Double-layered LC gels

As the voltage increases, the phase change occurs because of the electric field-induced LC director reorientation. At a very high voltage, the voltage-induced phase shift is reduced to  

To fairly compare the phase change of the orthogonal LC gel films with the nano-PDLC, let us use the same LC material. To achieve polarization independence, the LC gel needs two orthogonal layers, but nano-PDLC only needs one. Thus, d = Id. However, the LC concentration in the gel is two times higher than that in nano-PDLC, i.e. c = 2c. From Equation (12.17) and Equation (12.19), we find 

From Equation (12.20), the phase shift of the LC gel is three times higher than that of a nano-PDLC. 

The LC gel is made by mixing 28 wt% of photocurable rod-like LC diacrylate monomer (RM257) in a nematic LC (E48 = L523, An = 0.231 at i = 589nm). The mixture was injected into an empty cell in the nematic state. The inner surfaces of the ITO-glass substrates were coated with a thin polyimide layer and then rubbed in anti-parallel directions. The filled cell was exposed to UV (2 - 365 nm, / 10 mW/cm [2]) for 30 min. The cell gap was controlled at 8 pm by spacer balls. 

The rise time of the LC gel is -200 ps and decay time is -500 ps at room temperature (-22°C). Such a fast response time results from the small LC domain sizes and polymer stabUization. Due to the relatively high monomer concentration (28 wt%), the formed polymer networks arc quite dense so that the formed LC domains are in submicron size. Similar to a nano-PDLC, the contact interfaces between the polymer networks and the LC molecules are large. As a result, the anchoring force of polymer networks exerted on the LC is very strong. This is the primary reason for the observed fast response time and high threshold voltage. 

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In the following sections, we introduce two polarization-independent LC phase modulators (1) a double-layered structure with two ultra-thin anisotropic polymer films as cell separators, and (2) a double-layered LC gel without any separator. 

Figure 12.23 Measured phase shift of a 16-pm double-layered LC gel at different voltages. 2 = 633 nm. Ren 2006. Reproduced with permission from the American Institute of Physics.

Nano-sized polymer-dispersed liquid crystal (nano-PDLC), voltage-biased PDLC, double-layered homogeneous LC, and double-layered LC gels are aU polarization independent. Compare their pros and cons. 

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