International Journal of Physics and Research (IJPR) ISSN 2250 0030 Vol.2, Issue 2, Sep 2012 15-25 Š TJPRC Pvt. Ltd.,
SYNTHESIS AND STUDY OF NONLINEAR OPTICAL PROPERTIES OF A NEW AZO DYE BY Z-SCAN TECHNIQUE 1 1,* 2
HUSSAIN A. BADRAN & 2ADIL A. AL-FREGI
Department of Physics, College of Education, University of Basrah ,Basrah, Iraq
Department of chemistry, College of Science, University of Basrah ,Basrah, Iraq
ABSTRACT The single beam Z-scan technique was used to determine the nonlinear optical properties of the new azo dye [4-(2-hydroxy naphthylazo)phenyl][2-(2-methoxy benzylideneamino)-5-methylphenyl] tellurium dibromide dye in the solvent chloroform and a dye doped polymer film. The experiments were performed using cw SDL laser with a wavelength of 532 nm. This material exhibits a negative optical nonlinearity. The dye exhibited a nonlinear refractive index ( and
cm2/W in dye solution and polymer film, respectively),
nonlinear absorption coefficient ( and cm/W in dye solution and polymer film, respectively).Optical limiting characteristics of the dye solution and polymer film were studied. The result reveals that [4-(2-hydroxy naphthylazo)phenyl] [2-(2-methoxy benzylideneamino)-5-methylphenyl] tellurium dibromide dye can be a promising material for optical limiting applications. KEY WORDS: Nonlinear optics, nonlinear refractive index, Z-scan technique, optical limiting.
INTRODUCTION The field of nonlinear optics (NLO) has been developing for a few decades as a promising field with important applications in the domain of photoelectronics and photonics [1,2]. NLO materials can be used to manipulate optical signals in telecommunication systems and other optical signal processing applications [3,4]. Organic materials are considered as one of the important classes of third-order NLO materials because they exhibit large and fast nonlinearities and are, in general, easy to process and integrated into optical devices [5-7]. Moreover, a fine-tuning of the NLO properties of organic compounds can be achieved by rational modification of the chemical structure [8]. Various types of organic compounds have been studied to obtain materials with large third-order nonlinearity. On the other hand a wide range of techniques have been used to measure third-order nonlinearity: e.g. Z-scan [9,10], nonlinear interferometry [11], degenerate fourwave mixing [12], nearly degenerate three-wave mixing[13], ellipse rotation [14], beam distortion measurements [15], optical third harmonic generation (THG) [16] and frequency resolved optical gating [17].
16 Hussain A. Badran & Adil A. Al-Fregi
Among all these techniques the Z-scan is a simple but effective tool for the measurement of the nonlinear refractive index and has been used widely in material characterization. It provides not only the magnitudes of the real and imaginary parts of the nonlinear susceptibility, but also the sign of the real part [18]. Both nonlinear refraction and nonlinear absorption in solid and liquid samples can be rapidly measured by the Z-Scan technique, which utilizes self-focusing or self-defocusing phenomena in optical nonlinear materials. In the present work, for the first time we have synthesized [4-(2-hydroxy naphthylazo)phenyl] [2-(2methoxy benzylideneamino)-5-methylphenyl] tellurium dibromide. We have studied the nonlinear optical properties by using Z-scan technique to measure the magnitude and sign of the optical nonlinearity for new azo dye in the solvent chloroform and dye doped polymethylmethacrylate (PMMA) film. The optical limiting behavior of the dye solution and polymer film have been studied.
STRUCTURE CHARACTERIZATION The chemical structure of the new azo dye [4-(2-hydroxy naphthylazo)phenyl] [2-(2-methoxy benzylideneamino)-5-methylphenyl] telluriumdibromide is in Fig 1. O CH 3
C H =N
CH3 HO
Br
Te
N =N
Br
Figure .1. Chemical structure of the prepared azo dye. The UV-VIS (Ultraviolet-Visible) absorption spectra of the dye are recorded using an UV-VIS spectrophotometer (Cecil Reflecta-Scan CE -3055 Reflectance Spectrometer). The optical absorption of the azo dye in chloroform with 0.5 mM concentration shows an absorption peak (λmax) at 375 nm as shown in Table 1. Table 1. Ultraviolet-Visible data of azo dye.
wave length nm 210 225 375
molar extinction M-1 cm-1 16500 15860 16750
assignment π→π* (phenyl) π→π* (azomethine) π→π* (naphthyl)
The IR spectra for azo dye is listed in Table 2 and the spectra is shown in Fig 2. The compound show a strong band within the range 1623-1601 cm-1 which can be attributed to azomethine groups stretching vibration which are in agreement with previous reported data. A weak or shoulder band at 3072cm-1 which may assigned to the aromatic C-H stretching vibration. Furthermore the IR spectra of compound shows the
17
Synthesis and Study of Nonlinear Optical Properties of A New Azo Dye by Z-Scan Technique
asymmetrical νas (C-H) and symmetrical νs (C-H) stretching vibrations band of aliphatic C-H at 2959 cm-1 and 2929 cm-1 , respectively , and shows a weak band at 2866 cm-1 is due to methoxy group νOCH3. The bending vibration of aliphatic C-H bond appear as a medium band at 1381 cm-1 while aromatic C-H bending bands appear at a range of 837-751 cm-1. Strong bands at 1544-1460 cm-1 range can be attributed to aromatic (C=C) stretching. The IR spectra of compound shows a strong band at 1280 cm-1 due to C-N stretching while a strong bands appeared at 1127 cm-1 due to C-O stretching, Table 2.The IR spectra of compound show a broadly strong band at 3412 cm-1 corresponding to ν(O-H). It is worth noting that the stretching vibration of the (-N=N-) in the range 1601-1623 cm-1 was not observed in the IR spectra of all compounds. This is probably due to the presence of intense bands of ring vibration or to the presence of azomethine groups which masked these bands. Table 2. IR date of azo dye
νaro. (C-H) ν(OH)
3412 br,s
3072 w
Aliphatic
νa(CH) νs(CH)
νas(C=N)
Aromatic
Bending
Bending
νs(C=N)
ν (C=C)
νali(CH)
νaro.(CH)
ν (CN)
ν(OCH3)
2959 w 2929 w
1623 s 1601 s 2866 m
ν (CO)
1544 m 1495 s 1460 s
837 m 1381 m
Figure. 2 : IR spectrum of azo dye
1280 s
1127 s
816 m 751 s
18 Hussain A. Badran & Adil A. Al-Fregi
Fig. 3 displays 1H NMR spectra of azo dye (Table 3) in DMSO-d6 solution. From Fig. 3, it can be seen that the 1H NMR spectra of compound shows a singlet signal at 8.54 ppm due to azomethine proton. The 1H NMR spectra of compound show a singlet signal at 2.32 ppm which may attributed to methyl group attached to phenyl. Furthermore, the 1H NMR spectra of compound show a singlet signal at 3.49 ppm due to methoxy group. The 1H NMR spectra of compound shows a singlet signal at 4.10 ppm which may attributed to hydroxyl protons. On the other hand, the 1H NMR spectra of compound show complex signals between 7.95-6.54 ppm can be attributed to aromatic protons. Table 3: 1H NMR data of azo dye
chemical shift (ppm) compound structure TMS = 0 ppm
OCH3 HO
CH N
Br Te Br
N=CH
8.54 (s, 1H, CH=N) 7.95-6.54 (m, 17H, Ar-H) 4.10 (s,1H, OH) 3.49 (s, 3H, OCH3) 2.32 (s, 3H, CH3)
CH3
Figure. 3 . 1H NMR spectrum of azo dye
19
Synthesis and Study of Nonlinear Optical Properties of A New Azo Dye by Z-Scan Technique
NONLINEAR OPTICAL PROPERTIES The solution sample of the novel azo dye was dissolved in chloroform. The concentration of the dye solution is 0.5 mM. To prepare the solid films, PMMA was selected as the host material because it is hard, rigid and has a glass transition temperature of 125 oC. It also exhibits good linear optical transmittance, optical stability, thermal stability and moreover better compatibility with organics [18,19]. A known quantity of PMMA and azo dye were dissolved in chloroform separately, the concentration of the azo dye in chloroform is 0.5 mM, later both solutions were mixed and stirred for 2 hr using a magnetic stirrer. The ratio of PMMA solution and dye solution is 1:1. The film was prepared on a clean glass slide by the casting method and dried at 20 oC for 24 hrs. The thin film sample have a good purity and uniform thickness. The thickness of the thin film was measured using digital micrometer and is found to be 15 µm. The spectrum of the optical absorption was computed from the absorbance data. The absorption coefficient (α) has been obtained directly from the absorbance against wavelength curves using the relation [ 20 ]
α= 2.303A/d where d is the sample thickness and A is the absorbance. The values of absorption coefficient (α) at wavelength 532 nm for 0.5 mM concentration of azo dye in chloroform and polymer thin film are 4.29 cm-1 and 49.3 cm-1 , respectively. The Z-scan technique was applied for the measurements of nonlinear optical characteristics of investigated samples. We used a SDL laser beam with 532 nm wavelength and power of 40 mw , which was focused by (+5 cm) focal length lens. The laser beam waist ω0 at the focus measures 21.63 µm and the Rayleigh length measured ZR= 2.76 mm. A 1 mm thickness optical cell containing the azo dye in chloroform is translated across the region along the axial direction of the laser beam propagation. The transmission of the beam through an aperture placed in the far field was measured using a photo detector fed to the power meter. For the open aperture Z-scan, a lens was used to collect the entire laser beam transmitted through the sample replaced the aperture. The limiting effect of the samples was studied using the same laser in the z-scan technique. The experimental set-up for the demonstration of Z-scan and optical limiting is shown in Fig. 4.
Figure. 4. Experimental set-up for Z-scan and limiting effect.
20 Hussain A. Badran & Adil A. Al-Fregi
A 1 mm quartz cell containing dye solution is kept at the position where the transmitted intensity shows a valley in closed aperture Z-scan curve. A variable beam splitter (VBS) was used to vary the input power. The input laser intensity is varied systematically and the corresponding output intensity values are measured by the photo detector.
RESULTS AND DISCUSSIONS Fig. 4 shows the closed aperture Z-scan data for 0.5 mM concentration of [4-(2-hydroxy naphthylazo)phenyl][2-(2-methoxy
benzylideneamino)-5-methylphenyl]
tellurium
dibromide
dye
in
2
chloroform and polymer film at incident intensity I0=5.44 kW/cm .The scan of both the samples have peakvalley configuration, corresponding to a negative nonlinear refraction index i.e. self-defocusing occur. The defocusing effect is attributed to a thermal nonlinearity resulting from the absorption of a tightly focused beam traversing through the absorbing dye medium produces spatial distribution of the temperature in the sample and, consequently, a spatial variation of the refractive index that acts as thermal lens resulting in phase distortion of the propagating beam.
Normalized transmittance
1.2
1.0
0.8
0.6
0.4
0.2 -30
-20
-10
0
10
20
30
Z(mm)
Figure. 4. Closed Z-scan curve of the dye in solvent and polymer film. Let
Ø0 be the on-axis phase shift at the focus which is related to the difference in the peak and
valley transmission T p-v as [10]: T p-v= 0.406 (1-S)0.25 Ø0
(1)
Where S=1-exp (-2r02/ω02) is the aperture linear transmittance with r0 denoting the aperture radius and ω0 denoting the beam radius at the aperture in the linear regime. Then, the nonlinear refractive index, n2 , is given by [10] n2 = Ø0 λ /2πI0Leff
(2)
Where λ is the laser wavelength, I0 is the intensity of the laser beam at focus z =0, Leff =[1-exp(-αL)] /α is the effective thickness of the samples, L is the thickness of the sample.
21
Synthesis and Study of Nonlinear Optical Properties of A New Azo Dye by Z-Scan Technique
1.2
Dye solution
Normalizedtransmittance
Polymer film 1.0
0.8
0.6
0.4
0.2 -30
-20
-10
0
10
20
30
Z(mm)
Figure. 5. Open Z-scan curve of the dye in solvent and polymer film
Fig. 5 shows the measured Z-scan data for open aperture set-up for the dye in solvent and polymer film at 0.5 mM concentration. The typical Z-scan data with fully open aperture is insensitive to nonlinear refraction, therefore the data is expected to be symmetric with respect to the focus, but absorption in the sample enhances the valley and decreases the peak in the closed aperture Z-scan curve and results in distortions in the symmetric of the Z-scan curve about Z=0. The nonlinear absorption coefficient β can be estimated from the open aperture Z-scan data [21]. In general, the measurements of the normalized transmittance versus the sample position for the
β=
2 2 ∆T I 0 Leff
(3)
cases of the closed and open aperture allow for the determination of the nonlinear refractive index, n2, and the nonlinear absorption coefficient β, since the closed aperture transmittance is effected by the nonlinear refraction and absorption. The determination of n2 is less straight forword from the closed aperture scans. It is necessary to separate the effect of the nonlinear refraction from that of the nonlinear absorption. A method [10] to obtain a purely effective n2 is to divide the closed aperture transmittance by the corresponding open aperture scans. The data obtained in this way purely reflects the effects of the nonlinear refraction. The ratio of Figs 4 and 5 scans is shown in Fig. 6. The values of the nonlinear refractive index and nonlinear absorption for dye in solvent and polymer film at 0.5 mM concentration are given in Table 4. The value of n2 in a dye doped film is found to have large value than in the case of solutions, this may be attributed to three reasons, firstly absorption coefficient (α) for the dye doped film is large than in the case of dye solution,
22 Hussain A. Badran & Adil A. Al-Fregi
secondly the heat dissipation being faster in liquids as compared to that in a solid medium, thirdly it is due to Anderson localization of photons [22]. 2.2
Normalized transmittance
2.0 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 -30
-20
-10
0
10
20
30
Z(mm)
Figure. 6. Z-scan data obtained by dividing the closed aperture data by open aperture data for dye in solvent and polymer film. This is because of the strong scattering regime as the scattering mean free path of photons is less than in the case of liquids, so the localization of strong electromagnetic field inside the solid is responsible for the increase in nonlinearity in optical materials. Table 4: Nonlinear optical parameters for azo dye in chloroform solution and polymer film
β cm/W
n2 cm2/W
× 10 −3
× 10 −7
2.85
3.27
0.54
2.97
4.45
15.62
3.45
18.78
Concentration 0.5 mM
Ø0
Dye solution Polymer film
∆n × 10 −4
The optical limiting curve for the dye in solvent and polymer film at 0.5 mM concentration is shown in Fig.7
23
Synthesis and Study of Nonlinear Optical Properties of A New Azo Dye by Z-Scan Technique
Figure. 7. Optical limiting effects of the dye in solvent and polymer film The output power rises initially with an increase in input power for both samples, but after a certain threshold value the samples start defocusing the beam, resulting in a greater part of the beam cross-section being cut off by the aperture. Thus the transmittance recorded by the photo detector remained reasonably constant showing a plateau region. The limiting behavior observed in both samples is attributed mainly to nonlinear refraction. Since the samples were pumped with cw laser beam the arising nonlinearities are predominantly thermal in nature. The experiment was repeated for the pure solvent chloroform to account for its contribution, but no limiting was observed.
CONCLUSIONS We have synthesized a new azo dye and measured the nonlinear refraction, n2, and the nonlinear absorption coefficient, β, for [4-(2-hydroxy naphthylazo)phenyl][2-(2-methoxy benzylideneamino)-5methylphenyl] tellurium dibromide in solvent chloroform and polymer film at 0.5 mM concentration using the Z-scan technique with 532 nm of SDL laser. The results of Z-scan indicate that the samples have a large optical nonlinearity compared with other optical materials: n2 of the samples is the order of 10-7 cm2/W at 532 nm, while other nonlinear materials such as the poly (phenylacetylene) and poly (p-methoxyphenylacetylene) have n2 = 6x10-18 cm2/W and n2 = 1.1x10-18 cm2/W, respectively [23]; the averaged n2 values for crystals LN:Mg and potassium titanyl orthophosphate are 2.0x10-15 and
1.2x10-15 cm2/W, respectively, for eosin
doped in a polymer film, n2 = 5.58x10-18 cm2/W [24].The sample show good optical limiting behavior at 532 nm. All these experimental results show that the sample is a promising material for applications in nonlinear optical devices.
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Synthesis and Study of Nonlinear Optical Properties of A New Azo Dye by Z-Scan Technique
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