- Email: [email protected]
Third order optical nonlinearity and diffraction pattern of Ni nanoparticles prepared by laser ablation
Optics Communications 286 (2013) 318–321
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Third order optical nonlinearity and diffraction pattern of Ni nanoparticles prepared by laser ablation S. Alikhani a, H. Tajalli a,n, E. Koushki b a b
Department of physics, Science and Research Branch, Islamic Azad University, Tabriz, Iran Kharazmi (Tarbiat Moallem) University, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 May 2012 Received in revised form 31 July 2012 Accepted 1 August 2012 Available online 24 September 2012
In this article we studied all-optical Kerr effect of Ni nanoparticles immersed in ethanol using z-scan method. The nanoparticles were prepared by high frequency pulsed laser ablation. UV–Visible optical absorption spectroscopy and SEM observation were employed for characterization and studying the morphology of Ni nanoparticles. Analysis of scanning electron microscopy (SEM) images showed that the synthesized nanoparticles shape were dominantly spherical, varying from 19 nm to 40 nm for 1 mJ pulse energy. The nonlinear absorption and refraction indices were measured using open- and closedaperture z-scan techniques, with both CW and pulsed irradiations. In both regimes results were studied. Furthermore, diffraction rings pattern as a result of nonlinear refraction was observed. We suggested an opportunity to form a new nonlinear-optical media for nonlinear optical applications. & 2012 Elsevier B.V. All rights reserved.
Keywords: Ni nanoparticles Z-scan Ablation Diffraction rings pattern Third-order nonlinearity All-optical Kerr effect
1. Introduction The search for new materials has motivated scientists in the field of nano-materials to develop innovative approaches to synthesize nanostructures with novel morphologies in order to exploit their remarkable properties. Metal colloids as the prototypical metal nanoparticles have been the subject of numerous investigations due to their exceptional optical properties and high catalytic activities [1–5]. Laser ablation has been demonstrated as a promising approach to synthesize metal colloids [6,7]. The most outstanding advantage of this method compared with the common chemical routes is recognized to be the absence of chemical reagents in metal colloids. Hence, there would be no need to purify the colloids which were prepared by this method from residual ions [8–10]. In the nanoparticles, surface plasmon can be excited by an electromagnetic field of light. It has been found that the peak position and width of the absorption spectrum of a nano-colloid depend on the particle size as well as their shape and environment [11,12]. Mie was the first researcher who described plasmon interactions quantitatively by solving Maxwell’s equations with the appropriate boundary conditions of spherical particles and described the optical absorption by metal particles [12–15].
n
Corresponding author. Tel.: þ984113822203; fax: þ 984113822204. E-mail address: [email protected] (H. Tajalli).
0030-4018/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2012.08.016
In this paper we reported the nonlinear optical properties of Ni nanoparticles. UV–Visible optical absorption spectroscopy and SEM observation were employed for characterization and studying the morphology of Ni nanoparticles. Moreover the nonlinear refraction and absorption indices were measured by using openand closed-aperture z-scan techniques, respectively.
2. Experimental procedure 2.1. Sample preparation Nickel nanoparticles prepared by high frequency pulsed laser ablation of nickel metal plate (99.999%) in ethanol alcohol without any additive. As shown in Fig. 1, the target was located at the bottom of a glass vessel filled with 5 ml ethanol alcohol. To conduct the laser ablation process, the Q-switched Nd:YAG laser with 50 W maximum power and 240 ns pulse duration, and with the wavelength 1064 nm, was employed. It was adjusted to operate at 2 kHz repeat ion rate. The energy of incident laser pulse imposed on the target was 1 mJ with 70 mm spot diameter. The irradiation duration in our experiments was 10 min. Upon irradiation of the laser beam, the solution in the glass vessel gradually turned to dark grey, due to the creation of Ni nanoparticles. In order to have a uniform colloidal solution of Ni nanoparticles, the prepared mixture was subjected to ultrasound radiation. After carrying out the irradiation/ablation procedure, a small amount of solution was transferred into a quartz cell with 1 mm
S. Alikhani et al. / Optics Communications 286 (2013) 318–321
319
Scanner Mirrors
Ni target in ethanol alcohol
Fig. 3. UV/vis absorption of solution colloidal Ni nanoparticles.
BS
Lens
Sample
Aperture
Laser
Reference Detector
Close Aperture Detector
Fig. 4. SEM image of Ni nanoparticles.
Fig. 1. Schematic of the experimental set-up employed for the synthesis of Ni by a Q-switch Nd:YAG laser.
Absorption(1/cm)
25 20 15 10 5 0 200
400
600
800
1000
wavelength(nm) Fig. 2. Schematice of z-scan set-up to investigate the nonlinear absorption and nonlinear refraction of Ni.
diameter for UV–Visible absorption spectroscopy. The employed spectrophotometer was a Perkin Elmer Lambda 25, model. As it is presented at Fig. 2, we observed an intense peak around 202 nm which is attributed to the surface plasmon resonance (SPR) effect. The tail of a broad band extending towards the UV wavelength range is originating from an interband transition. Furthermore, we used SEM images, in order to measure the size and shape of synthesized nanoparticles (Fig. 3). Analysis of SEM images showed that the synthesized nanoparticles shape is dominantly spherical and the average diameter of Ni particles in colloidal solution is about 30 nm. The images indicate that the particles have a semi-normal size distribution with the mode of about 30 nm. For different experiments we prepared a colloidal solution with concentration of 1.71 10 4 g/cm3. 2.2. Z-scan setup The nonlinear absorption and nonlinear refraction of the sample were measured by using the open- and closed-aperture
z-scan technique. The sample was scanned around the waist of the focused Gaussian laser beam along the z axis and the irradiance on the sample varies as a function of z. The z-scan experimental trace of the sample was shown in Fig. 4. The light source was a CW He–Ne laser beam l ¼632.8 nm, with 48 mW maximum power, 30 mm beam waist radius, and the focal length of convergence lens was 8 cm. Using I0 ¼ 2P0 =pw20 the maximum intensity in the beam waist is obtained I0 ¼3.4 kW/cm2. The small amount of the colloid was poured into a quartz cell with 1 mm diameter. In this paper we studied nonlinear optical indexes of Ni nanoparticles. The work was repeated with a Q-switched Nd:YAG laser beam (30 W maximum power, 10 ns pulse duration, the wavelength of 532 nm and adjusted in 200 Hz repetition rate). It was focused at 30 mm beam waist radius (I0 ¼2.123 kW/cm2). The absorption bound is very wide and the absorption of the particles is almost similar in these two wavelengths and our comparison between their nonlinear indexes could be related only to the intensity (Fig. 2).
3. Experimental results and discussion 3.1. Z-scan results Figs. 5–7 show the z-scan curves at three different incident intensities which provide power transmittance as a function of sample position. As it was presented previously [16], pure closeaperture curve can be obtained by dividing the normalized closeaperture curve having nonlinear absorption by the normalized open-aperture. Experimental open and close aperture curves are fitted to theoretical ones (solid curves) which are evaluated with the Gaussian decomposition relations [17,18] and nonlinear optical indices are obtained. Also, nonlinear phase changes Dj0 and correspondingly n2 of the colloid could be obtained using [19]: 9Dj0 9 ¼
2pLeff Dn0
l
ð1Þ
320
S. Alikhani et al. / Optics Communications 286 (2013) 318–321
Open aperture curves have been fitted with [16];
2
P=48mW
a PðzÞ ¼ P 0 eaL
b
1.6
c 1.4
d
1.2
e
1 0.8 0.6 0.4 0.2
z(cm)
0 -4
-2
0
2
4
Fig. 5. Z-scan results with P0 ¼ 48 mW He–Ne laser. (a) Experimental closeaperture data. (b) Experimental open aperture data. (c) Division of experimental close data by open one. (d) Theoretical open aperture z-scan. (e) Theoretical close aperture z-scan.
1.8
a
P=30mW
b
Normalized Transmittance
1.6
c
1.4
d 1.2
ð3Þ
e
1
3.2. Diffraction rings pattern
0.8 0.6 0.4
z(cm)
0.2 0 -4
-2
0
2
4
Fig. 6. Z-scan results with P0 ¼30 mW He–Ne laser. (a) Experimental closeaperture data. (b) Experimental open aperture data. (c) Division of experimental close data by open one. (d) Theoretical open aperture z-scan. (e) Theoretical close aperture z-scan.
1.5
P=10.22mW
1.4
Normalized Transmittance
Lnð1 þq0 ðzÞÞ q0 ðzÞ
where q0(z)¼ bI0Leff/(1þ (z/z0)2). This relation is valid for a wide range of absorptive nonlinearity. Nonlinear optical indices have been brought in Table 1. In low power CW irradiations the main portion of nonlinearity is due to the thermo-optical effect [20]. In this regime, nonlinear phase shift and nonlinear refraction index (n2) are minus and inconsistent coefficients depend on thermo dynamical properties of the sample and the intensity of the beam [20,12]. As it is shown in Table 1, nonlinear refraction indices increase as the intensity decreases that indicate, thermal effect is become more prominent at lower intensities. In contrary nonlinear absorption indices decrease that are due to decreasing in two-photon absorption nonlinearity. In higher intensities, condition changes. Using the mentioned Nd:YAG laser (10 ns pulse duration and 200 Hz repetition rate), z-scan curves have been fitted with theoretical ones (Fig. 8). Numerical curve fittings give n2 ¼ 8 10 11 cm2/W (Dj0 ¼ 1.8 with S¼ 0.067) and b ¼7 10 5 cm/W. Because of high intensity irradiation, two-photon absorption nonlinearity is elevated. Nonlinear refraction index decreases indicating thermal-lensing effect reduction and n2 attains its absolute electronic amount.
Table 1 Third order nonlinear optical indices with CW He–Ne laser irradiation. n2 (cm2/W)
D j0
P0 (mW) 10.22 30 48
a
1.3
Diffraction rings pattern is a direct result of high optical nonlinearity in self-focusing (defocusing) samples [17,19,21–23]. Fig. 9a shows the diffraction pattern in the far-field where the sample is placed at z¼z0 ¼ þ0.5 cm and the irradiation power is48 mW. Using the Fresnel–Kirchhoff diffraction theory relations [17], theoretical curve (Fig. 9b) is obtained with putting the nonlinear refractive index obtained from close-aperture z-scan
b (cm/W)
7
2 4.1 5.4
0.79 10 3 1.16 10 3 1.5 10 3
3.04 10 2.12 10 7 1.75 10 7
b c
1.2
d e
1.1
1.4
1
Nd:Yag
0.9 0.8 0.7
z(cm)
0.6 0.5 -4
-2
0
2
4
Fig. 7. Z-scan results with P0 ¼10 mW He–Ne laser. (a) Experimental close-aperture data. (b) Experimental open aperture data. (c) Division of experimental close data by open one. (d) Theoretical open aperture z-scan. (e) Theoretical close aperture z-scan.
where Leff ¼ 1expðalÞ=a is the effective length of the sample and 9Dj0 9 ð1SÞ0:25 9DT pv 9 ¼ 3:06 1exp 5:81
for
9Dj0 9 o 25 ð2Þ
Normalized Transmittance
Normalized Transmittance
1.8
1.2 1 0.8 a b
0.6
c d
0.4
e 0.2 -3
-2
-1
0
1
2 z(cm) 3
Fig. 8. Z-scan results with P0 ¼30 W Nd:Yag laser. (a) Experimental close-aperture data. (b) Experimental open aperture data. (c) Division of experimental close data by open one. (d) Theoretical open aperture z-scan. (e) Theoretical close aperture z-scan.
S. Alikhani et al. / Optics Communications 286 (2013) 318–321
321
48 mW He–Ne laser beam gave the nonlinear refraction and absorption indexes in order of ( 1 10 7 cm2/W) and10 3 cm/W, respectively. In high intensity regime (30 W Nd:YAG laser) these indices were obtained ( 8 10 11 cm2/W)and (7 10 5 cm/W), respectively. The high nonlinearity of this nano-colloids, suggests that for all optical switchers, self defocusing devises and similar nonlinear optical devises.
References
Fig. 9. (a) Observed far-filed diffraction pattern. (b) Theoretical curve of the far-filed diffraction pattern.
(n2 ¼ 1.75 10 7 cm2/W) that is in good agreement with the observed pattern.
4. Conclusions In conclusion, we have reported the nonlinear optical coefficients of Ni nanoparticles immersed in ethanol using z-scan method. The nanoparticles were prepared by high frequency pulsed laser ablation. To conduct the laser ablation process, the Q-switched Nd:YAG laser with 50 W maximum CW power and 240 ns pulse duration, wavelength 1064 nm, adjusted to operate at 2 kHz repletion rate, was employed. Z-scan experiments with
[1] J.P Wilcoxon, J.E Matrin, F Parsapour, B Wiedenman, D.F Kelley, Chemical Physics 108 (1998) 9137. [2] M.L Steigerwaild, A.P Alivisatos, J.M Gibson, T.D Harris, R Kortan, A.J Muller, A.M ZThayer, T.M Duncan, D.C Douglass, L.E Brus, Journal of the American Chemical Society 110 (1988) 3046. [3] L Spanhel, M Haase, H Weller, A Jenglein, Journal of the American Chemical Society 109 (1987) 5649. [4] P.A Brugger, P Cuender, M.J Gratzel, Journal of the American Society 103 (1981) 2923. [5] P Mulvaney, T Linnert, A Henglein, Journal of Physical Chemistry 95 (1991) 7843. [6] F Mafune´, J Kohno, Y Takeda, T Kondow, Journal of Physical Chemistry B 105 (2001) 5114. [7] F Mafune´, J Kohno, Y Takeda, T Kondow, Journal of Physical Chemistry B 104 (2000) 9111. [8] C Petit, P Lixon, M.P Pileni, Journal of Physical Chemistry 97 (1993) 12974. [9] M.P Pileini, Langmuir 13 (1997) 3266. [10] M.P.V.M. Pileni, M Moskovis (Eds.), American Chemical Society, Washington, DC, 1997. [11] S Link, C Burda, Z.L Wang, M.A El-sayed, Journal of Chemical Physics 111 (1999) 1255. [12] E. Koushki, M.H. Majles Ara, S.H. Mousavi, H. Haratizadeh., Current Applied Physics 11 (2011) 1164. [13] Ji-Suk Kim, Kyeone-Seok Lee, Sang Sub Kim, Thin Solid Film 515 (2006) 2332. [14] R.J Gehr, R.W Boyd, Journal of Materials Chemistry 8 (1996) 1807. [15] F Hache, D Ricard, C Flytzanis, Journal of the Optical Society of America B 3 (1986) 1647. [16] M.H. Majles Ara, E. Koushki, Applied Physics B 107 (2012) 429. [17] E. Koushki, M.H. Majles Ara., Optics Communication 284 (2011) 5488. [18] G. Tsigaridas, M. Fakis, I. Polyzos, P. Persephonis, V. Giannetas, Applied Physics B 77 (2003) 71. [19] E Koushki, A Farzaneh, S.H Mousavi, Applied Physics B 99 (2010) 565. [20] E Koushki, A Farzaneh, Optics Communication 284 (2011) 5488. [21] G. Tsigaridas, M. Fakis, I. Polyzos, P. Persephonis, V. Giannetas, Applied Physics B 76 (2003) 83. [22] C.M Nascimento, M.A.R.K Alencar, S Chavez-Cerda, M.G.A Da Silva, M.R Meneghetti, J.M Hichmann, Journal of Optics A: Pure and Applied Optics 8 (2006) 947. [23] L Deng, K He, T Zhou, C Li, Journal of Optics A: Pure and Applied Optics 7 (2005) 409.
Contents lists available at SciVerse ScienceDirect
Optics Communications journal homepage: www.elsevier.com/locate/optcom
Third order optical nonlinearity and diffraction pattern of Ni nanoparticles prepared by laser ablation S. Alikhani a, H. Tajalli a,n, E. Koushki b a b
Department of physics, Science and Research Branch, Islamic Azad University, Tabriz, Iran Kharazmi (Tarbiat Moallem) University, Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 May 2012 Received in revised form 31 July 2012 Accepted 1 August 2012 Available online 24 September 2012
In this article we studied all-optical Kerr effect of Ni nanoparticles immersed in ethanol using z-scan method. The nanoparticles were prepared by high frequency pulsed laser ablation. UV–Visible optical absorption spectroscopy and SEM observation were employed for characterization and studying the morphology of Ni nanoparticles. Analysis of scanning electron microscopy (SEM) images showed that the synthesized nanoparticles shape were dominantly spherical, varying from 19 nm to 40 nm for 1 mJ pulse energy. The nonlinear absorption and refraction indices were measured using open- and closedaperture z-scan techniques, with both CW and pulsed irradiations. In both regimes results were studied. Furthermore, diffraction rings pattern as a result of nonlinear refraction was observed. We suggested an opportunity to form a new nonlinear-optical media for nonlinear optical applications. & 2012 Elsevier B.V. All rights reserved.
Keywords: Ni nanoparticles Z-scan Ablation Diffraction rings pattern Third-order nonlinearity All-optical Kerr effect
1. Introduction The search for new materials has motivated scientists in the field of nano-materials to develop innovative approaches to synthesize nanostructures with novel morphologies in order to exploit their remarkable properties. Metal colloids as the prototypical metal nanoparticles have been the subject of numerous investigations due to their exceptional optical properties and high catalytic activities [1–5]. Laser ablation has been demonstrated as a promising approach to synthesize metal colloids [6,7]. The most outstanding advantage of this method compared with the common chemical routes is recognized to be the absence of chemical reagents in metal colloids. Hence, there would be no need to purify the colloids which were prepared by this method from residual ions [8–10]. In the nanoparticles, surface plasmon can be excited by an electromagnetic field of light. It has been found that the peak position and width of the absorption spectrum of a nano-colloid depend on the particle size as well as their shape and environment [11,12]. Mie was the first researcher who described plasmon interactions quantitatively by solving Maxwell’s equations with the appropriate boundary conditions of spherical particles and described the optical absorption by metal particles [12–15].
n
Corresponding author. Tel.: þ984113822203; fax: þ 984113822204. E-mail address: [email protected] (H. Tajalli).
0030-4018/$ - see front matter & 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optcom.2012.08.016
In this paper we reported the nonlinear optical properties of Ni nanoparticles. UV–Visible optical absorption spectroscopy and SEM observation were employed for characterization and studying the morphology of Ni nanoparticles. Moreover the nonlinear refraction and absorption indices were measured by using openand closed-aperture z-scan techniques, respectively.
2. Experimental procedure 2.1. Sample preparation Nickel nanoparticles prepared by high frequency pulsed laser ablation of nickel metal plate (99.999%) in ethanol alcohol without any additive. As shown in Fig. 1, the target was located at the bottom of a glass vessel filled with 5 ml ethanol alcohol. To conduct the laser ablation process, the Q-switched Nd:YAG laser with 50 W maximum power and 240 ns pulse duration, and with the wavelength 1064 nm, was employed. It was adjusted to operate at 2 kHz repeat ion rate. The energy of incident laser pulse imposed on the target was 1 mJ with 70 mm spot diameter. The irradiation duration in our experiments was 10 min. Upon irradiation of the laser beam, the solution in the glass vessel gradually turned to dark grey, due to the creation of Ni nanoparticles. In order to have a uniform colloidal solution of Ni nanoparticles, the prepared mixture was subjected to ultrasound radiation. After carrying out the irradiation/ablation procedure, a small amount of solution was transferred into a quartz cell with 1 mm
S. Alikhani et al. / Optics Communications 286 (2013) 318–321
319
Scanner Mirrors
Ni target in ethanol alcohol
Fig. 3. UV/vis absorption of solution colloidal Ni nanoparticles.
BS
Lens
Sample
Aperture
Laser
Reference Detector
Close Aperture Detector
Fig. 4. SEM image of Ni nanoparticles.
Fig. 1. Schematic of the experimental set-up employed for the synthesis of Ni by a Q-switch Nd:YAG laser.
Absorption(1/cm)
25 20 15 10 5 0 200
400
600
800
1000
wavelength(nm) Fig. 2. Schematice of z-scan set-up to investigate the nonlinear absorption and nonlinear refraction of Ni.
diameter for UV–Visible absorption spectroscopy. The employed spectrophotometer was a Perkin Elmer Lambda 25, model. As it is presented at Fig. 2, we observed an intense peak around 202 nm which is attributed to the surface plasmon resonance (SPR) effect. The tail of a broad band extending towards the UV wavelength range is originating from an interband transition. Furthermore, we used SEM images, in order to measure the size and shape of synthesized nanoparticles (Fig. 3). Analysis of SEM images showed that the synthesized nanoparticles shape is dominantly spherical and the average diameter of Ni particles in colloidal solution is about 30 nm. The images indicate that the particles have a semi-normal size distribution with the mode of about 30 nm. For different experiments we prepared a colloidal solution with concentration of 1.71 10 4 g/cm3. 2.2. Z-scan setup The nonlinear absorption and nonlinear refraction of the sample were measured by using the open- and closed-aperture
z-scan technique. The sample was scanned around the waist of the focused Gaussian laser beam along the z axis and the irradiance on the sample varies as a function of z. The z-scan experimental trace of the sample was shown in Fig. 4. The light source was a CW He–Ne laser beam l ¼632.8 nm, with 48 mW maximum power, 30 mm beam waist radius, and the focal length of convergence lens was 8 cm. Using I0 ¼ 2P0 =pw20 the maximum intensity in the beam waist is obtained I0 ¼3.4 kW/cm2. The small amount of the colloid was poured into a quartz cell with 1 mm diameter. In this paper we studied nonlinear optical indexes of Ni nanoparticles. The work was repeated with a Q-switched Nd:YAG laser beam (30 W maximum power, 10 ns pulse duration, the wavelength of 532 nm and adjusted in 200 Hz repetition rate). It was focused at 30 mm beam waist radius (I0 ¼2.123 kW/cm2). The absorption bound is very wide and the absorption of the particles is almost similar in these two wavelengths and our comparison between their nonlinear indexes could be related only to the intensity (Fig. 2).
3. Experimental results and discussion 3.1. Z-scan results Figs. 5–7 show the z-scan curves at three different incident intensities which provide power transmittance as a function of sample position. As it was presented previously [16], pure closeaperture curve can be obtained by dividing the normalized closeaperture curve having nonlinear absorption by the normalized open-aperture. Experimental open and close aperture curves are fitted to theoretical ones (solid curves) which are evaluated with the Gaussian decomposition relations [17,18] and nonlinear optical indices are obtained. Also, nonlinear phase changes Dj0 and correspondingly n2 of the colloid could be obtained using [19]: 9Dj0 9 ¼
2pLeff Dn0
l
ð1Þ
320
S. Alikhani et al. / Optics Communications 286 (2013) 318–321
Open aperture curves have been fitted with [16];
2
P=48mW
a PðzÞ ¼ P 0 eaL
b
1.6
c 1.4
d
1.2
e
1 0.8 0.6 0.4 0.2
z(cm)
0 -4
-2
0
2
4
Fig. 5. Z-scan results with P0 ¼ 48 mW He–Ne laser. (a) Experimental closeaperture data. (b) Experimental open aperture data. (c) Division of experimental close data by open one. (d) Theoretical open aperture z-scan. (e) Theoretical close aperture z-scan.
1.8
a
P=30mW
b
Normalized Transmittance
1.6
c
1.4
d 1.2
ð3Þ
e
1
3.2. Diffraction rings pattern
0.8 0.6 0.4
z(cm)
0.2 0 -4
-2
0
2
4
Fig. 6. Z-scan results with P0 ¼30 mW He–Ne laser. (a) Experimental closeaperture data. (b) Experimental open aperture data. (c) Division of experimental close data by open one. (d) Theoretical open aperture z-scan. (e) Theoretical close aperture z-scan.
1.5
P=10.22mW
1.4
Normalized Transmittance
Lnð1 þq0 ðzÞÞ q0 ðzÞ
where q0(z)¼ bI0Leff/(1þ (z/z0)2). This relation is valid for a wide range of absorptive nonlinearity. Nonlinear optical indices have been brought in Table 1. In low power CW irradiations the main portion of nonlinearity is due to the thermo-optical effect [20]. In this regime, nonlinear phase shift and nonlinear refraction index (n2) are minus and inconsistent coefficients depend on thermo dynamical properties of the sample and the intensity of the beam [20,12]. As it is shown in Table 1, nonlinear refraction indices increase as the intensity decreases that indicate, thermal effect is become more prominent at lower intensities. In contrary nonlinear absorption indices decrease that are due to decreasing in two-photon absorption nonlinearity. In higher intensities, condition changes. Using the mentioned Nd:YAG laser (10 ns pulse duration and 200 Hz repetition rate), z-scan curves have been fitted with theoretical ones (Fig. 8). Numerical curve fittings give n2 ¼ 8 10 11 cm2/W (Dj0 ¼ 1.8 with S¼ 0.067) and b ¼7 10 5 cm/W. Because of high intensity irradiation, two-photon absorption nonlinearity is elevated. Nonlinear refraction index decreases indicating thermal-lensing effect reduction and n2 attains its absolute electronic amount.
Table 1 Third order nonlinear optical indices with CW He–Ne laser irradiation. n2 (cm2/W)
D j0
P0 (mW) 10.22 30 48
a
1.3
Diffraction rings pattern is a direct result of high optical nonlinearity in self-focusing (defocusing) samples [17,19,21–23]. Fig. 9a shows the diffraction pattern in the far-field where the sample is placed at z¼z0 ¼ þ0.5 cm and the irradiation power is48 mW. Using the Fresnel–Kirchhoff diffraction theory relations [17], theoretical curve (Fig. 9b) is obtained with putting the nonlinear refractive index obtained from close-aperture z-scan
b (cm/W)
7
2 4.1 5.4
0.79 10 3 1.16 10 3 1.5 10 3
3.04 10 2.12 10 7 1.75 10 7
b c
1.2
d e
1.1
1.4
1
Nd:Yag
0.9 0.8 0.7
z(cm)
0.6 0.5 -4
-2
0
2
4
Fig. 7. Z-scan results with P0 ¼10 mW He–Ne laser. (a) Experimental close-aperture data. (b) Experimental open aperture data. (c) Division of experimental close data by open one. (d) Theoretical open aperture z-scan. (e) Theoretical close aperture z-scan.
where Leff ¼ 1expðalÞ=a is the effective length of the sample and 9Dj0 9 ð1SÞ0:25 9DT pv 9 ¼ 3:06 1exp 5:81
for
9Dj0 9 o 25 ð2Þ
Normalized Transmittance
Normalized Transmittance
1.8
1.2 1 0.8 a b
0.6
c d
0.4
e 0.2 -3
-2
-1
0
1
2 z(cm) 3
Fig. 8. Z-scan results with P0 ¼30 W Nd:Yag laser. (a) Experimental close-aperture data. (b) Experimental open aperture data. (c) Division of experimental close data by open one. (d) Theoretical open aperture z-scan. (e) Theoretical close aperture z-scan.
S. Alikhani et al. / Optics Communications 286 (2013) 318–321
321
48 mW He–Ne laser beam gave the nonlinear refraction and absorption indexes in order of ( 1 10 7 cm2/W) and10 3 cm/W, respectively. In high intensity regime (30 W Nd:YAG laser) these indices were obtained ( 8 10 11 cm2/W)and (7 10 5 cm/W), respectively. The high nonlinearity of this nano-colloids, suggests that for all optical switchers, self defocusing devises and similar nonlinear optical devises.
References
Fig. 9. (a) Observed far-filed diffraction pattern. (b) Theoretical curve of the far-filed diffraction pattern.
(n2 ¼ 1.75 10 7 cm2/W) that is in good agreement with the observed pattern.
4. Conclusions In conclusion, we have reported the nonlinear optical coefficients of Ni nanoparticles immersed in ethanol using z-scan method. The nanoparticles were prepared by high frequency pulsed laser ablation. To conduct the laser ablation process, the Q-switched Nd:YAG laser with 50 W maximum CW power and 240 ns pulse duration, wavelength 1064 nm, adjusted to operate at 2 kHz repletion rate, was employed. Z-scan experiments with
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