General theory of asymmetrical polarization-dependent optics in functional material-doped 90° twisted nematic liquid crystals

Significance 

Over the past years, the optical behaviour of 90° twisted nematic liquid crystals (TNLCs) has been studied far and wide. This versatile optical liquid crystal can rotate the polarization of the input linearly polarized lights for 90° under certain conditions. As a result, the 90° TNLCs satisfying the Gooch–Tarry condition have been widely applied in LC display (LCD) industry. In addition, the 90° TNLCs have also been applying in several kinds of photonic devices, such as: variable optical attenuator and optical light shutter, and others. The advantages of utilization of the 90° TNLCs include ease of fabrication, excellent electrical control and being cheap. Unfortunately, despite being quite a versatile optical device, detailed research about their asymmetric optical properties are still scarce.

Recently, a team of researchers at National Central University and led by Professor Ko-Ting Cheng from the Department of Optics and Photonics investigated in detail the asymmetrical optics of guest–host LCDs based on dichroic dye-doped-90° (DDd-90°) TNLCs. They demonstrated that DDd-90° TNLCs could successfully perform asymmetrical transmission. In addition, they aspired to develop a general theory which could completely describe the asymmetrical optics in functional material-doped 90° TNLCs, based on Cayley-Hamilton theorem and Jones calculus. Their work is currently published in the research journal, Optics Express.

In brief, the research method employed commenced with the utilization of DDd-90° TNLCs as examples to construct the theory of functional material-doped 90° TNLCs. For the empty cells, two indium tin oxide-coated glass substrates coated with homogeneous planarly aligned layers were mechanically rubbed along two orthogonal directions. Lastly, the prepared homogeneous mixtures were filled into the empty cells so as to produce the desired liquid crystal cell, whose edges were then sealed with epoxy.

The authors observed that the functional material whose shape and size were similar to those of the adopted nematic liquid crystals, could be aligned along the long axes of the nematic liquid crystals. In addition, the researchers noted that their experimental results on asymmetrical transmission in DDd-90° TNLCs can be well described with the theoretical calculations. The asymmetric optics of DDd-90° TNLCs can refer to Fig. 1. The colourful letters “NCU”, displayed by a LCD, behind the DDd-90° TNLC cell was linearly polarized along the blue arrow. Under a suitable condition, Fig. 1(a) shows that the colourful letters “NCU” can be observed, while the “NCU” cannot be observed when the cell is rotated by 180°.

In summary, the study by National Central University scientists demonstrated the development of a general theory that could completely describe the asymmetrical optics in all functional material-doped 90° TNLCs by using Cayley–Hamilton theorem and Jones calculus. Experimental results on DDd-90° TNLCs were seen to support the theoretical calculations. This therefore confirmed that the proposed general theory could describe all functional material-doped 90° TNLCs and polymer network 90° TNLCs. Altogether, such asymmetrical characterization can be further used in the current applications based on 90° TNLCs in all fields to obtain new potential functions.

General theory of asymmetrical polarization-dependent optics in functional material-doped 90° twisted nematic liquid crystals - Advances in Engineering
Fig. 1 (a) The colourful letters “NCU”, displayed by a LCD, can be observed through the DDd-90° TNLC cell. (b) When the cell is rotated by 180°, the colourful “NCU” cannot be observed.

Reference

Cheng-Kai Liu, Ching-Yen Tu, Yi-Xuan Liu, Wei-Hsuan Chen, Ko-Ting Cheng. General theory of asymmetrical polarization-dependent optics in functional material-doped 90° twisted nematic liquid crystals. Volume 26, Number 13, page 17115 | 25 Jun 2018 | Optics Express.

Go To Optics Express

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