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Nonlinear Optical Properties, Optical Limiting and Optical Switching of Ag Nanoparticles Prepared by a Green Synthetic Method

Silver nanoparticles were synthesized using an aqueous extract of cinnamon berks (cinnamon cassia) as a reducer and stabilizer. The synthesized silver nanoparticles were characterized using UV-Vis spectrometry and Transmission Electron Microscopy (TEM), which revealed the nano nature of the particles. Nonlinear absorption and nonlinear refraction were measured using a Z-scan technique at different wavelengths with CW lasers. The third order nonlinear susceptibility was found to be between (1.5 to 22) × 10 m/V. The nonlinear property of the synthesized silver nanoparticles was used to demonstrate optical limiting and all-optical switching. An optical limiting threshold was found to be 0.1 mW at 632.8 nm.


Introduction
Metal nanoparticles such as silver and gold have attracted a significant interest relative to counterparts due to their wide range of applications in many fields, such as optoelectronic, medical diagnostic imaging and therapy, anti-bacterial and as a catalysis. Different methods have been reported for synthesizing silver nanoparticles (AgNPs), either physical or chemical methods [1] [2] [3]. Chemical reduction is the most common method which involves the reduction of the silver precursor in the liquid phase using sodium citrate. However, their disposal raises environmental issues due to cytotoxicity provoked by citrate. Hence, it is essential to develop environmentally friendly techniques for synthesis of nanoparticles. Nanoparticles made by employing green-chemistry based on the extract of compounds from plants and microbes potentially eliminate the environmental issues. These techniques are simple and less expensive and nanoparticles of different sizes and shapes can be produced in large scale. Green synthesis based on plants and plant extracts such as Hibiscus Cannabinus leaf [4], Curcumin [5], and Henna [6] have been successfully used for synthesizing metal nanoparticles. Extracts from plants usually contain sugars, terpenoids, polyphenols, alkaloids, phenolic acids, and proteins, are excellent reducing agents used in the synthesis of nanoparticles.
Metal nanoparticles such as Ag and Au synthesized by chemical reduction have been investigated for their large third order, nonlinear optical properties and their considerable fast time response, which are important for applications in optical devices such optical limiting and optical switching. The enhanced nonlinear optical properties of such materials are due to the amplification of local electric fields at the surface of the metal particles at the range of surface Plasmon resonance frequency. The nonlinear optical properties are strongly dependent on the size, shape and dielectric properties of metals and its surroundings [7].
This study reports on the synthetization of Ag NPS using cinnamon berks.
Two different methods were used: the normal heating method and the normal microwave heating. The nanoparticles were confirmed by UV-Vis spectrometry and Transmission Electron microscopy (TEM). The nonlinear absorption and nonlinear refractive index of silver nanoparticles were investigated at different CW laser wavelengths. The nonlinear optical properties were utilized to demonstrate optical limiting and optical switching.

Materials Synthesizes
Silver Nitrate was bought from Sigma Aldrich chemicals, Germany. Cinnamon berks were bought from the local shop. The cinnamon berks were grained with a blender and sieved. A 2.5 g of grained cinnamon was dissolved in 100 ml of double-distilled water. Then, this solution was boiled for 5 minutes and filtered twice with the help of what man No. 1 filter paper. Ag nano particles were synthesized according to the procedure given in [8]. Briefly, a 42.1 mg of AgNO 3 was dissolved in 100 ml of distilled water to prepare a 1 mM AgNO 3 solution. 1 ml of cinnamon was mixed into 50 ml of the AgNO 3 solution. The mixture was divided into two parts. One part of the solution was placed in a microwave oven for 20 seconds and the other part was stirred and boiled for 2 minutes. In both cases, the reduction took place as indicated by a yellowish colour of the solution. The solution was found to be stable for a month with no colour changes and showed little or no precipitation.

Characterization
Initial characterization of the silver nanoparticles was carried out using a band and a small shift in absorption peak of SPR (redshift) were observed over the time Figure 3. The shift in the absorption peak is an indication of the increase in sizes of nanoparticles [9]. It was observed that the SPR peak at 433 nm increased as a function of time up to 14 days. After that time, a very small variation in the SPR peak was observed. This indicates the stability of concentration and sizes of nanoparticles [4]. This stability may arise from a balance between    electrostatic repulsion force and a weak Van-der Waals attraction force [10]. Transmission Electron Microscopy (TEM) was performed for determining shapes and sizes of the synthesized Ag nanoparticles. The samples were prepared by dispersing a few drops of the solution on a copper grid and then at room temperature to dry. The TEM study reveals that the size and shape of most nanoparticles are nearly spherical. The size distribution of nanoparticles was analyzed using ImageJ 1.5J software. This was performed by converting pixels on the TEM images into nanometers by applying the scale of the image. Figure 3 shows TEM image and size distribution histogram of AgNPs. It can be seen that the nanoparticles have wide distribution range with a high yield of size, from 25 nm for nanoparticles synthesized by boiling method (a) to 15 nm-size for nanoparticles synthesized by Microwave method (b).

Z-Scan Measurements
The Z-scan technique was used to investigate the nonlinear optical properties of green synthesized Ag nanoparticles. This technique relies on the fact that the intensity varies along the optical axis of the convex lens and it is maximum at the focus. The Z-scan experiment was performed using Argon ion laser (488 nm, 514 nm, power 20 mW) and HeNe laser (632.8 nm, power 20 mW). A lens of 50 mm focal length was used to focus the laser beam, to roughly a beam waist of 20 μm. The sample can be moved along the z-axis by a computer-controlled moto-  Figure 4 shows the normalized transmittance for the open aperture case for the sample size of 15 nm at an excitation wavelength of 488 nm. The transmittance is symmetrical with respect to the focus (Z = 0), where it reaches a minimum transmission. This is an indication that the sample exhibits reverse saturation absorption, RSA. Similar characteristics were observed with the samples studied at 514 nm and 633 nm.
The origin of nonlinear absorption in nanoparticles can be attributed to different processes such as two-photon absorption, inter-band, intra-band transitions, excited state absorption and nonlinear scattering. In our Z scan experiment, the excitation of the wavelengths was 488 nm (2.54 eV), 514 nm (2.41 eV) and 633 nm (1.96 eV). The energies of these photons are much less than the electronic transition gap between the d-bands and the conduction band (4.1 eV) for Ag nanoparticles [11] [12]. Therefore, the only possible transition would be through two-photon absorption process; therefore, the observed RSA in our study is attributed to this process. A slight mismatch between conduction band gap of Ag nanoparticles and the total energy of the two photons will be compensated by phonon-assisted excitation [13].  For the open aperture case, the normalized transmittance is given by [14]: where 0 x z z = and where α is the linear absorption coefficient ( α = Absorbance × ln(10)/l), l is the thickness of the sample and I 0 is the peak laser irradiance at the focus  Table 1.
The nonlinear absorption coefficient β is related to the imaginary part of the third order susceptibility through: where n is the index of refraction taken here to be 1.33, 0  is the free space permittivity. The calculated values of imaginary third order susceptibility are shown in Table 1.
In the case of closed aperture configuration, the normalized transmittance is given by [14]:  The nonlinear refractive index was measured by the fitting of Equation (4) to the experimental data. The values of the nonlinear refractive index for all samples are shown in Table 1.
The refractive nonlinear coefficient is related to the real part of the third order susceptibility through: The absolute value of ( ) 3 χ is then given by: The calculated values of real and the absolute third order susceptibility are shown in Table 1.  band gap [10]. The wavelengths used in this experiment are close to SPR peak corresponding to 2.41 eV to 1.96 eV which is close to the resonance (one photon absorption) corresponding to SPR peak at 433 nm (2.87 eV), therefore one photon absorption cannot be ignored. As mentioned above, the nonlinearity observed, in this case, is due to the two-photon absorption which reveals that two-photon excitation cross section is enhanced due to resonant one-photon absorption. A mismatch between conduction band gap of Ag nanoparticles and one by photon energy will be compensated by phonon-assisted excitation [12].
The enhancement also arises due to an increased optical nonlinear interaction between the incident photon and the nanoparticles particles. This can be explained by the fact that, with an increase in particle size, there is an increase in the multiple scattering from the Ag nanoparticles which leads to a large effective interaction length whichin turn results in an enhancement in nonlinear absorption [12]. The nonlinear absorption and nonlinear refractive index values observed in this experiment are in the same order of magnitude with the reported values by Shahriari et al. [17] [18] [19] and one order of magnitude lower than those reported by Faraji et al. [20].

Optical Switching
A pump and probe experiment was used to demonstrate optical switching. A laser beam from an Argon ion laser (λ = 514 nm, 20 mW) was used as a pump beam and a very low power laser beam from a He-Ne laser (λ = 633 nm) was used as the probe beam. Both beams were overlapped at the focus of a lens. The probe beam was isolated from the pump beam using a narrow band filter. The choice of these two laser wavelengths is based on the fact that the ground-state absorption at 632.8 nm is weak (see absorption spectra above). The experiment was performed by moving the sample to a position beyond the valley as demon-  was detected by a photodiode and displayed on the oscilloscope. Figure 6 shows the traces of the waveforms for the pump beam (black trace) and the probe beam (red trace). It is clear from the figure that when the pump beam is in "on state", the probe beam is intensively absorbed and is in "off state. This phenomenon is probably related to the presence of a strong nonlinearity observed at 514.5 nm.

Optical Limiting
The observed nonlinear refractive index is used to demonstrate the power limiting performance. Silver nanoparticles in distilled water prepared boiling method are placed in 1 mm cuvette. In order to obtain the best performance, the sample is placed near post valley. The output power of the Argon Ion laser was recorded as the power of the incident beam is varied with neutral density filters. Figure 7 shows

Conclusion
Ag NP's were synthesized using aqueous extract cinnamon berks. Microwave heating and boiling methods were used for preparation. The stability of Ag NPS was monitored over a month using a UV-Vis spectrometer. The green synthesized nanoparticles showed nonlinear properties. These properties were used to demonstrate optical limiting and optical switching. The green synthesized Ag nanoparticles using cinnamon is quick, stable, and safe and can be used for optical devices and medical applications.