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Optical nonlinearities of Au nanoparticles and Au/ Ag coreshells Jae Tae Seo,1,* Qiguang Yang,1 Wan-Joong Kim,2 Jinhwa Heo,3,4 Seong-Min Ma,1 Jasmine Austin,1 Wan Soo Yun,5 Sung Soo Jung,5 Sang Woo Han,3,4 Bagher Tabibi,1 and Doyle Temple1 1
Department of Physics, Hampton University, Hampton, Virginia 23668, USA Bio-Photonic Device Team, Electronics and Telecommunications Research Institute, Deajeon 305-700, South Korea 3 Department of Chemistry, Research Institute of Natural Science, Gyeongsang National University, Jinju 660-701, South Korea 4 Environmental Biotechnology National Core Research Center, Gyeongsang National University, Jinju, 660-701, South Korea 5 Korea Research Institute of Standards and Science, Daejeon 305-600, South Korea *Corresponding author:
[email protected] 2
Received September 29, 2008; revised November 27, 2008; accepted November 28, 2008; posted December 19, 2008 (Doc. ID 102208); published January 26, 2009 Au nanoparticles exhibited both negative and positive nonlinear absorptions with ground-state plasmon bleaching and free-carrier absorption that could be origins of the saturable and reverse-saturable optical properties. Au/ Ag coreshells displayed only positive nonlinear absorption and reverse-saturable optical properties as a function of excitation intensity at the edge of surface-plasmon resonance, which implies no ground-state plasmon bleaching and the existence of two-photon absorption. © 2009 Optical Society of America OCIS codes: 190.4400, 240.6680, 160.4236, 160.4330.
Nonlinear optical properties of plasmonic nanometal at the surface-plasmon resonance (SPR) region have been extensively studied because of their resonant enhancement of the local electric field and their wide applications of optical, medical, and biological areas [1]. Recently, saturable absorption (SA), reversesaturable absorption (RSA) and optical powerlimiting properties [2,3] of plasmonic nanometals have been reported. The saturable and reversesaturable optical properties of nanometals are possibly utilized for the photonic applications of the saturable Q switch and the optical power limiter, respectively. Elim et al. reported SA and RSA properties of gold nanorods at the longitudinal SPR spectral region and demonstrated the SA process with moderate intensities up to ⬃4 GW/ cm2 and RSA behavior for intense optical fields [4]. Gao et al. published saturable and reverse-saturable optical properties of platinum nanoparticles at the spectral region away from the SPR band [2]. Qu et al. notified the SA and RSA of gold nanoparticles for different concentrations at the SPR region [3]. Both Gao’s and Qu’s groups remarkably described the origins of SA and RSA that were attributed by ground-state plasmon bleaching at moderate intensities and free-carrier absorption with significantly high excitation intensities, respectively. This article reports SA, RSA, and optical limiting properties at the SPR peak of Au nanoparticles and the nonlinear optical properties at the SPR edge of the Au/ Ag coreshell. Au nanoparticles [5] and Au/ Ag coreshells with indication of interdiffusions [6] were prepared according to the literature procedures. Morphology images and the linear absorption spectra of the Au nanoparticle and the Au/ Ag coreshells are shown in Fig. 1. 0146-9592/09/030307-3/$15.00
The morphology images of nanometals were taken using transmission electron microscopes (Hitachi, H-9000NAR, 300 kV; FEI, Technai G2 F30 SuperTwin, 300 kV), and the average radius of nanometals
Fig. 1. (Color online) Absorption spectra and images of transmission electron microscopy of (a) Au nanoparticles and (b) Au/ Ag coreshells. © 2009 Optical Society of America
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was estimated by the Gaussian fitting to their size distribution. The average radii of both nanometals were similar to each other, ⬃15 nm for Au nanoparticles and ⬃16 nm for Au/ Ag coreshells with ⬃7 nm core radius and ⬃9 nm shell thickness. The absorption spectra were collected using an UV–vis spectrophotometer (Agilent 8453) with 10 mm optical path for Au nanoparticles and 5 mm optical path for Au/ Ag coreshells. The concentrations and absorption cross sections were computed by Mie scattering theory for nanoparticles [7], its modification for coreshells [8], and Beer–Lambert’s law. The groundstate linear absorption cross sections and concentrations were roughly calculated to be ⬃3 ⫻ 10–17 m2 and ⬃6.3⫻ 10−7 mol/ m3 at both the SPR peak and the excitation wavelength for Au nanoparticles. Those parameters for Au/ Ag coreshells were estimated to be ⬃1.2⫻ 10–16 m2 and ⬃1.3⫻ 10−7 mol/ m3 at the SPR peak and ⬃1.1⫻ 10–17 m2 at the excitation wavelength. The absorption bands with peak at ⬃532 nm for Au nanoparticles and that with peak at ⬃408 nm for Au/ Ag coreshells are attributable to the SPR that is a result of transitions within the conduction band or intraband transitions. At the higher-energy region relative to the SPR band or the spectra by the intraband transitions, the spectra of interband resonance transitions of 5d-6s for Au and 4d-5s for Ag are located as depicted in Fig. 1 [9]. The interband and intraband spectra of Au nanoparticles are extensively overlapped, as shown in Fig. 1(a). Therefore the nonlinear absorptions with excitations at the SPR region can be attributed to both intraband and interband transitions for Au nanoparticles [3]. The dashed curve in Fig. 1(a) is the Lorentzian fit to the SPR, and the dashed-dotted line is a schematic sketch of the difference between the extinction spectrum and the Lorentzian fit. The difference is considered as the absorption through interband transitions. However, the interband and intraband transitions in Au/ Ag coreshells are well separated, as shown in Fig. 1(b), mainly because of optical contributions of Ag nanoshells. The nonlinear absorption with excitation at the edge of SPR of Au/ Ag coreshells is mostly attributed to the intraband transitions through possibly dominant two-photon absorption because of extremely weak SPR at the excitation region. The interband contribution to the nonlinear absorption on Au/ Ag coreshells is probably prohibited, because the Au nanocores are enfolded by Ag nanoshells with enough thickness to block optical contributions of Au core nanoparticles. Nonlinear optical characteristics of the colloidal Au nanoparticles and Au/ Ag coreshells in water are shown in Figs. 2 and 3 using Z-scan [10] and I-scan [11] techniques that provide nonlinear transmittance as a function of sample position and intensities, respectively. Any data changes after limited scanning times and the number of shots at the applied intensities were excluded, which could be possibly resulted by sample deformation [12], especially for Au/ Ag coreshells. Both open and closed Z-scan techniques were well described elsewhere [10]. Sample thickness
Fig. 2. (Color online) Normalized nonlinear transmittance as a function of sample position (Z) for (a), (b) Au nanoparticles and (c), (d) Au/ Ag coreshells with (a), (c) closed and (b), (d) open Z scans for the applied intensities (Ip in GW/ cm2).
of Au nanoparticles and Au/ Ag coreshells for the nonlinear optical experiments were 5 mm. The excitation source was a 6 ns pulsed and frequency-doubled laser (Continuum, Surelite II) at 532 nm wavelength with a 10 Hz repetition rate. The laser beam was focused on the sample by a positive lens with 25 cm focal length. The Rayleigh length was calculated to be ⬃9.4 mm, which was longer than the thicknesses of quartz cuvette wall 共2 ⫻ 1 mm兲 and optical sample 共5 mm兲. Quantitative analysis of nonlinear absorption and nonlinear refraction coefficients of Au or Au/ Ag nanometals from the open or closed Z scan were not attempted in this Letter; however, the negative and positive nonlinear optical properties of the nanometals by Z scan were described qualitatively to examine their saturable and reverse-saturable optical properties with various excitation intensities. The excitation intensities for were near or above the saturation intensities, from a few tens of MW/ cm2 to a few GW/ cm2 at the SPR region, for the nanometals. It is fundamentally known that the cubic nonlinearity can be well described only if the applied intensities are less than the saturation intensities. Otherwise, the polarization expression in terms of power series of the optical field does not converge, and it
Fig. 3. (Color online) Saturable and reverse-saturable optical properties of Au nanoparticles and Au/ Ag coreshells as a function of excitation intensity.
February 1, 2009 / Vol. 34, No. 3 / OPTICS LETTERS
ជ = 共共L兲 should be expressed in the simple form of P o 共NL兲 ជ + 兲E. Therefore the nonlinear optical properties of the nanometals were qualitatively described, instead of analyzing the quantitative values of multiorder nonlinearities, which is beyond the scope of this Letter. Figure 2 shows the normalized nonlinear transmittances as a function of sample position (Z) for Au nanoparticles [Figs. 2(a) and 2(b)] and Au/ Ag coreshells [Figs. 2(c) and 2(d)] with closed [Figs. 2(a) and 2(c)] and open [Figs. 2(b) and 2(d)] Z-scan techniques. The Au nanoparticles have positive nonlinearity or self-focusing property, because the valley leads the peak. As the excitation intensity is increased within the moderate region, the peak with the closed Z scan and the negative nonlinear absorption with the open Z scan are increased. As the intensity is increased up to the relatively intense region, the positive nonlinear absorption with the open Z scan appears, indicating the existence of SA with moderate excitation intensities and RSA with relatively high input intensities for Au nanoparticles. The changeover in the sign of nonlinear absorption of the Au nanoparticles is sensitive to the applied laser intensity and is related to the interplay of groundstate plasmon bleaching and free-carrier absorption at the excited state of the conduction band [13]. The SA is a result of the ground-state plasmon bleaching at moderate intensities, and the RSA is a consequence of free-carrier absorption with relatively high excitation intensities [3]. The Au/ Ag coreshells have negative nonlinearity or defocusing property, because the peak leads the valley. As the excitation intensity is increased, the depth of a positive valley with a closed Z scan and that of a positive absorption valley with an open Z scan are gradually increased, indicating the existence of a relatively influential twophoton absorption process rather than a two-step absorption process in the Au/ Ag coreshells, because of extremely weak SPR at the excitation region. Figure 3 shows the normalized nonlinear transmittance as a function of input intensity. Au nanoparticles exhibit both saturable property for the applied peak intensities from ⬃8 MW/ cm2 to ⬃ 600 MW/ cm2 at the SPR peak and reverse saturable property for the intensities from ⬃600 MW/ cm2 to ⬃ 8GW/ cm2. Au/ Ag coreshells display only reverse-saturable properties at the edge of SPR as a function of intensity from ⬃8 MW/ cm2 to ⬃ 8 GW/ cm2. The saturation absorption does not appear if the excitation wavelength is away from the SPR peak or the SPR absorption rarely exists, because the ground-state plasmon bleaching cannot occur. The SPR can also be quenched at high laser intensities. Possible physical origins of SA and RSA of Au nanoparticles are interband, intraband, hotelectron, and optical thermal processes [9,14]. Possible physical origins of reverse-saturable optical
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property of Au/ Ag coreshells as a function of intensity are the same as those of Au nanoparticles, excluding interband transition. In conclusion, the interband and intraband spectra of Au nanoparticles are extensively overlapped, and those spectra of Au/ Ag coreshells are well separated. This implies that the interband and intraband transitions in nanometal systems play important roles on their nonlinear optical properties. Saturable and reverse-saturable optical properties of Au nanoparticles and reverse-saturable optical properties of Au/ Ag nanoshells were qualitatively analyzed for nonlinear photonic applications of saturable Q switches and optical power limiters. The saturable and reverse-saturable optical properties of Au nanoparticles with resonant excitation near the SPR peak could be explained with the interplay of ground-state plasmon bleaching and free-carrier absorption at the excited state of conduction band. The reversesaturable optical properties as a function of intensity for the Au/ Ag coreshells at the SPR edge imply dominant two-photon absorption contributions to their optical nonlinearity. This work at Hampton University was supported by the National Science Foundation (NSF) (HRD0734635, HRD-0630372, ESI-0426328/002, and EEC0532472) and the U.S. Army Research Office (ARO) (W911NF-07-1-0608). References 1. G. Ma, W. Sun, S. H. Tang, H. Zhang, Z. Shen, and S. Qian, Opt. Lett. 27, 1043 (2002). 2. Y. Gao, X. Zhang, Y. Li, H. Liu, Y. Wang, Q. Chang, W. Jiao, and Y. Song, Opt. Commun. 251, 429 (2005). 3. S. Qu, Y. Gao, X. Jiang, H. Zeng, Y. Song, J. Qiu, C. Zhu, and K. Hirao, Opt. Commun. 224, 321 (2003). 4. H. I. Elim, J. Yang, J. Y. Lee, J. Mi, and W. Ji, Appl. Phys. Lett. 88, 083107 (2006). 5. K. C. Grabar, R. G. Freeman, M. B. Hommer, and M. J. Natan, Anal. Chem. 67, 735 (1995). 6. T. Shibata, B. A. Bunker, Z. Zhang, D. Meisel, C. F. Vardeman II, and J. D. Gezelter, J. Am. Chem. Soc. 124, 11989 (2002). 7. G. Mie, Ann. Phys. 25, 377 (1908). 8. R. D. Averitt, S. L. Westcott, and N. J. Halas, J. Opt. Soc. Am. B 16, 1824 (1999). 9. S. Qu, Y. Zhang, H. Li, J. Qiu, and C. Zhu, Opt. Mater. 28, 259 (2006). 10. M. Sheik-Bahae, A. Said, T. Wei, D. Hagan, and E. Van Stryland, IEEE J. Quantum Electron. 26, 760 (1990). 11. J. T. Seo, Q. Yang, S. Creekmore, D. Temple, K. P. Yoo, S. Y. Kim, A. Mott, M. Namkung, and S. S. Jung, Appl. Phys. Lett. 82, 4444 (2003). 12. R. Jin, Y. Cao, C. A. Mirkin, K. L. Kelly, G. C. Schatz, and J. G. Zheng, Science 294, 1901 (2000). 13. Y. Deng, Y. Sun, P. Wang, D. Zhang, X. Jiao, H. Ming, Q. Zhang, Y. Jiao, and X. Sun, Curr. Appl. Phys. 8, 13 (2008). 14. N. Rotenberg, A. D. Bristow, M. Pfeiffer, M. Betz, and H. M. van Driel, Phys. Rev. B 75, 155426 (2007).
