Liquid Crystals

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Transflective BPIII mode with no internal reflector Hui-Yu Chen, Sheng-Feng Lu, Pin-Hung Wu & Ching-Sheng Wang To cite this article: Hui-Yu Chen, Sheng-Feng Lu, Pin-Hung Wu & Ching-Sheng Wang (2017) Transflective BPIII mode with no internal reflector, Liquid Crystals, 44:3, 473-478, DOI: 10.1080/02678292.2016.1217569 To link to this article: http://dx.doi.org/10.1080/02678292.2016.1217569

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Date: 27 April 2017, At: 19:25

LIQUID CRYSTALS, 2017 VOL. 44, NO. 3, 473–478 https://doi.org/10.1080/02678292.2016.1217569

Transflective BPIII mode with no internal reflector Hui-Yu Chena, Sheng-Feng Lub, Pin-Hung Wua and Ching-Sheng Wanga a Department of Physics, National Chung Hsing University, Taichung, Taiwan; bDepartment of Photonics, Feng Chia University, Taichung, Taiwan

ABSTRACT

ARTICLE HISTORY

A transflective device without reflector using room temperature blue phase III (BPIII) material is demonstrated in this study. In this device, the coupling of an induced birefringence and fieldinduced BP, relating the ordered orientation of the double-helix cylinders, causes the reflection and transmission. Compared with other reported transflective liquid crystal devices, the BPIII device shown here does not need any type of internal reflector. Well-matched voltage-dependent transmittance and reflectance curves can be obtained easily without considering the cell gap and incident wavelength. The total response time is less than 2 ms, which is also independent of the cell gap. The experimental results exhibit a simple way to get a transflective device with good ability based on the electro-optical properties of BPIII.

Received 24 May 2016 Accepted 24 July 2016

Reflection mode

Transflective devices that incorporate both transmissive and reflective functions in the one device are attractive for outdoor image readability in portable displays. Their power consumption is lower than that of transmission displays because they use the sunlight as the light source in outdoor applications. Many methods have been offered to produce a transflective

© 2016 Informa UK Limited, trading as Taylor & Francis Group

Blue phase; liquid crystal; electro-optical property

Transmission mode

1. Introduction

CONTACT Hui-Yu Chen [email protected] Supplemental data for this article can be accessed here.

KEYWORDS

display based on liquid–crystal materials. The light switching mechanism of the nematic liquid–crystal (NLC) display is to change the orientation of the LC, as well as the effective birefringence, by applying an external field. It can control the light transmittance at a certain wavelength by tuning the electric field. However, the reflected light of the NLC display is relatively low. To increase the reflectance of the NLC

Department of Physics, National Chung Hsing University, Taichung, 402 Taiwan

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display, a reflector needs to be added to the bottom glass substrate. This causes the optical phase retardation in the reflection mode to be twice that in the transmissive mode. In order to balance the different optical phase retardations in the reflective and transmissive modes, double-cell-gap NLC and single-cellgap NLC combined with a fringe and in-plane switching (IPS) field have been suggested.[1,2] These approaches need a special design of the cell structure, including electrodes and cell gap, and need optical films to compensate for the optical performance. Polymer-stabilised blue phase (PSBP) [3] has been suggested as a replacement for NLC materials due to its fast response, optical isotropy, insensitivity to the cell gap with the application of an IPS field, and the fact that no alignment layers are needed.[4,5] As a transflective display, a single-cell-gap PSBP with bumpy reflectors on one of the glass substrates is made.[4,6] In order to let the electric field penetrate deeply into the BPLC bulk, the protrusion electrodes were used.[4] The well-matched voltage-dependent transmittance and reflectance curves can be obtained by designing the transmission and reflection regions to have different electrode gaps, and the wide viewing angle is achieved by employing two broadband wideview circular polariser and biaxial films.[4] Moreover, when the transflective device is operated in BPI or BPII, the hysteresis effect is obvious. Blue phase (BP) is usually seen when the chirality of the LC material is strong enough. Three BPs, BPI, BPII and BPIII, exist between the chiral nematic phase and the liquid phase. Because the network of disclination lines and the double twist cylinder (DTC) form bodycentred cubic and simple cubic structures, respectively, BPI and BPII can reflect bright and colourful light when the Bragg condition is satisfied. The reflection spectrum of BPIII is very weak and broad, so it has been suggested that the structure is amorphous. Compared with the electro-optical properties of BPI and BPII, BPIII is a fast response photonic device with no residual birefringence, and less hysteresis effect when an in-plane electric field is applied.[7] However, the thermal stability of BPs is usually low. In terms of the free energy of the BP, the thermal stability of the BP is decided by the balance between the free energies of the disclinations and the double-twisted cylinders. Of the three BPs, BPIII usually occupies a narrow temperature range of ~0.1°C just below the isotropic phase when the chirality is high enough.[8,9] There have been many suggestions for widening the BP temperature range,[3,10,11] among which the most effective for BPI or BPII is polymer stabilisation.[3] However, the polymer network cannot obviously

stabilise the amorphous BPIII in a wide temperature range. In this paper, our purpose is to demonstrate that BPIII can be a transflective device with no internal reflector. Thus, the experimental data presented here will focus on offering general electro-optical properties of the BPIII. In our BPIII material, the temperature range of BPIII in our sample is over 10°C and covers room temperature. Using a polymer network can widen the temperature range of BPIII. In Chien’s study, BPIII on polymer network stabilisation exhibits high thermodynamic stability with an extended temperature ranging from a few °C to more than 80°C.[12] Here, no reflectors are used in our cell. After we apply an in-plane electric field to the BPIII cell, the backlight can shine through the cell and ambient light is reflected. Measuring the transmission and reflection spectra, the reflecting intensity is higher than the transmitting intensity. The reflecting or transmitting intensity is almost equal in the visible wavelength. The total response time of the IPS-BPIII cell is less than 2 ms. These experimental results exhibit that BPIII can be a potential candidate for use in transflective devices, even for use on a transparent display.

2. Experimental The BPIII material used in this study consisted of 24wt% left-handed chiral dopant NC01 (from Daily Polymer and the helical twisting powers ~16.7 μm−1) and 76-wt% nematic liquid crystal (dielectric anisotropy Δε = 2.7, mean refractive index n , 1:6, birefringence Δn ~ 0.09 at 530 nm and clearing point~92°C. In order to prevent the BP being induced by a supercooled process or an electric field, the phase transition of the LC mixture was checked under slow cooling/ heating speeds (~0.5°C/min) in a null field. The phase sequence of the LC mixture is Iso. 32.5°C BPIII 21.7°C BPI −19°C, and the BPII is not observed in this mixture. The temperature range of the BPIII is 10.8°C and the BPIII cell can be operated at room temperature. The exact mechanism to induce the wide-temperaturerange BPIII is not clear yet. However, according to our previous experiment, a few issues should be considered when we choose the materials: (1) the dielectric anisotropy of the NLC should be small, and the elastic constants should be large [13]; (2) the solubility of the chiral dopant should be high and (3) the pitch length of the BP mixture does not vary apparently with temperature.[14] To drive the BPIII, we poured it into an empty cell consisting of interdigital electrodes made with transparent conductive film, on one of the glass substrates where the electrode width and gap were

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10 μm. The experimental setup was based on a transmission/reflection microscopic system with crossed polarisers, in which the transmission axes of the polarisers and analyser were at 45° and −45° with respect to the direction of the electric field. A 1-kHz square-wave AC signal was applied to drive the BPIII through the electrode pattern.

3. Results and discussion Figure 1 reveals that the IPS-BPIII cell can be a transflective device without placing any reflectors inside/ outside the cell, and exhibits that one can see the switching pattern at an oblique angle. When we applied a square wave with 100-V amplitude on the IPS-BPIII cell, the light can be transmitted and reflected by the cell in a very short time (~2 ms) (right pictures in Figure 1), and the transmission/reflection lights are blocked after turning off the external voltage. The IPS-BPIII cell returned to the initial dark state (left pictures in Figure 1) at about 1 ms, and no residual birefringence caused by field-induced N* in our sample was observed. Because the BPIII cell can reflect the incident light, it reveals that the periodically helical structure presents after applying the electric field. The field-dependence transmitting/reflecting intensities of the IPS-BPIII cell were measured, and the threshold voltage of the IPS-BPIII cell driven to the optical anisotropic state is about 31 V in Figure 2. The transmitting intensity is normalised by the lamp intensity through parallel polarisers, and the reflecting intensity is normalised by the lamp intensity reflected by a mirror, when the polariser and the analyser are parallel.

Figure 1. (Colour online) Pictures of the IPS BPIII cell before (left) and after (right) turning on the voltage (V = 100 V) in the reflection (upper, see Visualisation 1 (Reflection BPIII mode)) and transmission (bottom) modes.

Figure 2. (Colour online) Transmission and Reflection in a 57μm IPS BPIII cell at 25°C. The frequency of the square applied voltage is 1 kHz.

The transmission and reflection coefficients of the IPSBPIII cell are displayed in Figure 2. Because we did not use the reflector in the device, and the reflection was contributed by the BPIII itself, the reflection coefficient of the IPS-BPIII cell is more than 30% at 200 V, and the reflection is slightly higher than the transmission (~22%). When continuously increasing the applied voltage, both intensities in transmission and reflection modes increase. In this sample, we cannot see a saturated transmitting or reflecting intensity in such high applied voltage. As we know, the electro-optical response in BPI or BPII has been explained by the reorientation-induced Kerr effect,[15–20] lattice distortion, and phase transition. Turning to amorphous BPIII, because its structure is formed by random orientation of the DTC, one may only see the director orientation effect and phase transition effect induced by the applied voltage. The electrostriction effect does not occur in BPIII. From the microscopic observation, we can confirm that BPIII did not transit to chiral nematic or cubic blue when we applied the electric field. The director reorientation caused the changes in the refractive indices along and perpendicular to the direction of the electric field to be different, and then the optical property of the BPIII changed from isotropy to anisotropy. Through the fullwave retardation plate (U-TP530 nm, Olympus), we can confirm that the optical axis is induced along the direction of the electric field, as shown in Figure 3. As the light goes through the BP cell, one may measure the phase retardation caused by the induced birefringence if the direction of the electric field is perpendicular to the incident light.[7] In our previous study, we investigated the transmission electro-optical properties of IPS-BPIII cells,[7] and measured the induced birefringence via polarimetry. For our BPIII

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P

OA A

(a)

P OA C A

Figure 4. Microscopic pictures of the IPS-BPIII cell in the transmission mode with thicknesses of 9 and 57 μm, where a 50× objective lens was used. P OA A C

(b)

Figure 3. Microscopic observation of the induced optical axis in the 57-μm cell (a) before turning the applied voltage on, and (b) after applying 200 V. OA means the optical axis of the full wave retardation plate (U-TP530 nm, Olympus), and C means the induced optical axis of the BPIII.

sample, the induced birefringence is small (~0.02) and saturated when the applied voltage is higher than 10 V/μm.[7] The saturated induced birefringence in the high electric field has been explained by the extended Kerr effect.[20] For the transmission mode, the relation between light intensity and the induced birefringence (δn) is given in the following equation:   2 2 πδnd I / sin ð2φÞsin (1) λ From this equation, one can know that the light intensity also depends on the orientation of the optical axis φ, the light path (d), and the wavelength of the incident light (λ). The orientation of the optical axis (i.e. φ in Equation (1)) could be different with changes in the strength or direction of the electric field and the cell gap, due to the flexoelectric effect. The maximum rotation angle of the induced optical axis in BPIII is obtained in our previous study [7] and is less than 5°. Due to the electrode structure of the IPS cell, the induced birefringence gradually diminishes in the cell gap direction because the fringing field decreases rapidly, according to the Poisson equation. Therefore, the transmittance is insensitive to the cell gap, as long as the cell gap exceeds the penetration depth of the electric field. The detailed penetration depth depends on the electrode width and gap. Thus, we also observed the cell thickness-dependence transmission (/reflection) in

our IPS-BPIII cell. According to our cell geometry, when the applied voltage is 200 V, the detailed penetration depth of the electric field can be estimated and is around 31 μm along the cell gap direction. This means that when the cell gap is larger than 31 μm, the light intensity is insensitive to the cell gap due to the constant phase retardation. The microscopic pictures taken using a 50× objective lens to observe the spatial distribution of the light intensity after applying voltage are shown in Figure 4. Comparing the intensities of the 57-μm and 9-μm cells, the smaller intensity in the 9-μm cell is due to being partly driven in the thin cell, because the spatial distribution of the in-plane electric field created by the interdigital electrode is not uniform in the thin cell. Moreover, in both cells, one can see that the light intensity close to the IPS electrode is stronger than the intensity between the electrodes, due to the bigger out-of-plane component of the electric field. In order to explain why an IPS-BPIII cell can be a transflective device without any internal reflector in the experimental results displayed above, we should refer to the structure model of the BPIII. Some papers suggest that it is a phase consisting of randomly orientated DTCs and an amorphous network of disclination lines.[18,21] In a zero field, the reflecting peak of the BPIII is at a short wavelength and is very weak and broad. Thus, the BPIII has perfect optical isotropy in the visible wavelength in a zero field. When the BPIII is under the electric field, the local LC director rotates, and then the phase transition between the BPIII and BPE (fieldinduced blue phase) can be induced.[21,22] This phase transition can occur in BPIII samples with either positive or negative dielectric anisotropy. In this study, the positive-dielectric-anisotropy BPIII material is used, and the cylinder axes of the DTCs are perpendicular to the direction of the electric field due to the reorientation of the LC director. By considering the structure of the DTCs, the LC director

LIQUID CRYSTALS

rotates simultaneously on two helical axes perpendicular to the cylinder axis. When viewed along the inplane field, the helical axes of the LC director are parallel with the scattering vector, and then reflect a part of the incident light. According to the computer simulation of the BPIII under an application of the electric field, as discussed in another study,[21] as the electric field increases, the disclination lines lie along layers stacked perpendicular to the electric field, and the BPIII-BPE phase transition occurs. The BPE (field-induced BP) can enhance reflectance. At the same time, the reflecting layer formed by the ordered orientation DCT and the layers of the disclination lines cause slightly larger phase retardation in the reflection mode of the IPS-BPIII cell due to the longer optical path. The reflected signals in Figures 2 and 5 include the contribution of the phase retardation and the reflection caused by the induced-BPE. To discuss the wavelength-dependence transmission/reflection, we use a 57-μm cell to prevent the uncertainly from the non-uniform distribution of the electric field; these spectra are shown in Figure 5. Before the electric field was turned on, the reflecting/ transmitting intensities were close to null value under the crossed polarisers. In the visible-wavelength range, these spectra exhibit a very wide peak. We repeated the spectrum measurement by changing the cell gap from 3.4, 9, 28.4 to 57 μm, and found that the statement held true at different cell gaps which only affect the transmitting/reflecting intensity of the IPS-BPIII cell. The mechanisms of the electro-optical behaviour of the transflective BP cell includes (1) the ordered DTC orientation, which forms reflecting layers inside the cell and enhancement of the reflecting peak of BPIII; (2) the field-induced birefringence (extended Kerr effect), switching the BPIII from optical isotropic to

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anisotropic state and (3) the rotation of the optical axis, which suppresses the dependence of the intensity and wavelength. In the transmission mode, the last two mechanisms dominate the electro-optical behaviour; in the reflection mode, these three mechanisms need to be considered, and the first mechanism is the necessary condition. The reflecting layers formed by the ordered DTC could be at different positions along the normal direction of the cell. It causes the phase retardation in the reflection mode to not be twice as large as the transmission mode. The response times are recorded in Figure 6. The rise time decreases by increasing the applied voltage, but the decay time is almost constant and lower than 1 ms. It exhibits that the BPIII possesses greater potential to be a fast electro-optical device when the thermal stability and the driving voltage can be improved. The response times of these cells with different cell gaps are summarised in Table 1, and are independent of the cell gap. BPIII is an LC phase consisting of the randomly orientated DTC and does not like BPI or BPII with a lattice structure. When applying the voltage to the BPIII cell, the distortion of the lattice structure does not happen, but the DTC aligns uniformly. The rotation of the DTC is attributed to the local motion of the LC molecules. Thus, it is a fast response and is insensitive to the cell gap. The average rise time and decay time are 1.3, 0.06 ms and 0.94, 0.04 ms, respectively.

Figure 6. Response times of the 57-μm IPS-BPIII cell when applying various voltages. Table 1. Response times of the IPS-BPIII cell at 100 V. Cell gap (μm)

Figure 5. (Colour online) Reflection and transmission spectra of an IPS BPIII cell with a 57-μm cell gap where the applied voltage is 200 V.

3 9 34 57

Rise time (ms) 1.34 1.16 1.56 1.24

± ± ± ±

0.06 0.04 0.04 0.08

Decay time (ms) 0.92 0.96 0.96 0.96

± ± ± ±

0.04 0.04 0.04 0.04

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4. Conclusions In this study, a transflective BPIII device is demonstrated through the field-induced BP, without using any types of reflector. Because of the random DTC structure of BPIII, a perfect optically isotropic dark state can be obtained in a null field whether in transmission or in reflection. After we apply the electric field to the BPIII cell, the backlight can shine through the cell, and ambient light is reflected due to the coupling of the induced birefringence and BPE. The transflective BPIII device displays that the transmitting/reflecting intensities are independent of incident wavelength and cell gap. The response times can be lower than 2 ms in total. However, both transmittance and reflectance only reach 10% at 100 V. The required voltage is too high for practical applications. The possible ways to reduce the operation voltage of BPIII could be using the protrusion electrode, increasing the dielectric anisotropy of the host nematic LC, and using the high Kerr constant BP materials. The IPSBPIII exhibits great potential in the application of transflective devices and a transparent display. The IPS-BPIII cell can also work as a transparent display, as shown in Visualisation 2 (Transflective BPIII mode).

Acknowledgements The authors would like to thank the Ministry of Science and Technology of the Republic of China for financially supporting this research: [Grant Numbers MOST 101-2112-M-005006-MY3 and MOST 104-2112-M-005-004].

Disclosure statement No potential conflict of interest was reported by the authors.

Funding This work was supported by the Ministry of Science and Technology of the Republic of China: [Grant Numbers: MOST 104-2112-M-005-004 and MOST101-2112-M-005-006-MY3].

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