Studies of optical nonlinear properties of asymmetric ionic liquids

Optical Materials 84 (2018) 166–171

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Studies of optical nonlinear properties of asymmetric ionic liquids a

a

b

T

b

I. Severiano-Carrillo , E. Alvarado-Méndez , K.A. Barrera-Rivera , M.A. Vázquez , M. Ortiz-Gutierrezc, M. Trejo-Durána,∗ a b c

DICIS, Universidad de Guanajuato, Comunidad Palo Blanco s/n, 36885, Salamanca, Gto., Mexico DCNE, Universidad de Guanajuato, Noria Alta s/n, 36050, Guanajuato, Gto., Mexico FCFM, Universidad Michoacana de San Nicolás de Hidalgo, Morelia, Mich., Mexico

A R T I C LE I N FO

A B S T R A C T

Keywords: Nonlinear optics Nonlinear refraction Nonlinear absorption Z-scan Thermal nonlinear media Asymmetry ionic liquid

Nonlinear refraction and absorption properties to twelve ionic liquids were measured by the Z-scan technique. k/ dn/dT value reference was determinate by using only one measure. An Influence of anion and cation to nonlinear optical properties was analysed. Cations with mayor length chain and aromatic systems presented better nonlinear optical properties that the alicyclic cations. [NO3] anion presented the best contribution to these properties independently of cation used.

1. Introduction As it is well known, the Z-scan technique is used to determine the nonlinear optical properties such as nonlinear refraction index and nonlinear absorption index, due to its simplicity and accuracy [1]. It has been used to characterize hybrid materials [2,3], liquid crystals [4], organic dyes such as Hibiscus sabdariffa [5,6], gold and silver nanoparticles [7,8], among others. With this technique, it is possible to obtain the sign and magnitude of the nonlinear refractive index (n2) at far field and with closed aperture. Furthermore, the nonlinear absorption (β) is measured at near field with open aperture. The technique consists in displacing the sample along the optical axis (z direction) of a focused laser beam, usually with Gaussian distribution, and detecting the transmitted power at far field or at near field. Different mathematical models have been proposed to adjust the experimental Z-scan data where both, nonlinear refraction and nonlinear absorption, are presented simultaneously. The mathematical model for the characterization proposed by I. Severiano et al. [9] considered the nonlinear thick media as a photoinduced thin lens, where the focal length is represented by a constant (am) multiplied by high power beam radius (ω(z)m), where m represents the type of nonlinearity of the material. This modification allows us to have a better understanding of the nonlinear physical phenomena present in a thick material or liquids where the thermal effects are predominant. Furthermore, it is possible to obtain the nonlinear absorption and the nonlinear refraction with only one measurement at far field. In this work, we present the study of twelve ionic liquids using the Z-scan technique by Severiano's Model.

∗

Corresponding author. E-mail address: [email protected] (M. Trejo-Durán).

https://doi.org/10.1016/j.optmat.2018.06.063 Received 8 March 2018; Received in revised form 21 June 2018; Accepted 26 June 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.

Ionic liquids (ILs) are organic salts in liquid state at environment temperature. The IL's has been described as molecules with organic and inorganic moieties, which their physicochemical characteristics can be determined as viscosity, melting point, density, acid and basic behaviour among others. Due to this versatility, its use and application has been increased in different areas of science. For example, the catalysis process [10,11], chemistry industry [12,13], carbon dioxide capture [14], among others. There are several papers about their physical properties [15,16] or chemical properties [17,18], and one of the most interesting applications in optoelectronics is their nonlinear optical properties (NLO). However, there are few papers [19–26] about these NLO properties where the reported values of nonlinear refractive index to some ILs are 10−9 cm2/W order and 10−3 cm/W to nonlinear absorption coefficient. Santos [20,24] reports that using different derivatives of imidazolium ([Cnmim], n = 4,6,7,10) as cation, the [BF4] anion presents a better thermal nonlinear response comparing to [Tf2N] and [PF6] in the infrared region, and he refers that the best is [C4mim] [BF4]. In the ultraviolet region, the [Tf2N] anion is analysed with the same cations and changes are reported in thermal contribution to NLO. Novoa-Lopez [21] study methyl-imidazolium, pyridinium cations families with [BF4] and [Tf2N] anions, they confirm that the cations have a slight influence in the thermal NLO compared to the influence of the anion. In this work, the synthesis of twelve IL's and the influence of the cations and anions to the nonlinear optical properties are analysed. Only one measure was used to obtain the principal nonlinear optical parameters such as: β, n2, k/dn/dT, and σ. Then, the most of nonlinear

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I. Severiano-Carrillo et al.

Scheme 1. Structure of twelve different ionic liquids.

135.9. IR (cm−1): 3162, 3120 (ν C-H) aromatic stretching; 2992, 2954, 2886 (ν C-H) aliphatic stretching; 1634, 1575, 1467 (ν ring) symmetrical stretching; 1340 MeC-H asymmetrical, 1170 ring stretching; 1060, 850 and 756. [BMIM]+ [PF6]- (IL08). A 1-L, one-necked, roundbottomed flask was charged with 65.6 g (0.37 mol, 1 equiv) of 1-butyl3-methylimidazolium chloride, and 69.3 g (0.37 mol, 1 equiv) of potassium hexafluorophosphate in 70 mL of distilled water. The reaction mixture was stirred at room temperature for 2 h affording a two-phase system. The organic phase was washed with 3 × 50 mL of water and dried under reduced pressure (0.1 mbar, 0.001 mm). Then 100 mL of dichloromethane and 35 g of anhydrous magnesium sulfate were added. After 1 h, the suspension was filtered, and the volatile material was removed under reduced pressure (0.1 bar, 0.1 mm) at 30 °C for 2 h to afford 86.4 g (0.29 mol, 81%) of 1-butyl-3-methylimidazolium hexafluorophosphate as a light yellow viscous liquid. [BMIM]+ [PF6]- Yield: 95%. [1H NMR (acetone-d6) δ: 0.96 (t, 3H), 1.37 (m, 2H), 1.93 (m, 2H), 4.05 (s, 3H), 4.36 (t, 2H), 7.68 (s, 1H), 7.74 (s, 1H), 8.95 (s, 1H); 13C NMR (acetone-d6) δ: 13.0, 19.3, 32.1, 36.0, 49.6, 122.7, 124.1, 137.0; IR cm−1: 3171, 3125, 2965, 2939, 2878, 1571, 1167, 836.

absorption coefficients and nonlinear optical properties of IL with [NO3] – anion have not been reported to our knowledge. We found that the best NLO are due to influence of [NO3] anion despite the cation used. Aromatic cation presents better nonlinear response that alicyclic cation, and it is confirmed that the length of chain in aromatic cation have influence on NLO. 2. Experimental 2.1. Synthesis of ionic liquids The synthesis of ionic liquids was based on previous reports. [EMIM]+ [BF4]–, and [BMIM]+ [BF4]– [27], [BuPy]+ [CF3COO] –, and [BuPy]+ [BF4]– [28], [EMIM]+[NO3] –, and [BMIM]+[NO3] – [29], [EMIM]+[Tf2N] –, [BuPy]+[TF2N] –, [EMIM]+[CF3COO] –, and [BMIM]+[CF3COO] – [30], [MePyrr]+[HCOO] –[MePyrr]+[NO3] –, and [MePyrr]+[HSO4]– [31] (Scheme 1)-The spectroscopy of this ILs was reported in Ref. [32]. 2.1.1. Synthesis of IL01, IL05 and IL08 Synthesis of [EMIM]+[BF4]–(IL01). Tetrafluoroboric acid (22.34 mL, 0.171 mol, 48% solution in water) was slowly added to a stirred slurry of silver (I) oxide (19.83 g, 0.0855 mol) in 50 mL distilled water over a period of 10 min. To avoid silver (I) oxide photodegradation, the reaction of the mixture was fully covered with aluminium foil. After silver (I) oxide was completely consumed, a solution of 1-ethyl-3-methylimidazolium chloride (25.0 g, 0.171 mol) in 150 mL distilled water was added to the reaction mixture and stirred at room temperature for 2 h. The white precipitate of silver (I) chloride was filtered off, and the solvent was removed at 65 °C under vacuum. The resulting salt is a pale-yellow liquid. For the synthesis of [BMIM]+ [BF4]- (IL05) the same procedure was followed by using a 1-butyl-3methylimidazolium chloride (29.90 g, 0.171 mol) solution instead of 1ethyl-3-methylimidazolium chloride solution (25.0 g, 0.171 mol). [EMIM]+ [BF4]– Yield: 95%. [BMIM]+ [BF4]- Yield: 95%. 1H NMR (acetone-d6): δ 1.52 (t, 3H), 4.00 (s, 3H), 4.33 (q, 2H), 7.67 (t, 1.85), 7.74 (t, 1.94), 9.05 (s, 1H); 13C NMR: δ 15.2, 36.0, 45.1, 122.2, 123.8,

2.1.2. Synthesis of IL02 and IL06 Synthesis of [EMIM]+[NO3]– (IL02). An aqueous solution (30 mL) of silver nitrate (8.48 g, 0.05 mol) was added dropwise to a stirred icecooled aqueous solution (50 mL) of 1-ethyl-3-methylimidazolium chloride (2.14 g, 0.046 mol). The solution was slowly brought to room temperature and the white precipitate was removed by filtration. The filtrate was concentrated in vacuo. For the synthesis of [BMIM]+[NO3](IL06) the same procedure using an aqueous solution was carried out (50 mL) of 1-butyl-3-methylimidazolium bromide instead side 1-ethyl3-methylimidazolium chloride. [EMIM]+[NO3]–Yield: 93%. [BMIM]+[NO3]–Yield: 93%. 1H NMR (D2O): δ 1.4 (t, 3H), 3.8 (s, 3H), 4.1 (q, 2H), 7.3 (d, 2H), 8.5 (s, 1H); 13C NMR: δ 14.01, 35.13, 44.33, 121.4, 122.9, and 132.2. IR (cm−1): 3156, 3110 (ν C-H) aromatic stretching; 2990, 2946 (υC-H) aliphatic stretching; 1644, 1575 (ν ring) symmetrical stretching; 1360 MeC-H asymmetrical; 1170 ring stretching, symmetrical; 832 and 758. 167

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2.1.7. Synthesis of IL12 Tetrafluoroboric acid (30.0 mL, 0.230 mol, 48% solution in water) was slowly added to a stirred slurry of silver (I) oxide (26.66 g, 0.115 mol) in 60 mL distilled water over 10 min. To avoid photodegradation of silver (I) oxide, the reaction mixture was fully covered with aluminium foil. Until silver (I) oxide was completely reacted, a solution of 1-butylpyridinium bromide (49.71 g, 0.23 mol) in 150 mL distilled water was added to the reaction mixture and stirred at room temperature for 2 h. The yellow precipitate of silver (I) bromide was filtered off, and the solvent was removed at 65 °C under vacuum. The resulting salt is a light brown liquid. [BuPy]+ [BF4]– Yield: 92%.1H NMR (D2O): δ 0.86 (t, 7H), 1.26 (sextet, 6H), 1.92 (quintet, 5H), 4.54 (t, 4H), 8.0 (t, 2H), 8.43 (t, 1H), 8.74 (d, 3H). 13C NMR: δ 12.28, 18.24, 32.19, 61.36, 127.84, 143.8, 145.12. IR (cm−1): 3140, 3096, 3074 (υCH) aromatic stretching; 2966, 2939, 2878 (υC-H) aliphatic stretching; 1636, 1584, 1490 (υring) symmetrical stretching; 1431, 1382 MeC-H asymmetrical, 1174 ring stretching, symmetrical; 1062 and 772.

2.1.3. Synthesis of IL03 and IL07 Synthesis of [EMIM]+[CF3COO]– (IL03). A 4.342 g (22.72 mol) sample of 1-ethyl-3-methylimidazolium bromide and 5.019 g (22.72 mol) of silver trifluoroacetate were diluted in 50 mL of water and mixed. After 1 h of stirring at 70 °C, the silver bromide was filtered off, and the solution was concentrated under reduced pressure. For the synthesis of [BMIM]+[CF3COO]- (IL07). The same procedure was used by substituting 4.342 g (22.72 mol) sample of 1-ethyl-3-methylimidazolium bromide for 5.089 g (23.22 mol) and 5.019 g (22.72 mol) of silver trifluoroacetate for 5.089 g (23.03 mol). [EMIM]+[CF3COO]Yield 95%. [BMIM]+[CF3COO]- Yield 93%. 1H NMR (acetone-d6): δ 0.93 (t, H), 1.39 (sextet, H), 3.34 (s, H), 4.89 (t, H), 8.26 (t, H), 8.71 (t, 1H), 9.42 (d, 3H). 13C NMR: δ 13.72, 19.92, 34.19, 62.22, 115.74, 121.68, 129.3, 146.24; 159.33, 159.96, 160.58, 161.21 (due to carboxyl CF3COO group, JC-F = 32.4). IR (cm−1): 3138, 3087, 3067 (υC-H) aromatic stretching and (υC=O); 2968, 2939, 2878 (υC-H) aliphatic stretching; 1673 (υC=O), 1583, 1490 (υring) symmetrical stretching; 1200, 1170 ring stretching, symmetrical; 1125, 830 and 801.

3. Theory 2.1.4. Synthesis of IL04 Synthesis of [EMIM]+[Tf2N] – (IL04). A 20.54 g (107 mmol) sample of 1-ethyl-3-methylimidazolium bromide in 50 mL of distilled water and 30.85 g (107 mmol) of bistrifluoromethanesulfonimide lithium salt in 100 mL distilled water at 70 °C were mixed. The solution was extracted by 100 mL of CH2Cl2, and the extract was concentrated and dried for 2 h at 60 °C under reduced pressure. [EMIM]+[Tf2N]- Yield 92%. 1 H NMR (acetone-d6): δ 9.03 (s, 1H), 7.78 (t, 1H), 7.71 (t, 1H) 4.39 (q, 2H), 4.05 (s, 3H), 1.56 (t, 3H).

Traditionally, the Bahae model [1] measures nonlinear absorption to near field (open aperture) and the nonlinear refraction to far field (close aperture). Nevertheless, both effects can be measured with a single measurement to far field by using the mathematical model proposed by Severiano et al. [9], where the nonlinear absorption is considered in the formation of a new focal length f(z) of a thermal lens in the medium. This is expressed by

f (z ) = a

πω4 (z ) [αdπPω2 (z ) + 2βdP 2]

(1)

where a = κπ /(dn/ dT ) , P is the power to which the media is illuminated, κ is the thermal conductivity, dn/dT is the shift of refraction index due to temperature T, α is the linear absorption, d is the thickness of the media, ω is the beam radius and β is the nonlinear absorption coefficient. To our knowledge, there is no evidence of enough reports the κ values, and dn/dT for IL used in this paper. Therefore, to adjust the model with experimental data, it is necessary to modify numerically a = κπ /(dn/ dT ) and β. The normalized transmittance of the Z-scan curves was calculated numerically [33]. Focal length is related to nonlinear phase shift at the focus Δϕ by Ref. [34]:

2.1.5. Synthesis of IL09 andIL10 Preparation of methyl pyrrolidinium and pyridinium based ionic liquids with inorganic counter anion [MePyrr]+[NO3]- (IL09), [MePyrr]+[HSO4]- (IL10), were obtained through the same procedure. We report only the preparation of [MePyrr]+[NO3]-. Methyl pyrrolidine (26.78 g; 0.37 mol) is introduced in a two-necked round-bottom flask immerged in an ice bath, equipped with a dropping funnel to add nitric acid (68% in water) and a thermometer to control the temperature. Under vigorous stirring, nitric acid (34.54 g; 0.37 mol) is added dropwise to the flask for about 60 min (mixture temperature < 35 °C). Stirring is maintained during 4 h at room temperature before adding 120 mL of 1,2-dichloroethane. Then, this mixture is distilled under normal pressure until the water-DCE a heteroazeotropic boiling point is reached (73 °C) to get rid of residual water. 1,2-dichloroethane is finally evaporated from the mixture under reduced pressure in order to collect a pale yellow and viscous liquid. For the synthesis of [MePyrr]+[HSO4](IL10), the same procedure was followed using sulphuric acid instead of nitric acid.

Δϕ =

z0 2f (z )

(2)

In this way, the n2 values can be obtained using [35]:

n2 =

λω02 Δϕ 2PLeff

(3)

where Leff is the effective thickness of the sample, Leff = (1−exp (−αd))/α.

2.1.6. Synthesis of IL11 Methyl Pyrrolidine (38.55 g; 0.85 mol) is placed in a three-neck round-bottom flask immerged in an ice bath and equipped with a reflux condenser, a dropping funnel to add the acid, and a thermometer to monitor the temperature. Under vigorous stirring, formic acid (61.45 g; 0.85 mol) is added dropwise to the methyl pyrrolidine (60 min). As this acid-base reaction is strongly exothermic, the mixture temperature is maintained at less than 25 °C during the addition of the acid by using the ice bath. Stirring is maintained for 4 h at room temperature, and a low-viscous liquid is obtained. The residual methyl pyrrolidine or acid is evaporated under reduced pressure and the remaining liquid is further dried at 80 °C under reduced pressure (1–5 mmHg) to obtain the target ionic liquid (98.72 g; yield 98.7%). Neither crystallization nor solidification were observed on the liquid which was stored for several weeks at 20 °C [MePyrr]+[HCOO]- Yield 92%.

4. Experimental The experimental set up of Z-scan is shown in Fig. 1 (closed aperture). The parameters of the experimental set up are: ω0 = 28 μm, λ = 514 nm, z0 = 4.8 mm, focal lens of 7.5 cm and an Ar laser

Fig. 1. Z-scan typical setup to measure nonlinear refraction index. 168

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Fig. 2. Z-scan curves by nonlinear refraction to P = 25 mW, λ = 514 nm.

reported based on imidazolium cation and [BF4], [Tf2N] anions [19–25]. In addition, nonlinear absorption coefficient values of ILs agree with those reported by Novoa-Lopez [19]. Note that the IL06 has the |n2| with high value. IL04 presents high values of β and σ, which means that the thermal effect is major. Table 1 shows the linear refraction index measured (n0), the nonlinear absorption coefficient (β), the nonlinear refraction index (n2) (both obtained using eq. (1) in Fig. 2), the thermal response (k/dn/dT), σ (which shows the relation between nonlinear response and degree of nonlocality in nonlocal materials [36] defined by eq. (5)), and the correlation factor (r2); all of them were obtained at 25 mW of laser power. At this power we obtained the best correlation factor, and the behaviour of these values at the other powers are similar as it is denoted in Fig. 3.

continuous wave (CW) of variable power, and the thickness of cell 1 mm. Fig. 2 shows the experimental curves of ionic liquids obtained by Zscan technique at close aperture in which a 25 mW power of laser was used. In addition, the adjustment of the curve obtained by means of eq. (1) is also shown. We looked for the best experimental (red dashes) and theoretical fixing (black line), considering the difference in transmittance (ΔT). 5. Results and discussion The linear refractive index (n0), for each ionic liquid synthetized, was obtained to determine the nonlinear optical properties by using a multi-wavelength Abbe refractometer. The linear absorption coefficient (α) of ionic liquid pure at wavelength of 514 nm with, and it was calculated by using the Beer-Lambert law. The thickness of cell was 2 mm (lc). The transmitted power was measured across both the cell empty (Pce) and the cell fulled with the ionic liquid (Pcl), using the following equation:

α=

log10

σ=

(5)

It is important to mention that IL04 and IL07 present lower correlation factor, because these ILs have lower nonlinear refraction response, despite their high σ value; for example, [TF2N] anion is known to show lower nonlinear response [21]. To emphasize our affirmation we show, in Fig. 4, the Z-scan curves to IL07 at different powers. The Zscan curves will be defined as the laser power is increased. The ionic liquids data were analysed in agreement to anion or cation families. First, the [BMIM] family at 20 mW laser power is discussed. The dipolarity/polarizability theoretical given by π* for some ionic

( )

− lc

κ n2 α 0 dn/ dT ω02

Pcl Pce

(4)

The Z-scan curves were obtained to all ILs at 20, 25 and 30 mW power of laser. Fig. 3a–c shows |n2|, β and σ values to ILs. It is highlighted that the values are similar, which allows us to claim that our measurements of n2 values of all our ILs agree with others previously 169

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Fig. 3. Values to different power of laser to ILs. a) n2; b) β; c) σ.

[20,24]. However, k/dn/dT values for [BF4] are lower. Therefore, [NO3] family has the higher values for n2 and β comparing to [BF4] family. Moreover, the β value is also increased using [NO3] cation. However, [NO3] anion presents the best values for n2, κ/dn/dT, and β, in each cation family; this is possible due to the polarizability value (π*) is higher than the other anions. The nonlinear optical values were calculated to [Tf2N] anion; nonetheless, their correlation between theoretical and experimental data in Z-scan measure was lower. Hence, it is not possible to assert that the anion has the highest β and σ values.

liquids with [BMIM] cation and relative thermal lens sensitivity, is reported by Lungwitz [37] and Chieu [38]. The IL05 was employed to compare κ/dn/dT values previously reported [38] with the ones calculated in this research. We obtained 0.2367 [38] and 0.2705 [our] values which are very close. This shows that with our model is possible to obtain good results with only one measure. We analyse the κ/dn/dT values behaviour for IL05, IL06 and IL08 with polarizability (π*) values of anions reported in Ref. [37]. We localized the π* value of [CF3COO] for IL 07, following the order: NO3 > BF4 > PF6 > CF3COO of the cation family. Aromatic family (ILs01,02,03) and alicyclic derivatives family (ILs09,10,11) cations were analysed. The results showed that aromatic cation have better values of nonlinearity (n2) but alicyclic cations have better values of thermal nonlinearity (κ/dn/dT). Note how a π deficient system increases κ/dn/dT value but the nonlinearity n2 decreases. Finally, the best form to contribute to nonlinearity is using a π-excedent heterocycle cation. On the other hand, the contribution to nonlinearity of [BF4] (ILs01,05,12) and [NO3] (ILs02,06, 09) anions families were analysed. The [BMIM] cation showed higher values of n2 as Santos reported

6. Conclusions We have determined the nonlinear optical properties for β, n2, and k/dn/dT of twelve ILs at 20, 25 and 30 mW of power laser. Z-scan technique and the mathematical model presented by equation (1) allowed us to obtain a reference value for k/dn/dT of ILs. Anion NO3 gives the best nonlinear values for β, n2, since it has higher polarizability compared to others. We confirm that the influence of length chain in imidazolium cation contributes to increase the nonlinearity. The [BMIM]+[NO3]- has the highest value of nonlinearity. ILs have

Table 1 α, n0, β, n2, k/dn/dT, σ and r2 and values obtained to ionic liquids at 25 mW. Ionic liquid

Ionic liquid pure

α (cm−1)

n0 20 °C

β (10−4 cm/W)

n2 (10−9cm2/W) negative

k/dn/dT (104) negative

σ

Corr. factor r2

IL 01

([EMIM]+[BF4]-) 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM]+[NO3]-) 1-ethyl-3-methylimidazolium nitrate ([EMIM]+[CF3COO]-) 1-ethyl-3-methylimidazolium trifluoroacetate ([EMIM]+[Tf2N]-) 1-ethyl-3-methylimidazolium Bis((trifluoromethyl)sulfonyl) imide ([BMIM]+[BF4]-) 1-butyl-3- methylimidazolium tetrafluoroborate ([BMIM]+[NO3]-) 1-butyl-3- methylimidazolium nitrate ([BMIM]+[CF3COO]-) 1-butyl-3- methylimidazolium trifluoroacetate ([BMIM]+[PF6]-) 1-butyl-3- methylimidazolium hexafluorophosphate ([MePyrr]+[NO3]-) 1-methylpyrrolidinium nitrate ([MePyrr]+[HSO4]-) 1-methylpyrrolidinium hydrogen sulphate ([MePyrr]+[HCOO]-) 1-methylpyrrolidinium formate ([BuPy]+[BF4]-) 1-butyl pyridinium tetrafluoroborate

0.0635

1.4290

0.6

2.0811

2.2

0.8555

0.8538

0.8782

1.5396

15

4.2056

24.0

1.0801

0.9418

0.0275

1.4110

0.16

1.7379

5.0

1.7906

0.8772

0.1661

1.4275

96

0.20338

2400

5.4616

0.6235

0.0845

1.4235

0.32

2.8081

1.1

0.6664

0.9918

0.0175

1.4953

10

20.036

0.094

0.9649

0.9864

0.0932

1.4330

9

0.19006

250

2.2751

0.3755

0.0792

1.4125

2.5

0.924

15.7

1.3635

0.9068

0.5897

1.4275

10

1.8457

36

1.0694

0.9536

0.0713

1.4715

0.5

0.5474

7.8

0.7799

0.8944

0.0060

1.4377

0.1

1.5092

0.43

1.0443

0.9226

0.1049

1.4495

0.33

0.304

14.0

0.6417

0.7538

IL 02 IL 03 IL 04 IL 05 IL 06 IL 07 IL 08 IL 09 IL 10 IL 11 IL 12

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Fig. 4. IL 07 ([BMIM]+[CF3COO]-), Z-scan curves by nonlinear refraction to P = 20, 25 and 30 mW, λ = 514 nm.

potential applications in optoelectronic devices. [19]

Acknowledgments

[20]

We gratefully acknowledge the support of the CONACYT through grant 290908, and Guanajuato University DAIP-CII 234/2018.

[21]

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