Nonlinear optical response of liquid crystalline matrix doped with single walled carbon nanotube and graphene sheets

    Nonlinear optical response of liquid crystalline matrix doped with single walled carbon nanotube and graphene sheets Sepideh Naserbakht, Ali Maleki, Salman Mohajer, Keyvan Hesari, Mohammad Hossein Majlesara PII: DOI: Reference:

S0167-7322(16)30246-X doi: 10.1016/j.molliq.2016.07.080 MOLLIQ 6094

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

28 January 2016 11 July 2016 18 July 2016

Please cite this article as: Sepideh Naserbakht, Ali Maleki, Salman Mohajer, Keyvan Hesari, Mohammad Hossein Majlesara, Nonlinear optical response of liquid crystalline matrix doped with single walled carbon nanotube and graphene sheets, Journal of Molecular Liquids (2016), doi: 10.1016/j.molliq.2016.07.080

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Nonlinear optical response of liquid crystalline matrix doped with single walled carbon nanotube and graphene sheets

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Sepideh Naserbakht, Ali Maleki, Salman Mohajer, Keyvan Hesari, Mohammad Hossein Majlesara* Photonics Laboratory, Physics Department, Kharazmi University, Tehran, P. O. Box 15719114911, Iran

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Abstract

In this work, the nonlinear optical characterization of single-walled carbon nanotubes (SWCN) and graphene in guest-host liquid crystal system have been investigated using z-scan technique and far-field diffraction patterns. The guest materials have been

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dispersed in 8CB smectic liquid crystal at different concentration and specific thermal conditions. Z-scan measurements with continuous wave (CW) He:Ne laser beam at 632.8 nm have been used for determining the nonlinear refractive index (n2) of guesthost system. It was found that n2 depends substantially on the concentration of the SWCN and graphene layers doped in SLCs, and it led to a significant growth in the nonlinear refractive index. Furthermore, the results of far-field optical diffraction patterns

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under CW He-Ne laser illumination have been verified the result of nonlinear properties of Z-scan measurements.

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Keywords: liquid crystal; z-scan technique; nanotube; graphene

Corresponding author: Mohammad Hossein Majlesara

Tel.: +98 21 82233020; fax: +98 21 88575729.

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E-mail: [email protected]

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The z-scan method was introduced and reported by Sheik-Bahae et al.[2,17]and is a sensitive single beam tool for measuring the nonlinearity of optical materials. This method involves two experimental set-ups: 1. closed aperture and 2.open aperture, in order to resolve the nonlinear refraction index, n2, and absorption coefficient, β, [18]. In closed aperture (CA) z-scan measurement, an aperture is placed to prohibit fraction of the light from reaching the detector. The nonlinear refractive index, n2 in the expression n(I) =n0+n2I, would be calculated where n0 is the linear refractive index and I is the intensity of the incident laser light[18,19]. Indication of the sign or in other words, the type of nonlinearity right after the measurement is the boldest privilege of z-scan technique. The nonlinear behavior of the sample is equivalent to the formation of an induced positive or negative lens as self-defocusing (negative) or self-focusing (positive) [24–26](figure 1).

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1. Introduction During last decade, nonlinear optical properties of liquid crystals have attracted considerable attentions from numerous researchers. One of the most prominent cause of their popularity for the recent investigations is the highly birefringence [1] due to their large molecular anisotropy and intermolecular ordering [2]. The refractive indices of a liquid crystal are mainly determined by the molecular structure, operating wavelength and temperature [3,4]. Optical nonlinearity of these materials could be enhanced by the presence of small amounts of proper impurities [2,5]. Nonlinear properties of impurities doped in liquid crystal hosts have been studied in various experiments [6– 8]. Since Liquid crystals are significantly welcoming to other guest particles, they are used as host systems for new composites with improved characteristics. After the successful enhancement of the electro-optical properties of nematic liquid crystals, other candidates such as cholesteric and smectic liquid crystals has been considered for further studies [9–11]. Development of nonlinear optical behavior of liquid crystal compounds depends on several parameters such as size[10,12], type, doze and intrinsic characteristics of the nanoparticles used for doping[13], temperature and laser intensity[14,15]. The guest-host system should share similar attributes and size to not disrupt the order of the liquid crystal [16]. This reported investigation is concerned with the determination of the impacts of SWCNTs and graphene nanosheets as guest particles on the nonlinear refractive index of liquid crystalline material, 4-octyl4′-cyanobiphenyl (8CB).

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The nonlinear refraction index, n2, could be easily calculated from the peak-to-valley height (ΔTp–v) of the close z-scan curve by:

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n2= (λ ΔTp-v)/ (2πLeff (0.406)(1-S)(0.27) I0 )

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Where s and Leff are the linear transmission of the aperture and the effective length of sample, respectively:

2. Theory

S=1- exp(-(2r2a)/(w2a))

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Leff=(1-e-αL)/α

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And also:

Due to the anisotropy properties of liquid crystal, these materials have two refractive indices, ne and no which are the refractive indices for the extraordinary and ordinary rays, respectively. is defined as [3]:

α= (-Ln (I/I0 ))/L

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is the linear absorption[19,23]. In the second setup, which is open aperture (OA) measurement, aperture is totally removed and replaced by a lens to collect all the light into the detector (S=1, the total transmittance)[2]. Hence the measured transmittance only depends on nonlinear absorption coefficient (β)[19]. According to dependence of the detector signal intensity on the sample position, the magnitude of the nonlinear index is evaluated[24]. The normalized transmittance under open-aperture conditions is given by[2]:

= (n2e +2n2o )/3 (1) ne and no could be obtained as a function of and the birefringence (Δn = ne − no): ne= 〈n〉+2/3 Δn no=〈n〉 -1/3 Δn

(4)

(2) (3)

2.1 Z-scan Theory 2

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T(z)=m=0 [(-q0(z,0)]m /(m+1)3/2

temperature range of phases of 8CB is shown in figure 2[29].

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Where q0(z)= ( I0 Leff)/ (1+ z2/z02) for q0(0)<1.

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The samples were prepared via dissolving the dispersed SWCNT in ethanol and graphene in DMF (dimethylformamide) into the liquid crystal host separately. Approximately, 20 mg of SWNTs were immersed in 50 mL of absolute ethanol (99.99% purity) and ultrasound was applied for 30 min (9 Watts) with an interruption of 10 Sec. for every 30 Sec. using an ultrasonic probe (Sonics Vibra, model VCX 750) and centrifuged at 4000 rpm for 30 min[30]. Then, the dispersed nanotubes and graphene nano sheets were doped liquid crystal in the temperatures higher than nematic-isotropic phase and solvent was removed from the resulting mixture by evaporating in vacuum. The guest– host cells were made by sandwiching solutions between two optical glass plates (1×1.5 cm2) which coated with a thin Indium Tin Oxide (ITO) layer as transparent electrodes. The planer and homeotropic alignment of the cells were achieved by surface treatment of polyvinyl alcohol (Sigma) and Lecithin (Sigma). So, the LC molecules would lie with its long axis parallel and perpendicular to the rubbing direction, respectively. The cell thickness was fixed by placing a Mylar spacer (10μm) in between and was sealed together by a sealing material (epoxy resin glue). The prepared cells were placed on a polarizing microscope (Reichert) and their homogeneity confirmed using crossed polarizers. The assembled mixture was filled in samples by capillary method.[18,30]. Alignments of LC molecules in prepared cells and the orientations of graphene nanosheets and SWCNTs among the 8CB molecules are shown in a schematic way in figure 3.

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2.2 Diffraction Rings Pattern Theory The experimental setup for the diffraction ring patterns consists of a light source, laser, a convex lens, a sample holder and by the screen. The number of appeared ring patterns on the screen would introduce another way for defining the nonlinear refractive index. The laser beam used in the experiment has a Gaussian distribution, therefore the relative phase shift suffered by the beam while traversing the sample of thickness (L) can be written as:

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3.2 Sample Preparation

∆φ0= kL∆n

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In which k=2π/λ is the wave vector in vacuum. The on-axis nonlinear phase-shift, Δφ0, is also related to the number of rings, N, observed as: (10)

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∆φ0=2πN

n=n0+ Δn

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The number of rings grows as a result of laser power increase, thus they are intensity dependent [25]. On the other hand, the total refractive index of the medium (n) can be calculated by: (11)

Where n0 is the background refractive index and Δn= n2 I [26]. At the point that the laser beam illuminates the sample, the medium absorbs the light and its temperature rises. As a result of this, local refractive index is changes and will induce the self-diffraction [25]. 3. Experimental

3.3 z-scan setup The z-scan technique was implemented via using a linear polarized Gaussian continuous wave (CW) HeNe laser at λ=632nm. Power of the laser was reduced to 10mW by adequate filters. It is important to choose a low intensity for laser to avoid the thermal effects [2]. A photodetector and a power meter (Lab-Master, Coherent) placed behind a small aperture record the

3.1 Material Single-walled carbon nanotube that was obtained from US Research Nanomaterials [27] and graphene which was synthesized in Materials and Energy Research Centre of Iran[28], both had the role of the guest particles. The host material is 8CB liquid crystal bought from Merck. Chemical structure and 3

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The Red plots in figures belong to the pure 8CB sample which is mentioned by the title of 0%W/W of impurity. As it is explicit, the pure sample has the lowest peak-valley height and according to equation 4, the less TP-V, the less nonlinear refractive index. In figure 7, the cyan and blue curves pertain to the sample with 1%W/W, 5%W/W graphene nanosheets in 8CB, respectively. It is clear that the blue curve has a keen rise comparing to the red curve. Hence the utmost value of TP-V belongs to this curve. Upon increasing the dopant compositional percentage in host LC, TP-V experienced a rise, thus according to equation 4 it is expected that the sample with higher amounts of dopant owns maximum value of n2 in comparison with the other samples with the same dopant and different compositional percentage. The computed values of samples with different compositional percentage of graphene nanosheets in 8CB by equation 5 to 7 are written in tables 1 and 2 divided in the cells’ alignment. In tables 1 and 2 it could be seen that the pure 8CB has the maximum amount of effective length. Moreover, based on the equations 4 and 6, it also has the minimum amounts of linear absorption and nonlinear refractive index in constant intensity. All the samples have a nonlinear refractive index in order of 10−5cm2/W and as it was expected, the sample with 5%W/W of graphene nanosheets has the maximum value of the n2. By increasing the compositional percentage of the dopant the Leff has a gradual drop and hence the n2 and  experience a growth. In figure 9, the pink and purple curves belong to samples with 1%W/W and 5%W/W of SWCNTs in 8CB. Same as figure 8, there is a dramatic growth in the curve of sample with 5%W/W of SWCNTs in comparison with the pure sample and obviously it has the maximum value of TP-V. By equation 4, it is likewise expected to have a maximum n2 for this sample. Like first case, upon adding the compositional percentage of SWCNT, TP-V of curves has been surged and the result would be a rise in their nonlinear refractive indices. Therefore, the sample with more dopant have a larger n2 regarding to equation 4.

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transmittance intensity. The experimental setup of used z-scan technique is shown schematically in figure 4.

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In the experiment conducted, the sample is moved back or forth along the z-axis near the beam waist, which is focused by an 8 cm focal lens. When the sample is far from the beam waist, no transmission change is evident [2].

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The absorption spectrum of the guest materials in parallel-aligned liquid crystalline host was scanned by Shimadzu UV–Vis double-beam spectrophotometer (Model UV-2100) at room temperature. As it is shown in figure 5, the presence of guest materials has increased the absorbance of pure LC host. 8CB has a high polarizable rod-like molecular structure with a big  -electron system mainly due to the presence of biphenyl core. It also has a strong polar head group ( C  N ) which is capable of dipole-dipole interactions with other polar groups. However, as it is shown, there is not any interaction between guest and host media at the frequency of CW laser that was used in this experiment. The increase in absorbance could be as a result of the guest-host interactions which made an improvement in alignment of liquid crystal molecules and subsequently growth the nonlinear properties.

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4. Results and Discussions

4.1 Z-scan Measurements

The open aperture (OA) z-scan curve of the 0%, 1%, and 5% wt. graphene nanosheets and SWNTs doped in LC are shown in figure 6 and figure 7, respectively. It is seen that the normalized transmittance has a peak at Z=0. By increasing the compositional percentage of guest particles, the optical nonlinearity amplifies. Regarding to what just mentioned, the 5% wt. contains the highest amount of transmittance and shows the strong saturable absorption[18]. Plots of close aperture (CA) z-scan for different compositional percentage of graphene and SWCNT are shown in figures 8 and 9. The curves present valleypeak graphs, which obviously indicate the positive sign for nonlinear refractive index. Thus, graphene nanosheets and SWCNTs in 8CB exhibit self-focusing optical non-linearity due to the sign. 4

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Tables 3 and 4 demonstrate the calculated parameters of samples with different compositional percentages of SWCNT in 8CB by equation 5 to 7. All the composites have the nonlinear refractive index in order of 10−5cm2/W, similar to data in table 1.

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This work investigated the nonlinear refractive index of graphene nanosheets and SWCNTs doped in 8CB by the z-scan technique and also the results were verified by the diffraction ring pattern of samples. The values for n2 are obtained in the order of 10 −5 cm2/W which has its utmost amount in the compound with 5%W/W of guest particle (both graphene nanosheets and SWCNTs cases) as it was expected regarding to the z-scan plots. Diffraction rings patterns were also more distinct in mentioned samples in comparison with the other ones. A growth in the compositional percentage of both impurities until 5% W/W will cause a gradual rise in the amounts of nonlinear refractive indices, hence it could be expected that for the mixtures with more compositional percentage of dopant, there will be a larger refractive index.

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As it was interpreted from figure 9 and specifically the purple curve, the sample with 5%W/W of SWCNTs has the minimum amount of Leff and thus the maximum amounts of the n2 and. By increasing the compositional percentage of the dopant the Leff has a gradual drop and  has a rise, hence the n2 of these samples are increased. Comparing figures in this section declares that the valley-peak height has risen by increasing the compositional percentage of dopants, thus the maximum amount of n2 in both case belongs to the sample with 5%W/W of the doped impurities. In samples with higher compositional percentage of graphene or SWCNT, the nonlinear refractive index will exceed in comparison with the samples with lower percentage of each guest particles.

5. Conclusion

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4.2. Diffraction Rings Pattern Measurements

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Regarding to the nonlinear refractive indices of our samples, the He-Ne laser beam could act as a probe laser and detects a refractive index changes within the sample by determining the diffraction rings pattern. It is tried to use this method to compare the results of this technique with the data of z-scan method. According to the data in tables 1 to 4, it is expected that the diffraction rings patterns of the samples with 5%W/W of both dopants will be more distinct and clear than other samples with same dopant and also in comparison with the pure 8CB. The laser beam impinges the cell perpendicularly and the result image of this action would appear on the screen immediately. The diffraction rings pattern of each sample is shown in figures 10 and 11. It is shown that the number of rings increases with increasing concentration of the dopants and consequently the nonlinearity of liquid crystalline matrixes improves. The results from diffraction rings patterns verify the output data from z-scan measurements. Thus, both techniques indicate and confirm pure 8CB and the sample with 5%W/W of both dopants as maximum and minimum amount of n2 respectively in both cases. 5

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(Cm-1)

9.27 9.20 9.14

130.86 168.10 182

Leff(m)

(Cm-1)

9.37 7.83 7.18

152.7 510.83 701.10

n2(cm2/w) *10-5 0.50 1.43 3.92

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Leff(m)

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152.7 164.2 208.9

n2(cm2/w) *10-5 2.38 3.81 4.55

Leff(m)

(Cm-1)

9.27 9.26 9.03

130.86 145.08 151.99

n2(cm2/w) *10-5 0.50 1.01 3.53

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Table 4. Compositional percentage of SWCNT 0% 1% 5%

9.37 9.22 9.02

n2(cm2/w) *10-5 2.38 3.83 4.62

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Table 3. Compositional percentage of SWCNT 0% 1% 5%

(Cm-1)

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Table 2. Compositional percentage of Graphene 0% 1% 5%

Leff(m)

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Table 1. Compositional percentage of Graphene 0% 1% 5%

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Table captions: Table 1. Z-scan results at different compositional percentage of graphene in 8CB in homogeneous alignment Table 2. Z-scan results at different compositional percentage of graphene in 8CB in homeotropic alignment Table 3. Z-scan results at different compositional percentage of SWCNT in 8CB in homogeneous alignment Table 4. Z-scan results at different compositional percentage of SWCNT in 8CB in homeotropic alignment

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Captions:

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Fig. 1. Normalized closed-aperture z-scan data Fig. 2. Chemical structure of 8CB and its temperature ranges[29]

Fig. 3. Orientation of SWCNTs and graphene nanosheets among the 8CB molecules

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Fig. 4. Schematic diagram of the experimental arrangement for close aperture z-scan setup

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Fig. 5. Absorption spectrum of a) graphene nanosheets and b) SWCNTs in parallel-aligned liquid crystalline Fig. 6. The OA z-scan plot for different compositional percentage of graphene nanosheets doped in 8CB LC at constant intensity

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Fig. 7. The OA z-scan plot for different compositional percentage of SWCNTs doped in 8CB LC at constant

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intensity

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Fig. 8. Plots of the CA z-scan measurements of different compositional percentage of graphene nanosheets doped in 8CB LC at constant intensity Fig. 9. Plots of the CA z-scan measurements of different compositional percentage of SWCNTs doped in 8CB LC at constant intensity

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Fig. 10. Diffraction ring patterns of a) Pure 8CB, b) 8CB+1%W/W graphene, c) 8CB+5%W/W graphene

Fig. 11. Diffraction ring patterns of a) Pure 8CB, b)8CB+1%W/W SWCNTs, c) 8CB+5%W/W SWCNTs

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Fig. 1. Normalized closed-aperture z-scan data

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Fig. 2. Chemical structure of 8CB and its temperature ranges [28]

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Fig. 3. Orientation of SWCNTs and graphene nanosheets among 8CB molecules

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Fig. 4. Schematic diagram of the experimental arrangement for close aperture z-scan setup

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Fig. 5. The absorption spectrum of a) graphene nanosheets and b)SWCNTs in parallel-aligned liquid crystalline host

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Fig. 6. The OA z-scan plot for different compositional percentage of graphene nanosheets doped in 8CB LC at constant intensity

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Fig. 7. The OA z-scan plot for different compositional percentage of SWCNTs doped in 8CB LC at constant intensity

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Fig. 8. Plots of the CA z-scan measurements of different compositional percentage of graphene nanosheets doped in 8CB LC at constant intensity

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Fig. 9. Plots of the CA z-scan measurements of different compositional percentage of SWCNTs doped in 8CB LC at constant intensity

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Fig.10. Diffraction ring patterns of a) Pure 8CB, b) 8CB+1%W/W graphene, c) 8CB+5%W/W graphene

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Fig. 11. Diffraction ring patterns of a) Pure 8CB, b)8CB+1%W/W SWCNTs, c) 8CB+5%W/W SWCNTs

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The nonlinear optical characterization of guest-host liquid crystal system has been

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investigated.

Single-walled carbon nanotubes and graphene nanosheets are used as guest materials.

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The nonlinearity of samples was calculated using Z-scan technique and far-field

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Nonlinear refractive index depends substantially on the concentration of the SWCN and

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graphene nanosheets.

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diffraction pattern.

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