Electrically controlled lens and prism using nanoscale polymer-dispersed and polymer-networked liquid crystals

摘要

Inhomogeneous nanoscale polymer-dispersed liquid crystal (PDLC) devices having gradient nanoscale droplet distribution were fabricated. This gradient refractive index nanoscale (GRIN) PDLC film was obtained by exposing the LC/ monomer with a uniform ultraviolet (UV) light through a patterned photomask. The monomer and LC were mixed at 70:30 wt% ratio. The area exposed to a weaker UV intensity would produce a larger droplet size, and vice versa. Owing to the nanoscale LC droplets involved, the GRIN PDLC devices are highly transparent in the whole visible region. The gradient refractive index profile can be used as switchable prism gratings, positive and negative lenses, and Fresnel lens. Such a GRIN PDLC device is a broadband device and independent of light polarization. The diffraction efficiency of the lens is controllable by the applied voltage. The major advantages of the GRIN PDLC devices are in simple fabrication process, polarization-independent, and fast switching speed, although the required driving voltage is higher than 100 V_(rms). To lower the driving voltage, the technique of polymer-networked liquid crystal (PNLC) has been developed. The PNLC was also produced by exposing the LC/monomer mixture with a uniform UV light through a patterned photomask. However, the monomer concentration in PNLC is only around 2-5 wt%. The formed PNLC structure exhibits a gradient polymer network distribution. The LC in the regions stabilized by a higher polymer concentration exhibits a higher threshold voltage. By using this technique, prism grating, tunable electronic lens and Fresnel lens have been demonstrated. The driving voltage is around 10 V_(rms). A drawback of this kind of device is polarization dependence. To overcome the polarization dependence, stacking two orthogonal homogeneous PNLC lens is considered.