1

Low Driving Voltage, Full Phase Modulation Ferroelectric Liquid Crystal Device

Jian-Zhi Wen, Huang-Ming Philip Chen* Department of Photonics and Institute of Electro-Optical Engineering,

National Chiao Tung University, Hsinchu 30010, Taiwan Abstract The 2π full phase modulation of half-V ferroelectric liquid crystal (HV-FLC) was assembled and evaluated in this study. The response time is 1 ms under 10 V in a 1.58 μm cell. We applied this FLC material to prepare a high resolution LCoS panels with the cell gap uniformity error below 1 %

1. Introduction Ferroelectric liquid crystals (FLCs) have been vigorously studied since surface-stabilized FLC (SSFLC) was first reported in 1980. The characteristics of sub-millisecond response time and bistability in FLCs have greatly attracted the public attention for exploring numerous applications in fast response devices. The commercial ferroelectric liquid crystal modulators or digital micromirror device (DMD) can achieve sub-millisecond response time and low driving voltage. However, these devices provide only binary π phase modulation. The main problems with binary phase modulation are, such as significant quantization noise, reduced diffraction efficiency. In previous study, the dual frequency LC (DFLC), polymer network liquid crystal (PNLC) and polymer-stabilized blue phase LC (PS-BPLC) are able to achieve fast-response time of 2π phase modulation. The obvious drawback is the high operation voltage for the phase modulation (V2π > 15 V). Therefore, we reported the low driving voltage (V2π = 5V) with 2π phase modulation at ~1 ms response time and further applied in the high resolution LCoS panels. 2. Experiment and Result The FLC cells were prepared starting from clean ITO glass. The FLC polyimide (PI) and polyvinyl alcohol (PVA) was coated on the ITO glass substrates, and then PI and PVA hard baked at 200oC for two hours. The hard baked PI and PVA substrates were treating under rubbing process. The cells were assembled with the UV glue mixed with 1.6 μm spacers. We prepared two type of empty cells PI-PVA and PI-PI for molecular alignment properties study. A HV-FLC material R3206 mixture was used in this study. The pitch length shown in Fig. 1 was generated by applying a parallel rubbed cell with a cell gap of 60 µm. The distance between two stripes indicates the FLC’s pitch length. The pitch length of R3206 mixtures is

linearly reduced as the amount of R3206 is increased. See Fig. 1 (b)

(a) (b)

Fig. 1. (a) Microscopic texture of R3206-50 in a parallel rubbed cell with the cell thickness of 60 µm and (b) pitch lengths of R3206 mixtures; R3206-30: 9.3±0.2μm, R3206-50: 6.3±0.2μm, and R3206-70: 3.9±0.2μm The response time were measured by reflectance at 532 nm under 100 Hz square wave. The different cell gap R3206 mixtures with the phase retardation are list in Table 1. In the series of R3206 mixtures, only the cell gap more than 1.6μm can achieve 2π phase modulation. In order to achieve fast-response time (~1ms) and defect free FLC cell, we choose R3206-50 1.6μm cell in this study.

Table 1.R3206 mixtures cells phase retardation R3206 mixtures

R3206-30

R3206-50

R3206-70

R3206-80

1.0 μm 1.0π 1.4π 1.38π 1.5 μm 1.8π 1.88π 1.9π 1.88π 1.6 μm 2.0 π 2.05π 2.03π

The horizontal chevron domain defects were shown when the HV- FLC R3206-50 injected into the PI-PI cell. In the PVA-PI asymmetrical surface polarity hybrid cell, the layer structure of R3206-50 appeared in uniform direction and the horizontal chevron defects were completely suppressed, as shown in Fig 2. After utilizing asymmetrical hybrid cell, contrast ratios and horizontal chevron defects of R3206–50 were greatly improved.

2

(b) (b)

Fig. 2. Polarizing optical micrographs of (a) (b) R3206-50 PVA-PI, (c)(d) R3206-50 PI-PI cells bright and dark state in the 1.5μm The E-O properties were measured by reflectance at 532 nm under 100 Hz square wave. R3206–50 possesses threshold voltage under 0.8 V with saturation voltages ~8 V, analogue gray scales are presented as shown in Fig.3. The full 2.0π is under V2π = 5 Vrms.

Fig.3. Voltage-dependent phase modulation curve at 532nm Temperature-insensitive and continuous electro-optical responses were presented in Fig. 4. The phase rising and decay response time of R3206-50 at the cell gap d=1.58 ±1% μm, under λ = 532 nm is 0.3 ms and 0.71 ms, respectively, as shown in Fig 5. The 2π phase modulation under 1 ms response time is able to reach the good diffraction efficiency than other higher spatial resolution phase-only nematic LCoS-SLM panel.

Fig.4 Temperature-insensitive were presented.

Fig. 5. Measured phase rise and decay times of R3206–50 in the 1.58μm under λ = 532nm In order to get the continuously gray scale, we changed the input waveform measure the R3206-50 cell E-O properties at 532 nm under 10V and 10khz square wave. When we changed the square wave duty cycle from 50% to 95%, the R3206-50 cell E-O properties had also changed the transmittance corresponds to duty cycle continuously as shown in Fig 6. With this result, we can drive the FLC cell in analogue or digital. Furthermore, we can make the LCoS with continuously 2π phase modulation or binary phase modulation.

(a)

(b)

(c)

(d)

3

Fig. 6. Duty cycle-dependent phase modulation curve at 532 nm We were measured the response time again but changed the duty cycle from 10% to 90% and we got the response time is about 1ms as shown in Fig 7. It’s similarly to the measurement under 100hz square wave.

(a)

(b)

Fig. 7. Response time (a) from 10% to 90% duty cycle (b) from 90% to 10% duty cycle

The LCoS panel filled with R3206–50 before bonding, as shown in Fig 8. The average cell gap value is 1.71 μm, standard deviation 15 nm and cell gap max.-min. value is 53 nm. The uniformity error is below 0.83 % (i.e. uniformity error % = standard deviation / average cell gap value).

Fig. 8. Defect free LCoS panel filled with R3206-50 and uniformity error 3. Conclusions In this study, we reported the asymmetrical hybrid cell to improve the horizontal chevron defects and the reflective device filled with R3206-50 can reach 2π phase modulation. The 2π phase modulation of defect free half-V ferroelectric liquid crystal device with low driving voltage and 1 ms response time will be a potential material to apply in LCoS-SLM and we already made on LCoS panel with the uniformity error is below 1 %. In the end, we also show up how to get the continuous gray scale by changing the duty cycle and the driving frequency of the square wave. 4. Acknowledgements The authors are grateful for the financial support provided by National Science Council of the Republic of China under Grant No. MOST105-2221-E-009-090 and Jasper Display Corp. under Grant No. MOST105-2622-E-009-012-CC2.

References 1. Huang-Ming Philip Chen and and Chi-Wen Lin

“Free alignment defect, low driving voltage of half-V ferroelectric liquid crystal device” Appl. Phys. Lett. 95 (2009)”

2. Jhou-Pu Yang, and Huang-Ming Philip Chen “A 3-msec Response-time Full-Phase-Modulation 1080p LCoS-SLM for Dynamic 3D Holographic

1

2

3

4

5

6

7 10

8

9 12

11

4

Displays”SID (2017) 3. K. Kondo, F. Kobayashi, H. Takezoe, A.

Fukuda, and E. Kuze, Jpn. J. Appl. Phys., 19, 2293 (1980)

4. Huang-Ming Philip Chen and and Chih-I Hsu “Application of Fast Response Liquid Crystal Modes in Reflective Display Technology”

5. Huang-Ming Philip Chen and and Chi-Wen Lin “Defect-free half-V-mode ferroelectric liquid-crystal device” SID (2010)