Measurement of the infrared complex Faraday angle in semiconductors and insulators

We measure the infrared (wavelength λ = 11 - 0.8 μm; 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 measurements also provide a rigorous test of the accuracy and sensitivity of our unique magnetopolarimetry 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 broadband system is approximately 10 μrad. Our measurements reveal that the Verdet coefficients of these materials are proportional to λ^-2, which is expected when probing with photon energies below the band gap. Reproducible ellipticity signals are also seen, which is unexpected since the photon energy 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 of the photoelastic modulator 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 photoelastic modulator and/or other optical components.