c Allerton Press, Inc., 2018. ISSN 8756-6990, Optoelectronics, Instrumentation and Data Processing, 2018, Vol. 54, No. 1, pp. 1–7. c A. Azarpour, S. Sharifi, M.K. Nezhad, 2018, published in Avtometriya, 2018, Vol. 54, No. 1, pp. 38–45. Original Russian Text
OPTICAL INFORMATION TECHNOLOGIES
Study of Nonlinear Optical Properties of Saffron in Nano-Droplet and Solvent by the Single Beam Z-Scan Technique A. Azarpour, S. Sharifi, and M. K. Nezhad Department of Physics, Faculty of Science, Ferdowsi University of Mashhad, 9177948974, Iran, Mashhad, Azadi Square, Rasavi Khorasan Province E-mail:
[email protected] [email protected] Received September 12, 2017 Abstract—Molecules with strong two-photon absorption have applications in photodynamic therapy and photonic devices. The effect of polarity of the medium and microemulsion on nonlinear optical properties of saffron is studied by the Z-scan technique with a CW laser at the 532 nm wavelength. The values of the nonlinear refractive index and two-photon absorption of saffron are found to increase with a decrease in the medium polarity and with an increase in the droplet size in the microemulsion. . Keywords: nonlinear optical properties of saffron, Z-scan. DOI: ?
INTRODUCTION Physical factors responsible for the emergence of nonlinear optical (NLO) properties, which are versatile and occur in various types of materials. Materials with strong two-photon absorption (TPA), e.g., saffron, have applications in medicine for imaging and in photodynamic therapy (PDT) [1–4]. Saffron is extracted from Crocus sativus flowers, and its quality depends on three components: crocin, picrocrocin, and safranal, which are responsible for color, taste, and odor, respectively. Saffron is cultivated mainly in Iran, Italy, India, Greece, and Spain. The north-east of Iran has one of the largest saffron farms: Robat-e-Sang, where saffron used in the present study was produced. In this work, the novel materials were synthesis mixtures of different solvents and microemulsion (ME) with saffron to improve nonlinear optical properties. Microemulsions are mixtures of a surfactant, oil, and water because they are stable and transparent, which makes them suitable for optical studies [5, 6]. It was shown in the previous study [7, 8] that the optical properties (fluorescence intensity and wavelength) of fluorescein sodium salt (FSS) and rhodamine B (RhB) depend on ME formation. TPA was studied in this work in a water-soluble molecule located in distyrylbenzene. The results indicated that TPA reduced in saffron owing to an increase in the dielectric constant of the solvent [9]. The TPA cross section is defined as σTPA =
4π 2 a50 α ω 2 g(ω) δTPA . 15c0 Γf
(1)
Here a0 , c0 , and α are the Bohr radius, the speed of light in vacuum, and a constant; g(ω) and Γf are the profile of the spectral line and the level broadening; finally, δTPA is the TPA cross section defined as δTPA = 24
(μ − μ )μ 2 e g ge , ωf /2
(2)
where μge and ωf are the dipole moment of the transition and the difference between the energy of the excited and ground states; μg and μe are the dipole moments of the ground and excited states, respectively. 1
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AZARPOUR et al.
It follows from Eqs. (1) and (2) that the TPA cross section depends on the molecular dipole moment, which, in turn, depends on the solvent polarity and interaction of the molecule with the medium. For example, in 4-trans-[p-N, N-di-n-butylamino-p-stilbenylvinyl]-pyridine (DBASVP), TPA changes with the dielectric constant or solvent polarity [10]. Other studies indicated that the NLO properties of rhodamine 6G depend on the amount of the surfactant and on the micelle concentration in water [11]. Z-scan experiments used for studying the TPA and nonlinear refractive (NLR) index were described in [12, 13]. In the Z-scan instrument, the NLR index was measured by a closed-aperture setup. The NLR index n2 can be calculated from the difference between the maximum and minimum of light transmission trough the closed aperture ahead of the detector ΔTP − V [14]: n2 = (λΔTP − V )/(2πLeff I0 0.406(1 − S)0.27 ).
(3)
Here S is the linear transmission of the aperture, Leff is the effective length, and I0 is the light intensity. The normalized transmittance under open-aperture conditions is T (z) =
[(−q0 )]m /(m + 1)3/2 ,
(4)
m=0
where q0 = (βI0 Leff )/(1 + (z/z0 )2 ) (z0 is the Rayleigh length and β is the TPA coefficient) [15]. The nonlinear susceptibility is given as (3)
(3)
χ(3) = χR + iχI .
(5)
The real part of the nonlinear susceptibility defines the NLR index and is estimated by the equation (3)
χR = 2n2 n20 ε20 c,
(6)
where n0 and c are the refractive index and light velocity, respectively. The real part of molecular hyperpolarizability γR is (3)
γR = χR /(L4 N ).
(7)
where L is the Lorentz correction factor and N is number density of molecules [16]. The effects of the solvent and ME on enhancement of the NLO properties of saffron was studied in this work by using the Z-scan technique. EXPERIMENT The surfactant Aerosol OT (Bis(2-ethylhexyl) sulfosuccinate sodium salt, AOT) and hexane used in the present study were produced by the Sigma-Aldrich company (USA). The concentrations of saffron extracts were measured by weighing the remaining solute after evaporation of the solvents (water, ethanol, or methanol). All the solutions were kept in a refrigerator. Two types of solutions were prepared: (a) mixtures of natural red saffron with different dye concentrations Cdye in water, ethanol, or methanol, and (b) ME in a mixture of AOT and hexane with an aqueous solution of saffron. The ME was prepared by mixing volumes at a fixed molar ratio of the surfactant to water, X = [H2 O]/[AOT] (X = 5, 10, and 20) and different mass fractions of droplets (mf, drop = (mass of droplet)/(total mass)). The droplet size was controlled by the molar ratio X. The Z-scan experiments were performed with a laser having an output power of 80 mW and operating at a wavelength of 532 nm. OPTOELECTRONICS, INSTRUMENTATION AND DATA PROCESSING Vol. 54 No. 1
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STUDY OF NONLINEAR OPTICAL PROPERTIES OF SAFFRON IN NANO-DROPLET (a)
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1.5
1.0
0.5
0
1 0.5 2
0
0 300 350 400 450 500 550 Wavelength, nm
(c)
1.0 Absorption, rel. units
Absorption, rel. units
Absorption, rel. units
(b)
2.0
2
3
300 350 400 450 500 550 Wavelength, nm
300 350 400 450 500 550 Wavelength, nm
Fig. 1. Absorption spectra of saffron in three solvents: (a) methanol with 0.01%; (b) ethanol with 0.01%; (c) water with 0.01% curve 1) and 0.005% (curve 2).
(a) 1.1
1.0
1.0
0.9 0.7 0.6 0.5 0.4 0.5 0.4 0.3 0.2
0.3 0.2 0.1 -80
-40
0 Z, mm
40
80
Open aperture
0.9
0.8
Open aperture
Open aperture
(c)
(b)
1.1
0.8 0.7 0.6 0.5 0.4
0.2 0.3 0.4
0.3 0.2 -80
-40
0 Z, mm
40
80
1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -80
0.04 0.05 0.07 0.10
-40
0 Z, mm
40
80
Fig. 2. Normalized open-aperture curves demonstrating the minimums near Z = 0 for different percentage of saffron in water (a), methanol (b), and ethanol (c).
RESULTS AND DISCUSSION Effect of the Solvent The absorption spectra of saffron in three solvents are presented in Fig. 1. The absorption spectra of saffron in water show a peak at λ = 440 nm, which remains constant as the saffron amount increases from 0.005 to 0.01%. Saffron in ethanol and methanol shows two peaks at λ = 433 and 458 nm. Saffron absorption in water is less intense than in ethanol and methanol. A red shift is observed in the absorption spectra with a change in the solvent from water to ethanol and methanol. The intensity of the peak at λ = 433 nm decreases, and the broadband grows with a change in the solvent from ethanol and methanol to water. Probably, this difference between the absorption spectra is due to saffron aggregation in water because the solubility of saffron in ethanol and methanol is higher than in water. The NLO properties of saffron were studied for three solvents (water, ethanol, and methanol) by the Z-scan technique (Figs. 2 and 3). The normalized open-aperture curve shows a minimum for saffron in the solvents (see Fig. 2). The normalized closed-aperture curve shows a peak followed by a valley for saffron OPTOELECTRONICS, INSTRUMENTATION AND DATA PROCESSING Vol. 54 No. 1
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AZARPOUR et al. (b)
1.5
1.0
(c) 0.07 0.08 0.01
1.8
1.2
0.012 0.008 0.007
1.8 Closed aperture
0.06 0.05 0.01
Closed aperture
Closed aperture
(a)
0.6
1.2
0.6
0.5 _50
0 Z, mm
50
-50
0 Z, mm
50
-50
0 Z, mm
50
Fig. 3. Normalized closed-aperture curves demonstrating the peaks followed by the valleys for saffron solutions in water (a), methanol (b), and ethanol (c) with different saffron percentage.
(a)
(b)
9
8
C = 0.1 %
8 7 b . 10 , W/cm
5
-7
b . 10-7, W/cm
7 6
4 3
6 5 4 3
2 2 1 10-5
2 .10-5
3.10-5
4.10-5
5.10-5 C, %
1 20
30
40
50
60
70
80
e
Fig. 4. TPA coefficient (a) as a function of the saffron percentage in water (), methanol (•), and ethanol (); (b) as a function of the dielectric constant of the solutions for a constant saffron percentage in the solution.
in water, methanol, and ethanol with different percentage of saffron (see Fig. 3). The values of the NLR index n2 and TPA coefficient β were calculated from the closed-aperture and open-aperture data by means of fitting with the use of Eqs. (3) and (4). Figure 4a shows the TPA coefficient β as a function of the saffron percentage in water, methanol, and ethanol. The results indicate that the TPA values of saffron in water are smaller than in ethanol. Figure 4b shows that β as a function of the dielectric constant of the solvents at a constant percentage of saffron decreases with an increase in the dielectric constant of the solutions. The TPA reduction due to an increase in the medium polarity is associated with a change in the molecular dipole moment. The dye aggregation can affect the dipole moment of the molecule [17–19]; hence, it can change the TPA magnitude. The NLR index n2 as a function of the saffron percentage in different solvents is presented in Fig. 5a. The value of n2 for ethanol samples is higher than that for water. The value of n2 as a function of the dielectric constant of the solvents is plotted in Fig. 5b. The results indicate than n2 increases with an increase in the saffron percentage in the solvent and decreases with an increase in the solvent polarity. OPTOELECTRONICS, INSTRUMENTATION AND DATA PROCESSING Vol. 54 No. 1
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(b)
12
C = 0.01 %
10
n2 . 10 , W/cm2
8
-12
n2 . 10-12, W/cm2
10
6
8
6
4
4 2 2 2 .10-6
0
4 .10-6
6 .10-6
8 .10-6 C, %
20
30
40
50
60
70
e
80
Fig. 5. NLR index (a) as a function of the saffron percentage in water (), methanol (•), and ethanol (); (b) as a function of the dielectric constant of the solvents for a constant saffron percentage in the solution.
(b)
0.20 0.10 0.08 0.06
-40
0 40 Z, mm
80
(c)
1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 -80
1.1 1.0 0.9 Open aperture
Open aperture
Open aperture
(a) 1.1 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -80
0.15 0.10 0.20 0.30
-40
0 Z, mm
40
80
0.8 0.7 0.6 0.5 0.30 0.25 0.20 0.10
0.4 0.3 -80
-40
0 Z, mm
40
80
Fig. 6. Normalized open aperture transmission intensity demonstrating the minimums near Z = 0 for saffron MEs with different values of X: (a) X = 20; (b) X = 10; (c) X = 5; the points and solid curves are the experimental data and the results of their fitting, respectively.
Effect of the Microemulsion Figure 6 shows the normalized open-aperture transmission intensity as a function of Z, which was obtained for saffron mixed with MEs (saffron-MEs) with three droplet molar ratios (X = 20, 10, and 5). The normalized closed-aperture Z-scan plots for saffron-MEs for different droplet concentrations are shown in Fig. 7. The peak-valley curves show the negative sign of n2 in the solutions. Figure 8 shows the values of n2 and β as functions of the saffron percentage in MEs. The results reveal TPA and NLO enhancement with an increase in the molar ratio from 5 to 20. The thermal effects in the Z-scan experiments are manifested if a high-power CW laser is used; due to light absorption, the energy is transferred into the materials, and the refractive index decreases. The thermal effects depend on the solution density and thermal expansion coefficient (dn/dT ). By varying the droplet size or saffron concentration, one can change the density and thermal expansion coefficient in the solution, with corresponding changes in the NLR index and TPA coefficient. Thus, the TPA coefficient β and the NLR index n2 increase with an increase in the droplet size, which is due to saffron aggregation and to the change in the molecular dipole moment. OPTOELECTRONICS, INSTRUMENTATION AND DATA PROCESSING Vol. 54 No. 1
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1.4
1.8
1.2 1.0 0.8
1.4 1.2 1.0 0.8
0.6
0.6
0.4
0.4 -20
0 20 Z, mm
60
0.08 0.06 0.04
2.0 1.6 1.2 0.8 0.4
0.2 -60
2.4
0.01 0.02 0.04
1.6 Closed aperture
Closed aperture
1.6
(c)
(b) 0.006 0.015 0.020
Closed aperture
1.8
-60
-20 0 20 Z, mm
60
0 -60
-20
0 20 Z, mm
60
Fig. 7. Normalized closed-aperture transmission curves for saffron-MEs at X = 20 (a), X = 10 (b), and X = 5 (c).
(a)
12
(b)
8 7
10 8
b . 10 , W/cm
6
5 4
-7
n2 . 10-12, W/cm2
6
4
3 2 1
2
0 0 0
0.02
0.04
0.06
0.08
mf
0.05
0.10
0.15
0.20
0.25
0.30
mf
Fig. 8. NLR index n2 (a) and TPA coefficient β (b) as functions of the saffron percentage in saffron-MEs for X = 5 (•), X = 10 (), and X = 20 ().
It is well known that the AOT/water/hexane mixture has three different phases: oligomeric, transient, and monomeric phases. The microemulsion with X < 8 is oligomeric, and that with X ≥ 20 is monomeric. Moreover, the hydrocarbon chain length of hexane is small; therefore, the transition phase is smaller and is around X = 10. CONCLUSIONS The values of the NLO parameters of saffron in three solvents (water, methanol, and ethanol) and saffron-MEs were studied by using the Z-scan technique. The NLO values are measured at a wavelength of 532 nm. A negative nonlinear refractive index of the order of 10−12 cm2 /W and the TPA coefficient of the order of 10−7 cm2 /W are obtained for all the samples. The nonlinear effects are enhanced with a decrease in the dielectric constant and with an increase in the droplet size. A change in the droplet size (molar ratio) is the best way to improve the nonlinear properties of saffron. We believe that the saffron-doped droplet (microemulsion) is suitable for further fundamental studies on molecule interaction in solutions and photodynamic applications.
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