ОПТИКА И СПЕКТРОСКОПИЯ, 2015, том 118, № 6, с. 981-986
^ НЕЛИНЕЙНАЯ
И КВАНТОВАЯ ОПТИКА
УДК 535.2
NONLINEAR OPTICAL CHARACTERIZATION OF THE Ag NANOPARTICLES
DOPED IN POLYVINYL ALCOHOL FILMS
© 2015 г. Mahshad Ghanipour and Davoud Dorranian
Laser Laboratory, Plasma Physics Research Center, Science and Research Branch, Islamic Azad University, Tehran, Iran
E-mail: doran@srbiau.ac.ir Received October 8, 2014
The effect of silver nanoparticles doped in polyvinyl alcohol (PVA) on the nonlinear optical properties of composite films is studied experimentally. Samples are PVA films of 0.14 mm thickness doped with different concentrations of silver nanoparticles. Nonlinear optical properties of doped polymer films are studied experimentally employing Z-scan techniques. Experiments are performed using the second harmonic of a continuous Nd-Yag laser beam at 532 nm wavelength and 45 mW power. The effect of nonlinear refractive index of samples is obtained by measuring the profile of propagated beam through the samples and their nonlinear refractive index is found to be negative. The nonlinear absorption coefficient is calculated using open aperture Z-scan data while its nonlinear refractive index is measured using the closed aperture Z-scan data, leads to measuring the third order susceptibility |x(3)|. Real and imaginary parts of the third-order nonlinear optical susceptibility |x(3)| are decrease with increasing the concentration of Ag nanoparticles in the films. The values of thermo-optic coefficient are determined at different concentrations of silver nanoparticles for samples.
DOI: 10.7868/S0030403415060136
1. INTRODUCTION
Noble metal nanoparticles exhibit wide range of applications in different fields of science and engineering such as catalysts, bactericides and optical materials [1—4]. Recently, there has been growing interest in the nonhnear optical properties of metal nanocomposite films, particularly of metal nanoparticles doped in polymer films [5, 6]. Material with large third order nonlinear susceptibilities and nonlinear optical response are attractive for many applications such as electronic and optical devices, chemical and biological sensors, optical energy transport, and thermal therapy [7-12].
The nonlinear properties of metal nanoparticles can be modulated by variation of their concentration, size and shape. Metal nanoparticles are usually dispersed in host matrices such as polymers, colloidal solutions, LB films, glasses and zeolites [13]. When the nanoparticles of noble metals are embedded in dielectric matrices, the films will exhibit specific optical absorption and large third order optical nonlinearity, which have potential application in nonlinear optical devices and optical limiters. Furthermore, it is proven that nonlinear optical properties of dielectric matrices can enhance by mixing with metal nanoparticles.
The Z-scan technique was developed to measure the magnitude and sign of nonlinear refraction (NLR) by the Mansoor Sheik-Bahae et al. (1998). It is also useful for characterizing nonlinear absorption (NLA). The Z-scan method has gained rapid acceptance by
the nonlinear optics community as a standard technique for separately determining the nonlinear changes in index refraction and changes in absorption. This acceptance is primarily due to the simplicity of the technique as well as the simplicity of the interpretation. In most experiments the index change, An, and absorption change, Aa, can be determined directly from the data without resorting to computer fitting. However, it must always be recognized that this method is sensitive to all nonlinear optical mechanisms that give rise to a change of the refractive index and/or absorption coefficient, so that determining the underlying physical processes present from a Z-scan is not in general possible [14]. This method, utilizes a tightly focused laser beam that is intense enough to access nonlinearities in a sample. The sample with thickness smaller than the diffraction length of the focused beam (a thin medium) passes through the focal point of the beam and we measure the transmittance of a nonlinear medium as a function of the sample position z measured with respect to the focal plane (Fig. 1). These changes in its transmittance due to NLA and NLR are measured by an open aperture and closed aperture, respectively. For NLR we measure the transmittance of a nonlinear medium through a finite aperture in the far field as a function of the sample position z measured with respect to the focal plane. The converging and diverging of the beam (allowing more and less of the beam to pass through the aperture, respectively) made changes in the refractive index. In fact, nonlinear refractive index of the sample when its thickness is
Beamsplitter
Lens
Nd:YAG laser
at 532 nm _
J
I
Reference detector
Sample
Lens
a t
o p o o on
Fig. 1. Schematic diagram of experimental open Z-scan setup.
smaller than the diffraction length of the focused beam, makes it to act as a thin lens with variable focal length. A prefocal valley and postfocal peak are observed for a positive change in refraction and a prefo-cal peak and a post-focal valley are observed for a negative change in refraction. In the open aperture technique after the beam is passed through the sample it is focused directly into a detector. As the sample travels through the focus of the initial beam, the transmit-tance either increases or decreases (depending on the nonlinearity of the sample) and the detector receives more or less light than the linear transmittance, yielding a hump or dip in the curve of transmittance as a function of sample position [7, 14].
In this work, we presented our studies of nonlinear optical properties of silver nanoparticles with various concentration which were prepared by laser ablation, doped in PVA polymer. The optical nonlinearity of the polymer films doped with silver nanoparticles is measured by Z-scan technique. The nonlinear refractive index and nonlinear absorption coefficient are investigated using a continuous wave laser beam with wavelength X = 532 nm. Z-scan measurements are carried out in three different concentrations of the same thickness of samples. The negative refractive index and two photon absorption coefficient are measured for these films. The third order nonlinear refractive index and absorption coefficient are found to be dependent on the concentration within the range of study. It is well known that concentration of Ag nanoparticles dependence plays a very important role in the nonlinear optical action.
This manuscript is organized as follows: following the introduction in Sec. 1, experimental details are presented in Sec. 2, Sec. 3 is devoted to results and discussion and Sec. 4 includes conclusion.
2. EXPERIMENTAL DETAILS
Nanoparticles (NPs) were prepared by ablation of a high purity silver bulk in distilled water, using the fundamental harmonic of a Nd : YAG laser operating at 1064 nm with pulse width of 7 ns and 10 Hz repetition rate. Silver bulk was placed at the bottom of a water contain with its surface at the focal point of an 80 mm convex lens. Height of water on the silver target was
12 mm. Laser beam diameter was 2 mm before the lens and has been calculated to be 30 ^m on the surface of the target. The volume of the water in the ablation contain was 20 ml and silver target was ablated with 500 laser pulses at different energies. Samples 1—3 (S1 to S3) were prepared with laser pulse fluencies of 1.5, 2, 3 J/cm2, respectively.
By weighting the dried target before and after ablation process the mass of ablated Ag nanoparticles were measured to be 3.7 x 10-4, 4 x 10-4, 6.5 x 10-4 g for S1, S2, and S3, respectively. PVA films were prepared by dissolving 1 g of PVA powder in 20 ml distilled water at 57°C. Mixture was stirred for two hours continuously to form a viscous solution. The PVA powder was provided by Merck Co. (Germany). After complete desolation, 8 ml of silver nanoparticles suspension was added to the 20 ml aqueous PVA solution and finally samples was left to dry on a plane surface for 24 h at room temperature in close atmosphere to produce 3 samples of 0.14 mm thickness uniform silver nanoparticles doped PVA films. S1 to S3 are PVA films which are doped with sample 1 to 3 nanoparticles.
TEM micrographs were taken using CM120 system form PHILIPS Co. The transmission spectrum of samples was recorded on a UV-Wis-NIR spectrophotometer from Varian Cary-500 Scan. After characterizing the nanoparticles solution, Z-scan experiments were carried out to study the nonlinear optical properties of samples as a function of concentration. Indeed, the optical properties of Ag nanoparticles doped PVA films were studied by means of transmittance and Z-scan measurements using a 45 mW continuous wave laser operate at 532 nm wavelength with single mode TEM00 Gaussian beam. The M2 factor of beam was 1.2. The beam was focused onto the sample by using a 6.5 mm focal length lens. The spot size of the beam was 0.25 cm before the lens, and Rayleigh length was 3.68 mm. The film was moved in the z direction by using a translation system along the propagation direction through the focusing area. The transmitted power was measured by a Gentec-Eo laser power meter.
3. RESULTS AND DISCUSSION
TEM images of nanopartciles are presented in Figs. 2a—2c. In this set of images, the interbrain struc-
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Fig. 2. TEM images: (a) S1 (1.5 J/cm2), (b) S2 (2 J/cm2), (c) S3 (3 J/cm2); (d) average size distribution of Ag nanoparticle generated in distilled water with laser ablation method.
ture can be observed. Produced nanoparticles are spherical without any aggregation. The average size distribution of nanoparticles for all three samples can be observed in Fig. 2d. These graphs are plotted using the "measurement" software. It can be seen that the average size for these nanoparticles was estimated to b
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