In a liquid crystal device with a microstrip-line structure, microwave-band insertion losses make it impossible to use an extremely thin liquid crystal layer as in liquid crystal display applications. On the other hand, the decay time of a liquid crystal device increases in proportion to the square of the layer thickness. Therefore, with a liquid crystal layer thickness of approximately 100 µm, the decay time ends up being approximately 1000 times greater than the rise time. It is thus hoped that a way can be found to substantially reduce the decay time even if this entails a slight increase in rise time and deterioration of the dielectric properties. A promising means of achieving this involves substituting the liquid crystal layer with a so-called polymer dispersed liquid crystal consisting of a polymer dispersed into a nematic liquid crystal at a suitable concentration.

In this paper, our chief aim is to demonstrate quantitatively how the decay times of devices using polymer dispersed liquid crystals can be improved in order to address a serious issue - namely the large value of the decay time in plain liquid crystal devices. In order to take direct measurements of the liquid crystal device characteristics in the microwave-band when measuring the response time characteristics and dielectric properties of microstrip line structure polymer dispersed liquid crystal devices, we used a microwave resonance method based on an inductive coupled ring resonator that we developed.

We measured the response time characteristics with respect to changes in the polymer concentration in the microwave-band with a fixed layer thickness of 50 µm and a bias voltage of 100 V. The measurement results are shown in Figure. In these results, the decay time t_d was about 20s for the plain liquid crystal, decreasing to 1.6 s with the addition of 5 wt% polymer, approximately 700 ms at 7 wt%, approximately 650 ms at 9 wt% and approximately 500 ms at 14 wt%. The rise time of the plain liquid crystal was about 10 ms, but this increased to 22 ms with the addition of 5 wt% polymer, 25 ms at 7 wt%, about 28 ms at 9 wt%, and about 36 ms at 14 wt%, which is an acceptable range. Thus as the polymer concentration increases, t_r becomes larger and t_d becomes smaller. This is because a higher polymer concentration results in a denser polymer network so that the liquid crystal molecules drifting through this network are subjected to a stronger aligning force from the neighboring polymer interfaces. Thus by using a polymer dispersed liquid crystal it is possible to greatly improve t_d but at the expense of an increase in t_r.

Fig. Response time (t_r,t_d) performances of polymer-dispersed liquid crystal for polymer concentration change.