Study on the linear & nonlinear optical properties of

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productive tools for the understanding the band structure and band gap. (Eg) of both ..... [9] Dawood O. Altaify, Muhammad A. Hazaa( Study of Nonlinear. Refraction and ... [15] M. R. Islam and J. Podder, "Optical properties of ZnO nano fiber thin films ... [23]Rawaa A. Faris, Zainab F. Mahdi, Hussein A. Jawad and Dawood O.
Study on the linear & nonlinear optical properties of prepared silver nanoparticles Abstract Noble metal silver NPs was synthesized by pulsed (Q-switched, 1064 nm Nd:YAG) laser ablation of silver metal plates immersed in double distilled and de-ionized water DDDW and ethanol. The formation efficiency of PLAL process was quantified in term of the surface Plasmon extinction SPE peaks. The SPE spectra show a sharp and single peak around 400 nm, indicating the production of pure and spherical Ag. UVVisible absorption results confirmed formation of silver particles prepared and atomic force microscope (AFM) indicates the size in nanometer (nm) range. The Z-scan technique was used to measure the nonlinear, the results showed that the nonlinear refractive index is directly proportional to the input intensities, which caused by the selffocusing of the material. The UV-Visible show a blue shift was recognized with a larger number of laser pulses while the photoluminescence result gives a blue shift with the decrease in number of laser pulses. Keywords Silver Nanoparticles, Z-scan, Nonlinear refractive index, Nonlinear Absorption Introduction In the last two decades, the term of nanoscience has inserted into the scientific vocabulary such as nanomaterial, nanoparticle, nanostructure, nanocolloids and nanocluster Nanomaterials are defined as a solid material characterized by at least one dimension in the nanometer range can be classified into nanocrystalline materials and nanoparticles. The term “Nanoparticle” (NP) refers to a particle with dimensions in nanometer scale [1]. Nanoparticles are defined as particulate matter with at least one dimension that is smaller than 100 nm However, aggregates that are nanoparticles held together by weak chemical bounds (van der Walls, capillary, electrostatic or other physical forces) and agglomerates that are bound together by strong

chemical bounds (metallic, ionic, covalent bounds) can be larger than 100nm [2]. Nanoparticles of a wide range of materials have been prepared by both chemical and physical methods, the formation of nanoparticle during the pulsed laser ablation process is not fully understood, therefore, it is important to understand the effect of various (wavelength, pulse, energy...) Parameters on the size and morphology of the nanoparticles with an aim to achieve narrow particle-size distribution [3]. Laser ablation in liquids is promising as a rapid, simple and most versatile technique to prepare noble metal nanoparticles for analytical chemical and biological sensing applications. Metal nanoparticles could be prepared by laser ablation in clean liquids without contamination [4]. Nanoparticles have been prepared by a wide variety of techniques such as pulsed laser deposition, chemical reduction, photo reduction, electrochemical reduction,…etc. Among them, the pulsed laser ablation in liquid medium (PLAL) has become an increasingly popular top-down approach ,for producing nanoparticles[5].The laser ablation of metallic targets in liquids is without any contamination of chemical reagent and parasitic ions compared to the chemical ways to produce metallic nanoparticles in solution As for laser ablation in vacuum laser ablation in liquids is a quite easy method for the production of mono-, bi- and multi-metallic component nanoparticles that can replace chemical preparations[6]. Noble metal nanoparticles exhibit characteristic size and shapedependent electronic structure leading to unique optical and nonlinear optical _NLO_ properties [7]. There is considerable interest in finding materials having large yet fast nonlinearities. This interest, that is driven primarily by the search for materials for all-optical switching and sensor protection applications, concerns both nonlinear absorption (NLA) and nonlinear refraction (NLR) [8]. The Z-scan method provides a sensitive and straightforward method for the determination of the nonlinear refractive index and the nonlinear absorption coefficient. The simplicity of both the

experimental set-up and the data analysis has allowed the technique to become widely used by many research groups [9]. Metal nanoparticles and nano composites have, in recent years, received significant attention owing to their unique nonlinear optical (NLO) properties, such as two-photon absorption (TPA), saturable absorption(SA), reverses at urable absorption (RSA), and selffocusing/defocusing arising from non- linear refraction[10]. In this work, preparation of colloidal solution of Ag Nps synthesized by method PLAL with different pulses and study linear optical properties and, Non-Linear optical properties. Z-scan technique, which has emerged as a powerful method to determine the non-linear refraction of colloidal metal NPs, has been employed to study the non-linear refractive response from Ag NPs.

Linear optical properties The optical constants are very important parameters because they describe the optical behavior of the materials. The absorption coefficient of the material is a very strong function of the photon energy and band gap energy. Absorption coefficient represents the attenuation that occurs in incident photon energy on the material for unit thickness, and the main reason for this attenuation is attributed to the absorption processes [11, 12]. Analysis of optical absorption (A) spectra is one of the most productive tools for the understanding the band structure and band gap (Eg) of both crystalline and amorphous materials [13]. Absorbance (A) Is the ratio between the intensity-absorbed beam with absorbed by the matter to the original intensity incident beam on it, and gives the ratio: A=IA / I◦ ……. (1)

Transmittance Transmittance is defined as the ratio between the intensity of the transmitted beam from the matter to intensity incident beam, and can be expressed as T=It /I◦

. … (2)

absorption coefficient: was calculated from Beer-Lambert law, (α) which is expressed in terms of the intensity of the incident light (Io ) and the intensity of transmitted light (I) after passing through a thickness (t) [ 14 ,15] as: I=I◦ e-αt……..

(3)

Where I˳ is the intensity incident beam, I is the transmittance of matter.α̥ is the absorption coefficient, t is the thickness material, and after the Re-form the equation (3), we get: α̥ = 2.303A/t …. (4) Extinction coefficient: is the quantity of energy that absorbed by electronics valance band at the incident of beam on the matter it could be calculated as the following: K= α̥λ/4π …… (5) Where k is the extinction coefficient, α̥ is the linear absorption coefficient, λ is the wavelength. Refractive Index (n) Refractive Index is defined as the ratio between the speed of the light in the vacuum to its speed in any other material medium. It also can be found through knowing extinction coefficient and magnitude of refraction value of matter.it could be calculated as the following n˳ = (1+ √R)/(1-√R) …. (6) Non- Linear optical properties Silver nanoparticles have received increased attention in the past few years due to its large third order nonlinearity. A large number of investigations have been carried out to study nonlinear optical properties of metal nanoparticles dispersed in different optically trans-

parent solid matrices and liquids [17]. There are a several characterization techniques available for measuring the third-order optical nonlinearities these includes degenerate four-wave mixing, nearly degenerate three-wave mixing, optical Kerr effect, ellipse rotation, interferometric methods, two-beam coupling, beam selfbending and third harmonic generation[18] .In 1990 M. Sheik-Baha, established a measurement method with the purpose of determining the nonlinear refraction of thin samples. This is called the Z-scan technique [19]. Z-scan technique is based on the principle of spatial beam distortion. Using z-scan technique, the magnitude of nonlinear absorption and the sign and magnitude of nonlinear refraction can be determined simultaneously[20]. When a high intensity laser beam propagates through a material, induced refractive index changes leads to selffocusing or defocusing of the laser beam. Z-scan method is also used to determine the coefficient of nonlinear absorptionβ.The whole absorptionis defined: α= α̥+βI ….. (7) Where α̥ is the linear absorption coefficient and I is the intensity of the beam [21]. While NLA can be determined using a two parameter fit to a closed aperture Z-scan (i.e. fitting for both Δn and Δα), it is more directly (and more accurately) determined in an open aperture Z-scan. For small third-order nonlinear losses, i.e. ΔαL=βILeff