Open Access
Add to BibSonomyAbstract
We present the linear and nonlinear optical properties of Ag nanocubes. The surface plasmon resonance absorption peak shows a red shift with increasing cube size. The nonlinear absorption and nonlinear refraction were measured by using Z-scan technique. The Ag nanocubes with the edge size of 60 nm show the largest third-order optical nonlinearity, which is supported by the largest field enhancement factor through the FDTD simulations. In the wavelength range from 720 nm to 920 nm, the one-photon and two-photon figures of merit, W and T satisfy the demand of W > 1 and T < 1, which implies it is a promising candidate for the optical switching devices.
© 2015 Optical Society of America
1. Introduction
In the past decades, noble metal nano-materials have attracted vast amount of interests owing to their large third-order optical nonlinearity and ultrafast response [1–5 ]. Among which, Au and Ag nanoparticles [6–9 ], and Au (or Ag)/dielectric composite materials [10–13 ] have been extensively studied because they showed large optical nonlinearities due to their large localized field enhancement, as a consequence of collective oscillation of electron gas in metal that couples with electromagnetic fields. These properties enable them to have potential applications especially for making all-optical switching devices [14–16 ].
The requirements of all-optical switching with high switching efficiency, low loss have driven people to search for a nonlinear material with large nonlinear refraction (NLR) and small linear and nonlinear absorption (NLA) [17–20 ]. Two key parameters, namely, the one-photon figure of merit (FOM) W and two-photon FOM T are used to evaluate the quality of the nonlinear optical materials,
where γ is nonlinear refraction index, α and β are linear and nonlinear absorption coefficients, I and λ are the intensity and wavelength of the laser. A suitable candidate for all-optical switching requires W > 1 and T < 1 [21].Silver nano-materials have the potential to obtain the largest enhancement factors and large third-order nonlinearity. Compared to gold and copper, silver shows lower intrinsic loss of plasmonic energy at visible frequencies [22], which gives rise to SPR (surface plasmonic resonance). Further, the SPR energy of silver is far from the interband transition energy, which makes it easy to analyze the origin of the optical nonlinearities [23]. On the other hand, the nonlinear optical properties of metal nano-materials are strongly dependent on the particle size, shape, and the dielectric constant of the host medium [24–27 ]. So nanocube, as a novel shape which has large local electric field enhancement at its corner, would show unique SPR absorption and large local field enhanced optical nonlinearities. In recent years, Ag nanocubes with the size of tens nanometers have been synthesized and the researches mainly focus on catalytic activity of oxygen reduction reaction [28], improvement of solar cell performance [29] and surface enhanced Raman scattering.
In this paper, the linear and nonlinear optical properties of Ag nanocubes have been investigated. The NLA, NLR, and FOMs of the Ag nanocubes are found to be dependent with the cube size. At a wide wavelength range, the demanded FOMs were obtained for the smallest Ag nanocube we could synthesize.
2. Experiment
The Ag nanocubes were prepared through a modified reduction of Ag+ by pentanediol in the presence of Cl-. Firstly, NaCl (10 mg) was dissolved in glycol solution (10 mL) to obtain Cl- solution. Secondly, AgNO3 (48 mg) were added to pentanediol (3 mL) and PVP-29 (48 mg) were added to the pentanediol (3 mL) containing Cl- solution (80-120 μL), respectively, to prepare precursor solutions. Thirdly, the reaction solution was prepared by heating pentanediol (5 mL) at 150 °C for 1 h with continuous stirring. Finally, both of the precursor solutions were simultaneously injected in the hot pentanediol at a rate of 600 μL/min. Then such prepared mixed solutions were heated for 2, 3, and 5 hours, respectively, to acquire different sizes of products (sample A, B, and C). After heating, they were allowed to cool to room temperature under constant stirring. The products were centrifuged and washed repeatedly, and dispersed in DMF [30].
The microscopic images of the nanocubes were taken by using a scanning electron microscope (SEM, Zeiss Auriga-39-34). The absorption spectra were recorded by a UV-VIS-NIR spectrophotometer (PerkinElmer Lambda 950). The nonlinear optical measurements were performed by open- and closed-aperture Z-scan techniques, in which a Ti: Sapphire laser (Coherent, Mira 900) with the pulse duration of 130 fs and the repetition rate of 76 MHz was used as the laser source.
3. Results and discussion
The as-prepared samples are comprised of homogeneous distributed Ag cubes and very few nano-cones, as shown by the SEM images in Fig. 1(a)-(c) . The histograms in Fig. 1(d)-(f) show the edge length (S) distribution of the Ag nanocubes. The percentages of Ag nanocubes with the edge length within the range of 0.9S-1.1S nm are estimated to be 90%, 95%, and 80% for the three samples A (S = 60 nm), B (S = 75 nm), and C (S = 110 nm), respectively.
Fig. 1 (a)-(c) SEM images of sample A, B, and C, respectively. The length of the scale bar is 100 nm. (d)-(f) show the size distributions of the corresponding cubes.
The optical extinction spectra of the Ag nanocubes show a tendency of red shift of the peak when the cube size increases, as shown in Fig. 2 . The dominant absorption peaks due to SPR are located at 460 nm, 490 nm and 680 nm, respectively. An additional peak appears at around 500 nm for sample C, which should be caused by the multipolar mode and could be simulated by the finite difference time domain (FDTD, commercial software FDTD Solutions 8.0) method. But for sample A and B, although the FDTD results indicate the existence of the sharp peaks at around 430 nm, this multipolar effect is not observed experimentally. These are consistent with previous reports which stated that the effect is suppressed for nano-particles with the size smaller than 100 nm [29, 31 ]. The absorption peak for sample C is broader than the other two, which should be caused by the much widely distributed cube size indicated by the SEM images in Fig. 1(f).
Fig. 2 Optical extinction spectra of sample A, B and C. Solid curves are experimental data and dash curves are the FDTD simulations.
In order to obtain the nonlinear optical parameters, the imaginary and real part of the third-order optical nonlinearity due to NLA and NLR are measured by the open- and closed-aperture Z-scan experiments, respectively. The variety of the nonlinear absorption coefficient and refractive index with the laser wavelength and irradiance are extracted from this measurement.
The normalized open-aperture transmittance T OP as a function of the sample position Z for a standard Gaussian laser beam is given by [32],
where q 0 = βI 0 L eff, L eff = [1-exp(-αL)]/α is effective thickness of the sample, I 0 is the peak irridiance at the focus point (z = 0), z 0 = nω 0 2/2 is Rayleigh length and ω 0 is radius of the beam spot at focus (z = 0). A typical open-aperture Z-scan data is shown in Fig. 3(a) , the very small peak indicates a small negative value of the coefficient β. This is contrary to most previous results and calculations on Ag nanomaterials in the off-resonant region [8, 33 ]. One possible reason is related to the two-photon process which was used to intepret the positive optical nonlinear refractive index in [34]. Note that the imaginary part and the real part of the nonlinear polarization shows the same frequency dependence (the fundamental and double laser frequency, the SPR frequency) [35], Eq. (2) in [34] could also be used to evaluate the sign of the nonlinear absorption. In this study, the possible two-photon transitions at the laser wavelength (360-460 nm) is in the vicinity of the SPR region (400-550 nm), which may induce the resonant contributions to the nonlinear absorption and caused the negative nonlinear absorption coefficient.Fig. 3 (a) and (c) show the open- and closed- aperture z-scan results at λ = 800 nm for sample A, open symbols are experiment data and the solid lines are fitting curves using Eq. (2)-(3) . (b) Optical NLA coefficient β and (d) NLR index γ versus laser wavelength at I 0 = 0.25 GW/cm2 for the three samples. (e), (f), and (g) are the electric field distribution of a single cube with the edge size of 60 nm, 75 nm, and 110 nm, respectively.
The ratio of the closed-aperture transmittance T CL to the corresponding open-aperture one T OP was taken to evaluate γ. A typical peak-to-valley curve of sample A is shown in Fig. 3(c). The data can be fit by the expression [21]:
where ΔΦ0 = γkI 0 L eff, γ is the nonlinear refraction index and k is the wave number. The peak-to-valley lineshape indicates γ is negative and self-defocusing of the laser beam occurs near the focus. One may note that the thermal effect is highly depressed because of this self-defocusing, and a very low peak irradiance (I 0 = 0.25 GW/cm2) was applied during the measurements. Figure 3(d) shows in the wavelength region of 720-920 nm, the |γ| value generally increases with decreasing cube size. Additionally, this value does not change very much for sample A and B, while for sample C, a small increase of |γ| at λ > 850 nm was observed. The behaviors of the optical nonlinearity results could be intepreted by the local field enhancement effect [23].In order to evaluate this local field enhancement, the electric field distributions in the vicinity of the Ag nanocubes are evaluated by the FDTD method and shown in Fig. 3(e)-(g). The laser is set propagating along z-axis with the electric field parallel to x direction and wavelength of 800 nm. With increasing S, the field enhanced factor f(ω) becomes smaller, which are 2.2, 1.7, 1.4 for sample A, B, and C respectively. As χ (3) µ f 2(ω)|f(ω)|2 [20], the factors of χ (3) become 6.1: 2.2: 1, which are in accord with the experimental results = 7.24 × 10−19 m2/V2, = 2.3 × 10−19 m2/V2, and = 9.76 × 10−20 m2/V2 evaluated by the equation , in which and , are the real and imaginary part of χ (3), ε 0 is the dielectric constant in vaccum, n 0 is the linear refractive index of the solution, and ω is the circular frequency of the laser. One may note that the susceptibility is mainly contributed by the nonlinear refraction. The ratio of |γ| from Fig. 3(d) approximates to that of the above |χ (3)|, indicating the local field enhancement plays a dominant role on the value of the nonlinear refraction. This is consistent with our data: smaller cubes show larger nonlinear refraction. The increase at 850 nm for sample C may result from the local field enhancement from the multi-polar mode through the two-photon process. Although we didn’t make the simulation for it, the increasing optical extinction for sample C at around 420 nm in Fig. 2 would be an indication.
For making optical switching devices, the conditions W > 1 and T < 1 should be satisfied. From Eq. (1) one may know that sample A should have the best performance. As shown in Fig. 4(a) , with increasing laser irradiance, the values of W for sample A firstly increase linearly with I 0, and then saturate at I0 = 1.48 GW/cm2. In contrast, the values of T are generally much smaller than 1. At the wavelength range from 720 nm to 920 nm and irradiance of I 0 = 0.25 GW/cm2, W is 1.2~1.8 and T is smaller than 0.3, as shown in Fig. (b). Compared with the previous reported FOMs of Au nanoparticles [25], the value of W for the Ag nanocube is much larger, as indicated in Table 1 .
Table 1. FOMs value of the Ag cubes in the current study and the Au nanoparticlesa
4. Conclusion
In summary, we have synthezied Ag nanocubes with the average size of 60 nm, 75 nm and 110 nm and investigated their linear and nonlinear optical properties. The smallest cube shows the largest nonlinear refraction and smallest linear and nonlinear absorption in the wavelength range from 720 nm to 920 nm, due to local field enhancement. The conditions of W > 1 and T < 1 are satisfied which make the Ag cube a candidate for all-optical switching applications in a broad wavelength and weak irradiance region. The results were supported by the FDTD simulations. Smaller Ag nanocubes with the size below 60 nm are therefore highly required to further improve the performance although aggregation of the Ag nanocubes would often occur.
Acknowledgment
The authors thank to Mr. Zhiwei Wang for the synthesis of the Ag nanocubes, to Prof. Shaoding Liu, Dr. Zhangkai Zhou, and Mr. Jiancai Xue for the assistances in the FDTD simulations. The work is supported by the National Natural Science Foundation of China (NSFC) (Nos.: 11104097 and 11404124), the Fundamental Research Funds for the Central Universities (No: 2015TS014), and the Natural Science Foundation of Jiangsu Province (No. BK2011694).
References and links
1. L. Polavarapu, V. Mamidala, Z. P. Guan, W. Ji, and Q. H. Xu, “Huge enhancement of optical nonlinearities in coupled Au and Ag nanoparitcles induced by conjugated polymers,” Appl. Phys. Lett. 100(2), 023106 (2012). [CrossRef]
2. S. Toroghi, C. Lumdee, and P. G. Kik, “Heterogeneous plasmonic trimers for enhanced nonlinear optical absorption,” Appl. Phys. Lett. 106(10), 103102 (2015). [CrossRef]
3. Y. Yu, S. S. Fan, H. W. Dai, Z. W. Ma, X. Wang, J. B. Han, and L. Li, “Plasmon resonance enhanced large third-order optical nonlinearity and ultrafast optical response in Au nanobipyramids,” Appl. Phys. Lett. 105(6), 061903 (2014). [CrossRef]
4. C. Y. Wang, H. Y. Chen, L. Sun, W. L. Chen, Y. M. Chang, H. Ahn, X. Li, and S. Gwo, “Giant colloidal silver crystals for low-loss linear and nonlinear plasmonics,” Nat. Commun. 6, 7734 (2015). [CrossRef] [PubMed]
5. N. N. Lepeshkin, A. Schweinsberg, G. Piredda, R. S. Bennink, and R. W. Boyd, “Enhanced nonlinear optical response of one-dimensional metal-dielectric photonic crystals,” Phys. Rev. Lett. 93(12), 123902 (2004). [CrossRef] [PubMed]
6. V. Singh and P. Aghamkar, “Surface plasmon enhanced third-order optical nonlinearity of Ag nanocomposite film,” Appl. Phys. Lett. 104(11), 111112 (2014). [CrossRef]
7. M. Nidya, M. Umadevi, P. Sankar, R. Philip, and B. J. M. Rajkumar, “L-Phenylalanine functionalized silver nanoparticles: photocatalytic and nonlinear optical applications,” Opt. Mater. 42, 152–159 (2015). [CrossRef]
8. Q. Q. Wang, J. B. Han, H. M. Gong, D. J. Chen, X. J. Zhao, J. Y. Feng, and J. J. Ren, “Linear and nonlinear optical properties of Ag nanowire polarizing glass,” Adv. Funct. Mater. 16(18), 2405–2408 (2006). [CrossRef]
9. Z. Chen, H. Dai, J. Liu, H. Xu, Z. Li, Z. K. Zhou, and J. B. Han, “Dipole plasmon resonance induced large third-order optical nonlinearity of Au triangular nanoprism in infrared region,” Opt. Express 21(15), 17568–17575 (2013). [CrossRef] [PubMed]
10. S. Toroghi, C. Lumdee, and P. G. Kik, “Heterogeneous plasmonic trimers for enhanced nonlinear optical absorption,” Appl. Phys. Lett. 106(10), 103102 (2015). [CrossRef]
11. C. Torres-Torres, N. Peréa-López, J. A. Reyes-Esqueda, L. Rodríguez-Fernández, A. Crespo-Sosa, J. C. Cheang-Wong, and A. Oliver, “Ablation and optical third-order nonlinearities in Ag nanoparticles,” Int. J. Nanomedicine 5, 925–932 (2010). [CrossRef] [PubMed]
12. K. Ogusu, J. Yamasaki, S. Maeda, M. Kitao, and M. Minakata, “Linear and nonlinear optical properties of Ag-As-Se chalcogenide glasses for all-optical switching,” Opt. Lett. 29(3), 265–267 (2004). [CrossRef] [PubMed]
13. L. Karvonen, J. Ronn, S. Kujala, Y. Chen, A. Saynatjoki, A. Tervonen, Y. Svirko, and S. Honkanen, “High non-resonant third-order optical nonlinearity of Ag-glass nanocomposite fabricated by two-step ion exchange,” Opt. Mater. 36(2), 328–332 (2013). [CrossRef]
14. H. Kind, H. Q. Yan, B. Messer, M. Law, and P. D. Yang, “Nanowire ultraviolet photodetectors and optical switches,” Adv. Mater. 14(2), 158–160 (2002). [CrossRef]
15. Y. H. Lee, Y. L. Yan, L. Polavarapu, and Q. H. Xu, “Nonlinear optical switching behavior of Au nanocubes and nano-octahedra investigated by femtosecond Z-scan measurements,” Appl. Phys. Lett. 95(2), 023105 (2009). [CrossRef]
16. X. Y. Hu, P. Jiang, C. Xin, H. Yang, and Q. H. Gong, “Nano-Ag: polymeric composite material for ultrafast photonic crystal all-optical switching,” Appl. Phys. Lett. 94(3), 031103 (2009). [CrossRef]
17. R. A. Ganeev, M. Baba, A. I. Ryasnyansky, M. Suzuki, and H. Kuroda, “Characterization of optical and nonlinear optical properties of silver nanoparticles prepared by laser ablation in various liquids,” Opt. Commun. 240(4–6), 437–448 (2004). [CrossRef]
18. A. Sarkar, A. Thankappan, and V. P. N. Nampoori, “Effect of silver nanoparticles on fluorescence and nonlinear properties of naturally occurring betacyanin dye,” Opt. Meter. 39, 211–217 (2015). [CrossRef]
19. F. Chen, J. Cheng, S. Dai, Z. Xu, W. Ji, R. Tan, and Q. Zhang, “Third-order optical nonlinearity at 800 and 1300 nm in bismuthate glasses doped with silver nanoparticles,” Opt. Express 22(11), 13438–13447 (2014). [CrossRef] [PubMed]
20. G. Yang, W. T. Wang, Y. L. Zhou, H. B. Lu, G. Z. Yang, and Z. H. Chen, “Linear and nonlinear optical properties of Ag nanocluster/BaTiO3 composite fims,” Appl. Phys. Lett. 81(21), 3969–3971 (2002). [CrossRef]
21. Y. Hamanaka, A. Nakamura, S. Omi, N. Del Fatti, F. Vallée, and C. Flytzanis, “Ultrafast response of nonlinear refractive index of silver nanocrystals embedded in glass,” Appl. Phys. Lett. 75(12), 1712–1714 (1999). [CrossRef]
22. A. Boltasseva and H. A. Atwater, “Materials science. Low-loss plasmonic metamaterials,” Science 331(6015), 290–291 (2011). [CrossRef] [PubMed]
23. R. A. Ganeev, A. I. Ryasnyansky, A. L. Stepanov, and T. Usmanov, “Saturated absorption and nonlinear refraction of silicate glasses doped with silver nanoparticles at 532 nm,” Opt. Quantum Electron. 36(10), 949–960 (2004). [CrossRef]
24. S. L. Luo, Y. Chen, G. H. Fan, F. Q. Sun, and S. L. Qu, “Saturable absorption and reverse saturable absorption on silver particles with different shapes,” Appl. Phys., A Mater. Sci. Process. 117(2), 891–894 (2014). [CrossRef]
25. P. M. Santiago, L. T. Rivera, A. C. Sosa, A. Oliver, and J. A. R. Esqueda, “Size-and shape-depenent nonlinear optical response of Au nanoparticles embedded in sapphire,” Opt. Mater. Express 4(1), 92–100 (2014). [CrossRef]
26. Q. Zhang, W. Li, L. P. Wen, J. Chen, and Y. Xia, “Facile synthesis of Ag nanocubes of 30 to 70 nm in edge length with CF3COOAg as a precursor,” Chemistry 16(33), 10234–10239 (2010). [CrossRef] [PubMed]
27. R. Sato, M. Ohnuma, K. Oyoshi, and Y. Takeda, “Experimental investigation of nonlinear optical properties of Ag nanoparticles: effects of size quantization,” Phys. Rev. B 90(12), 125417 (2014). [CrossRef]
28. Q. Y. Wang, X. Q. Cui, W. M. Guan, L. Zhang, X. F. Fan, Z. Shi, and W. T. Zhang, “Shape-dependent catalytic activity of oxygen reduction reaction (ORR) on silver nanodecahedra and nanocubes,” J. Power Sources 269, 152–157 (2014). [CrossRef]
29. S. Linic, P. Christopher, and D. B. Ingram, “Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy,” Nat. Mater. 10(12), 911–921 (2011). [CrossRef] [PubMed]
30. Z. L. Huang, G. W. Meng, Q. Huang, B. Chen, F. Zhou, X. Y. Hu, Y. W. Qian, H. B. Tang, F. M. Han, and Z. Q. Chu, “Polyacrylic acid sodium salt film entrapped Ag-nanocubes as molecule traps for SEMS detection,” Nano Res. 7(8), 1177–1187 (2014). [CrossRef]
31. D. Seo, C. I. Yoo, J. C. Park, S. M. Park, S. Ryu, and H. Song, “Directed surface overgrowth and morphology control of polyhedral gold nanocrystals,” Angew. Chem. Int. Ed. Engl. 47(4), 763–767 (2008). [CrossRef] [PubMed]
32. M. Sheik-Bahae, A. A. Said, T. H. Wei, D. J. Hagan, and E. W. Van Stryland, “Sensitive measurement of optical nonlinearities using a single beam,” IEEE J. Quantum Electron. 26(4), 760–769 (1990). [CrossRef]
33. Y. Hamanaka, A. Nakamura, N. Hayashi, and S. Omi, “Dispersion curves of complex third-order optical susceptibilities around the surface palsmon resonance in Ag nanocrystal-glass composites,” J. Opt. Soc. Am. B 20(6), 1227–1232 (2003). [CrossRef]
34. R. A. Ganeev, A. I. Ryasnyansky, A. L. Stepanov, C. Marques, R. C. da Silva, and E. Alves, “Application of RZ-scan technique for investigation of nonlinear refraction of sapphire doped with Ag, Cu, and Au nanoparticles,” Opt. Commun. 253(1–3), 205–213 (2005). [CrossRef]
35. J. F. Reintjes, Nonlinear Optical Parametrical Processes in Liquids and Gases (Academic, 1984).
