Far-infrared spectroscopy in large magnetic fields

Full text: A far-infrared spectroscopy facility has recently been established at the National Pulsed Magnet Laboratory. The radiation source consists of a mid-infrared CO2 laser (>80 lines, peak power >40 W) optically pumping a far-infrared cavity (peak power >150 mW). Choice of suitable lasing gas, pressure, and pump line, allow a variety of lasing lines of wavelengths from 40 to 1200 μm to be utilised. The laser radiation is conducted into the magnet room through metal light pipes. A dedicated top-loading probe, incorporating suitable optical filtering, conducts the light to the sample in the bore of the magnet. This arrangement permits photoconductivity measurements in fields up to 60 T and at temperatures down to 300 mK. A second probe which will permit absorption and reflection measurements in fields up to 30 T has been designed and is under construction. A variety of materials have been investigated using this facility. A preliminary report concerning organic conductors has already been given. Earlier work on the far-infrared photoconductivity of these failed to find any spectral features which shifted systematically with field, apart from an overall change in intensity. Our results are similar and indicate the importance of heating effects in these materials. Similar bolometric effects were evident in a multiple quantum well sample we have investigated. The present material being studied is high-mobility bulk n-type GaAs. We have observed sharp impurity transitions in complementary low-field (<7 T) far-infrared absorption measurements. The polarisation features are in agreement with the established selection rules and the magnitude of the Zeeman splitting in accord with that measured by others