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We report a new nematic liquid crystal (LC) mixture with an ultra-low rotational viscosity (γ1 = 53.4 mPas @ 35°C), relatively high birefringence (Δn ≈0.15), and moderate dielectric anisotropy (Δε = −2.80 @ 35°C). When employed in a liquid-crystal-on-silicon (LCoS) projector with RGB light-emitting diodes (LEDs), a sub-millisecond response time is obtained without the need for complicated overdrive circuitry. Such a fast response time enables field sequential color display, which not only triples the optical efficiency and resolution density, but also greatly suppresses the image blur and color breakup. Moreover, the required cell gap is 1.2 µm, which is still manageable for high-yield manufacturing. We believe this mixture would find widespread applications for the emerging augmented reality displays.
© 2016 Optical Society of America
1. Introduction
Liquid-crystal-on-silicon (LCoS) display has found new applications in augmented reality (AR) such as Google Glass and Microsoft HoloLens, and vehicular head-up displays because of its high resolution density, low power consumption, lightweight and low cost [1–5]. In a wearable AR device, the displayed images are superimposed with the see-through real world. Thus, if the ambient light is bright, then the contrast ratio would be reduced dramatically. To enhance ambient contrast ratio, two approaches can be considered: (1) to use a smart dimmer to dynamically tune the intensity of incident ambient light [6], and (2) to boost the brightness of the LCoS display. To achieve high brightness, an effective approach is to use field sequential color (FSC) with red, green and blue LEDs [7]. By removing the spatial color filters, both optical efficiency and resolution density are improved significantly. Although FSC offers tremendous advantages, it demands liquid crystal (LC) response time to be faster than 1 ms in order to suppress color breakup [7, 8]. Therefore, the burden to achieve sub-millisecond response time falls on the employed LC material. On the device side, there are two additional constraints for a high-resolution LCoS backplane: (1) its maximum voltage is 6 V, limited by the wafer foundry’s process, and (2) the commonly used overdrive and undershoot driving circuitry for improving LC response time [9] is usually not implemented because it could affect the accuracy of the pixel voltage. These constraints exert extra difficulties for the LC to achieve sub-millisecond response time.
To achieve sub-millisecond response time, several approaches have been investigated, including ferroelectric LCs [10, 11] and blue-phase LCs [12, 13]. Each technology has its own merits and demerits. For example, ferroelectric LC offers 100-μs response time, but it is bistable. Thus to obtain gray levels, we have to apply pulse width modulation, which dramatically increases the driving frequency and power consumption. On the other hand, blue phase requires a relatively high voltage, which is beyond what an LCoS device can afford.
To overcome the abovementioned technical barriers, in this paper, we formulate a new nematic LC mixture, designated as UCF-N5, for LCoS display to achieve fast response time without the need of overdrive circuitry. UCF-N5 exhibits a relatively high birefringence (Δn = 0.15), ultra-low viscosity, and moderate negative dielectric anisotropy (Δε = −2.80 @ 35°C). Its on-state voltage (Von) is below 6 V. Meanwhile, high Δn enables the use of thin cell gap (d = 1.2 µm) to obtain high reflectance and sub-millisecond gray-to-gray (GTG) response time. Such a cell gap is still manageable for high-yield manufacturing. UCF-N5 is expected to find widespread applications for the emerging AR displays.
2. LC mixture formulation
Table 1 lists the compositions of UCF-N5. The base host is ZOC-7003 (JNC, Japan) [14]. It is a commercial mixture with Δn = 0.103 @ 550 nm, Δε = −4.36, visco-elastic constant γ1/K33 = 5.60 ms/µm2, and clearing temperature Tc = 79°C. Its Δε is reasonably large, but the Δn and γ1/K33 require further improvement. To boost Δn, we added two lateral difluoro-terphenyl homologs (#2 & #3) [15], while to reduce viscosity we added ~25% non-polar diluters (#4 & #5).
Table 1. Chemical structures and compositions of UCF-N5.
3. Material characterization
In experiment, we measured the dielectric anisotropy, birefringence, visco-elastic constant, and activation energy of UCF-N5. The obtained results are summarized in Table 2. For an LCoS projector using a high-brightness LED lamp, the panel temperature is about 35°C due to thermal effect [16, 17]. Therefore, all the data listed here are intended for 35°C operation. On the other hand, for an LCoS data projector using a high power arc lamp, the chassis temperature would be about 50°C~55°C [16, 18]. Here, we focus on the LCoS display using an LED lamp for wearable AR displays.
Table 2. Measured physical properties of UCF-N5 at T = 35°C, and f = 1 kHz.
3.1 Dielectric anisotropy
Dielectric anisotropy affects the operation voltage because Von is proportional to . For a high resolution LCoS display, its maximum voltage is limited to 6 V. To lower the operation voltage, a large Δε helps, but the lateral polar groups dramatically increase the viscosity [19]. Thus, there exist contradicting requirements on Δε between low operation voltage and fast response time. A compromised Δε value is around −3. As a matter of fact, our UCF-N5 has Δε = −2.80 at 35°C. Later, we will prove that this Δε value is adequate to meet our voltage requirement, which is Von < 6 V. The compositions in Table 1 represent a delicate balance between low operation voltage, manufacturable cell gap, and fast response time.
3.2 Temperature dependent birefringence
The cell gap of an LCoS panel can be made much thinner than that of a conventional large screen LCD panel (d ~3 µm). The main reason is LCoS device employs reflective mode, so that its optical path is doubled. In theory, the required cell gap is 2x thinner than its transmissive counterpart with the same LC mixture. Small cell gap leads to faster response time (τ), as it is proportional to d2. Using a high Δn LC mixture is another effective approach to lower the cell gap, which will further accelerate LC’s relaxation process. For example, Chen et al. developed a VA mixture with Δn = 0.191 (at 550 nm and 50°C) for an LCoS panel with d = 0.93 µm [20]. It enables submillisecond response time, but such a thin cell gap is quite challenging for practical fabrications. To release this burden, we choose the cell gap of 1.2 µm, which is compatible to the current LCoS fabrication facility. Under such condition, the required Δn is 0.15 in order to obtain 100% normalized reflectance, i.e. dΔn ~180 nm. By adjusting the weight percentage of each component in Table 1, our UCF-N5 shows Δn = 0.154 at λ = 550 nm and T = 35°C.
Next, we measured the temperature dependent birefringence of UCF-N5 because our intended operating temperature is 35°C. In experiment, we filled UCF-N5 into a commercial VA cell with d = 9.3 µm. Then it was placed on a Linkam heating stage controlled by the temperature program (Linkam TMS94). Birefringence at each temperature was measured through phase retardation of the VA cell sandwiched between two crossed polarizers [21]. A He-Ne laser (λ = 633 nm) was used as the probe beam. The obtained results are plotted in Fig. 1, where black dots represent the measured data and red solid line is the fitting curve using Haller’s semi-empirical equation [22]:
where Δn0 is the extrapolated birefringence at T = 0, S is the order parameter, and β is a material constant. Through fitting, we found Δn0 = 0.246 and β = 0.259 for UCF-N5.
Fig. 1 Temperature dependent birefringence of UCF-N5 at λ = 633 nm. Dots are experimental data and solid line is fitting curve with Eq. (1).
3.3 Wavelength dependent birefringence
To investigate the electro-optical performances at the specified RGB colors, we measured the birefringence dispersion at 35°C. During the measurement, a tunable Argon ion laser (457 nm, 488 nm and 514 nm) and a He-Ne laser were used as probe beams. Figure 2 shows the obtained results, in which black dots represent the measured data and red solid line is the fitting curve using single-band birefringence dispersion equation [23]:
where G is a proportionality constant and λ* is the mean resonance wavelength. Through fitting, we found G = 1.8936 µm−2 and λ* = 0.2574 µm. From Eq. (2), we can calculate the birefringence at the RGB wavelengths we want. Results (at 35°C) are Δn = 0.1780 at λ = 450 nm, 0.1537 at 550 nm, and 0.1423 at 650 nm. These data will be used later for calculating the voltage-dependent reflectance.Fig. 2 Wavelength dependent birefringence of UCF-N5 at T = 35°C. Dots are experimental data and solid line is fitting curve with Eq. (2).
3.4 Temperature dependent visco-elastic constant
By measuring the time-dependent relaxation process of a VA cell, we can extract the visco-elastic constant (γ1/K33). The LC response time is linearly proportional to γ1/K33, which is highly dependent on the temperature, as described by the following equation [24]:
where a is a proportionality constant, Ea is the activation energy and kB is the Boltzmann constant. From Eq. (3), γ1/K33 increases exponentially as the temperature decreases. This explains why LCoS response time at 35°C (with an LED lamp) is much slower that that at 50°C (with a high power arc lamp). In Eq. (3), activation energy (Ea) plays the key role; it determines the rising rate of γ1/K33 [25, 26].In experiment, we measured γ1/K33 at different temperatures, and fitted the measured data. Results are plotted in Fig. 3. The obtained activation energy is 264.0 meV. At 35°C, its γ1/K33 is only 3.83 ms/µm2, which originates from high concentration of diluters. The corresponding rotational viscosity (γ1) is 53.4 mPas and bend elastic constant (K33) is 13.9 pN.
Fig. 3 Temperature dependent γ1/K33 of UCF-N5. Dots are experimental data and solid line is fitting curve with Eq. (3).
4. Simulation results
The electro-optical performance of UCF-N5 in a VA LCoS device is characterized using a commercial LCD simulator DIMOS 2.0. In simulation, we set the cell gap at 1.2 µm, to be compatible with current fabrication capability. We used the material parameters (at 35°C) listed in Table 2. As for VA LCoS, the pretilt angle is controlled at 88°, and the azimuthal angle is 45° w.r.t. the optic axis of the polarizing beam splitter (PBS). An aluminum reflector is placed on the inner surface of the VA cell. Figure 4 shows the simulated voltage-dependent reflectance (VR) curves for the RGB primary colors. For green light (550 nm), its peak voltage is 5.7 V, but if we are willing to sacrifice 2% reflectance (98.3%), we can get Von ≈5 V. Low power consumption is expected.
To study the GTG response time of our VA LC device, we divided the VR curve (550 nm) into eight gray levels equally and calculated the response time between different gray levels. Table 3 lists the detailed reflectance and driving voltage of each corresponding gray level. As usual, the response time is defined as the time interval between 10% and 90% reflectance. Results are summarized in Table 4. During our calculations, no overdrive and undershoot voltage was applied. From Table 4, we find the rise time is 0.32 ms and decay time is 0.47 ms between gray levels 1 and 8. The average GTG rise time is 0.86 ms and the decay time is 0.91 ms. With such a fast response time, image blur and color breakup can be greatly suppressed. As far as we know, this is the first demonstration for achieving sub-millisecond response time at 35°C without the assistance of overdrive and undershoot effects. Besides, it is realized using a practical cell gap (d = 1.2 µm).
Table 3. Reflectance and driving voltage for each corresponding gray level.
Table 4. Calculated (left) and measured (right) GTG response time of the reflective VA cell at T = 35°C. (unit: ms)
To validate the simulation results listed in Table 4, in experiment we prepared a VA test cell with cell gap d = 2.4 µm, and measured its response time at transmissive mode. In a reflective LCoS device, its cell gap is 2x thinner than that of our test cell. As a result, the response time should be 4x faster than the measured one. Taking this factor into consideration, we obtained the GTG response time at 35°C experimentally. Results are also included (at the right side) in Table 4. Overall speaking, the agreement with simulation results is reasonably good. We find that the measured average GTG rise time is 0.84 ms and decay time is 0.86 ms. We believe UCF-N5 would find widespread applications for the emerging wearable displays, vehicle displays, as well as pico-projectors.
5. Conclusion
We have developed a new LC mixture UCF-N5 with an ultra-low viscosity (γ1 = 53.4 mPas @ 35°C) and moderate dielectric anisotropy (Δε = −2.80 @ 35°C). When employed into a VA LCoS projector with a LED lamp, we can achieve sub-millisecond response time without the complicated overdrive circuitry. Moreover, the operation voltage is below 6 V. With such a fast response time, image blur and color breakup can be greatly suppressed. Moreover, its 1.2-µm cell gap is relatively easy to fabricate in mass production. Potential application of UCF-N5 for the emerging wearable AR displays is foreseeable.
Funding
Air Force Office of Scientific Research (AFOSR) (FA9550-14-1-0279).
Acknowledgments
The authors would like to thank Dr. Simon Fan-Chiang of Himax Display and Fenglin Peng for helpful discussion, and AFOSR for partial financial support.
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