Experimental observation of the far field diffraction patterns of functionalization single and multi-walled carbon nanotubes using nonlinear diffraction technique

Nonlinear diffraction pattern can be induced by focusing CWlaser into a thin quartzes cuvette containing nanofluid. The numberof revealed pattern rings indicates to the nonlinear behavior of fluid.Here, the nonlinear refractive index of each of functionalized singlewall carbon nanotube (F-SWCNTs) suspention and multi wall carbonnanotube (F-MWCNTs) suspention have been investigatedexperimentally .Each of CNTs suspention was at volume fraction of13×10−5 and 6×10−5. Moreover the laser source at wavelength of473 nm was used. The results show that SWCNTs suspentionpossesses higher nonlinearty than other at the same volume fraction


Introduction
Nonlinear optics has been a section of optics that was studied the interactions between the media and electromagnetic radiations [1,2]. The interaction means, this issue respond in a non-linear way incident radiation fields. The nonlinear optical effects take much attention in the last decade because of their useful application in science and industry such as frequency conversion phase modulation, and multi absorption [2]. Therefore, the nonlinear pattern concentric rings can be detected as far field diffraction [3] when a Gaussian laser beam irradiates a thin slice of nanofluid [4][5][6]. This phenomenon had been revealed in many nonlinear materials like organic substance [7][8][9] nanofluids [9,10], dye [11], graphene and CNTs [12]. The most application of nonlinear diffraction effects as optical power limiting, optical switching, and beam modulation [13,14].

Experimental work
The

Results and discusses
Absorption spectra of (F-SWCNTs) suspension at volume fraction 13x10 -5 was about 405 nm checked by using UV‫ــ‬VIS spectrometer. The absorption peaks were detected at 415 nm as shows in Figs.3 and 4 show the absorption spectra to SWCNTs suspension at volume fraction (6x10 -5 ) was about 454 nm.

Fig.4: The UV‫ــ‬VIS absorption spectra of S-WCNTs DI water suspension at volume fraction 6x10 -5 .
While Fig.5 shows that the peak of MWCNTs suspension at volume fraction 13x10 -5 was about (405 nm). It can be concluded from all previous absorption spectrum of both of two suspensions that the laser source at 473 nm was suitable for all the process of this experiment, where the peak was around 485 at the volume fraction 6x10 -5 , as shows in Fig.6. For all the previous absorption peaks the laser source at wavelength 473 nm was suitable for the process.  Table 1 and 2 show that the nonlinear refractive index change ∆ has been increased for both F-SWCNT's and F-MWCNT's suspensions by increasing of their volume fraction using the same value of laser power density. This can be attributed to fact that increasing of suspension volume fraction means increasing its density and finally leads its nonlinear refractive index to be increased. Also it can be noticed that F-MWCNTs suspension possess higher nonlinear refractive index change then that of F-MWCNT's suspension at the same volume fraction (Table 1 and 2). The calculation of the maximum nonlinear refractive index change (∆ * ) was from the following equation [15].
where λ is the laser wavelength, λ and L materials are the cuvette thickness and N number of diffraction rings respectively, for each volume fraction value.
Figs. 7 and 8 show the relationship between the incident laser intensity and the maximum change the nonlinear refractive index of nanofluids in different volume fractions.

Fig.8: Maximum change of non-linear refractive index created using F-MWCNTs suspension in DI water of the volume fraction (13X10 -5 ), and (6 X10 -5 ) irradiated by laser intensity.
At high laser beam intensity and high volume fraction the maximum change of the nonlinear refractive index Δnnl, max will be increases as seen in previous table and figure.
Different density values were used to obtain different numbers of nonlinear pattern rings that lead to different values of nonlinear refractive indices and maximum change in previous Nano liquids, as shown in Figs. 9 and 10.

Fig.10: The diffraction rings of DI water (MWCNTs) suspension at different laser densities at volume fraction 13 × 10 -5 and 6×10 -5 .
The impact of nonlinear refractive index of the functionalization single and multi-wall Carbone nanotube at two volume fraction can be measured from the following equation [15]: It has been shown from the Figs.11 and 12 that the nonlinear refractive index increase with decreases lasers intensity according g to the Eq. (2), and increase with increased volume fraction

Conclusions
From this experiment in short, were found that by increasing the intensities, the number of rings will be increased until they reach a specific number, after which they will remain constant even in increasing density. This leads to increasing the nonlinear refractive index change (Δn nl,max ) to both of (SWCNTs and MWCNTs) diffuse in DI water at two volume fraction. By determining the nonlinear parameters of the jaginocytes prepared in DI water. The results show that F-SWCNTs suspended fluid possess higher nonlinearity than other at each volume fraction. The material shows non-linear good grade III(third order non linearity), on the other hand the nonlinearity of the F-SWCNTs and F-MWCNTs at volume fraction 13x10 -5 it was more efficiently than other and also it was noted that the minimum threshold of F-SWCNTs at all volume fraction was lower than F-MWCNTs.