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.

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