Untwisted Nematic Frederick's Transition

[SamBam77]

I am interested in understanding the behavior of an untwisted nematic liquid crystal cell undergoing a Frederick's transition.It would seem that, by far, the twisted nematic structure is used and studied more than the untwisted/homogeneous nematic configuration. I can find lots of examples that show how the intensity of light, transmitted through an twisted nematic LCD viewed between crossed polarizers, changes with applied voltage. Once the applied voltage crossed a minimum threshold, the liquid crystal molecules are induced to orient normal to the alignment surface. In doing so, they no longer rotate the polarization of light as it passes through the cell, thereby reducing the transmitted intensity down to zero. But what about untwisted nematic cells? The same transition, from planar to normal orientation, should occur with applied voltage. How does the transmission curve for those cells behave?As I said, I cannot find many examples of people studying/publishing these curves, but I have found a few. The few I found seem to indicate that the transmission oscillates – goes up and down - with increased voltage. For example, this article (open access) describes their LCDs made by sandwiching the liquid crystals between alignment layers rubbed in anti-parallel directions, inducing homogeneous planar alignment. Then, in Figure 11, you can see that the transmission varies up and down with increasing voltage, eventually decreasing toward zero (assuming tha is the final “swing” of the oscillations). What is causing those oscillations?

In that article, the align the optical axis of the cell at 45 degrees away from the polarizers, but I have also seen this behavior for cells aligned parallel/perpendicular to a set of crossed polarizers. Naively, I would not expect to see any change in the transmission in this case. What's going on instead?

 

[gtoguy97]

The most likely explanation I can come up with is that the oscillations are caused by a combination of surface anchoring strength and the tilt angle of the molecules. As the voltage is increased, the tilt angle of the molecules increases, but the surface anchoring strength prevents them from completely aligning normal to the surface. Thus, the molecules’ optical axis is no longer parallel or perpendicular to the polarizers, and the transmission begins to oscillate. In summary, it appears that an untwisted nematic liquid crystal cell undergoing a Frederick's transition will display oscillations in the transmitted light intensity with applied voltage. These oscillations are likely caused by a combination of surface anchoring strength and the tilt angle of the molecules.