Variable Ultra-broadband and Narrowband Composite Polarization Retarders
  We propose and experimentally demonstrate novel types of composite sequences of half-wave and quarter-wave polarization retarders, either permitting operation at ultra-broad spectral bandwidth or narrow bandwidth. The retarders are composed of stacked standard half-wave retarders and quarter-wave retarders of equal thickness. To our knowledge, these home-built devices outperform all commercially available compound retarders, made of several birefringent materials. 
Thorsten Peters, Svetoslav S. Ivanov, Daniel Englisch, Andon A.   Rangelov, Nikolay V. Vitanov and Thomas Halfmann
physics.optics