- Email: [email protected]
Non-linear optical properties of silver nanoparticles prepared by hydrogen reduction method
Optics Communications 283 (2010) 1650–1653
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Non-linear optical properties of silver nanoparticles prepared by hydrogen reduction method M.H. Majles Ara a,*, Z. Dehghani b, R. Sahraei c, G. Nabiyouni d a
Department of Physics, Tarbiat Moallem University, P.O. Box 15614, Tehran, Iran Department of Physics, Tarbiat Modares University, Tehran, Iran c Department of Chemistry, University of Ilam, P.O. Box 65315-516, Ilam, Iran d Department of Physics, University of Arak, Arak, Iran b
a r t i c l e
i n f o
Article history: Received 19 May 2009 Received in revised form 11 September 2009 Accepted 11 September 2009
Keywords: Nanomaterials Optical materials and properties z-Scan technique
a b s t r a c t Silver nanoparticles have been prepared using hydrogen gas as the reducing agent for silver nitrate and poly(vinyl pyrrolidone) as the capping agent; the reaction was carried out at 70 °C for 3 h. The size of the nanoparticles was found to be about 20 nm as analyzed using transmission electron micrographs. The Xray diffraction pattern revealed the face-centered cubic (fcc) structure of silver nanoparticles. The linear absorption of Ag nanoparticles, a, is obtained about 3.71 cm1. The non-linear refractive indices of silver nanoparticles were defined by the z-scan technique using CW He–Ne laser (k = 632.8 nm) at different incident intensities. The magnitude of non-linear refractive index (n2) was measured to be in the order of 107 (cm2/W) with a negative sign. Therefore self-defocusing phenomena is taking placed for Ag nanoparticles. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Research on silver (Ag) nanoparticles has been triggered by their potential applications in optics, photography, catalysis, biological labeling, photonics, optoelectronics, and surface-enhanced Raman scattering (SERS) detection [1–4]. Additionally, silver nanoparticles have a surface plasmon resonance absorption in the UV–Vis region. The surface plasmon band arises from the coherent existence of free electrons in the conduction band due to the small particle size effect [5]. The band shift is dependent on the particle size, surrounding chemical and dielectric constant [6]. The formation of metal nanoparticles dispersed in solid dielectric materials, which can result in novel optical properties, have been of increasing interest because of their potential applications in non-linear optics [7]. The use of the silver nanoparticles as non-linear optical materials seems to open a new area for such materials [8]. The aim of this work is to develop a simple method to modify the synthesis of long-time stable Ag nanoparticles and study the non-linear optical properties of these nanoparticles under low power continuous wave (CW) He–Ne laser irradiation. Non-linear refractive index, n2, for three input intensities is easily measured by the z-scan method. The z-scan technique has been extensively used to measure non-linear optical properties of materials because of its simplicity and high accuracy [9–11]. The ob-
* Corresponding author. Tel.: +98 21 8158 3313; fax: +98 21 8830 9294. E-mail address: [email protected] (M.H. Majles Ara). 0030-4018/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2009.09.025
tained nanoparticles were also characterized by means of UV–Vis absorption spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM). 2. Experimental Colloidal Ag nanoparticles were prepared in a 1.0 L Pyrex round-bottom three-neck flask equipped with a thermometer, a condenser, and a spout for drawing samples from the third neck. Each synthesis followed the same basic procedure: 160 mg AgNO3 was dissolved in 200 ml of doubly distilled water and then mixed with 5 g of poly(vinyl pyrrolidone) denoted PVP dissolved in 200 ml hot pure water. The mixture was then poured into the flask for reduction by H2 gas. The flask and its contents were then heated to 70 °C, and this temperature was maintained throughout the synthesis. Aliquots can be taken through the spout as the reaction progresses. Removing the aliquots from the reducing environment stops further growth of the particles. Likewise, the reaction can be stopped at any time simply by releasing the gas from the vessel. The UV–Vis absorption data were collected on a Perkin–Elmer Lambda 25 spectrophotometer using a quartz cell with an optical path of 1 cm. X-ray diffraction (XRD) pattern was recorded with an automated Philips X’ Pert X-ray diffractometer with Cu Ka radiation (40 kV and 30 mA) for 2h values over 20–70°. TEM image was obtained using a JEOL model JEM-100CX microscope at an acceleration voltage of 80 kV. The specimens were prepared by dropping the nanoparticle dispersion onto an amorphous carbon-coated
M.H. Majles Ara et al. / Optics Communications 283 (2010) 1650–1653
300 mesh copper grid and allowing the solvent to evaporate. The investigation of the non-linear optical characteristics of silver nanoparticles suspensions was carried out using the CW He–Ne laser and the transmission z-scan technique [12]. In order to describe the non-linear optical properties at the colloid at low concentrations it is used an extension of the MaxwellGamet model [13]. In this model the imaginary part of effective dielectric constant e of a composite medium in the limit of longwavelength and neglecting interactions among particles can be used to express the absorption coefficient a as 3
a¼
18pqe2k e0m 0 k ðem þ 2ek Þ2 þ e002 m
ð1Þ
where q is the volume fraction, em and ek are the dielectric constant of the metallic particle and the host respectively, em = e0m þ ie0m and k is the light wavelength. The z-scan experimental set-up for the CW laser is shown in Fig. 1. The samples were moved along the z-axis through the focal plane of 8 cm focal length lens. It is well known that the relationship of the effective non-linear refractive index, n2, and the total index of refraction, n, is n = n0 + n2I0, where n0 is the linear index of refraction, I0 = p2pxin2 is 0
the incident illumination intensity at focal point, Pin is the laser power, and x0 is the radius of the waist of the illumination beam inside the sample. For the following case of (a) k = 632.8 nm, x0 = 42 lm, the condition z0 ¼ px20 k L can be satisfied. Here the sample thickness, L is 1 mm, thus the sample can be considered a thin medium. The phase shift |D/0| on the optical axis can be obtained from Eq. (2).
DT pv ¼ f jD/0 j
for jD/0 j 6 p
ð2Þ
where DTpv is the difference between the normalized peak transmittance and valley transmittance and f = 0.406(1S)0.25 is an experimental constant, where S is the aperture’s linear transmittance. |D/0| relates to n2 through the following expression:
2p 2p n2 I0 Leff jD/0 j ¼ DnLeff ¼ k k
1651
Absorption spectra measurements were extended to much longer times than the 3 h shown in Fig. 2. The results show that the surface plasma resonance absorption peaks are located in the 400 nm and as the reaction proceeds, no displacement occurs in the position of the extinction maximum of peaks. In other words, as the time of reaction increases, the new nanoparticles are formed and no further growth of already existing nanoparticles occurs. The evolution of the maximum in the absorption spectra starts from the initial peak position around 400 nm representing the dipole component of the plasmon resonance of small silver nanoparticles. As the number of nanoparticles increases, the intensity of the resonance increases and its position remain at the same wavelength. The nanoparticles synthesized by hydrogen reduction appear to be crystalline as crystal faces are evident in TEM image (Fig. 4). XRD pattern of silver nanoparticles powder is shown in Fig. 3. The pattern exhibits peaks at 2h angles of 38.17, 44.21, 64.32, and 77.12 that correspond to the [1 1 1], [2 0 0], [2 2 0], and [3 1 1] crystal planes of a cubic lattice structure of silver nanoparticles, respectively [18]. From the full-width at half-maximum of diffraction peaks, the average size of the silver nanoparticles has been calculated using the Debye–Scherrer equation [19,20]. The calculated average size of Ag nanoparticles was around 22 nm. Crystallinity of the nanoparticles was further studied by TEM. Fig. 4 shows the TEM image of silver nanoparticles along with their electron diffraction pattern. The solutions were contained in 1 mm thick quartz cells. The concentration of silver nanoparticles in suspension was 0.05 mg/ ml. The laser radiation was focused using an 80 mm focal length lens. The linear absorption a = 3.71 cm1 was calculated by measuring the output power for a sample as a function of the power without the sample at low power. The linear absorption in low incident powers can be found from Eq. (4).
ð3Þ
where the effective thickness Leff of the sample is defined as a L Leff ¼ ð1ea Þ; where a is the linear absorption coefficient [14]. The sample itself acts as a thin lens with varying focal length as it moves through the focal plane [15]. The non-linear behavior of the sample is equivalent to the formation of an induced positive or negative lens for self-focusing (positive) or self-defocusing (negative) [16]. 3. Results and discussion Silver nanoparticles absorb radiation in the 380–450 nm regions of the electromagnetic spectrum due to the excitation of surface plasmon vibrations and this is responsible for the striking yellow–brown color of silver nanoparticles in various media [17].
Fig. 1. Close aperture z-scan technique setup.
Fig. 2. Evolution of absorption spectra of the Ag nanoparticles taken at 15 min intervals following the initiation of the reaction for the first 3 h.
Fig. 3. X-ray diffractogram pattern of silver nanoparticles powder.
1652
M.H. Majles Ara et al. / Optics Communications 283 (2010) 1650–1653
Fig. 4. (a) TEM image of the silver nanoparticles, (b) selected-area electron-diffraction pattern from an ensemble of silver nanoparticles.
1 L
a ¼ Ln
P P0
ð4Þ
The high linear absorption results the thermal effect and the non-linear optical properties. The non-linear refraction is induced from thermo optical effect and the values of n2 in the focal point obtained using z-scan curves. The non-linear optical properties of Ag nanoparticles were investigated using a single Gaussian beam of continuous-wave (CW) He:Ne laser at a wavelength of 632.8 nm. The effective non-linear coefficient, (n2) of Ag nanoparticles was determined by close aperture z-scan technique. An aperture was fixed at the distance of 120 cm from the focal plane (closed-aperture scheme). This technique is a simple and sensitive method to make simultaneous measurements of the sign and magnitude of refractive nonlinearities of optical materials [12]. By a = 3.71 cm1, x0 = 42 lm and the values of S, after some simple calculation, the n2 of this suspension can be determined as reported in Table 1. In the thermal effect process the non-linear refraction index is not a constant value and depends to the incident intensity [21]. Moreover, the decrease of n2 with increasing incident laser intensity may ascribe to thermal NLO effect contribution, which is negative, to non-linear refractive index of nanoparticles, because the thermal NLO effect could not be eliminated completely using CW laser [22]. Table 1 Calculated values of linear absorption coefficient and non-linear refractive index at three incident intensities. I0 (kW/cm2)
a (cm1)
D U0
n2 (cm2/W)
DN
P0 (mW)
1.80 0.721 0.641
3.71 3.71 3.71
3.76 2.26 1.21
2.53 107 3.79 107 4.04 107
1.4 0.85 0.35
50 20 15
0
10
20
30
40
50
60
Fig. 6. Open aperture z-scan experimental curve of silver nanoparticles suspension.
The normalized curves of close aperture z-scan have a pair of sharp peak and valley which can be identified as a self-defocusing material (Fig. 5). Since the peak of the transmittance precedes the valley, the sign of the refractive nonlinearity of Ag nanoparticles is negative (i.e., the negative lens effect). Fig. 6 shows open aperture result for silver nanoparticles. The measured open-aperture z-scans exhibit that the z-scan trace at three incident intensities is nearly flat, indicating that no two photon absorption occurs in the sample by CW He–Ne laser. It should be pointed that non-linear absorption was not observed for Ag nanoparticles. As we can see in Refs. [23,24] that consider non-linear optical properties of Ag nanoparticles using a CW laser, the behavior of close aperture z-scan is similar to our sample. In Refs. [23,24] the sign of non-linear refraction is negative and the order of n2 is about 108 (cm2/W) [23] and 107 (cm2/W) [24]. In our sample the sign of n2 is negative too and the order of n2 is about 107 (cm2/W). In this paper, the open aperture z-scan did not result and the sample had a negligible non-linear absorption which is similar to the result of Ref. [23]. These results suggest that the Ag nanoparticles could be considered as a promising candidate for optical device applications such as optical limiting and switching. 4. Conclusion
Fig. 5. Close aperture z-scan experimental curves of silver nanoparticles suspension under He–Ne laser beam with 632.8 nm wavelength at 3 input intensities.
Silver nanoparticles in stable aqueous suspensions were prepared via reduction of silver nitrate (AgNO3) by hydrogen gas. The nanoparticles were characterized by their UV–Vis absorption spectra, by X-ray diffraction, and by transmission electron microscopy. The average particle size was obtained from TEM analysis is 20 ± 5 nm that agreed fairly well with XRD results. The non-linear optical properties of the Ag nanoparticles were also studied using the z-scan technique. No non-linear absorption, b, was measurable
M.H. Majles Ara et al. / Optics Communications 283 (2010) 1650–1653
for these nanoparticles by CW He–Ne laser at 632.8 nm. The nonlinear refraction, n2, is obtained to be in the order of 107. The negative sign of non-linear refraction indicate self-defocusing phenomena for Ag nanoparticles and the values of the refractive nonlinearities of silver nanoparticles are of interest from the application point of view. References [1] Y. Deng, G. Dang, H. Zhou, X. Rao, C.H. Chen, Mater. Lett. 62 (2008) 1143. [2] D. Philip, K.G. Gopchandran, C. Unni, K.M. Nissamudeen, J. Spectrochim. Acta A: Mol. Biomol. Spectrosc. 70 (2007) 780. [3] G.J. Kears, E.W. Foster, J.E. Hutchison, Anal. Chem. 78 (2006) 298. [4] L. Rivas, C.S. Sanchez, J.V. Garcia-Ramos, G. Morcillo, Langmuir 17 (2001) 574. [5] A.M. Smith, H. Duan, M.N. Rhyner, G. Ruan, S.A. Nie, Phys. Chem. Chem. Phys. 8 (2006) 3895. [6] L.M. Liz-Marzan, Langmuir 22 (2006) 32. [7] A. Babapour, A.B. Akhavan, A.Z. Moshfegh, A.A. Hosseini, Thin Solid Films 515 (2006) 771. [8] I.V. Kityk, J. Ebothé, K. Ozgad, K.J. Plucinskie, G. Changc, K. Kobayashi, M. Oyama, Physica E 31 (2006) 38. [9] M. Sheik-bahae, A.A. Said, T.H. Wei, D.J. Hagan, E.W. Van Stryland, IEEE J. Quant. Electron. 26 (1990) 760.
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[10] M.H. Majles Ara, E. Koushki, S. Salmani, S.H. Mousavi, Opt. Commun. 278 (2007) 418. [11] S.Q. Chen, W.P. Zang, Z.B. Liu, W.Y. Zhou, Y.F. Kong, J.G. Tian, Opt. Commun. 274 (2007) 213. [12] R.A. Ganeev, A.I. Ryasnyanskiy, T. Usmanov, Opt. Commun. 272 (2007) 242. [13] D. Ricard, Ph. Roussignol, C. Flytzanis, Opt. Lett. 10 (1985) 511. [14] B. Yu, C. Zhu, F. Gan, Physica E 8 (2000) 360. [15] L. Ning-ning, S. Jia-ming, P. Shao-hua, Ch. Zheng-hao, Sh. Wen-sheng, W. Rongping, W. Xiao-guang, Opt. Commun. 176 (2000) 239. [16] A.Y. Fasasi, M. Maaza, Ch. Theron, P. Neethling, U. Buttner, A. Leitch, A.K. Chaudhary, Thin Solid Films 516 (2008) 6233. [17] H. Huang, X. Yang, Carbohydr. Res. 339 (2004) 2627. [18] X. Wang, Y. Chen, Mater. Lett. 62 (2008) 4366. [19] M. Habib Ullah, I.I. Kim, Chang-Sik Ha, Mater. Lett. 61 (2007) 4267. [20] A. Goudarzi, G.M. Aval, R. Sahraei, H. Ahmadpoor, Thin Solid Films 516 (2008) 4953. [21] Palfalvi L. Hebling, J. Appl. Phys. B. 78 (2004) 775. [22] C. Li, G. Shi, Y. Song, X. Zhang, Sh. Guang, H. Xu, Phys. Chem. Solids 69 (2008) 1829. [23] Jia Tingjian, He Tingchao, Li Pengwei, Mo Yujun, Cui Yuting, Opt. Laser Technol. 40 (2008) 936. [24] M. Torres-Cisneros, Yanagihara Naohisa, B. Gonzalez-Rolon, M.A. MenesesNava, O.G. Ibarra-Manzano, D.A. May-Arrioja, J. Sánchez-Mondragón, E. Aguilera-Gómez, L.A. Aguilera-Cortés, Microelectron. J. 40 (2009) 621.
Contents lists available at ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Non-linear optical properties of silver nanoparticles prepared by hydrogen reduction method M.H. Majles Ara a,*, Z. Dehghani b, R. Sahraei c, G. Nabiyouni d a
Department of Physics, Tarbiat Moallem University, P.O. Box 15614, Tehran, Iran Department of Physics, Tarbiat Modares University, Tehran, Iran c Department of Chemistry, University of Ilam, P.O. Box 65315-516, Ilam, Iran d Department of Physics, University of Arak, Arak, Iran b
a r t i c l e
i n f o
Article history: Received 19 May 2009 Received in revised form 11 September 2009 Accepted 11 September 2009
Keywords: Nanomaterials Optical materials and properties z-Scan technique
a b s t r a c t Silver nanoparticles have been prepared using hydrogen gas as the reducing agent for silver nitrate and poly(vinyl pyrrolidone) as the capping agent; the reaction was carried out at 70 °C for 3 h. The size of the nanoparticles was found to be about 20 nm as analyzed using transmission electron micrographs. The Xray diffraction pattern revealed the face-centered cubic (fcc) structure of silver nanoparticles. The linear absorption of Ag nanoparticles, a, is obtained about 3.71 cm1. The non-linear refractive indices of silver nanoparticles were defined by the z-scan technique using CW He–Ne laser (k = 632.8 nm) at different incident intensities. The magnitude of non-linear refractive index (n2) was measured to be in the order of 107 (cm2/W) with a negative sign. Therefore self-defocusing phenomena is taking placed for Ag nanoparticles. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction Research on silver (Ag) nanoparticles has been triggered by their potential applications in optics, photography, catalysis, biological labeling, photonics, optoelectronics, and surface-enhanced Raman scattering (SERS) detection [1–4]. Additionally, silver nanoparticles have a surface plasmon resonance absorption in the UV–Vis region. The surface plasmon band arises from the coherent existence of free electrons in the conduction band due to the small particle size effect [5]. The band shift is dependent on the particle size, surrounding chemical and dielectric constant [6]. The formation of metal nanoparticles dispersed in solid dielectric materials, which can result in novel optical properties, have been of increasing interest because of their potential applications in non-linear optics [7]. The use of the silver nanoparticles as non-linear optical materials seems to open a new area for such materials [8]. The aim of this work is to develop a simple method to modify the synthesis of long-time stable Ag nanoparticles and study the non-linear optical properties of these nanoparticles under low power continuous wave (CW) He–Ne laser irradiation. Non-linear refractive index, n2, for three input intensities is easily measured by the z-scan method. The z-scan technique has been extensively used to measure non-linear optical properties of materials because of its simplicity and high accuracy [9–11]. The ob-
* Corresponding author. Tel.: +98 21 8158 3313; fax: +98 21 8830 9294. E-mail address: [email protected] (M.H. Majles Ara). 0030-4018/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.optcom.2009.09.025
tained nanoparticles were also characterized by means of UV–Vis absorption spectroscopy, X-ray diffraction (XRD), and transmission electron microscopy (TEM). 2. Experimental Colloidal Ag nanoparticles were prepared in a 1.0 L Pyrex round-bottom three-neck flask equipped with a thermometer, a condenser, and a spout for drawing samples from the third neck. Each synthesis followed the same basic procedure: 160 mg AgNO3 was dissolved in 200 ml of doubly distilled water and then mixed with 5 g of poly(vinyl pyrrolidone) denoted PVP dissolved in 200 ml hot pure water. The mixture was then poured into the flask for reduction by H2 gas. The flask and its contents were then heated to 70 °C, and this temperature was maintained throughout the synthesis. Aliquots can be taken through the spout as the reaction progresses. Removing the aliquots from the reducing environment stops further growth of the particles. Likewise, the reaction can be stopped at any time simply by releasing the gas from the vessel. The UV–Vis absorption data were collected on a Perkin–Elmer Lambda 25 spectrophotometer using a quartz cell with an optical path of 1 cm. X-ray diffraction (XRD) pattern was recorded with an automated Philips X’ Pert X-ray diffractometer with Cu Ka radiation (40 kV and 30 mA) for 2h values over 20–70°. TEM image was obtained using a JEOL model JEM-100CX microscope at an acceleration voltage of 80 kV. The specimens were prepared by dropping the nanoparticle dispersion onto an amorphous carbon-coated
M.H. Majles Ara et al. / Optics Communications 283 (2010) 1650–1653
300 mesh copper grid and allowing the solvent to evaporate. The investigation of the non-linear optical characteristics of silver nanoparticles suspensions was carried out using the CW He–Ne laser and the transmission z-scan technique [12]. In order to describe the non-linear optical properties at the colloid at low concentrations it is used an extension of the MaxwellGamet model [13]. In this model the imaginary part of effective dielectric constant e of a composite medium in the limit of longwavelength and neglecting interactions among particles can be used to express the absorption coefficient a as 3
a¼
18pqe2k e0m 0 k ðem þ 2ek Þ2 þ e002 m
ð1Þ
where q is the volume fraction, em and ek are the dielectric constant of the metallic particle and the host respectively, em = e0m þ ie0m and k is the light wavelength. The z-scan experimental set-up for the CW laser is shown in Fig. 1. The samples were moved along the z-axis through the focal plane of 8 cm focal length lens. It is well known that the relationship of the effective non-linear refractive index, n2, and the total index of refraction, n, is n = n0 + n2I0, where n0 is the linear index of refraction, I0 = p2pxin2 is 0
the incident illumination intensity at focal point, Pin is the laser power, and x0 is the radius of the waist of the illumination beam inside the sample. For the following case of (a) k = 632.8 nm, x0 = 42 lm, the condition z0 ¼ px20 k L can be satisfied. Here the sample thickness, L is 1 mm, thus the sample can be considered a thin medium. The phase shift |D/0| on the optical axis can be obtained from Eq. (2).
DT pv ¼ f jD/0 j
for jD/0 j 6 p
ð2Þ
where DTpv is the difference between the normalized peak transmittance and valley transmittance and f = 0.406(1S)0.25 is an experimental constant, where S is the aperture’s linear transmittance. |D/0| relates to n2 through the following expression:
2p 2p n2 I0 Leff jD/0 j ¼ DnLeff ¼ k k
1651
Absorption spectra measurements were extended to much longer times than the 3 h shown in Fig. 2. The results show that the surface plasma resonance absorption peaks are located in the 400 nm and as the reaction proceeds, no displacement occurs in the position of the extinction maximum of peaks. In other words, as the time of reaction increases, the new nanoparticles are formed and no further growth of already existing nanoparticles occurs. The evolution of the maximum in the absorption spectra starts from the initial peak position around 400 nm representing the dipole component of the plasmon resonance of small silver nanoparticles. As the number of nanoparticles increases, the intensity of the resonance increases and its position remain at the same wavelength. The nanoparticles synthesized by hydrogen reduction appear to be crystalline as crystal faces are evident in TEM image (Fig. 4). XRD pattern of silver nanoparticles powder is shown in Fig. 3. The pattern exhibits peaks at 2h angles of 38.17, 44.21, 64.32, and 77.12 that correspond to the [1 1 1], [2 0 0], [2 2 0], and [3 1 1] crystal planes of a cubic lattice structure of silver nanoparticles, respectively [18]. From the full-width at half-maximum of diffraction peaks, the average size of the silver nanoparticles has been calculated using the Debye–Scherrer equation [19,20]. The calculated average size of Ag nanoparticles was around 22 nm. Crystallinity of the nanoparticles was further studied by TEM. Fig. 4 shows the TEM image of silver nanoparticles along with their electron diffraction pattern. The solutions were contained in 1 mm thick quartz cells. The concentration of silver nanoparticles in suspension was 0.05 mg/ ml. The laser radiation was focused using an 80 mm focal length lens. The linear absorption a = 3.71 cm1 was calculated by measuring the output power for a sample as a function of the power without the sample at low power. The linear absorption in low incident powers can be found from Eq. (4).
ð3Þ
where the effective thickness Leff of the sample is defined as a L Leff ¼ ð1ea Þ; where a is the linear absorption coefficient [14]. The sample itself acts as a thin lens with varying focal length as it moves through the focal plane [15]. The non-linear behavior of the sample is equivalent to the formation of an induced positive or negative lens for self-focusing (positive) or self-defocusing (negative) [16]. 3. Results and discussion Silver nanoparticles absorb radiation in the 380–450 nm regions of the electromagnetic spectrum due to the excitation of surface plasmon vibrations and this is responsible for the striking yellow–brown color of silver nanoparticles in various media [17].
Fig. 1. Close aperture z-scan technique setup.
Fig. 2. Evolution of absorption spectra of the Ag nanoparticles taken at 15 min intervals following the initiation of the reaction for the first 3 h.
Fig. 3. X-ray diffractogram pattern of silver nanoparticles powder.
1652
M.H. Majles Ara et al. / Optics Communications 283 (2010) 1650–1653
Fig. 4. (a) TEM image of the silver nanoparticles, (b) selected-area electron-diffraction pattern from an ensemble of silver nanoparticles.
1 L
a ¼ Ln
P P0
ð4Þ
The high linear absorption results the thermal effect and the non-linear optical properties. The non-linear refraction is induced from thermo optical effect and the values of n2 in the focal point obtained using z-scan curves. The non-linear optical properties of Ag nanoparticles were investigated using a single Gaussian beam of continuous-wave (CW) He:Ne laser at a wavelength of 632.8 nm. The effective non-linear coefficient, (n2) of Ag nanoparticles was determined by close aperture z-scan technique. An aperture was fixed at the distance of 120 cm from the focal plane (closed-aperture scheme). This technique is a simple and sensitive method to make simultaneous measurements of the sign and magnitude of refractive nonlinearities of optical materials [12]. By a = 3.71 cm1, x0 = 42 lm and the values of S, after some simple calculation, the n2 of this suspension can be determined as reported in Table 1. In the thermal effect process the non-linear refraction index is not a constant value and depends to the incident intensity [21]. Moreover, the decrease of n2 with increasing incident laser intensity may ascribe to thermal NLO effect contribution, which is negative, to non-linear refractive index of nanoparticles, because the thermal NLO effect could not be eliminated completely using CW laser [22]. Table 1 Calculated values of linear absorption coefficient and non-linear refractive index at three incident intensities. I0 (kW/cm2)
a (cm1)
D U0
n2 (cm2/W)
DN
P0 (mW)
1.80 0.721 0.641
3.71 3.71 3.71
3.76 2.26 1.21
2.53 107 3.79 107 4.04 107
1.4 0.85 0.35
50 20 15
0
10
20
30
40
50
60
Fig. 6. Open aperture z-scan experimental curve of silver nanoparticles suspension.
The normalized curves of close aperture z-scan have a pair of sharp peak and valley which can be identified as a self-defocusing material (Fig. 5). Since the peak of the transmittance precedes the valley, the sign of the refractive nonlinearity of Ag nanoparticles is negative (i.e., the negative lens effect). Fig. 6 shows open aperture result for silver nanoparticles. The measured open-aperture z-scans exhibit that the z-scan trace at three incident intensities is nearly flat, indicating that no two photon absorption occurs in the sample by CW He–Ne laser. It should be pointed that non-linear absorption was not observed for Ag nanoparticles. As we can see in Refs. [23,24] that consider non-linear optical properties of Ag nanoparticles using a CW laser, the behavior of close aperture z-scan is similar to our sample. In Refs. [23,24] the sign of non-linear refraction is negative and the order of n2 is about 108 (cm2/W) [23] and 107 (cm2/W) [24]. In our sample the sign of n2 is negative too and the order of n2 is about 107 (cm2/W). In this paper, the open aperture z-scan did not result and the sample had a negligible non-linear absorption which is similar to the result of Ref. [23]. These results suggest that the Ag nanoparticles could be considered as a promising candidate for optical device applications such as optical limiting and switching. 4. Conclusion
Fig. 5. Close aperture z-scan experimental curves of silver nanoparticles suspension under He–Ne laser beam with 632.8 nm wavelength at 3 input intensities.
Silver nanoparticles in stable aqueous suspensions were prepared via reduction of silver nitrate (AgNO3) by hydrogen gas. The nanoparticles were characterized by their UV–Vis absorption spectra, by X-ray diffraction, and by transmission electron microscopy. The average particle size was obtained from TEM analysis is 20 ± 5 nm that agreed fairly well with XRD results. The non-linear optical properties of the Ag nanoparticles were also studied using the z-scan technique. No non-linear absorption, b, was measurable
M.H. Majles Ara et al. / Optics Communications 283 (2010) 1650–1653
for these nanoparticles by CW He–Ne laser at 632.8 nm. The nonlinear refraction, n2, is obtained to be in the order of 107. The negative sign of non-linear refraction indicate self-defocusing phenomena for Ag nanoparticles and the values of the refractive nonlinearities of silver nanoparticles are of interest from the application point of view. References [1] Y. Deng, G. Dang, H. Zhou, X. Rao, C.H. Chen, Mater. Lett. 62 (2008) 1143. [2] D. Philip, K.G. Gopchandran, C. Unni, K.M. Nissamudeen, J. Spectrochim. Acta A: Mol. Biomol. Spectrosc. 70 (2007) 780. [3] G.J. Kears, E.W. Foster, J.E. Hutchison, Anal. Chem. 78 (2006) 298. [4] L. Rivas, C.S. Sanchez, J.V. Garcia-Ramos, G. Morcillo, Langmuir 17 (2001) 574. [5] A.M. Smith, H. Duan, M.N. Rhyner, G. Ruan, S.A. Nie, Phys. Chem. Chem. Phys. 8 (2006) 3895. [6] L.M. Liz-Marzan, Langmuir 22 (2006) 32. [7] A. Babapour, A.B. Akhavan, A.Z. Moshfegh, A.A. Hosseini, Thin Solid Films 515 (2006) 771. [8] I.V. Kityk, J. Ebothé, K. Ozgad, K.J. Plucinskie, G. Changc, K. Kobayashi, M. Oyama, Physica E 31 (2006) 38. [9] M. Sheik-bahae, A.A. Said, T.H. Wei, D.J. Hagan, E.W. Van Stryland, IEEE J. Quant. Electron. 26 (1990) 760.
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