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Illumination & Displays

Fast-switching technology for nematic liquid-crystal cells

A three-terminal-electrode structure reduces gray-to-gray response time, enabling high-speed operation of liquid-crystal cells.
17 January 2011, SPIE Newsroom. DOI: 10.1117/2.1201012.003409

LCDs are widely used because of their low power consumption, light weight, and small thickness. Their performance characteristics (such as their viewing angle, contrast ratio, and color shift) have been greatly enhanced over the past decade. However, their slow response is still a major drawback and makes display of quickly moving pictures difficult. This limitation is caused by inherently slow relaxation and the commonly used hold-type driving of liquid crystals (LCs).

Several technologies have been proposed to realize fast switching of LCs. However, they are all associated with certain disadvantages, such as decreased light transmittance, additional bias-voltage requirements, and complicated fabrication processes.1–3 Moreover, their gray-to-gray (GTG) response, an important feature for displaying moving pictures, has not been taken into account.

Here, we introduce a novel method for fast switching of a homogeneously aligned LC cell using a three-terminal (3T)-electrode structure (see Figure 1). This allows simultaneous application to the LC layer of both in-plane and vertical electric fields. Just before each frame is displayed, homogeneously aligned LCs are briefly vertically aligned by application of a vertical electric field. Subsequently, an in-plane field is applied to rotate the LCs, so that the cell shows either the bright state or gray levels.

Figure 1.Structure of a homogeneously aligned liquid-crystal (LC) cell with three-terminal (3T)-electrode structure for fast switching.

Figure 2 shows the operational principle of our method. Initially, LCs are homogeneously aligned along the transmission axis of the bottom polarizer, which displays the dark state. A vertical electric field is applied temporarily to align the LCs vertically before an in-plane field is applied. Both the initial and vertically aligned configurations display the dark state. As soon as the vertical field is removed, the LCs relax to their initial state. An in-plane electric field is then applied to obtain either the bright state or gray levels by rotating the LCs. We suspect that LCs in the transient state could easily respond to an applied in-plane field, so that a fast turn-on response may be observed.4

Figure 2.Operational principle of the proposed method. Pol: Polarization. E: Electric.

To switch to the dark state, a vertical electric field is again briefly applied. As soon as this field is removed, the LCs relax to their initial state. During relaxation, the azimuthal angle of the LC director remains parallel to the rubbing direction, i.e., aligned with the transmission axis of the bottom polarizer. Thus, the LC cell maintains the dark state during relaxation from the vertically aligned state.5

To confirm the fast-switching characteristics of the proposed LC cell, we measured its GTG responses (see Figure 3). For comparison, we also show typical GTG responses of a conventional two-terminal (2T)-electrode LC cell. We considered five gray levels, corresponding to 0, 25, 50, 75, and 100% of the maximum transmittance. The slowest GTG response time for the 2T-electrode cell was 38.7ms, while that of the 3T-electrode cell was 9.6ms. Thus, the slowest GTG response of the latter was four times faster than that of the former. Moreover, all response times from any gray level to the zero level were less than 1ms.

Figure 3.Measured gray-to-gray responses—expressed in percentages of maximum transmittance—of (a) a conventional two-terminal (2T)-electrode and (b) our proposed 3T-electrode LC cell.

In summary, we have introduced a fast-switching method for a homogeneously aligned LC cell with 3T-electrode structure. By briefly applying a vertical electric field just before the onset of in-plane switching, all GTG responses were accelerated, so that we could realize high-speed operation of the LC cell. The proposed LC mode could potentially achieve efficient fast switching. We continue our research efforts toward this aim.

Tae-Hoon Yoon, Ki-Han Kim, Dong Han Song, Jae Chang Kim
School of Electrical Engineering, Pusan National University
Busan, Republic of Korea