BDPI-2Y, by THG spectroscopy. The three-photon resonant |χ(3)| value reaches 3.0× 10−11 esu, which is comparable to that of typical π-conjugated materials like poly (3-hexylthiophenes). Its figure of merit, |χ(3)|/α, in which the difference in density of π-electrons is cancelled, is also large in comparison with
Photon Energy (eV) 1
2
3
4
3
3 2 2 1
α (105 cm-1)
|χ(3)| (10-11 esu)
0 4
1
0
0
0.5
1.0
0
Excitation Photon Energy (eV) Fig. 2. Absorption coefficient α (solid line, upper and right axes) and |χ(3)| (dots, lower and left axes) spectra of BDPI-2Y.
Fig. 3. Max |χ(3)| versus peak values of absorption coefficients α for various conjugated systems.
1030
H. Kishida et al. / Thin Solid Films 519 (2010) 1028–1030
polythiophenes. This strongly suggests that the open-shell electronic states and singlet biradical character enhance third-order optical nonlinearity. Acknowledgement This work was supported by a Grant-in-Aid for Science Research in a Priority Area, “Super-Hierarchical Structures”, from the Ministry of Education, Culture, Sports, Science and Technology, Japan. References [1] C. Sauteret, J.–P. Hermann, R. Frey, F. Pradère, J. Ducing, R.H. Baughman, R.R. Chance, Phys. Rev. Lett. 36 (1976) 956. [2] H.S. Nalwa, Adv. Mater. 5 (1993) 341. [3] F. Meyers, S.R. Marder, B.M. Pierce, J.L. Brédas, J. Am. Chem. Soc. 116 (1994) 10703. [4] M. Nakano, T. Nitta, K. Yamaguchi, B. Champagne, E. Botek, J. Phys. Chem. A 108 (2004) 4105. [5] M. Nakano, R. Kishi, T. Nitta, T. Kubo, K. Nakasuji, K. Kamada, K. Ohta, B. Champagne, E. Botek, K. Yamaguchi, J. Phys. Chem. A 109 (2005) 885. [6] M. Nakano, R. Kishi, N. Nakagawa, S. Ohta, H. Takahashi, S. Furukawa, K. Kamada, K. Ohta, B. Champagne, E. Botek, S. Yamada, K. Yamaguchi, J. Phys. Chem. A 110 (2006) 4238. [7] S. Ohta, M. Nakano, T. Kubo, K. Kamada, K. Ohta, R. Kishi, N. Nakagawa, B. Champagne, E. Botek, A. Takebe, S. Umezaki, M. Nate, H. Takahashi, S. Furukawa, Y. Morita, K. Nakasuji, K. Yamaguchi, J. Phys. Chem. A 111 (2007) 3633.
[8] M. Nakano, R. Kishi, S. Ohta, H. Takahashi, T. Kubo, K. Kamada, K. Ohta, E. Botek, B. Champagne, Phys. Rev. Lett. 99 (2007) 033001. [9] K. Kamada, K. Ohta, T. Kubo, A. Shimizu, Y. Morita, K. Nakasuji, R. Kishi, S. Ohta, S. Furukawa, H. Takahashi, M. Nakano, Angew. Chem. Int. Ed. 46 (2007) 3544. [10] U. Mayer, H. Baumgärtel, H. Zimmermann, Angew. Chem. Int. Ed. 5 (1966) 311. [11] A. Kikuchi, F. Iwahori, J. Abe, J. Am. Chem. Soc. 126 (2004) 6526. [12] A. Kikuchi, H. Ito, J. Abe, J. Phys. Chem. B 109 (2005) 19448. [13] A. Kikuchi, J. Abe, Chem. Lett. 34 (2005) 1552. [14] D. Kato, K. Inoue, J. Akimitsu, J. Abe, Chem. Lett. 37 (2008) 694. [15] H. Kishida, K. Hirota, T. Wakabayashi, H. Okamoto, H. Kokubo, T. Yamamoto, Appl. Phys. Lett. 87 (2005) 121902. [16] F. Kajzar, J. Messier, C. Rosilio, J. Appl. Phys. 60 (1986) 3040. [17] R.C. Miller, Appl. Phys. Lett. 5 (1964) 17. [18] C.G.B. Garrett, F.N.H. Robinson, IEEE J. Quantum Electron. 2 (1966) 328. [19] B. Buchalter, G.R. Meredith, Appl. Opt. 21 (1982) 3221. [20] T. Yamamoto, H. Kokubo, M. Kobashi, Y. Sakai, Chem. Mater. 16 (2004) 4616. [21] T. Morikita, I. Yamaguchi, T. Yamamoto, Adv. Mater. 13 (2001) 1862. [22] H. Kishida, K. Hirota, H. Okamoto, H. Kokubo, T. Yamamoto, Appl. Phys. Lett. 92 (2008) 033309. [23] T. Sato, H. Kishida, A. Nakamura, T. Fukuda, T. Yamamoto, Synth. Met. 157 (2007) 318. [24] T. Yamamoto, T. Sato, T. Iijima, M. Abe, H. Fukumoto, T. Koizumi, M. Usui, Y. Nakamura, T. Yagi, H. Tajima, T. Okada, S. Sasaki, H. Kishida, A. Nakamura, T. Fukuda, A. Emoto, H. Ushijima, C. Kurosaki, H. Hirota, Bull. Chem. Soc. Jpn 82 (2009) 896. [25] P.N. Butcher, D. Cotter, The elements of Nonlinear Optics, Cambridge University Press, Cambridge, 1990.