  Light-matter interactions are key in providing fundamental information about
materials. Even in the linear-response regime, the spectroscopic response of a
material encodes in it many properties of the ground state as well as of its
excitations. This knowledge has been critical in our understanding of novel
quantum materials, and the further improvement and extensions of linear
spectroscopy will continue to be key in the exploration of novel states of
matter. We report the development of broadband circular polarization
spectroscopy in the terahertz range of the electromagnetic spectrum. We take
advantage of a recent design of a broadband quarter wave plate, based on the
Fresnel rhomb concept, and use it in conjunction with a polarization modulation
technique to provide direct information of the response of a material to
circularly polarized THz radiation; a new capability shown here for the first
time. As an example of this technique we study the cyclotron resonance of a 2D
electron gas from a AlGaAs-GaAs quantum well. We demonstrate the unique
advantages that this technique will bring in the study of novel quantum
materials.
