Liquid crystal phase retarder with broad spectral range

Abstract

Phase retarders normally show a strong wavelength dependence of retardance. For example even without taking birefringence dispersion into account a quarterwave retarder designed for 600 nm acts as a halfwave retarder at 300 nm. Achromatic phase retarders with retardance changes of only ±5% in ≈200 nm broad spectral regions can be assembled from two layers of different birefringent materials the fast axes of which are crossed. Further improvement is possible by combining three achromatic phase retarders based on a recipe given by Pancharatnam. A quarterwave plate is realized with an inner halfwave plate the fast axis of which is positioned at a certain angle with respect to the fast axes of the outer retarders. With this design phase retardance can be kept constant within ±2% in the entire visible spectrum from 400 to 900 nm. We report on a first realization of such a broad band phase retarder using liquid crystals instead of crystals as the birefringent materials. Although the performance is not as good as calculated it is well comparable to practically realized crystalline `superachromatic' phase retarders.