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Optical nonlinearities from transverse plasmon resonance in gold nano-rods
Materials Letters 58 (2004) 1427 – 1430 www.elsevier.com/locate/matlet
Optical nonlinearities from transverse plasmon resonance in gold nano-rods Shiliang Qu a,b,*, Huajun Li a, Tianyou Peng c, Yachen Gao a, Jianrong Qiu b, Congshan Zhu b a
b
Department of Physics, Harbin Institute of Technology at Weihai, Weihai 264209, China Photon Craft Project Lab, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China c Department of Chemistry, Wuhan University, Wuhan 430072, China Received 11 June 2003; received in revised form 16 September 2003; accepted 29 September 2003
Abstract The nonlinear refraction of two gold nano-rods with different sizes was investigated by using Z-scan technique with nanosecond pulses at 532 nm. Their optical limiting (OL) effect was also studied at the wavelength. The experimental results show that they possess strong optical nonlinearities when excited near the transverse plasmon resonance, depending critically on the size of nano-rods. The nonlinear refractions arise mainly from the strong excited-state nonlinearity of conduction band electrons. In contrast, the OL effect originates from the transverse plasmon resonance absorption, excited-state absorption and absorption-induced nonlinear scattering. D 2003 Elsevier B.V. All rights reserved. Keywords: Optical nonlinearities; Transverse plasmon resonance; Gold nano-rods
Noble metal nanoparticles have been known to possess a large third-order nonlinear optical coefficient and ultrafast time response in the surface plasmon absorption region. [1 – 3] Thus, the particles have been subjected to intensive studies in optical nonlinearities. The surface plasmon resonance (SPR) of noble metal particles in composites occurs in the UV –visible to near-IR region, depending on the metal species, shape, and dielectric medium. For spherically shaped metal nanoparticles, the peak position can be predicted by the Mie’s theory. [4– 6] Typical nanocomposites studied most are Au, Ag nanoparticles in glass or solution, showing the SPR band around 550 and 400 nm, respectively. However, rod-type metal nanoparticles exhibit two characteristic SPR bands, the transverse SPR and the longitudinal SPR. The peak position of the transverse SPR is similar to that of spherical particles. That of the longitudinal SPR depends on the axial ratio of the longitudinal to transverse sizes. [7] In this letter, we investigated the nonlinear refraction and optical limiting (OL) effect of two gold nano-rods at the wavelength of 532 nm. The
* Corresponding author. Department of Physics, Harbin Institute of Technology at Weihai, Weihai 264209, China. Tel.: +86-21-59929373; fax: +86-631-568-7036. E-mail address: [email protected] (S. Qu). 0167-577X/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2003.09.039
experimental results observed are significantly different for the two nano-rods. The two kinds of gold nano-rods are prepared by a seeding growth approach in the presence of an aqueous miceller template. Citrate-capped gold particles, prepared by the reduction of HAuCl4 with borohydride, are used as the seed. The aspect ratio of the nano-rods is controlled by varying the ratio of seed to metal salt. The long rods are isolated from spherical particles by centrifugation. [8] By TEM, we estimate their average diameters to be 25 and 35 nm, and aspect ratios to be 4.6 and 18. The two samples are denoted as R1 and R2. Their linear absorption spectra are shown in Fig. 1. We can observe that both R1 and R2 exhibit two SPR absorptions, the transverse and longitudinal, at 532 and 738 nm for R1, and 527 and 685 nm for R2, respectively. This is consistent with the published result that the resonance peak exhibits redshift with increasing particle size. [9] For optical excitation close to the linear band (532 nm), the transverse SPR absorption may be rather strong. Of course, the light scattering of gold nano-rods with such sizes can occur, but the weak linear scattering cannot change the peak position of SPR. The nonlinear optical properties of the two samples were measured with laser pulses from a frequency-doubled, Qswitched, mode-locked Continuum ns/ps Nd:YAG laser,
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S. Qu et al. / Materials Letters 58 (2004) 1427–1430
Fig. 1. Absorption spectra of two kinds of nano-rods.
which produces linearly polarized 8 ns (FWHM) laser pulses at 532 nm with a repetition rate of 1 Hz. The spacial profile of the pulse is nearly Gaussian. Z-scan experiment setup is similar to that in the literature. [10] The laser pulse energy used in our experiment is 160 AJ. The linear transmittance of the two samples in a quartz cell of 1-mm thickness is all 81%, corresponding to the linear absorption coefficient of 261 m 1. In the OL experiments, the two samples in quartz cells with a thickness of 5 mm were placed at focus of a lens with a focal length of 30 cm. Both the incident and transmitted laser pulses were monitored simultaneously by using two energy detectors D1 and D2 (Rjp-735 energy probes, Laser Precision), respectively. Apertures with different transmittances were placed in front of D2 in order to analyze the OL origins. Fig. 2 gives the experimental results of the closed aperture Z-scan data divided by the open aperture Z-scan data for R1 and R2 with a same transmittance of 81%. We
Fig. 2. Nonlinear refractive properties of R1 and R2.
find that both exhibit the strong self-focusing behaviour as revealed in the valley-peak shaped curves. In addition, we have known that strong nonlinear absorption and scattering can induce an asymmetric curve. However, the curves (see Fig. 2) are symmetrical with respect to the focus (z = 0). This means that the influence of the nonlinear absorption and nonlinear scattering on the nonlinear refraction can be ruled out by using Z-scan data of closed aperture divided by that of the open aperture. As can be seen in Fig. 2, R1 exhibits stronger nonlinear refraction as compared with R2. In term of Z-scan theory [10], we estimate their nonlinear refractive indexes n2 = 1.65 10 4 and 0.77 10 4 cm2/GW for R1 and R2, respectively. The different n2 can be due to the differences in size of gold nano-rods. As is known, the physical origin of the light absorption by metallic nano-particles is the coherent oscillation of the conduction band electrons induced by the interacting electromagnetic field. Once these electrons are excited by a pulse close to absorption peak, they do not oscillate at the
Fig. 3. OL results of R1 (a) and R2 (b).
S. Qu et al. / Materials Letters 58 (2004) 1427–1430
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Table 1 NAF’s for different aperture transmittances Samples
S (Aperture transmittance)
Tlin (Normalized linear transmittance)
Emax/mJ (Max input energy)
TNL (Normalized transmittance at Emax)
NAF (Tlin/TNL)
R1
1 0.55 0.26 1 0.64 0.50
1 1 1 1 1 1
1 1 1 1 1 1
0.24 0.20 0.16 0.28 0.25 0.23
4.2 5.0 6.2 3.6 4.0 4.3
R2
same frequency as that of the unexcited electrons, thus causing the SPR. Since the transverse SPR peak of R1 is just situated at the excitation wavelength of 532 nm, almost all electrons in the conduction band are pumped to the excited-state, and thus, resulting in the strong excited-state refraction and absorption. [11] Refractive and absorptive cross-sections in the excited-state are larger than those in the ground state, inducing the enhanced nonlinearity. For R2, the peak locates at 527 nm, slightly away from the excitation wavelength, giving birth to the weaker nonlinearity than R1. We also studied the OL effect of the two nano-rods by using several apertures with different linear transmittances, S, in front of detector D2. The experimental results are shown in Fig. 3. For quantitatively description of the OL effect of samples, we introduce the nonlinear attenuation factor (NAF), namely, the ratio of a linear transmittance Tlin to a nonlinear transmittance TNL at the maximum input energy. These opto-physical parameters were shown in Table 1. Evidently, R1 is stronger than R2 in OL effect. Also, both of them show the increasing OL effect with decreasing S. Since 532-nm irradiated gold nano-rods should result in the strong transverse plasmon absorption followed by the excited-state absorption, the transmittance of the sample would decrease with increasing laser intensity. Namely, OL effect arises. In addition, the nonlinear scattering can result from the strong nonlinear absorptions for metal nanoparticles in solution [12,13]. Actually, the aperture-size-dependent results indicate the nonlinear scattering. We estimate that the nonlinear scattering is stronger than the plasmon and excited-state absorptions. If not so, the nonlinear absorption will cover up the nonlinear scattering. We should not observe the aperture-size-dependent OL effect. We have known that the energy absorbed by the strong plasmon absorption of metal nanoparticles can transfer to solvents in the time scope of 100 – 200 ps [14] and lead to local heating of the solvent at the focus, then creating microbubbles when close to boiling temperatures of solvents with nanosecond pulses excitation. [15] These micro-bubbles serving as scattering centers can result in the nonlinear scattering and accordingly enhance the OL effect. In order to confirm the above view, we carried out the OL experiments of gold nano-rods in ethanol and methanol. We observed that the OL effect of gold nano-rods in the two
solvents was different. The OL effect of gold nano-rods in solution, related to the boiling point of solvents, shows that micro-bubbles result in stronger nonlinear scattering. The micro-bubble is not essentially linked to optical properties of gold nano-rods, but induced from the transverse plasmon absorption of gold nano-rods. Therefore, we can infer the strength of plasmon absorption of gold nano-rods from the OL effect. Summarizing, we report, with the excitation of nanosecond pulses at 532 nm, the nonlinear refraction and OL effect of two gold nano-rods with different aspect-ratios. The nonlinear refraction results from the excited-state refraction of electrons in the conduction band as excited near the transverse plasmon resonance peak. The OL effect is inferred to include the contributions of the plasmon absorption, excited-state absorption and nonlinear scattering resulted from micro-bubbles in solution. The optical nonlinearities from transverse plasmon resonance in gold nanorods depend upon their aspect-ratios, surrounding media and excitation wavelength. This means that these nano-rods can be used as optoelectronic devices, e.g. optical switches, optical limiters. Acknowledgements This work is supported by Foundation of HIT at Weihai, China(HIT(WH).2002.28), and Shanghai Committee of Science and Technology, China (0259nm055). References [1] L. Yang, K. Becker, F.M. Smith, et al., J. Opt. Soc. Am. B 11 (1994) 457. [2] F. Gonella, G. Matti, P. Mazzoldi, et al., Appl. Phys. Lett 69 (1996) 3101. [3] J. Sasai, K. Hirao, J. Appl. Phys. 89 (2001) 4548. [4] S. Link, M. A. El-Sayed, J. Phys. Chem., B 103 (1999) 8410. [5] U. Kreibig, M. Vollmer, Optical Properties of Metal Clusters, Springer, Berlin, 1995, pp. 35 – 67. [6] S.L. Qu, C.M. Du, Y.L. Song, et al., Chem. Phys. Lett. 356 (2002) 403; S.L. Qu, Y.L. Song, H.F. Liu, et al., Opt. Commun. 203 (2002) 283. [7] M.J. Bloemer, M.C. Buncick, R.J. Warmack, et al., J. Opt. Soc. Am. B 5 (1988) 2552. [8] N.R. Jana, L. Gearheart, C.J. Murphy, J. Phys. Chem., B 105 (2001) 4065.
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S. Qu et al. / Materials Letters 58 (2004) 1427–1430
[9] U. Kreibig, L. Genzel, Surf. Sci. 156 (1985) 678. [10] M. Sheik-Bahae, A.A. Said, E.W. Van Stryland, IEEE J. Quantum Electron. 26 (1990) 760. [11] R. Philip, G.R. Kumar, Phys. Rev., B 62 (2000) 13160. [12] S.R. Mishra, H.S. Rawat, S.C. Mehendale, et al., Phys. Lett. 317 (2000) 510.
[13] L.W. Tutt, T.F. Boggess, Prog. Quantum Electron. 17 (1993) 299. [14] J. H. Hodak, A. Henglein, M. Giersig, et al., J. Phys. Chem., B 104 (2000) 11708. [15] R.G. Ispasoiu, L. Balogh, O.P. Varnavski, et al., J. Am. Chem. Soc. 122 (2000) 11005.
Optical nonlinearities from transverse plasmon resonance in gold nano-rods Shiliang Qu a,b,*, Huajun Li a, Tianyou Peng c, Yachen Gao a, Jianrong Qiu b, Congshan Zhu b a
b
Department of Physics, Harbin Institute of Technology at Weihai, Weihai 264209, China Photon Craft Project Lab, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences, Shanghai 201800, China c Department of Chemistry, Wuhan University, Wuhan 430072, China Received 11 June 2003; received in revised form 16 September 2003; accepted 29 September 2003
Abstract The nonlinear refraction of two gold nano-rods with different sizes was investigated by using Z-scan technique with nanosecond pulses at 532 nm. Their optical limiting (OL) effect was also studied at the wavelength. The experimental results show that they possess strong optical nonlinearities when excited near the transverse plasmon resonance, depending critically on the size of nano-rods. The nonlinear refractions arise mainly from the strong excited-state nonlinearity of conduction band electrons. In contrast, the OL effect originates from the transverse plasmon resonance absorption, excited-state absorption and absorption-induced nonlinear scattering. D 2003 Elsevier B.V. All rights reserved. Keywords: Optical nonlinearities; Transverse plasmon resonance; Gold nano-rods
Noble metal nanoparticles have been known to possess a large third-order nonlinear optical coefficient and ultrafast time response in the surface plasmon absorption region. [1 – 3] Thus, the particles have been subjected to intensive studies in optical nonlinearities. The surface plasmon resonance (SPR) of noble metal particles in composites occurs in the UV –visible to near-IR region, depending on the metal species, shape, and dielectric medium. For spherically shaped metal nanoparticles, the peak position can be predicted by the Mie’s theory. [4– 6] Typical nanocomposites studied most are Au, Ag nanoparticles in glass or solution, showing the SPR band around 550 and 400 nm, respectively. However, rod-type metal nanoparticles exhibit two characteristic SPR bands, the transverse SPR and the longitudinal SPR. The peak position of the transverse SPR is similar to that of spherical particles. That of the longitudinal SPR depends on the axial ratio of the longitudinal to transverse sizes. [7] In this letter, we investigated the nonlinear refraction and optical limiting (OL) effect of two gold nano-rods at the wavelength of 532 nm. The
* Corresponding author. Department of Physics, Harbin Institute of Technology at Weihai, Weihai 264209, China. Tel.: +86-21-59929373; fax: +86-631-568-7036. E-mail address: [email protected] (S. Qu). 0167-577X/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2003.09.039
experimental results observed are significantly different for the two nano-rods. The two kinds of gold nano-rods are prepared by a seeding growth approach in the presence of an aqueous miceller template. Citrate-capped gold particles, prepared by the reduction of HAuCl4 with borohydride, are used as the seed. The aspect ratio of the nano-rods is controlled by varying the ratio of seed to metal salt. The long rods are isolated from spherical particles by centrifugation. [8] By TEM, we estimate their average diameters to be 25 and 35 nm, and aspect ratios to be 4.6 and 18. The two samples are denoted as R1 and R2. Their linear absorption spectra are shown in Fig. 1. We can observe that both R1 and R2 exhibit two SPR absorptions, the transverse and longitudinal, at 532 and 738 nm for R1, and 527 and 685 nm for R2, respectively. This is consistent with the published result that the resonance peak exhibits redshift with increasing particle size. [9] For optical excitation close to the linear band (532 nm), the transverse SPR absorption may be rather strong. Of course, the light scattering of gold nano-rods with such sizes can occur, but the weak linear scattering cannot change the peak position of SPR. The nonlinear optical properties of the two samples were measured with laser pulses from a frequency-doubled, Qswitched, mode-locked Continuum ns/ps Nd:YAG laser,
1428
S. Qu et al. / Materials Letters 58 (2004) 1427–1430
Fig. 1. Absorption spectra of two kinds of nano-rods.
which produces linearly polarized 8 ns (FWHM) laser pulses at 532 nm with a repetition rate of 1 Hz. The spacial profile of the pulse is nearly Gaussian. Z-scan experiment setup is similar to that in the literature. [10] The laser pulse energy used in our experiment is 160 AJ. The linear transmittance of the two samples in a quartz cell of 1-mm thickness is all 81%, corresponding to the linear absorption coefficient of 261 m 1. In the OL experiments, the two samples in quartz cells with a thickness of 5 mm were placed at focus of a lens with a focal length of 30 cm. Both the incident and transmitted laser pulses were monitored simultaneously by using two energy detectors D1 and D2 (Rjp-735 energy probes, Laser Precision), respectively. Apertures with different transmittances were placed in front of D2 in order to analyze the OL origins. Fig. 2 gives the experimental results of the closed aperture Z-scan data divided by the open aperture Z-scan data for R1 and R2 with a same transmittance of 81%. We
Fig. 2. Nonlinear refractive properties of R1 and R2.
find that both exhibit the strong self-focusing behaviour as revealed in the valley-peak shaped curves. In addition, we have known that strong nonlinear absorption and scattering can induce an asymmetric curve. However, the curves (see Fig. 2) are symmetrical with respect to the focus (z = 0). This means that the influence of the nonlinear absorption and nonlinear scattering on the nonlinear refraction can be ruled out by using Z-scan data of closed aperture divided by that of the open aperture. As can be seen in Fig. 2, R1 exhibits stronger nonlinear refraction as compared with R2. In term of Z-scan theory [10], we estimate their nonlinear refractive indexes n2 = 1.65 10 4 and 0.77 10 4 cm2/GW for R1 and R2, respectively. The different n2 can be due to the differences in size of gold nano-rods. As is known, the physical origin of the light absorption by metallic nano-particles is the coherent oscillation of the conduction band electrons induced by the interacting electromagnetic field. Once these electrons are excited by a pulse close to absorption peak, they do not oscillate at the
Fig. 3. OL results of R1 (a) and R2 (b).
S. Qu et al. / Materials Letters 58 (2004) 1427–1430
1429
Table 1 NAF’s for different aperture transmittances Samples
S (Aperture transmittance)
Tlin (Normalized linear transmittance)
Emax/mJ (Max input energy)
TNL (Normalized transmittance at Emax)
NAF (Tlin/TNL)
R1
1 0.55 0.26 1 0.64 0.50
1 1 1 1 1 1
1 1 1 1 1 1
0.24 0.20 0.16 0.28 0.25 0.23
4.2 5.0 6.2 3.6 4.0 4.3
R2
same frequency as that of the unexcited electrons, thus causing the SPR. Since the transverse SPR peak of R1 is just situated at the excitation wavelength of 532 nm, almost all electrons in the conduction band are pumped to the excited-state, and thus, resulting in the strong excited-state refraction and absorption. [11] Refractive and absorptive cross-sections in the excited-state are larger than those in the ground state, inducing the enhanced nonlinearity. For R2, the peak locates at 527 nm, slightly away from the excitation wavelength, giving birth to the weaker nonlinearity than R1. We also studied the OL effect of the two nano-rods by using several apertures with different linear transmittances, S, in front of detector D2. The experimental results are shown in Fig. 3. For quantitatively description of the OL effect of samples, we introduce the nonlinear attenuation factor (NAF), namely, the ratio of a linear transmittance Tlin to a nonlinear transmittance TNL at the maximum input energy. These opto-physical parameters were shown in Table 1. Evidently, R1 is stronger than R2 in OL effect. Also, both of them show the increasing OL effect with decreasing S. Since 532-nm irradiated gold nano-rods should result in the strong transverse plasmon absorption followed by the excited-state absorption, the transmittance of the sample would decrease with increasing laser intensity. Namely, OL effect arises. In addition, the nonlinear scattering can result from the strong nonlinear absorptions for metal nanoparticles in solution [12,13]. Actually, the aperture-size-dependent results indicate the nonlinear scattering. We estimate that the nonlinear scattering is stronger than the plasmon and excited-state absorptions. If not so, the nonlinear absorption will cover up the nonlinear scattering. We should not observe the aperture-size-dependent OL effect. We have known that the energy absorbed by the strong plasmon absorption of metal nanoparticles can transfer to solvents in the time scope of 100 – 200 ps [14] and lead to local heating of the solvent at the focus, then creating microbubbles when close to boiling temperatures of solvents with nanosecond pulses excitation. [15] These micro-bubbles serving as scattering centers can result in the nonlinear scattering and accordingly enhance the OL effect. In order to confirm the above view, we carried out the OL experiments of gold nano-rods in ethanol and methanol. We observed that the OL effect of gold nano-rods in the two
solvents was different. The OL effect of gold nano-rods in solution, related to the boiling point of solvents, shows that micro-bubbles result in stronger nonlinear scattering. The micro-bubble is not essentially linked to optical properties of gold nano-rods, but induced from the transverse plasmon absorption of gold nano-rods. Therefore, we can infer the strength of plasmon absorption of gold nano-rods from the OL effect. Summarizing, we report, with the excitation of nanosecond pulses at 532 nm, the nonlinear refraction and OL effect of two gold nano-rods with different aspect-ratios. The nonlinear refraction results from the excited-state refraction of electrons in the conduction band as excited near the transverse plasmon resonance peak. The OL effect is inferred to include the contributions of the plasmon absorption, excited-state absorption and nonlinear scattering resulted from micro-bubbles in solution. The optical nonlinearities from transverse plasmon resonance in gold nanorods depend upon their aspect-ratios, surrounding media and excitation wavelength. This means that these nano-rods can be used as optoelectronic devices, e.g. optical switches, optical limiters. Acknowledgements This work is supported by Foundation of HIT at Weihai, China(HIT(WH).2002.28), and Shanghai Committee of Science and Technology, China (0259nm055). References [1] L. Yang, K. Becker, F.M. Smith, et al., J. Opt. Soc. Am. B 11 (1994) 457. [2] F. Gonella, G. Matti, P. Mazzoldi, et al., Appl. Phys. Lett 69 (1996) 3101. [3] J. Sasai, K. Hirao, J. Appl. Phys. 89 (2001) 4548. [4] S. Link, M. A. El-Sayed, J. Phys. Chem., B 103 (1999) 8410. [5] U. Kreibig, M. Vollmer, Optical Properties of Metal Clusters, Springer, Berlin, 1995, pp. 35 – 67. [6] S.L. Qu, C.M. Du, Y.L. Song, et al., Chem. Phys. Lett. 356 (2002) 403; S.L. Qu, Y.L. Song, H.F. Liu, et al., Opt. Commun. 203 (2002) 283. [7] M.J. Bloemer, M.C. Buncick, R.J. Warmack, et al., J. Opt. Soc. Am. B 5 (1988) 2552. [8] N.R. Jana, L. Gearheart, C.J. Murphy, J. Phys. Chem., B 105 (2001) 4065.
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S. Qu et al. / Materials Letters 58 (2004) 1427–1430
[9] U. Kreibig, L. Genzel, Surf. Sci. 156 (1985) 678. [10] M. Sheik-Bahae, A.A. Said, E.W. Van Stryland, IEEE J. Quantum Electron. 26 (1990) 760. [11] R. Philip, G.R. Kumar, Phys. Rev., B 62 (2000) 13160. [12] S.R. Mishra, H.S. Rawat, S.C. Mehendale, et al., Phys. Lett. 317 (2000) 510.
[13] L.W. Tutt, T.F. Boggess, Prog. Quantum Electron. 17 (1993) 299. [14] J. H. Hodak, A. Henglein, M. Giersig, et al., J. Phys. Chem., B 104 (2000) 11708. [15] R.G. Ispasoiu, L. Balogh, O.P. Varnavski, et al., J. Am. Chem. Soc. 122 (2000) 11005.