A Dye-Dispersed Liquid Crystal Smart Reflector
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An is the birefringence of the CLC, the difference between the indices of refraction parallel and perpendicular to the local nematic director. The second property of the CLC is the strong temperature dependence of the pitch length of its helicoidal structure near the phase transition from the cholesteric phase to the smectic A phase. As temperature T is lowered toward the transition temperature TAC, the pitch length diverges toward infinity, with a power law dependence, P xC(T - TAC)-' 7 [4]. This device works in the following way. At ambient temperature, the liquid crystal is in the smectic A phase, and incident light passes through it to the absorbing dye layer, where the light is converted to heat. This raises the temperature of the liquid crystal, converting it to the cholesteric phase, and the pitch of the helicoidal structure of the CLC is initially in the infrared. With further heating, the pitch grows shorter, until the reflection band of the CLC begins to reflect part of the incident light. The resulting reduction of optical power reaching the absorbing layer constitutes a negative feedback mechanism, controlling the temperature of the CLC and allowing just enough light to reach the absorbing layer to maintain the device 97 Mat. Res. Soc. Symp. Proc. Vol. 360 0 1995 Materials Research Society
at its correct operating temperature. This negative feedback stabilizes the reflector for a given light intensity. Any increase in light intensity results in a compensating increase in temperature and reflectivity, and conversely, any decrease in incident light intensity results in a corresponding decrease in temperature and reflectivity. In this paper a different kind of absorptive mechanism is used. Dichroic dye was dissolved in the CLC directly, so light is absorbed by the dye molecules distributed everywhere in the liquid crystal. Once the light enters the cholesteric layer, some light will be reflected, some will be absorbed because of the dye molecules, and the rest of it will be transmitted. The light absorbed in the sample is converted to heat and raises the average temperature of the mixture and the glass substrates. Again, just as in the previous case, the negative feedback mechanism will stabilize the system. The advantage of this system is that the heat transfer is expected to be more efficient because the liquid crystal absorbs the incident light, rather than a separate backing layer. On the other hand, the maximum reflectivity will be lower due to the absorption in the reflecting layer. EXPERIMENT The liquid crystal used in this experiment belongs to the cyanobiphenal group [5]. Details can be found in [1]. The dichroic dye, which has an elongated structure similar to liquid crystal molecules, has been studied and used in liquid crystal applications for many years. Because of the similar shapes, all the molecules are compatible with each other, and the dye molecules are expected to follow the alignment of liquid crystal molecules, a relationship known as the guest-host effect. Dichroic dye D37 (2.
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