Ag Nanoparticles: Experimental Study of Sign Identification of Nonlinear Refractive Index by Moiré Deflectometry and Z-Scan Methods

Two different methods are presented for the sign identification of nonlinear refractive index ( ) of Ag colloidal nanoparticles which are based on nonscanning Moiré deflectometry and Z-scan. In the Moiré deflectometry setup, two lasers are used, one is used as pump laser which causes thermal nonlinear effects in the sample, and the second one is used as the probe laser for monitoring these effects by Moiré deflectometry system. By observing the deflected Moiré fringes, we can determine the sign of nonlinear refractive index in real time, and there would be no need for calibration or complicated calculations. The second technique for sign identification is Z-scan. In this technique, a CW 532？nm second harmonic Nd:YAG laser with a beam power of 55？mW is used as the excitation source. Results show that the nonlinear refractive index is negative for Ag nanoparticles in pure water by both methods. 1. Introduction Measurements of nonlinear optical parameters of colloidal metallic nano-particles have drawn a lot of attentions because of their fast nonlinear optical response and high nonlinearity ability [1]. Colloidal metallic solutions are frequently used in the design of optical instruments and photonic limiters because of their photoinduced nonlinear properties [2, 3]. There are two standard and usual methods for sign identification of nonlinear refractive index of nanoparticles, Z-scan [4–7] and Moiré deflectometry techniques [8–13]. In Z-scan method, the refractive index and the sign are found by drawing the diagram of the dependence of the transmitted beam intensity but in Moiré deflectometry technique, these are found by observing the Moiré fringes patterns [14]. It has also been shown that Moiré method is simpler and more robust than other methods [15]. In recent years, sign identification of third nonlinear refractive index of materials by Moiré deflectometry technique is found by observing size of Moiré fringes and Moiré fringes spacing curves [16, 17]. This technique avoids the requirement of highly calibrated detectors, but still, the need to use a scanner that is highly sensitive to movements exists. In our recent work, we have proposed a novel Moiré deflectometry technique that not only avoids the use of highly sensitive calibrated detectors but also omits the need for scanner [18]. The presented methods in this paper which are based on scanning (Z-scan) and nonscanning (Moiré deflectometry) techniques have been used to identify the sign of the nonlinear refractive index of Ag nano-particles suspended in water. 2. Experimental 2.1. Material