- Email: [email protected]
Nonlinear optical properties in three novel nanocomposites with gold nanoparticles
1 September 2001
Optics Communications 196 (2001) 317±323
www.elsevier.com/locate/optcom
Nonlinear optical properties in three novel nanocomposites with gold nanoparticles Shiliang Qu a,*, Yinglin Song a,*, Chimin Du b, Yuxiao Wang a, Yachen Gao a, Shutian Liu a, Yuliang Li b,*, Daoben Zhu b a
Department of Physics, Harbin Institute of Technology, Harbin 150001, China Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China
b
Received 15 March 2001; received in revised form 14 June 2001; accepted 14 June 2001
Abstract Nonlinear optical properties of three novel nanocomposites with zerovalent noble metal gold nanoparticles were investigated by using Z-scan technique. Optical limiting eects has been measured with 8 ns pulses at 532 nm. The crosssections of nonlinear absorption were obtained by the simulation with a simpli®ed model in which the eective excitedstate absorptions of three ligands in nanocomposites were considered. The nonlinear refractive indices were calculated from the data of Z-scan measurement. The experimental results were found to be signi®cantly dierent in these nanocomposites. Optical nonlinearities in these nanocomposites can be attributed to the strong excited-state absorptions of the ligands and the surface plasmon resonance of gold nanoparticles. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Nonlinear refraction; Optical limiting; Surface plasmon resonance; Nanocomposite
1. Introduction The physical and chemical properties of nanocomposites have attracted much attention for their promising applications in electronic and photonic technologies. Carbon nanotubes, a typical nanocomposite, have been extensively investigated in various kinds of physical processes. As one of nonlinear optical eects, for instance, optical limi-
*
Corresponding authors. E-mail addresses: [email protected] [email protected] (Y. Li).
(Y.
Song),
ting (OL) eect in both single-walled and multiwalled carbon nanotubes has been explored [1±4]. The OL behavior in carbon nanotubes is similar to that of fullerene C60 solution in toluene and carbon black aqueous suspension. Mechanistically, OL behaviors are generally considered to result from reverse saturable absorptions and subsequently induced nonlinear scatterings. Silver colloid nanoparticles and gold nanoparticle solution [5,6] have also been investigated for such nonlinear optical eect. The bleaching of the plasmon band, which depends on the particle size, was observed. The size-dependent spectral change can be attributed to the reduction of the density. The rapid progress in the synthetic chemistry of organic
0030-4018/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 ( 0 1 ) 0 1 3 7 5 - X
318
S. Qu et al. / Optics Communications 196 (2001) 317±323
dendrimers has also led to the fabrication of metaldendrimer nanocomposites. It has been demonstrated that the zerovalent transition metals can be encapsulated inside polydendrimers [7,8]. Large OL eects in silver-dendrimer nanocomposite were reported. The nanocomposite displays a drastic optical extinction, by a factor of 115, which arises from nonlinear scattering [9]. In spite of above reports, the investigation of optical nonlinearities in these nanostructured materials is relatively little. In particular, in the nanocomposites with zerovalent transition metal nanoparticles. In this paper we report, for the ®rst time to our knowledge, the nonlinear optical responses of the three novel nanocomposites composed of gold nanoparticles and the ligands based on C60 . The cross-sections of nonlinear absorption and the nonlinear refractive indices of these materials have been calculated from a simple model with the experimental data of Z-scan measurement. We have also observed strong OL behaviors in these nanocomposites. 2. Materials and experiments The three materials measured in our experiments are nanocomposites with zerovalent noble metal gold nanoparticles protected by the ligands. The ligands in three nanocomposites are all based on fullerene C60 but, with the dierent substituent groups of terpyridine, bipyridines, denoted as C60 tpy, C60 bpy-1 and C60 bpy-2, respectively. Their molecular structures are illustrated in Fig. 1. The average size of the gold nanoparticle domains in three nanocomposites were estimated by transmission electron microscopy to be of the order of 5±15 nm. Here, the three nanocomposites are also denoted as C60 tpy±Au and C60 bpy-1±Au and C60 bpy-2±Au for convenience, respectively. All the three samples are dissolved in chloroform. Their linear absorption spectra are shown in Fig. 2. The nonlinear responses of the three nanocomposite chloroform solutions were measured with Nd:YAG laser, which produces 8 ns (FWHM) laser pulses at 532 nm with a repetition rate of 1 Hz. The spacial pro®le of the pulses is nearly Gaussian. Z-scan experiments, similar to that in
the literature [10], were performed. The pulse energy of 150 lJ is used in the experiment. All the three samples in chloroform were placed in 1 mm thick quartz cells. In our OL experiments, the C60 tpy±Au and C60 bpy-2±Au chloroform solutions in 2 mm thick quartz cells were placed at focus of a lens with focal length of 30 cm, while C60 bpy-1±Au solution in the same cell was placed at the position of the valley of the normalized transmittances in Z-scan experiment. Both the incident and transmitted laser pulses were monitored simultaneously by using two energy detectors (Rjp-735 energy probes, Laser Precision), respectively. 3. Results and discussion 3.1. Nonlinear absorption To determine the nonlinear absorption, we performed Z-scan experiments without an aperture. The measured normalized energy transmissions are shown in Fig. 3. We ®nd that all the three samples display strong nonlinear absorptions. Apparently, C60 bpy-2±Au, C60 bpy-1±Au and C60 tpy± Au increase in turn in the nonlinear absorptions due to the corresponding decreasing in valley values of the normalized transmissions which are 0.48, 0.38 and 0.28 for the three samples, respectively. The physical origin of the light absorption by metallic nanoparticles is the coherent oscillation of the conduction band electrons induced by the interacting electromagnetic ®eld. These resonances are known as surface plasmons and are indeed a small particle eect, since they are absent in the individual atoms as well as in the bulk. While the density of states and the spatial length scale of the electronic motion are reduced with decreasing size [11,12]. The energy eigenstates are now determined by the system's boundaries, and therefore, surface eects become very important [13]. The plasmon absorption of metal nanoparticles is size dependent and related to the number density of the particles [14]. From Fig. 2, we can observe that both C60 bpy1±Au and C60 bpy-2±Au exhibit the surface plas-
S. Qu et al. / Optics Communications 196 (2001) 317±323
319
Fig. 1. Molecular structure schemes of the three ligands: (a) C60 tpy, (b) C60 bpy-1 and (c) C60 bpy-2.
mon resonance peaks in the visible region at around 570 and 560 nm, respectively. C60 tpy±Au does not exhibit an absorption peak in visible spectrum, but rather a shoulder is found in the range of 500±600 nm. The wavelength of 532 nm used in our experiments is within their surface plasmon absorption spectral range, which implies that C60 bpy-1±Au and C60 bpy-2±Au may possess relatively strong surface plasmon absorptions, as compared to C60 tpy±Au at 532 nm. We believe that the smaller size and the lower number density
of the gold particles are responsible for the relatively weak surface plasmon absorption in C60 tpy± Au. Besides, the delocalization of p electrons has been shown to be a major contributor to the large o-resonant third-order optical susceptibility [15± 18]. Similar to C60 , the ligands based on C60 also possess a highly delocalized p conjugated electron system. The nonlinear optical responses of these samples are probably associated with the excitedstate population of the ligands. Therefore, we can consider that the strong nonlinear absorptions
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Fig. 2. The linear absorption spectra of C60 tpy±Au, C60 bpy1±Au and C60 bpy-2±Au chloroform solutions.
result from the excited-state absorption of the three ligands and the surface plasmon absorption of the gold nanoparticles in the three nanocomposites. However, the former is relatively much strong compared with the latter. This is because C60 tpy±Au displays the stronger nonlinear absorption as mentioned above, despite its relatively weak surface plasmon absorption. According to the above analysis, we assume that the nonlinear absorption mainly arises from the excited-state absorption including the twofold contribution of the singlet and triplet excitedstates in the three ligands, C60 tpy, C60 bpy-1 and C60 bpy-2. Indeed, the excited-state absorption is in¯uenced by the surface plasmon absorption. But any theory that can handle these materials is not currently available, requiring further theoretical studies. At the present, it may be sucient to use the eective excited-state absorption theory to describe the nonlinear absorption while the surface plasmon absorption is relatively weak in these nanocomposites. Thus, the equations governing the light absorption are [19] dI I
a0 reff Nex dz0
dNex a0 I hm dt
1 2
Fig. 3. The normalized energy transmissions of Z-scan experiments without an aperture for three samples with a linear transmission of 87%. Solid curves are calculated results: (a) C60 tpy±Au in chloroform, (b) C60 bpy-2±Au in chloroform and (c) C60 bpy-1±Au in chloroform.
where z0 is the depth in the sample, I the laser intensity, a0 the linear absorption coecient, reff the eective excited-state absorption cross-section, Nex
S. Qu et al. / Optics Communications 196 (2001) 317±323
321
the number density of charges in the excited-states, and hm the photon energy. Eqs. (1) and (2) can be combined to yield dF dz0
a0 F
a0 reff 2 F 2hm
3
Solving this equation for the ¯uence F (energy per unit area) and integrating over the spatial extent of the beam, the normalized energy transmission Tnorm can be written as q0 q0 Tnorm ln 1 4 1 x2 1 x2 where q0
reff a0 F0 Leff ; 2 hm
Leff
1
e a0
a0 L
with L the sample length, F0 e= px20 =2 on-axis ¯uence, e is the incident pulse energy, x z=z0 , z0 px20 =k, x0 is the radius of the waist spot at the focus, z is the distance of the sample from the focus. The solid curve in Fig. 3 is the best ®t of the open aperture Z-scan data by Eq. (4), which yields the eective excited-state absorption cross-sections of 3:06 10 17 , 1:16 10 17 and 0:93 10 17 cm2 for C60 tpy±Au, C60 bpy-1±Au and C60 bpy-2±Au, respectively. These obtained values indicate that the ligands, C60 tpy, C60 bpy-1 and C60 bpy-2, possess the distinct dierent excited-state absorption behaviors. We think that p electronic orbits in C60 molecules can be aected by these substituent groups, terpyridine, bipyridines, joined up with C60 . Thus, the size of molecular orbits is changed. Terpyridine could increase the larger delocalization of p electron systems than bipyridines. So C60 tpy shows the stronger excited-state absorption than C60 bpy-1 and C60 bpy-2. 3.2. Nonlinear refraction The nonlinear refraction can be measured from the normalized energy transmission of Z-scan with an aperture (linear transmission of S 0:1). The pure nonlinear refractive results by Z-scan data with an aperture divided by that without an aperture are shown in Fig. 4. Apparently, the nonlinear refractive indices of the three samples are
Fig. 4. The pure refractive normalized energy transmissions by Z-scan data with an aperture S 0:10 divided by that without an aperture for three samples with a linear transmission of 87%: (a) C60 tpy±Au in chloroform, (b) C60 bpy-2±Au in chloroform and (c) C60 bpy-1±Au in chloroform.
positive. All of them exhibit the strong self-focusing eects due to the large peak±valley dierence
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values. C60 bpy-1±Au is the strongest, C60 tpy±Au is relatively weak and C60 bpy-2±Au is the weakest in the nonlinear refraction from the peak±valley difference values of 0.55, 0.34 and 0.29 for the above three samples, respectively. C60 and its derivatives generally possess the relatively weak excited-state refraction (weak selfdefocusing eects) though they display the strong excited-state absorption in visible region [18,20,21]. These ligands in three nanocomposites, similar to C60 derivatives, could also possess the weak self-defocusing behaviors. However, the strong nonlinear refractions (self-focusing eects) are observed in three samples from Fig. 4, for which the surface plasmon resonance due to light excitation in the noble metal gold nanoparticles could be responsible. We ®nd that the smaller peak and larger valley occur in Fig. 4(a), which could arise from the nonlinear scattering in C60 tpy±Au chloroform solution. In this case, the stronger nonlinear absorption may induce the formation of scattering centers due to local heating of chloroform solution at focus, most probably microbubbles, when close to boiling point temperatures. Indeed, the time needed for heat transfer from the metal to the solvent and the development of the bubbles around the particles is usually 100±200 ps [22]. It should be emphasized that the nonlinear absorption measured in Section 3.1 includes the strong absorption-induced nonlinear scattering for C60 tpy±Au chloroform solution. Thus, the eective absorption cross-section of 3:06 10 17 cm2 obtained is rough. It is indeed an extinction crosssection. The approximate symmetry in Fig. 4(b) and (c) implies no distinct nonlinear scattering appearing in C60 bpy-1±Au and C60 bpy-2±Au, which is relevant for the relatively weak nonlinear absorptions in comparison with that of C60 tpy±Au. Based on the above analysis, we mainly consider the contribution of the surface plasmon to the nonlinear refractions in three samples, and use the approximate solution of Z-scan theory to calculate the nonlinear refractive indices of the three samples by the following equations [23], despite the probable in¯uencing of nonlinear scattering on the nonlinear refraction in C60 tpy±Au.
DTP±V 0:406 1
S0:25 jDU0 j;
DU0 2p=kn2 I0 Leff and Leff 1 exp a0 L=a0
5
where DTP±V is the dierence between the peak and the valley of the normalized transmittances, I0 the pulse irradiance, n2 the nonlinear refractive index, and Leff the eective length of the sample. The value of DTP±V was taken from Fig. 4. Applying Eq. (5), we obtain n2 1:11 10 4 , 1:30 10 4 and 2:11 10 4 cm2 /GW for C60 bpy-2±Au, C60 tpy± Au and C60 bpy-1±Au, respectively. Direct comparison between those data is not so meaningful because the number density of gold nanoparticles in the nanocomposites is unknown. Our results may show that the nonlinear refractions of these nanocomposites result from the interband transition, to the large extent, due to the surface plasmon resonance. The dierent n2 can be due to the dierences of the number density and sizes of gold particles. As well known, the surface plasmon resonance can be strengthened as the number density and the size of metal nanoparticles increase in the nanocomposites [14]. In this case, the nonlinear scattering can be induced by the strong excited-state absorption of the ligands based on C60 and the strong surface plasmon absorption of metal nanoparticles in the nanocomposites. Therefore, the optical nonlinearities could be much enhanced. 3.3. Optical limiting We now investigate the OL properties in these three nanocomposites due to the strong optical nonlinearities of three samples. For utilizing adequately the nonlinear properties in these materials, we performed the refractive OL experiment of C60 bpy-1±Au which, namely, was placed in the position of the valley of the normalized transmittances in the Z-scan experiment. While C60 tpy±Au and C60 bpy-2±Au were still placed at the focus of the lens, as usual. As for comparison, we also measured the OL properties of C60 in toluene which was also placed at the focus for its strong excited-state absorption. The OL results of these samples with an identical linear transmission of
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the relatively weak OL properties, for which the substituent group may be accountable.
Acknowledgements This work is supported by Organic Solid Laboratory of Chinese Academy of Sciences.
References Fig. 5. The OL experimental results of the three nanocomposites chloroform solutions and C60 toluene solution with an identical linear transmission of 81%.
81% are shown in Fig. 5. Apparently, the OL properties of C60 tpy±Au and C60 bpy-1±Au are stronger than that of C60 , but C60 bpy-2±Au is weaker. We think that the stronger excited-state absorption of the ligand, C60 tpy, and subsequently induced nonlinear scattering in C60 tpy±Au solution are responsible for the stronger OL behavior. While the OL performances of C60 bpy-1±Au is enhanced mainly by the surface plasmon resonance. 4. Conclusions We investigated the optical nonlinearities of three nanocomposites with noble metal gold nanoparticles by Z-scan technique and OL experiments. Both eective nonlinear absorptive cross-sections and nonlinear refractive indices were obtained. The results show the signi®cant dependence of the nonlinear absorptions of these nanocomposites on the ligands. While the surface plasmon resonance could mainly be responsible for the stronger selffocusing eects. We also observe the stronger OL performances of C60 tpy±Au and C60 bpy-1±Au than that of C60 in toluene. The former originates from the stronger nonlinear absorption and subsequently induced nonlinear scattering, the latter from the stronger nonlinear absorption and the surface plasmon resonance. C60 bpy-2±Au shows
[1] L. Vivien, E. Anglaret, D. Riehl, et al., Opt. Commun. 174 (2000) 271. [2] S.R. Mishra, H.S. Rawat, S.C. Mehendale, et al., Chem. Phys. Lett. 317 (2000) 510. [3] J.E. Riggs, D.B. Walker, D.L. Carroll, Y.-P. Sun, J. Phys. Chem. B 104 (2000) 7071. [4] Z.X. Jin, X. Sun, G.Q. Xu, S.H. Goh, W. Ji, Chem. Phys. Lett. 318 (2000) 505. [5] Y.P. Sun, J.E. Riggs, H.W. Rollins, R.J. Guduru, J. Phys. Chem. B 103 (1999) 77. [6] L. Francois, M. Mostafavi, J. Belloni, J. Phys. Chem. B 104 (2000) 6133. [7] L. Balogh, K.S. Laverdure, S.P. Gido, et al., Mater. Res. Soc. Symp. Proc. 756 (1999) 69. [8] J.A. He, R. Valluzzi, K. Yang, et al., Chem. Mater. 11 (1999) 3268. [9] R.G. Ispasoiu, L. Balogh, O.P. Varnavski, D.A. Tomalia, T. Goodson III, J. Am. Chem. Soc. 122 (2000) 11005. [10] M. Sheik-Bahae, A.A. Said, E.W. Van Stryland, IEEE J. Quant. Electron. 26 (1990) 760. [11] A. Henglein, J. Phys. Chem. 97 (1993) 8457. [12] A. Henglein, Chem. Rev. 89 (1989) 1861. [13] A.P. Alivisatos, J. Phys. Chem. 100 (1996) 13226. [14] S. Link, M.A. El-Sayed, J. Phys. Chem. B 103 (1999) 8410. [15] V.V. Golovlev, W.T. Garrett, C.H. Chen, J. Opt. Soc. Am. B 13 (1996) 2801. [16] Y.P. Sun, J.E. Riggs, B. Liu, Chem. Mater. 9 (1997) 1268. [17] Y.P. Sun, B. Ma, C.E. Bunker, J. Phys. Chem. A 102 (1998) 7580. [18] J.R. He¯in, K.Y. Wong, O. Zamani-Khamiri, A.F. Garito, Phys. Rev. B 38 (1988) 1573. [19] G.L. Wood, M.J. Miller, A.G. Mott, Opt. Lett. 20 (1995) 973. [20] T.Q. Zhang, J.L. Li, P. Gao, et al., Opt. Commun. 150 (1998) 201. [21] B. Ma, C.E. Bunker, R. Guduru, et al., J. Phys. Chem. A 101 (1997) 5625. [22] J.H. Hodak, A. Henglein, M. Giersig, G.V. Hartland, J. Phys. Chem. B 104 (2000) 11708. [23] M. Sheik-Bahae, A.A. Said, E.W. Van Stryland, IEEE J. Quant. Electron. 26 (1990) 760.
Optics Communications 196 (2001) 317±323
www.elsevier.com/locate/optcom
Nonlinear optical properties in three novel nanocomposites with gold nanoparticles Shiliang Qu a,*, Yinglin Song a,*, Chimin Du b, Yuxiao Wang a, Yachen Gao a, Shutian Liu a, Yuliang Li b,*, Daoben Zhu b a
Department of Physics, Harbin Institute of Technology, Harbin 150001, China Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China
b
Received 15 March 2001; received in revised form 14 June 2001; accepted 14 June 2001
Abstract Nonlinear optical properties of three novel nanocomposites with zerovalent noble metal gold nanoparticles were investigated by using Z-scan technique. Optical limiting eects has been measured with 8 ns pulses at 532 nm. The crosssections of nonlinear absorption were obtained by the simulation with a simpli®ed model in which the eective excitedstate absorptions of three ligands in nanocomposites were considered. The nonlinear refractive indices were calculated from the data of Z-scan measurement. The experimental results were found to be signi®cantly dierent in these nanocomposites. Optical nonlinearities in these nanocomposites can be attributed to the strong excited-state absorptions of the ligands and the surface plasmon resonance of gold nanoparticles. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Nonlinear refraction; Optical limiting; Surface plasmon resonance; Nanocomposite
1. Introduction The physical and chemical properties of nanocomposites have attracted much attention for their promising applications in electronic and photonic technologies. Carbon nanotubes, a typical nanocomposite, have been extensively investigated in various kinds of physical processes. As one of nonlinear optical eects, for instance, optical limi-
*
Corresponding authors. E-mail addresses: [email protected] [email protected] (Y. Li).
(Y.
Song),
ting (OL) eect in both single-walled and multiwalled carbon nanotubes has been explored [1±4]. The OL behavior in carbon nanotubes is similar to that of fullerene C60 solution in toluene and carbon black aqueous suspension. Mechanistically, OL behaviors are generally considered to result from reverse saturable absorptions and subsequently induced nonlinear scatterings. Silver colloid nanoparticles and gold nanoparticle solution [5,6] have also been investigated for such nonlinear optical eect. The bleaching of the plasmon band, which depends on the particle size, was observed. The size-dependent spectral change can be attributed to the reduction of the density. The rapid progress in the synthetic chemistry of organic
0030-4018/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 0 - 4 0 1 8 ( 0 1 ) 0 1 3 7 5 - X
318
S. Qu et al. / Optics Communications 196 (2001) 317±323
dendrimers has also led to the fabrication of metaldendrimer nanocomposites. It has been demonstrated that the zerovalent transition metals can be encapsulated inside polydendrimers [7,8]. Large OL eects in silver-dendrimer nanocomposite were reported. The nanocomposite displays a drastic optical extinction, by a factor of 115, which arises from nonlinear scattering [9]. In spite of above reports, the investigation of optical nonlinearities in these nanostructured materials is relatively little. In particular, in the nanocomposites with zerovalent transition metal nanoparticles. In this paper we report, for the ®rst time to our knowledge, the nonlinear optical responses of the three novel nanocomposites composed of gold nanoparticles and the ligands based on C60 . The cross-sections of nonlinear absorption and the nonlinear refractive indices of these materials have been calculated from a simple model with the experimental data of Z-scan measurement. We have also observed strong OL behaviors in these nanocomposites. 2. Materials and experiments The three materials measured in our experiments are nanocomposites with zerovalent noble metal gold nanoparticles protected by the ligands. The ligands in three nanocomposites are all based on fullerene C60 but, with the dierent substituent groups of terpyridine, bipyridines, denoted as C60 tpy, C60 bpy-1 and C60 bpy-2, respectively. Their molecular structures are illustrated in Fig. 1. The average size of the gold nanoparticle domains in three nanocomposites were estimated by transmission electron microscopy to be of the order of 5±15 nm. Here, the three nanocomposites are also denoted as C60 tpy±Au and C60 bpy-1±Au and C60 bpy-2±Au for convenience, respectively. All the three samples are dissolved in chloroform. Their linear absorption spectra are shown in Fig. 2. The nonlinear responses of the three nanocomposite chloroform solutions were measured with Nd:YAG laser, which produces 8 ns (FWHM) laser pulses at 532 nm with a repetition rate of 1 Hz. The spacial pro®le of the pulses is nearly Gaussian. Z-scan experiments, similar to that in
the literature [10], were performed. The pulse energy of 150 lJ is used in the experiment. All the three samples in chloroform were placed in 1 mm thick quartz cells. In our OL experiments, the C60 tpy±Au and C60 bpy-2±Au chloroform solutions in 2 mm thick quartz cells were placed at focus of a lens with focal length of 30 cm, while C60 bpy-1±Au solution in the same cell was placed at the position of the valley of the normalized transmittances in Z-scan experiment. Both the incident and transmitted laser pulses were monitored simultaneously by using two energy detectors (Rjp-735 energy probes, Laser Precision), respectively. 3. Results and discussion 3.1. Nonlinear absorption To determine the nonlinear absorption, we performed Z-scan experiments without an aperture. The measured normalized energy transmissions are shown in Fig. 3. We ®nd that all the three samples display strong nonlinear absorptions. Apparently, C60 bpy-2±Au, C60 bpy-1±Au and C60 tpy± Au increase in turn in the nonlinear absorptions due to the corresponding decreasing in valley values of the normalized transmissions which are 0.48, 0.38 and 0.28 for the three samples, respectively. The physical origin of the light absorption by metallic nanoparticles is the coherent oscillation of the conduction band electrons induced by the interacting electromagnetic ®eld. These resonances are known as surface plasmons and are indeed a small particle eect, since they are absent in the individual atoms as well as in the bulk. While the density of states and the spatial length scale of the electronic motion are reduced with decreasing size [11,12]. The energy eigenstates are now determined by the system's boundaries, and therefore, surface eects become very important [13]. The plasmon absorption of metal nanoparticles is size dependent and related to the number density of the particles [14]. From Fig. 2, we can observe that both C60 bpy1±Au and C60 bpy-2±Au exhibit the surface plas-
S. Qu et al. / Optics Communications 196 (2001) 317±323
319
Fig. 1. Molecular structure schemes of the three ligands: (a) C60 tpy, (b) C60 bpy-1 and (c) C60 bpy-2.
mon resonance peaks in the visible region at around 570 and 560 nm, respectively. C60 tpy±Au does not exhibit an absorption peak in visible spectrum, but rather a shoulder is found in the range of 500±600 nm. The wavelength of 532 nm used in our experiments is within their surface plasmon absorption spectral range, which implies that C60 bpy-1±Au and C60 bpy-2±Au may possess relatively strong surface plasmon absorptions, as compared to C60 tpy±Au at 532 nm. We believe that the smaller size and the lower number density
of the gold particles are responsible for the relatively weak surface plasmon absorption in C60 tpy± Au. Besides, the delocalization of p electrons has been shown to be a major contributor to the large o-resonant third-order optical susceptibility [15± 18]. Similar to C60 , the ligands based on C60 also possess a highly delocalized p conjugated electron system. The nonlinear optical responses of these samples are probably associated with the excitedstate population of the ligands. Therefore, we can consider that the strong nonlinear absorptions
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Fig. 2. The linear absorption spectra of C60 tpy±Au, C60 bpy1±Au and C60 bpy-2±Au chloroform solutions.
result from the excited-state absorption of the three ligands and the surface plasmon absorption of the gold nanoparticles in the three nanocomposites. However, the former is relatively much strong compared with the latter. This is because C60 tpy±Au displays the stronger nonlinear absorption as mentioned above, despite its relatively weak surface plasmon absorption. According to the above analysis, we assume that the nonlinear absorption mainly arises from the excited-state absorption including the twofold contribution of the singlet and triplet excitedstates in the three ligands, C60 tpy, C60 bpy-1 and C60 bpy-2. Indeed, the excited-state absorption is in¯uenced by the surface plasmon absorption. But any theory that can handle these materials is not currently available, requiring further theoretical studies. At the present, it may be sucient to use the eective excited-state absorption theory to describe the nonlinear absorption while the surface plasmon absorption is relatively weak in these nanocomposites. Thus, the equations governing the light absorption are [19] dI I
a0 reff Nex dz0
dNex a0 I hm dt
1 2
Fig. 3. The normalized energy transmissions of Z-scan experiments without an aperture for three samples with a linear transmission of 87%. Solid curves are calculated results: (a) C60 tpy±Au in chloroform, (b) C60 bpy-2±Au in chloroform and (c) C60 bpy-1±Au in chloroform.
where z0 is the depth in the sample, I the laser intensity, a0 the linear absorption coecient, reff the eective excited-state absorption cross-section, Nex
S. Qu et al. / Optics Communications 196 (2001) 317±323
321
the number density of charges in the excited-states, and hm the photon energy. Eqs. (1) and (2) can be combined to yield dF dz0
a0 F
a0 reff 2 F 2hm
3
Solving this equation for the ¯uence F (energy per unit area) and integrating over the spatial extent of the beam, the normalized energy transmission Tnorm can be written as q0 q0 Tnorm ln 1 4 1 x2 1 x2 where q0
reff a0 F0 Leff ; 2 hm
Leff
1
e a0
a0 L
with L the sample length, F0 e= px20 =2 on-axis ¯uence, e is the incident pulse energy, x z=z0 , z0 px20 =k, x0 is the radius of the waist spot at the focus, z is the distance of the sample from the focus. The solid curve in Fig. 3 is the best ®t of the open aperture Z-scan data by Eq. (4), which yields the eective excited-state absorption cross-sections of 3:06 10 17 , 1:16 10 17 and 0:93 10 17 cm2 for C60 tpy±Au, C60 bpy-1±Au and C60 bpy-2±Au, respectively. These obtained values indicate that the ligands, C60 tpy, C60 bpy-1 and C60 bpy-2, possess the distinct dierent excited-state absorption behaviors. We think that p electronic orbits in C60 molecules can be aected by these substituent groups, terpyridine, bipyridines, joined up with C60 . Thus, the size of molecular orbits is changed. Terpyridine could increase the larger delocalization of p electron systems than bipyridines. So C60 tpy shows the stronger excited-state absorption than C60 bpy-1 and C60 bpy-2. 3.2. Nonlinear refraction The nonlinear refraction can be measured from the normalized energy transmission of Z-scan with an aperture (linear transmission of S 0:1). The pure nonlinear refractive results by Z-scan data with an aperture divided by that without an aperture are shown in Fig. 4. Apparently, the nonlinear refractive indices of the three samples are
Fig. 4. The pure refractive normalized energy transmissions by Z-scan data with an aperture S 0:10 divided by that without an aperture for three samples with a linear transmission of 87%: (a) C60 tpy±Au in chloroform, (b) C60 bpy-2±Au in chloroform and (c) C60 bpy-1±Au in chloroform.
positive. All of them exhibit the strong self-focusing eects due to the large peak±valley dierence
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values. C60 bpy-1±Au is the strongest, C60 tpy±Au is relatively weak and C60 bpy-2±Au is the weakest in the nonlinear refraction from the peak±valley difference values of 0.55, 0.34 and 0.29 for the above three samples, respectively. C60 and its derivatives generally possess the relatively weak excited-state refraction (weak selfdefocusing eects) though they display the strong excited-state absorption in visible region [18,20,21]. These ligands in three nanocomposites, similar to C60 derivatives, could also possess the weak self-defocusing behaviors. However, the strong nonlinear refractions (self-focusing eects) are observed in three samples from Fig. 4, for which the surface plasmon resonance due to light excitation in the noble metal gold nanoparticles could be responsible. We ®nd that the smaller peak and larger valley occur in Fig. 4(a), which could arise from the nonlinear scattering in C60 tpy±Au chloroform solution. In this case, the stronger nonlinear absorption may induce the formation of scattering centers due to local heating of chloroform solution at focus, most probably microbubbles, when close to boiling point temperatures. Indeed, the time needed for heat transfer from the metal to the solvent and the development of the bubbles around the particles is usually 100±200 ps [22]. It should be emphasized that the nonlinear absorption measured in Section 3.1 includes the strong absorption-induced nonlinear scattering for C60 tpy±Au chloroform solution. Thus, the eective absorption cross-section of 3:06 10 17 cm2 obtained is rough. It is indeed an extinction crosssection. The approximate symmetry in Fig. 4(b) and (c) implies no distinct nonlinear scattering appearing in C60 bpy-1±Au and C60 bpy-2±Au, which is relevant for the relatively weak nonlinear absorptions in comparison with that of C60 tpy±Au. Based on the above analysis, we mainly consider the contribution of the surface plasmon to the nonlinear refractions in three samples, and use the approximate solution of Z-scan theory to calculate the nonlinear refractive indices of the three samples by the following equations [23], despite the probable in¯uencing of nonlinear scattering on the nonlinear refraction in C60 tpy±Au.
DTP±V 0:406 1
S0:25 jDU0 j;
DU0 2p=kn2 I0 Leff and Leff 1 exp a0 L=a0
5
where DTP±V is the dierence between the peak and the valley of the normalized transmittances, I0 the pulse irradiance, n2 the nonlinear refractive index, and Leff the eective length of the sample. The value of DTP±V was taken from Fig. 4. Applying Eq. (5), we obtain n2 1:11 10 4 , 1:30 10 4 and 2:11 10 4 cm2 /GW for C60 bpy-2±Au, C60 tpy± Au and C60 bpy-1±Au, respectively. Direct comparison between those data is not so meaningful because the number density of gold nanoparticles in the nanocomposites is unknown. Our results may show that the nonlinear refractions of these nanocomposites result from the interband transition, to the large extent, due to the surface plasmon resonance. The dierent n2 can be due to the dierences of the number density and sizes of gold particles. As well known, the surface plasmon resonance can be strengthened as the number density and the size of metal nanoparticles increase in the nanocomposites [14]. In this case, the nonlinear scattering can be induced by the strong excited-state absorption of the ligands based on C60 and the strong surface plasmon absorption of metal nanoparticles in the nanocomposites. Therefore, the optical nonlinearities could be much enhanced. 3.3. Optical limiting We now investigate the OL properties in these three nanocomposites due to the strong optical nonlinearities of three samples. For utilizing adequately the nonlinear properties in these materials, we performed the refractive OL experiment of C60 bpy-1±Au which, namely, was placed in the position of the valley of the normalized transmittances in the Z-scan experiment. While C60 tpy±Au and C60 bpy-2±Au were still placed at the focus of the lens, as usual. As for comparison, we also measured the OL properties of C60 in toluene which was also placed at the focus for its strong excited-state absorption. The OL results of these samples with an identical linear transmission of
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the relatively weak OL properties, for which the substituent group may be accountable.
Acknowledgements This work is supported by Organic Solid Laboratory of Chinese Academy of Sciences.
References Fig. 5. The OL experimental results of the three nanocomposites chloroform solutions and C60 toluene solution with an identical linear transmission of 81%.
81% are shown in Fig. 5. Apparently, the OL properties of C60 tpy±Au and C60 bpy-1±Au are stronger than that of C60 , but C60 bpy-2±Au is weaker. We think that the stronger excited-state absorption of the ligand, C60 tpy, and subsequently induced nonlinear scattering in C60 tpy±Au solution are responsible for the stronger OL behavior. While the OL performances of C60 bpy-1±Au is enhanced mainly by the surface plasmon resonance. 4. Conclusions We investigated the optical nonlinearities of three nanocomposites with noble metal gold nanoparticles by Z-scan technique and OL experiments. Both eective nonlinear absorptive cross-sections and nonlinear refractive indices were obtained. The results show the signi®cant dependence of the nonlinear absorptions of these nanocomposites on the ligands. While the surface plasmon resonance could mainly be responsible for the stronger selffocusing eects. We also observe the stronger OL performances of C60 tpy±Au and C60 bpy-1±Au than that of C60 in toluene. The former originates from the stronger nonlinear absorption and subsequently induced nonlinear scattering, the latter from the stronger nonlinear absorption and the surface plasmon resonance. C60 bpy-2±Au shows
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