Spectral and Third Non-Linear Properties for Mixture Solutions of (R6G , RB, and RC) Dyes

In this research, the spectral characteristics and the nonlinear optical properties for the mixing of Rhodamine dyes (Rh6G, RC, and RB) have been determined at different concentrations ( 1×10 -5 ,2×10 -5 ,5×10 -5 ,7×10 -5 , and 1×10 -4 mole/L) at room temperature. The spectral characteristics were studied by recording their absorption and fluorescence spectra. The intensity of absorption increased and fluorescence decreased when increasing concentration which in agreement with Beer – Lambert Law. It was observed that this mixing had a wide spectral range. The quantum efficiency decreased while the radiative life time and the fluorescence life time increased when increasing the concentration. Nonlinear optical properties were measured by using Z-Scan technique, using (CW) continuous Nd: YAG laser with frequency doubled wavelength (532nm) with output power (100 mW). The obtained nonlinear properties results of the mixture (R6G, RC, and RB) showed a negative nonlinear refractive index for concentrations (7×10 -5 , and 1×10 -4 mole/L ) while the concentrations (1×10 -5 ,2×10 -5 , and 5×10 -5 mole/L ) showed a positive nonlinear refractive index, also this mixing showed two photon absorption behavior for all concentrations. The origin of optical nonlinearity in the dye may be attributed to laser-heating induced nonlinear effect.

. Organic dye molecules have been widely used in solutions as amplifying media in a tunable lasers [3]. There are large amount of data about laser dyes from many researchers. In (2001) H. M. Mekhlif studied the effect of oxygen on absorption and fluorescence spectrum of two organic dye laser (RB, R6G) in different solvents ( methanol, ethanol , and a binary instance sulfur dioxide ) and found that the present of oxygen effect on fluorescence spectrum, quantum efficiency, fluorescence life time [4]. In (2011) Ali Hadi Al-Hamdani, Shaima Khyioon, Rafah Abdul Hadi studied the spectral properties of two dyes (RC, and R6G) separately and also for the mixing of these dyes for different concentration, they found that the best concentration was (1×10 -5 mole/L) which quantum efficiency 96% so this concentration of mixtures can be used to improve solar cell conversion efficiency [5]. In (2013) R. M. Ahmed, and M. Saif studied the spectral characteristics of the prepared samples of dye (RB) doped in the different transparent polymer hosts (PVAc, PMMA, PVAc/PMMA 50/50) by absorption and fluorescence spectroscopy. They concluded that the absorption peaks values as well as the fluorescence intensity were increased by increasing the concentration [6]. Organic material can show very high nonlinear coefficients, because of the large variety of these compounds at high intensities [7]. Many of researchers studied nonlinear properties for many dyes. In (2010) Zainab Fadhil studied the spectral characteristics and nonlinear optical properties of the mixed donor (C480) acceptor (R6G) by using Z-Scan technique, using Q-switched Nd:YAG laser with 1064 nm wavelength. The obtained nonlinear properties results of the mixture C-480/ Rh-6G showed a negative nonlinear refractive index and reverse saturation absorption results show that mixture of laser dyes are effective nonlinear optical materials as compared to individual laser dyes [8]. In (2013) Amal F. Jaffar studied solvents effect on the nonlinear optical properties of oxazine dyes doped films in PMMA at concentration of 10-5 M in three solvents (Trichloroethan, Chloroform ,THF) by using a high sensitive method known as Z-Scan technique The nonlinear refractive index was found to be of the order of 10 -6 cm 2 /W. The magnitude of third order susceptibility was of the order of 10 -9 cm/watt [9]. Z-scan technique is a simple and sensitive method introduced in 1990 to measure the nonlinear refractive index of optical materials. It is based on a single beam method. It refers to the process of inserting a sample in a focused Gaussian beam and translating it along beam axis D through a focal region. The wave front distortion from self-focusing or selfdefocusing will cause the kerr nonlinearity. The beam power propagating through a small aperture at far field varies with a sample position. Measuring the output versus position sample allows to determine nonlinearity. There are two methods of z-scan, the closed aperture and open aperture system [10]. A closed-aperture z-scan measures the change in power intensity of a beam, focused by lens L in Fig.(1). Photo-detector PD collects the light that passes through an axially centered aperture A in the far field. The change in on-axis intensity is caused by self-focusing or self-defocusing by the sample S as it travels through the beam waist. A TEM00 Gaussian beam has greatest intensity at the center and will create a change in index of refraction forming a lens in a nonlinear sample as shown in Fig. (1) [11].

Figure(1): The scheme of the closed aperture z-scan
An open-aperture z-scan measures the change in power intensity of beam, focused by lens L in Fig.(2). In the far field at detector PD, which captures the entire beam. The change in power intensity is caused by multi-photon absorption in the sample S as it travels through the beam waist. In the focal plane where the intensity is greatest, the largest nonlinear absorption is observed. At the "tails" of the z-scan signature, where | z | >> zo, the beam intensity is too weak to elicit nonlinear effects. The higher order of multi-photon absorption present in the measurement depends on the wavelength of light and the energy levels of the sample [11]

Experimental work Materials and Instrument
Three well known groups of laser dyes, (R6G, RB, and RC) belong to xanthene family, The molecular formula of (R6G) dye (C 28 H 31 N 2 O 3 Cl), it is observed as dark reddish purple, brown or black crystalline solid. The molecular formula of (RC) (C 28 H 31 O 3 N 2 Cl) and molecular weight (MW= 479.02gm/mole), available as solid crystalline powder. The molecular formula of (RB) (C 28 H 31 N 2 O 3 Cl) and molecular weight ( M W = 479.02gm/mole). These dyes were supplied from (HIMEDIA) India company. These three laser dyes were dissolved in chloroform solvent whose scientific name (Trichloro Methane) with a Chemical formula (CHCl 3 ), molecular weight (Mw=119.38gm/mole) %), refractive index (1.4459), and Productive company Analar company (England) at different concentrations The analytic concentrations of the five solutions examined are 1×10 -5 , 2×10 -5 ,5× 10 -5 , 7× 10 -5 , and 1×10 -4 mole/L . A 2:2:2 (v/v) solution was mixed to produce five series. The absorption and fluorescence spectra of mixture of Rhodamine dyes were recorded by a UV-VIS-NIR spectrophotometer (SP 8001) supplied from Metertech company which working with wave length (190-1100nm) and SL 147 spectrophotometer supplied from (ELICO limited) Indian company.

Z -Scan experiment
The principle of the Z-scan technique is based on using a Gaussian laser beam in a tight focusing geometry, and moving the sample under investigation along the beam (along the z-axis) through the focal point. The transmittance in the far field is measured and normalized to 1 for linear absorption, and plotted as a function of sample position along the z-axis, with the focus of the laser beam chosen to be at z = 0. The Z-scan experiments were performed using continuous (CW) Nd:YAG laser with frequency doubled wavelength (532 nm) focused by a lens of 12 cm focal length. The laser beam waist w o at the focus was measured to be 0.0675 mm and the Rayleigh length ZR was 26.9 mm (where ZR is the diffraction length of the laser beam, (ZR=Kw o 2 /2), and K=2π/λ¸ is the wave number. The schematic of the experimental setup used is shown in Fig.(3). A 1 mm width quartz cell containing the aqueous solution of mixture of Rhodamine dyes was translated across the focal region along the axial direction, which was the direction of the propagation of the laser beam. The transmission of the beam through an aperture placed in the far field was measured using photo detector operating at wavelength(400-1100) and it is supplied from (Changchun) company and its type is (S121C), with power (1-500mW). For an open aperture Z-scan, a lens replaced the aperture to collect the entire laser beam transmitted through the sample. When the sample was moved from negative Z into focus, initially the beam irradiance was low and negligible nonlinear refraction occurred. Hence the transmittance remained relatively constant. As the sample was brought closer to focus the, beam irradiance increased leading to selflensing in the sample. A negative self-lensing prior to focus collimated the beam and caused beam narrowing at the aperture, which resulted in an increase in transmittance. As the sample was moved away from the focus i.e., towards positive Z the beam divergence caused a decrease in transmittance at the aperture. A pre-focal transmittance maximum (peak) followed by a post-focal transmittance minimum (valley) is the z scan nature of negative nonlinearity. The opposite effect is the nature of positive nonlinearity. The sensitivity to nonlinear refraction is entirely due to the aperture, and the removal of aperture completely eliminates the effect. The third order Part (A), No.1, 201 , 3 3 Eng. &Tech.Journal, Vol.
nonlinear refraction index of the sample (Dye solution) was evaluated from the Zscan data. The Z-scan was performed for different polarity of organic solvents at different concentrations of the dye solution.

Quantum efficiency
It is the ratio of the number of fluorescence photons emitted by a system of molecules in dilute solution to the number of molecules excited into S 1 (the number of absorbed photons). Quantum efficiency is given by the following equation:

The Radiative life time (τ fm )
The raditive life time (τ fm ) is given by using the following relation: where n:refractive index of a medium ύ: wave number at the maximum absorption ∫ε(ύ)d ύ:the area under the absorption spectrum curve as a function of the wave number [12].

Nonlinear absorption coefficient
Nonlinear absorption coefficient was measured by open aperture z-scan technique from transmittance curves according to equation [11,14] .

Results and Discussion Spectral result
The spectra of absorption and fluorescence was studied for mixture of dyes (R6G, RC, and RB) dissolved in chloroform to concentrations (1x10 -5 , 2x10 -5 , 5x10 -5 , 7x10 -5 , and 1x10 -4 mole / L) as shown in fig. (5) and (6) and found that the increasing of concentration shifted the absorption spectrum towards shorter wavelengths (Blue Shift) and shifted the fluorescence spectrum towards longer wavelengths (Red Shift) because of non-radiative process (Internal conversion, Inter system crossing. Fig.(4) indicates the absorption and fluorescence spectra of the chloroform did not absorb at the same rang of dye solution. Stock's shift increased as the dye concentrations was increased for all dyes. The tables (1) show the important parameters, the raditave τ fm (nsec) and fluorescence life time τ f (nsec) and the quantum efficiency Ф fm of the mixture. The quantum efficiency decreased as the dye concentrations was increased for dyes because of decrease the probability of non-radiative transition (I.S.C and I.C. ).The raditave τ fm (nsec) and fluorescence life time τR f R (nsec) increased as the dye concentrations was increased for dyes. These results in agreement with results of [5, Part (A), No.1, 201 , 3 3 Eng. &Tech.Journal, Vol. 16, and 17] These results indicate that mixing the dyes may improve the characteristics properties of the RC,R6G,RB dye and can be used in different applications (luminous solar concentrators and laser dye medium).

Z -Scans results
Experiment was performed for different concentrations of the dye solution, A typical closed aperture Z-scan curve of the dye solution at concentrations (1x10 -5 , 2x10 -5 , 5x10 -5 , 7x10 -5 , and 1x10 -4 mole/L) exhibiting the normalized transmittance is shown in the Fig. (7) at incident intensity Io= 1.39 KW/cm 2 .The curves are characterized by a pre-focal peak followed by a post-focal valley for some concentration (7x10 -5 ,1x10 -4 mole/L) and revers for other concentrations (1×10 -5 ,2×10 -5 and 5×10 -5 mole/L), which implies that the nonlinear refractive index is negative (n 2 <0) for concentration(7x10 -5 ,1x10 -4 mole /L) and positive for concentration(1×10 -5 ,2 ×10 -5 and 5×10 -5 mole/L). The transmittance profile at openaperture (OA) z-scan experiment shows (Two Photon Absorption) for concentrations (1x10 -5 ,2x10 -5 , 5x10 -5 ,7x10 -5 , and1x10 -4 mole /L ) as in Fig.(8). To avoid any discrepancy caused by deviations from an ideal Gaussian profile, all the measurements were taken with the experimental configuration kept identical for all the concentrations of the dye. Investigation has been carried out for the dependence of nonlinear refractive index on concentration. The nonlinear refractive index n 2 increased with increasing concentration as in Fig.(9). This may be attributed to the fact that the number of the dye molecules increases when the concentration increases and more particles get thermally agitated resulting in an enhanced effect . The nonlinear refractive index (n 2 ) and nonlinear absorption coefficient (β) values of the dye solution are given in Table (2) and the corresponding χ (3) values calculated from data of nonlinear refractive index and nonlinear absorption coefficient. The linear dependence of nonlinear absorption coefficient β on the concentrations of mixture of (R6G, RC, and RB) as shown in Fig.  (9). The third order nonlinear susceptibility χ (3) increases with increasing concentration as in Fig. (10).All results of n 2 , β and χ (3) show a direct relation with concentration