Enhancing the thermo-optical response of nematic liquid crystal with a polymer network

Abstract

As nematic liquid crystals are heated, the change in the nematic order parameter affects the physical properties of the phase such as dielectric permittivity, indices of refraction, and elastic constants. This enables nematic liquid crystals to be used as temperature sensors. Of particular interest is their use as an uncooled thermal imager. In this paper, an optical sensor for this application is considered where the change in birefringence of the liquid crystal with temperature is probed by a visible wavelength. One problem with this type of sensor is the large change in birefringence as the liquid crystal that undergoes a first-order nematic-isotropic phase transition cannot be used. To solve this problem, confinement of the liquid crystal to obtain a second-order phase transition is considered. Confinement of the liquid crystal is accomplished with a polymer network. Through numerical Landau-de Gennes calculations and experimental measurements, it is demonstrated that the average domain size of an appropriate polymer network is ∼250 nm. This confinement results in a doubling of the thermo-optical response of the nematic liquid crystal. Additionally, it is demonstrated that the polymer network suppresses twist fluctuations of the nematic director, which halves the noise of the sensor. The results of this work allow for improved uncooled thermal imagers.