Effect of UV curing conditions on polymerized tunable chiral nematic liquid crystals

Chiral nematic liquid crystals have attracted substantial interest. They spontaneously self-organize to form a helical structure with no complex fabrication procedure required and exhibit a reflection band for a certain wavelength interval. Since the photonic band gap can be tuned by applying external factors (heat, voltage, light, elasticity) chiral nematic liquid crystals are potentially interesting for large area optical filters and shutters, reflective displays and tunable lasers. In this work, a device which consists of a mixture of photo-polymerizable liquid crystal, non-reactive nematic liquid crystal and a chiral dopant is fabricated. By selecting the appropriate chiral dopant concentration, it is possible to make devices for different operation wavelengths. The influence of UV illumination on a partially polymerized chiral liquid crystal is investigated. A blue-wavelength shift of the photonic band gap is obtained as a function of power, duration time of UV illumination and the thickness of the cells. Interestingly the width and depth of the photonic band gap is unaffected by the change in UV curing conditions, which indicates that there is no degradation by the UV light