Polarization-Independent Liquid-Crystal Fresnel Lenses Based on Surface-Mode Switching of 90 $^{\circ}$ Twisted-Nematic Liquid Crystals

A simple polarization-independent liquid-crystal Fresnel lens is developed using the surface-mode switching of 90?? twisted-nematic liquid crystals (TN-LCs). In the surface-mode switching of LCs, the major reorientation of LCs occurs near the substrates. Optical calculations indicate that 90?? TN-LCs are polarization-independent under surface-mode switching, when the applied voltage exceeds four times the threshold voltage for director reorientation. The maximum tunable phase shift in this polarization-independent state of 90?? TN-LCs is ~11% of LC cell retardation. An LC Fresnel lens that is polarization-independent and has a fast response with a response time of <10 ms is demonstrated using the surface-mode switching of 90?? TN-LC.