Tuning of birefringence, response time, and dielectric

Applied Physics A (2018) 124:463 https://doi.org/10.1007/s00339-018-1878-9

Tuning of birefringence, response time, and dielectric anisotropy by the dispersion of fluorescent dye into the nematic liquid crystal Govind Pathak1 · Kaushlendra Agrahari1 · Geeta Yadav1 · Atul Srivastava1 · Olga Strzezysz2 · Rajiv Manohar1 Received: 25 January 2018 / Accepted: 29 May 2018 © Springer-Verlag GmbH Germany, part of Springer Nature 2018

Abstract In this study, fluorescent dye benzo 2,1,3 thiadiazole has been dispersed into the pure nematic liquid crystal (NLC) 2020, which is composed of fluorinated 4′-alkylphenyl-4-isothiocyanatotolanes, in three different concentrations. Electro-optical and dielectric parameters have been investigated in the present work. Birefringence has been calculated by transmittance technique for pure and dye-dispersed system, and found to be increased for dispersed system. This is the key finding of the present investigation. In this work, response time has also been calculated by optical switching method and found to be decreased after the dispersion of fluorescent dye into the pure NLC 2020. Contrast ratio has also been measured here and found to be increased for the dispersed system. Relative permittivity has been found to be decreased after the dispersion of dye. Dielectric anisotropy has also been calculated for pure NLC and dye–NLC dispersed system, and found to be decreased for dispersed system. The present investigation may be helpful in the improvement of response time of liquid crystal-based devices.

1 Introduction Liquid crystals have attracted much attention from researchers because of their optical non linearity and very rapid optical response [1–5]. The nature of the liquid crystals (LC) is very complex and their electro-optical properties also play an important role on optical processing systems and photonic devices such as mobile phones, smart cards, and integrated displays [6, 7]. Nematic liquid crystals (NLC) are the most commonly used LC materials in the modern display industry [8]. NLCs are preferred in many different applications due to their anisotropic structure, which relates to molecular orientation and temperature variation [9–13]. It is well known that a laser beam passing through a transparent nematic liquid crystal (NLC) produces the reorientation of the nematic director. This effect is explained by the optical torque acting on NLC molecules due to their anisotropic optical polarizability [14]. Recently, it was demonstrated that a small amount (less than 1%) of a dye dissolved in * Rajiv Manohar [email protected] 1

Liquid Crystal Research Lab, Department of Physics, University of Lucknow, Lucknow 226007, India

Liquid Crystal Group, Military University of Technology, S. Kaliskiego 2, 00‑908 Warsaw, Poland

2

NLC can enhance the reorientation by almost two orders of magnitude and, in some cases, can even change its sign [15]. The explanation proposed by Janossy [16] is connected with the electronically excited metastable states appearing in the molecules of absorbing dye. The interaction of the excited dye molecule with the host LC molecules is different from the one of the dye molecules in the ground state. A single LC compound cannot fulfil all the requirements of suitable parameters for the displays. Therefore, guest–host mixtures of LCs have been used because of their potential application in displays devices [17]. Dye-doped liquid crystalline systems have been the subject of intense studies in recent decades [18, 19]. The addition of absorbing dyes to NLCs introduces new orienting mechanisms [20, 21]. In a mixture of LC and dye, the collective orientation of the LC molecules under the action of an electric field influences the dye molecules. This phenomenon is called guest–host interaction. Some work on guest nematic devices has already been published by Sims et al. [22]. In guest host displays, the liquid crystalline materials used are the solutions of dye in pure liquid crystal. Work on the photoswitching of diazulene dyes in nematic hosts has also been published by Petersen et al. [23]. Dyes have an elongated molecular structure similar to the molecules of liquid crystals. When dyes are dissolved in the LCs, they acquire an orientation, such that all the long axes lie considerably in the same direction as that

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of the molecules of liquid crystal. The detail study of preparation of dye mixtures for use in nematic hosts has already been reported by Cowling et al. [24]. Therefore, the presence of dye molecules in the liquid crystal host influences many properties of the liquid crystals. The study of dye-doped systems is, hence, important to understand the molecular dynamics of such systems and the effect of presence of dye molecules on the molecular properties of the pure LC. There are many work have been done recently on improving host material for dye-doped display [25, 26]. Optical study of ferroelectric liquid crystal (FLC) dispersed with fluorescent dye has already been published by our group [27]. In the present paper, we report the electro-optical and dielectric parameters of the pure NLC and fluorescent dyedoped NLC. Birefringence has been measured for pure and dye-dispersed system. Response time has been calculated for pure NLC and dye-dispersed system. Contrast ratio (CR) has also been calculated in the present study. In the present work, dielectric anisotropy has also been measured. Calculation of threshold voltage after dispersion of dye in the pure matrix is also a promising target of this investigation.

2 Experimental details and methods 2.1 Liquid crystalline material The liquid crystal material used in the present study is NLC 2020 which is composed of fluorinated 4′-alkylphenyl4-isothiocyanatotolanes. This NLC 2020 has been obtained from Institute of Chemistry, Warsaw Poland. This nematic liquid crystal is a high birefringent material [28, 29]. It has ∆n = 0.45 (589 nm), ∆ε = 15.5, and η = 24.6 mPas [30]. The phase sequence of the material is as follows: − 20 ◦ C

107.7 ◦ C

Cr ��→ � N ��→ � Iso, where “Cr” represents the crystal phase of liquid crystal, “N” represents the nematic phase of liquid crystal, and “Iso” represents the isotropic phase of the liquid crystal.

2.2 Dopant The dopant material used for doping in the present investigation is fluorescent dye benzo 2,1,3 thiadiazole, which has been purchased from Sigma-Aldrich with the chemical structure, as shown in Fig. 1 [31]. This fluorescent dye is a disc shape dye. In the molecular structure of dye, we can see that one sulphur atom (S) is attached with two nitrogen atoms (N) and these two nitrogen atoms are attached with a hexagonal benzene ring. Fluorescent dye has the unique property of absorbing in the UV range and emitting in the visible range of the colour spectrum [27]. Toxic rating is low

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Fig. 1 Molecular structure of the fluorescent dye benzo 2,1,3 thiadiazole

in the fluorescent dyes. It has excellent fastness properties and also it has high colour strength.

2.3 LC cell preparation and instruments used Current investigation is focused on the electro-optical and dielectric parameters of nematic liquid crystal dispersed with fluorescent dye. For this investigation, we have dispersed the dye into pure NLC in two different concentration with weight percentage of dye: 0.5% (mix 1) and 1% (mix 2). First, we have mixed the dye properly into the toluene, and then, a definite volume concentration has been mixed with pure NLC. Now, we have heated this sample to evaporate the solvent. For this study, we have used LC planar aligned cell having thickness of 8 µm. The method for preparation of liquid crystal planar aligned cell has already been described by us in recent articles [28, 29]. LC cells have been filled with pure and dye-dispersed NLC by capillary action. We have used HP4194A Impedance/gain phase analyser (frequency range 100 Hz–40 MHz) for the dielectric measurements. For the electro-optical measurements, we have used a function generator (Tektronix AFG-3021B), a digital storage oscilloscope (Tektronix TDS-2024C), and a photo detector (Instec-PD02-L1), which is directly fitted to digital storage oscilloscope. We have also used INSTEC (mK 2000) hot plate for the temperature variation. Response time has been calculated by optical switching method. Detailed experimental information of electro-optical and dielectric studies has already been described by our groups [32–34].

2.4 Birefringence measurement Birefringence measurement has been performed by transmittance technique method. In this method, first, we have set the polariser and analyser in cross position. Then, we have put the LC cell between the crossed polariser and analyser, and applied a He–Ne laser source onto the LC cell. Now, we have rotated the liquid crystal cell at the angle of 45°. Then, we have recorded the output intensity with the variation of

Tuning of birefringence, response time, and dielectric anisotropy by the dispersion of…

temperature. With the help of this transmitted output intensity, we have plotted the graph between output intensity and temperature, and calculated the phase difference (∆ϕ), and then, we have calculated the birefringence (∆n) by applying the following formula [29]:

Δn =

Δ𝜙𝜆 , 2𝜋d

(1)

where d is the thickness of the liquid crystal cell and λ is the wavelength of laser (He–Ne laser: 632 nm wavelength and 2 mW power). Laser power used to measure birefringence does not induce sufficient local heating to perturb the sample in this investigation. Birefringence is inversely proportional to the cell thickness; therefore, we can reduce the cell gap of any display by increasing the birefringence. The detail measurement of birefringence has already been reported by us in the recent publication [29].

2.5 Response time measurement The response time has been measured by optical switching method [35, 36] and measurement mechanism for response time has been shown in Fig. 2a–d. For the measurement of response time, a square wave of 10 V and 5 Hz frequency has been applied to the LC cell using a function generator. The cell is set at angle 45° crossed polarizer and analyzer for ensuring maximum optical transmittance. Thus, the cell works

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as a phase retarder, thereby altering the polarization of light. He–Ne laser as the input signal is detected by a photodetector which is directly connected to a digital storage oscilloscope. Then, we have calculated the rise time and fall time by the output waveform. Initially, when there is no electric field (Fig. 2a), all molecules are aligned in parallel to the surface. Now, when we applied the electric field and when applied voltage is greater than the threshold voltage (Fig. 2b), then molecules try to align in a direction perpendicular to the surface. Here, dipole–dipole interactions of molecules and electric field force the molecules to align in the direction perpendicular to the surface. Now, when the field is turned off (Fig. 2c), molecules return back to their original position due to elastic forces. These mechanisms can be illustrated by the Fig. 2d. Here, we are applying a square wave as input signal to the LC cell, and then, we are getting output waveform. By this output waveform, we are calculating rise time (τon) and fall time (τoff) values. Here, rise time is the time required for the transmittance to rise from 10 to 90%, and fall time is the time required for the transmittance to fall from 90 to 10% (Eqs. 2a, 2b). The total response time is given by τtotal = τrise + τfall [37]:

𝜏rise = 𝜏90 − 𝜏10 ,

(2a)

(2b) where “τrise” and “τfall” represent the rise time and fall time, respectively. If we can reduce the value of response time

𝜏fall = 𝜏90 − 𝜏10 ,

Fig. 2 Response time measurement mechanism for nematic liquid crystal and dye-dispersed system

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In the present investigation, birefringence of pure and dyedispersed system has been measured by transmittance technique method [29]. Figure 3a shows the variation of birefringence (∆n) with respect to temperature for pure NLC and dye-dispersed system. In Fig. 3, the filled squares represent the ∆n values for pure NLC, filled circles represent the ∆n values for mix 1 (pure NLC + 0.5% fluorescent dye), and filled triangles represent the ∆n values for mix 2 (pure NLC + 1% fluorescent dye). Here, ∆n values are decreasing gradually as temperature was increased. This is because, as the temperature is increased, the liquid crystal molecules tend to go to a less ordered state. In this study, birefringence has been increased after the dispersion of dye into pure NLC 2020. Fluorescent dye easily adjusts into the NLC geometry and, therefore, alignment of the system improved. The dye used in the present study is a disc shape dye, and when the disc shape dye is dispersed into the NLC, the dye molecules try to fit in the sample geometry and follow the intrinsic geometry constraints. This dye affects the LC molecular orientation around the dye molecule and enhances the alignment of the system, which may be due to the π–π interaction between dye molecules and rigid core of LC molecules. This type of behaviour of increasing order parameter by the dispersion of dye in LC material has already been reported by many researchers [38, 39]. As we increase the concentration of the dye into NLC, alignment gets better continuously. Due to the improved alignment, birefringence has also been improved after the dispersion of dye. Figure 3b shows the variation of birefringence with respect to concentration of dopant at 35 °C temperature. As we can see from this figure that the birefringence is increasing continuously as we increase the concentration of dye into pure NLC. For pure NLC 2020, birefringence has been found to be 0.44. For mix 1 and mix 2, the values of birefringence have found to be 0.46 and 0.51, respectively. Therefore, we can say that, for mix 2, we have achieved almost 16% enhanced birefringence after the dispersion of dye as compared to pure NLC. High birefringence material can be very useful in display as well as photonic devices and phase shifters. High birefringent material is also applicable in making of flat panel displays. Therefore, our present investigation about birefringence study may play a very important role in the field of NLC–dye research, which is widely performing in all around the world. In addition, Fig. 3c shows the variation of output transmittance intensity with respect to temperature for pure NLC and dye-dispersed system. As we increase the temperature, the intensities go maxima and minima due to

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0.48

Birefringence (∆n )

3 Results and discussion

(A) 0.52

0.44

0.40

0.36

Pure NLC 2020 Pure NLC 2020+0.5% dye Pure NLC 2020+1% dye

0.32 30

40

50

60

70

80

90

100

110

Temperature C O

(B)

0.52 0.51

Birefringence (∆n)

by any method, then this will improve the features of many devices which are based on liquid crystal.

G. Pathak et al.

0.50

At 35OC

0.49 0.48 0.47 0.46 0.45 0.44 0.43

(C) Output Intensity (Volt)

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0.0

0.2

0.4

0.6

0.8

1.0

Concentration of dopant % wt/wt Pure NLC 2020 Pure NLC 2020+0.5% dye Pure NLC 2020+1% dye

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0

30

40

50

60

70

80

Temperature C O

90

100

110

Fig. 3 a Variation in birefringence with respect to temperature for pure NLC and dye-dispersed system. b Variation of birefringence with respect to concentration of dopant at 35 °C temperature. c Variation of output intensity (V) with respect to temperature for pure NLC and dye-dispersed system

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change in molecular orientation, and, therefore, decrease the order parameter. This changed molecular orientation causes change in phase difference. Then, with the help of phase difference, we can calculate the birefringence by applying Eq. 1. In the present investigation, we have reduced the value of response time of nematic liquid crystal by the dispersion of fluorescent dye. Figure 4 shows the variation of response time with respect to temperature for pure NLC and dyedispersed system. The trend of response time graph shows that it is decreasing as we increase the temperature. After the dispersion of fluorescent dye, the value of response time has been decreased as compare to pure NLC. This shows that we have achieved fast optical response time by the dispersion of fluorescent dye. Fluorescent dye has the tendency to easily

0.019

Response Time (Seconds)

Response Time (Seconds)

0.020 0.018 0.016 0.014 0.012

0.018 O

At 35 C

0.017 0.016 0.015 0.014 0.013 0.012 0.011

0.010

0.0

0.2

0.4

0.6

0.8

1.0

Concentration of dopant % wt/wt

0.008 0.006 Pure NLC 2020 Pure NLC 2020+0.5% dye Pure NLC 2020+1% dye

0.004 0.002 0.000

At 10 Volt and 5 Hz

30

40

50

60

70

80

90

100

110

Temperature OC Fig. 4 Variation of response time with respect to temperature for pure NLC 2020 and dye-dispersed system. Inset of this figure shows the variation of response time with respect to concentration of dopant

(A)

adjust into the NLC geometry; therefore, response time value of the dispersed system reduces. After the addition of dye into pure NLC, rotational viscosity has been decreased in the present investigation (Fig. 5b), which is described in the later part of this investigation, and therefore, response time also decreases here. Response time is directly proportional to rotational viscosity; therefore, decrement in the rotational viscosity cases decrement in the response time. The inset of Fig. 4 shows the variation of response time with respect to concentration of dopant at 35 °C temperature. This figure clearly shows that the value of response time for both the mixtures has been decreased as compare to pure NLC. The value of response time for pure NLC 2020 has been found to be 0.0182 s (18.2 ms), but when we dispersed 0.5% wt/wt fluorescent dye (mix 1) and 1% wt/wt fluorescent dye (mix 2) into the pure NLC, the response time values for mix 1 and mix 2 come to 0.0134 s (13.4 ms) and 0.0111 s (11.1 ms), which is very significant. In the present study, order parameter is increasing and response time is decreasing simultaneously. The dispersion of the dye causes the enhancement in the alignment of the system which increases the order parameter due to π–π interaction between dye molecules and rigid core of LC molecules. Response time depends upon the rotational viscosity. In the present investigation, rotational viscosity has been decreased after the dispersion of dye into the liquid crystal, and therefore, response time also decreases. In addition, self-assembly of dyes causes the decrease in response time. Self-assembly of dye increases the anchoring energy of the dispersed system. Here, selfassembly of dye means that they are arranging themselves in the NLC matrix. The dye molecules may be supporting to increase the elastic interactions between the LC layer and alignment layer which results into the increase of anchoring energy of dispersed system. Since anchoring energy

(B) O

Rotational Viscosity (Pa.s)

Splay Elastic Constant (K 11 )

1.00E-010

At 35 C 9.00E-011

8.00E-011

7.00E-011

6.00E-011

5.00E-011

0.0

0.2

0.4

0.6

0.8

1.0

Concentration of Dopant % wt/wt

1.00E-011 9.00E-012 O

At 35 C

8.00E-012 7.00E-012 6.00E-012 5.00E-012 4.00E-012 3.00E-012

0.0

0.2

0.4

0.6

0.8

1.0

Concentration of Dopant % wt/wt

Fig. 5 a Variation of splay elastic constant with respect to concentration of dopant at 35 °C temperature; b variation of rotational viscosity with respect to concentration of dopant at 35 °C temperature

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is inversely proportional to the response time, therefore, response time reduces [37, 40]. To support the behaviour of response time, we have also calculated the rotational viscosity in this investigation. Figure 5a, b represents the variation of splay elastic constant and rotational viscosity with respect to concentration of dopant at 35 °C temperature. Splay elastic constant (K11) has been calculated using Eq. 3 [28] and rotational viscosity (γ) has been calculated using equation given in Ref. [33, 37] using fall time value: (3) We can see from Fig. 5a that the splay elastic constant has been decreased after the dispersion of dye into the pure NLC. K11 directly depends upon the threshold voltage and dielectric anisotropy. In the present investigation, we can see from Figs. 8b and 9 that the dielectric anisotropy and threshold voltage both parameters have been reduced for the dispersed system, and therefore, splay elastic constant also reduces. Figure 5b shows that the rotational viscosity has also been reduced after the dispersion of dye into pure NLC. As we can see from Fig. 4 that the response time has been reduced for the dispersed system, and therefore, due to the combined effect splay elastic constant and response time, rotational viscosity also reduces. High contrast is also a very important parameter for display devices. Nowadays, displays having good quality of contrast are in demand. Therefore, if we can improve the contrast of liquid crystal material, then this will be very beneficial for display industries. In the present investigation, we have improved the contrast ratio by the dispersion of fluorescent dye in the nematic liquid crystal. Figure 6 represents the variation of contrast ratio with respect to concentration of dopant at 35 °C temperature. The CR has been calculated by the following equation [34]:

K11 = (Vth )2 D𝜀𝜀0 ∕p2 .

CR =

Transmax − Transmin , Transmax

where Transmin and Transmax are the transmittance at extinction position and the corresponding maximum transmittance during the switching of NLC molecules, respectively. The calculated value of CR for pure NLC has been found to be 50, whereas, for mix 1 and mix 2, the CR values have been found to be 67 and 74, respectively. This indicates that we have improved the contrast ratio of the NLC system to 48% by dispersing the dye. The dye used in the present investigation is a fluorescent dye; therefore, it is increasing the contrast of NLC system. This increment in the contrast ratio may also be due to the plane switching for the dye-dispersed system. Figure 7 shows the variation of relative permittivity with respect to temperature at 1 kHz frequency for pure NLC and dye-dispersed system. Here, relative electric permittivity has been decreased after the dispersion of fluorescent dye into the pure NLC 2020. The mutual interaction between NLC molecules and dye causes to reduce the relative electric permittivity. The shape of fluorescent dye is of disc shape which is different from the NLC shape; therefore, these different shapes of both the guest and host may be responsible in the fall of relative electric permittivity. There are several interactions taking place when we dispersed the dye into NLC. The dominating interaction is between the NLC and dye molecules, but there are two more interactions taking place in the system, which are interaction between NLC–NLC molecules and interactions between dye–dye molecules. These two additional interactions may also be the reason to reduce the relative permittivity of the system, because this disc shape fluorescent dye may be reducing the charge storage capacity of the dispersed system. It is well known Pure NLC 2020 Pure NLC 2020+0.5% dye Pure NLC 2020+1% dye

5.6 5.4

Contrast Ratio

Relative permittivity

75 O

70

At 35 C

65 60 55

5.2 5.0 4.8 At 1 KHz

4.6 4.4 4.2 4.0

50

3.8 0.0

0.2

0.4

0.6

0.8

1.0


Fig. 6 Variation of contrast ratio with respect to concentration of dopant at 35 °C temperature

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(4)

30

40

50

60

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80

90

100

110

Temperature OC Fig. 7 Variation of relative permittivity with respect to temperature at 1 kHz frequency for pure NLC and dye-dispersed system


(A) 20

Pure NLC 2020 (εII)

Dielelctric Anisotropy (∆ ε)

18

Pure NLC 2020 (ε⊥)

εII

16

Mix 1 (εII)

Mix 1 (ε⊥) Mix 2 (εII)

14

Mix 2 (ε⊥)

12

At 1 KHz

10 8

ε⊥

6 4 30

40

50

60

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90

100 110 120

O

Temperature C

Dielectric Anisotropy (∆ε)

(B)

14.5 14.0

At 35 OC At 1KHz

13.5 13.0 12.5 12.0

0.0

0.2

0.4

0.6

0.8

1.0


Fig. 8 a Variation of dielectric anisotropy with respect to temperature for pure and dispersed system; b variation of dielectric anisotropy with respect to concentration of dopant at 35 °C temperature and 1 kHz frequency

Threshold Voltage (Volt)

2.8 2.7 O At 35 C

2.6 2.5 2.4 2.3 2.2 0.0

0.2

0.4

0.6

0.8


1.0

Fig. 9 Variation of threshold voltage with respect to concentration of dopant at 35 °C temperature

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that relative permittivity = C/C0 [34], which shows that the relative permittivity depends directly upon the capacity of the system that stores the charge. In this study, dielectric anisotropy (∆ε) has also been investigated for pure NLC 2020 and dye-dispersed NLC. Figure 8a shows the variation of dielectric anisotropy with respect to temperature at 1 kHz frequency. We can see from this figure that the ∆ε has been reduced after the dispersion of dye into pure NLC. Here, parallel component of relative electric permittivity (ε∥) is decreasing as we increase the concentration of dye. Perpendicular component of relative electric permittivity (ε⊥) is also decreasing as we increase the concentration of dye, but decrement fraction in perpendicular component of relative electric permittivity is less as compared to parallel component. Since dielectric anisotropy is the difference of parallel and perpendicular components of relative electric permittivity (∆ε = ε∥ − ε⊥), therefore, overall dielectric anisotropy has been reduced for dye-dispersed system. Figure 8b shows the variation of dielectric anisotropy with respect to concentration of dopant at 1 kHz frequency and 35 °C temperature. This figure clearly shows that the ∆ε has been decreasing continuously as we increase the concentration of dopant. For pure NLC 2020, dielectric anisotropy has been found to be 14.2, whereas for mix 1 and mix 2, the values of ∆ε are found to be 12.79 and 12.37, respectively. Threshold voltage is also a very important parameter for liquid crystal devices [28]; therefore, in the present investigation, we have measured the threshold voltage of the pure and dye-dispersed system. Figure 9 shows the variation of threshold voltage (Vth) with respect to concentration of dopant at 35 °C temperature. We can clearly see that threshold voltage has been decreased after the dispersion of fluorescent dye into the pure NLC 2020, because dye molecules easily adjust into the NLC geometry. The value of threshold voltage for pure NLC 2020 comes to 2.8 V, whereas, for mix 1 and mix 2, the values of threshold voltages are 2.4 and 2.2 V, respectively. Here, Vth decreases continuously as we increase the concentration of dye into the NLC matrix. In the present investigation, we have measured some important electro-optical and dielectric parameters for NLC and dye-dispersed system. NLC–dye studies have very much importance nowadays. Therefore, our present investigation may play a key role in this area of research. We have given here all the investigated parameters in tabulated form. Table 1 shows the variation in the values of birefringence, response time, contrast ratio, dielectric anisotropy, and threshold voltage for pure NLC 2020 and NLC–dye composite system at 35 °C temperature. Birefringence has been enhanced almost 16% for mix 2 (pure NLC 2020 + 1% dye) as compared to pure NLC. This prominent increment of birefringence parameter has very much applicability in display as well as non-display devices. As we can also see from Table 1 that the response time has been decreased from

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Table 1 Variation in the values of birefringence, response time, contrast ratio, dielectric anisotropy, and threshold voltage for pure NLC 2020 and NLC–dye composite systems at 35 °C temperature Liquid crystal samples

Birefringence (∆n) at 35 °C temperature

Response time (ms) Contrast ratio (CR) at 35 °C temperature at 35 °C temperature

Dielectric anisotropy (∆ε) at 35 °C temperature and 1 kHz frequency

Threshold voltage (Vth) (V) at 35 °C temperature

Pure NLC 2020 Pure NLC 2020 + 0.5% dye Pure NLC 2020 + 1% dye

0.44 0.46

18.2 13.4

50 67

14.2 12.79

2.8 2.4

0.51

11.1

74

12.37

2.2

0.0182 to 0.011 s for mix 2, and this fast response time is very much needed in display devices. Here, CR has also been increased from 50 to 74 for mix 2. We have achieved almost 48% high CR after the dispersion of dye into pure NLC 2020. In the dielectric study, dielectric anisotropy has also been increased for dispersed system at 35 °C temperature and 1 kHz frequency, as we can see from Table 1. Threshold voltage decreases almost 21% for mix 2 as compared to pure NLC at 35 °C temperature.

4 Conclusions The effect of fluorescent dye on the electro-optical and dielectric parameters of pure nematic liquid crystal (NLC) has been investigated in the present study. Dispersion of the dye has enhanced many important electro-optical parameters of liquid crystal in this study. This paper may develop new way to the dye–NLC composites studies which are nowadays performing very much by the researchers. Birefringence has been enhanced in this study after the dispersion of dye. Enhanced birefringence has vital applications in displays as well as non-display devices. High birefringence reduces the cell thickness, so it is very useful in making of flat panel displays. Decrement in the value of response time after dispersion of dye is the main finding of present work. Fast optical response time is a critical issue for the dye-doped NLC system; therefore, the present investigation about response time measurement may play a very important role to improve the features of liquid crystal display devices. In the present investigation, enhancement in contrast ratio has been found for dye-dispersed NLC system. This is also a one of the promising results of the current investigation. In the present study, dispersion of the dye into NLC reduces the relative permittivity of the NLC system. Dielectric anisotropy has been increased for all dispersed system in comparison to pure system. Threshold voltage has also been reduced for the dispersed system in comparison to the pure NLC. In the current article, we have dispersed fluorescent dye into NLC and calculated some important electro-optical properties.

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Birefringence has been enhanced after the dispersion of dye into NLC and high birefringent materials are very useful in the making of flat panel displays. In addition, we have achieved fast optical response time in this NLC-fluorescent dye system and fast response time is an important feature for LC displays. Therefore, the present reported study of NLC–dye composite system may play a very important role in the field of display as well as non-display technology; hence, it is very interesting for investigation. The outcome of present study may be useful to enhance the electro-optical parameters of liquid crystal displays (LCDs). Acknowledgements The authors are thankful to DST for grant of IndoPolish Project and UPCST for grant of project. We are also thankful to Dr. Swadesh Kumar Gupta, working at Hong Kong University of Science and Technology and Dr. Satya Prakash Yadav, working at Banaras Hindu University, for informative discussion about experiments.

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