  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 1/4 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
different handednesses for GaAs in a magnetic field. This 1/4 wave retarder is
expected to be useful for far-infrared spectroscopy of circular dichroism in
many materials.
