MgO nanoparticle effect on nonlinear refractive index of nematic liquid crystal doped with Sudan black B dye using Z-scan method

In this paper, by capitalizing on z-scan technique and using cw He–Ne laser at 632.8 nm, we investigated the impact of added MgO nanoparticles on nonlinear optical properties of 6CHBT liquid crystal which was doped with Sudan black B dye. The applied cell had homogeneous alignment. MgO nanoparticles were added to 6CHBT liquid crystal which was doped with 0.3 wt.% Sudan black B dye at three concentrations of 0.1, 0.3 and 0.6 wt.%. It was found that the added MgO nanoparticles did not significantly change the nonlinear refractive index of the compound at 0.1 and 0.6 wt.% concentrations; however, they enhanced the nonlinear refractive index of the compound at 0.3 wt.% concentration. The accumulation of nanoparticles in high concentrations causes high scattering of light which can reduce the intensity of transmitted light and, as a result, nonlinear effects.


Introduction
In recent years, a vast number of photonic applications have been developed and proposed on the basis of nonlinear optical properties of materials. A wide variety of media have been investigated and engineered so as to be applied in photonic devices. Given the myriad of examined compounds, organic materials, especially liquid crystals, thanks to their great optical nonlinearities and fast responses, are of high significance (Meier et al. 1975;. They demonstrate remarkable light-induced nonlinearity. This notable feature is attributed to their large dielectric anisotropy which is caused by molecules' collective reorientation (Engström et al. 2009;Hisakado et al. 2005). As a case in point, given a reorientating optical field, easy susceptibility and collective response of liquid crystals (LCs) result in a very strong cubic nonlinearity of medium which is referred to as the so-called nematic optical orientational nonlinearity. During the past 30 years, research studies on LC nonlinear optical properties (NLO) have led to outstanding developments. Studies in this domain investigate a wide spectrum of issues starting from isotropic state study to later nematic phase, discovery of dye-enhanced effect, trans-cis isomerization, orientational photorefractive effect and surface mediated reorientation. Increasingly strong nonlinear response of Nematic liquid crystal (NLC) has been reported. In addition, their optical properties can be easily controlled using a weak electric or magnetic field. The physical properties of crystals such as phase transition temperatures, viscosity, etc. can be easily changed by mixing them together. The above-mentioned features make them appropriate for numerous applications, namely the production of displays, optical storage devices, optical computers, etc. Accordingly, the optimization and enhancement of NLCs' nonlinear optical properties via doping with different materials such as dyes, nanoparticles is regarded as a hot research issue which has recently attracted researchers' attention (Iranizad et al. 2014;. Despite the fact that liquid crystals are inherently highly nonlinear materials, their molecular reorientation response is greatly enhanced by adding a small amount of a dichromatic, light-absorbing dye (Khoo et al. 1998(Khoo et al. , 2003Lee and Chiu 2001). Nonlinearity of liquid crystals may be enhanced by means of adding dye which is attributed to the effect of the additional optical torque and the director orientation process carried out with the participation of another torque induced by the dye. The dye-induced torque is often notably stronger than the optical torque. The addition of dye to a liquid crystal has a significant impact on its nonlinearity feature including nonlinear refractive index and nonlinear absorption coefficient which depend on the concentration of dopants (Jafari et al. 2006).
For demonstrating the impact of nanoparticles on the nonlinear refractive index of liquid crystal, we added a very small amount of Sudan black B dye (about 0.3%wt.) to the liquid crystal. Then, we investigated the impact of adding nanoparticles with different concentrations on the nonlinear refractive index of the desired composition. In this experiment, we investigated the effect of adding magnesium oxide nanoparticles on the nonlinear properties of dye-stained liquid crystals in a homogeneous alignment of its molecules with cw laser. Dyeing the liquid crystal with dye provides the opportunity for observing nonlinearity in samples with low-power homogeneous alignment.

Experimental
Z-scan technique is regarded as one of the primary tools which is aimed at specifying nonlinear properties of materials. It was firstly invented and reported in 1989 by Sheik-Bahae et al. Based on this technique, the normalized transmittance of a tightly focused Gaussian beam is changed through a finite aperture in the far field as a function of sample position, z, when the sample is displaced along beam propagation (z-direction) (Sheik-Bahae et al. 1989, 1990 (Fig. 1). The change in the normalized transmittance is attributed to the selffocusing effect in the sample environment. Fig. 1 The experimental arrangement of the closed Z-scan set-up. D, detector; L, convex lens; S, sample A typical illustration of the normalized transmittance through a finite aperture (closed aperture) is given in Fig. 2. A valley followed by a peak is depicted in the above-mentioned diagram as the sample with a positive nonlinear refraction index is moved away from the lens. The difference between the peak and valley of the normalized transmittance is given by the following equation: In this equation, s = 1 − e −2 r 2 a ω 2 a , r a and ω a denote radius of the aperture and the beam radius in the aperture plane in the linear regime (Δ • = 0) , respectively. The induced onaxis phase shift at the beam waist is defined by the following equation: where λ, α, L and I refer to the laser wavelength, the linear absorption coefficient, the thickness of sample and the on-axis intensity at focus in the sample, respectively. Having (1), we can obtain n 2 from Eq. (2). 6CHBT-nematic liquid crystalline materials, used in the present experiment, were purchased from the institute of chemistry of the military technical academy, Warsaw, Poland. 6CHBT-nematic liquid crystal molecules include a polar group (-NCS) at one end and a hexyl group at the other end (-C6H 13 ) (Fig. 3a). The polar end has a remarkable effect on the nonlinear property including its dielectric anisotropy. One more useful property of 6CHBT liquid crystal is that it has a wide temperature range but its transition temperature is relatively low. The clearing points of this material were specified via the DSC curves drawn up by DSC822e-model of the DSC system, the Swiss Mettler Toledo company (Fig. 4). Based on these curves, during heating and cooling cycles, the clearing temperatures were 43.23 and 42.47 • C respectively. Some outstanding properties and merits of Sudan black B dye are solubility, stability, dichroic ratio and high-order; such properties are functions of a long molecular structure and the absence of lateral groups in these molecules. High-order feature of Sudan Black B might be somewhat attributed to the molecules' high polarizability and long molecular axis (Jafari et al. 2006). Furthermore, as shown in Fig. 3c, Sudan black B dye has a broad linear absorption spectrum with its peak centered at 600 nm. Sudan black B dye was applied as an additive to enhance mixture nonlinearity so much that the nonlinear refraction index of the final mixture can be measured. The absorption spectra of Sudan black B dye and the chemical structure of these compounds are depicted in Fig. 3. MgO nanopowders with a diameter smaller than 50 nm were supplied from US research nano-materials institute. Like NaCl, magnesium oxide has a regular structure of the same  type where the bonds are strongly ionic. Thanks to the electrostatic nature of the binding forces and the spherical symmetry of charge distribution, single ions in the crystalline network are surrounded by as many large number of ions as the opposite charge. Accordingly, MgO is considered to be an inert oxide. Atoms on the surface, edges and corners have lower coordination than the atoms of other locations. Nevertheless, regarding nano-MgO, atoms in analogous locations demonstrate a high density of catalytically active sites. This phenomenon indicates that, like a heterogeneous catalyst, magnesium oxide as a nanoparticle is useful (Pilarska et al. 2017). Stability, antibiotic property and nontoxicity are regarded as the practical merits of MgO. It is relatively easy to obtain MgO; there is a lack of its sedimentation and it is easily dispersed in 6CHBT. MgO nanoparticles have spherical shape with diameter values of approximately ≲ 50 nm and surface areas greater than 50 m 2 /g. They demonstrate high purity and appear in a white powder form.
In addition to its catalytic properties, thanks to its small particle size and large surface areas, synthetic magnesium oxide demonstrates excellent adsorption features (Martra et al. 2001). MgO reactivity is attributed not so much to its relatively high surface area but to the number of defect sites per unit area; this feature supports adsorption process (Razouk and Mikhail 1975;Nowak and Carter 2009). Moreover, magnesium oxide has high dielectric constant and electrical resistance (Julkapli and Bagheri 2016;Roessler and Huffman 1991;Li et al. 2014;Ramachandran et al. 2016). Figure 5 illustrates transmission electron microscopy (TEM) image and particle size distribution of MgO nanoparticles (https:// www. us-nano. com/ inc/ sdeta il/ 7708).
The cells applied in the experiments were made of two glass plates. The glass substrates were coated by dipping them in PVA solution for homogeneous alignment. The quality of the alignment layers was acknowledged by optical polarising images of the liquid crystal (Fig. 6). The nematic liquid crystal and the SBB (Sudan black B) dye were combined at the 0.3 wt.% concentration. Then, the resulting combination was doped with MgO nanoparticles at three different concentrations of 0.1, 0.3 and 0.6 wt.%. The obtained mixture was agitated with an ultrasonic stirrer so that nanoparticles were uniformly dispersed for 45 min. The empty cells were filled with the final mixture by capillary action. He-Ne laser at λ = 632.8 nm with 2.2 mw power was used as a light source for recording z-scan data. The power of z-scan data was reduced by 0.5-0.6 mw by using a polarizer for determining Transmission electron microscopy (TEM) image and particle size distribution of the applied MgO nanoparticles (refer to (https:// www. us-nano. com/ inc/ sdeta il/ 7708) the polarization of incident light on the cell. The focal length of the lens applied in the experiment setup was 50 mm. A precision stepper motor drove the sample with 0.5 mm steps along the beam. Aperture diameter in the closed mode was 2 mm. The intensity of laser beam was reduced through a polaroid pair. The linear absorption coefficient was computed via the well-known I = I • e − L equation. An edge scan method was applied for measuring the radius of the laser Gaussian beam.

Results and discussion
As shown in Fig. 7, the z-scan data of the prepared samples was obtained using the abovementioned setup at the room temperature. The curves related to the normalized transmittance changes versus sample position (z) were drawn for all samples by Originpro 2018 software. The values regarding linear refractive index of the pure 6CHBT liquid crystal for homeotropic and homogenous alignment configuration were obtained by A. Jafari and M. H. Majles Ara, +10 × 10 −5 and −0 ⋅ 71 × 10 −5 cm 2 w . Furthermore, the nonlinear refractive index of 6CHBT-liquid crystal composition with high concentrations of Sudan black B dye was investigated (Jafari et al. 2006;Majles Ara et al. 2008). The change in the nonlinear refractive index of the liquid crystal (Jafari et al. 2006) which is attributed to adding dye can be justified by the Janossy effect (Janossy 1999). The basic outline of this method can be summarized as follows. In explaining dye-enhanced reorientation mechanism, the interaction between liquid-crystal molecules and the excited dye molecules is considered as the starting point. When dye molecules are in their ground state or in the excited state, it is assumed that the mean field acting upon the dye molecules is different. Such an assumption leads to a modified version of optical torque, which is introduced in the following manner: In this equation, τ dye and τ em refer to dye and the electric field of the light-induced torque, respectively; a refer to the characteristic parameter of the dye which depends only on dye structure. The angular momentum of dye molecules, in the excited state, is enhanced by the laser beam. Hence, the host molecules minimize their angular momentum by means τ dye = a τ em Fig. 6 a Optical polarising images and b microscopic photo illustrating the quality of the homogeneous alignment of cells filled with 6CHBT liquid crystal doped with MgO nanoparticals at 0.6 wt.% concentration of reorientation. Consequently, the total angular momentum remains constant. The reorientation occurs in the radiated region of the sample which may lead to self-focusing of the laser beam. The decreased linear absorption of the samples is attributed to adding nanoparticles which is related to a decrease in the impact of dye molecules ( τ dye ) . The impact of nanoparticles on the nonlinear refractive index of the samples is evident from the shape differences of the above-mentioned curves. It demonstrates the difference between the peak and valley of the normalized transmittance. The nonlinear refractive index (n 2 ) can be computed by the related relationships given in Sect. 2. The nonlinear refractive indices of the composition with different percentages are reported in Table 1. Based on the values given in this table, it was observed that adding nanoparticles to the 0.3 wt.% concentration compound significantly enhances its refractive index; however, the same result was not achieved at the other concentrations. Moreover, such performance may be observed in the values of dielectric permittivity anisotropy and refractive index anisotropy which is explained by dilution theory (Gorkunov and Osipov 2011). Based on this theory, Nanoparticles reduce the ordering of the nanocomposite systems by declining the interaction between liquid crystal molecules. The strong dipole-dipole interaction between NPs and nematic molecule pushes NPs towards aligning themselves along the direction of nematic director; as a result, a strong elastic coupling is produced. In contrast, at the 0.6 wt.% concentration of MgO doped in liquid crystal and dye composition, an abnormal performance

Conclusion
The investigation of the results obtained from the experiments indicated that adding MgO nanoparticles does not significantly change the nonlinear refractive index of the combination at 0.1 wt.% and at 0.6 concentrations. However, at 0.3 wt.% concentration, this index is enhanced. It should be reiterated that dye concentration was the same in all the samples. Hence, the observed changes may not be attributed to dye addition. The justification for the different behavior of the combination and the enhanced nonlinear refractive index of the 0.3 wt.% concentration compound can be attributed to the accumulation of nanoparticles at high concentrations. In other words, the change in the refractive index of the liquid crystal doped with nanoparticles can be attributed to the reorientation of the electric dipole of the nanoparticles which results from the interaction of the intense light electric field and the induced dipole of the nanoparticles. Thus, the orientation of nematic molecules increases; also, along with an increase in the concentration of nanoparticles, the interaction of adjacent nanoparticles increases. Consequently, the alignment of the liquid crystal molecules is disorienting in the areas where the accumulation of nanoparticles occurs. By increasing the compositional percentage of the nanoparticle, the locally orientated LC media decreases which leads to the reduced nonlinear indices. According to the results obtained in Vafaee, MgO nanoparticles accumulate at concentrations higher than 0.5 wt.% (Vafaie et al. 2021). The accumulation of nanoparticles within the composition reduces the number of single free nanoparticles; this may reduce their impact on nonlinear properties such as nonlinear refractive index. On the other hand, the accumulation of nanoparticles in high concentrations leads to vast scattering of light, which can potentially reduce the intensity of transmitted light; in this way, nonlinear effects are not produced.
Doping the combination of liquid crystal and dye with a certain concentration of nanoparticles significantly changes their linear absorption coefficient. Such a change is observed Table 1 The linear absorption coefficient (α), the difference between the peak and valley of the normalized transmittance (ΔT p-v ), the induced on-axis phase shift at the beam waist (Δ • ) , the nonlinear refractive index (n 2 ), and third order susceptibility (χ (3) ) of different percent composition Compositions α (cm −1 ) ΔT p-v Δ •(rad) n 2 (10 −5 cm 2 /w) χ (3) (10 −3 esu) in the values of non-linear refractive index. Given the different reported concentrations in this paper, 0.3 wt.% concentration can be potentially regarded as the optimal concentration. To add Sudan black B dye to 6CHBT liquid crystal changed the sign of its nonlinear refractive index (Majles Ara et al. 2008). All the samples have a negative refractive index which includes a sample lacking nanoparticles. The liquid crystal combined with Sudan black B dye with a low concentration has a negative refractive index (Jafari et al. 2006;Majles Ara et al. 2008).