Title: Tunable broadband polarization retarders

Abstract: We theoretically propose a type of tunable polarization retarder, which is composed of sequences of half-wave and quarter-wave polarization retarders, allowing operation at broad spectral bandwidth. The constituent retarders are composed of stacked standard half-wave retarders and quarter-wave retarders rotated at designated angles relative to their fast-polarization axes. The proposed composite retarder can be tuned to an arbitrary value of the retardance by varying the middle retarder alone, while maintaining its broadband spectral bandwidth intact.