A broadband silicon quarter-wave retarder for far-infrared spectroscopic circular dichroism

•We describe the design of a silicon quarter retarder working in the far infrared.•The device produces far-infrared light with a high degree of circular polarization.•We demonstrate its performance by photo-induced magnetospectroscopy of GaAs.

The high brightness, broad spectral coverage and pulsed characteristics of infrared synchrotron radiation enable time-resolved spectroscopy under throughput-limited optical systems, as can occur with the high-field magnet cryostat systems used to study electron dynamics and cyclotron resonance by far-infrared techniques. A natural extension for magnetospectroscopy is to sense circular dichroism, i.e. the difference in a material’s optical response for left and right circularly polarized light. A key component for spectroscopic circular dichroism is an achromatic 14 wave retarder functioning over the spectral range of interest. We report here the development of an in-line retarder using total internal reflection in high-resistivity silicon. We demonstrate its performance by distinguishing electronic excitations of differing handedness for GaAs in a magnetic field. This 14 wave retarder is expected to be useful for far-infrared spectroscopy of circular dichroism in many materials.