  We measure the infrared (wavelength 11 - 0.8 microns; energy E = 0.1 - 1.5
eV) Faraday rotation and ellipticity in GaAs, BaF2, LaSrGaO4, LaSrAlO4, and
ZnSe. Since these materials are commonly used as substrates and windows in
infrared magneto-optical measurements, it is important to measure their Faraday
signals for background subtraction. These measurement also provide a rigorous
test of the accuracy and sensitivity of our unique magneto-polarimetry system.
The light sources used in these measurements consist of gas and semiconductor
lasers, which cover 0.1 - 1.3 eV, as well as a custom-modified prism
monochromator with a Xe lamp, which allows continuous broadband measurements in
the 0.28 - 1.5 eV energy range. The sensitivity of this broad-band system is
approximately 10 micro-rad. Our measurements reveal that the Verdet
coefficients of these materials are proportional to $1/\lambda^2$, which is
expected when probing with radiation energies below the band gap. Reproducible
ellipticity signals are also seen, which is unexpected since the radiation is
well below the absorption edge of these materials, where no magnetic circular
dichroism or magnetic linear birefringence should occur. We suggest that the
Faraday ellipticity is produced by the static retardance (Rs) of the
photoelastic modulator (PEM) and other optical elements such as windows, which
convert the polarization rotation produced by the sample into ellipticity. This
static retardance is experimentally determined by the ratio of the Faraday
rotation and ellipticity signals, which are induced by either applying a
magnetic field to a sample or mechanically rotating the polarization of light
incident on the PEM and/or other optical components.
