Optimization of Interferometer Design for Measurement of Spin Injection Dynamics in Semiconductors

Abstract

This report presents the results of an experimental study on noise limitations when measuring nanoradian polarization rotations by three different methods. The ability to measure these rotations within the photon shot noise limit was checked for partially crossed polarizers, a Sagnac interferometer, and an optical bridge. Faraday rotation was achieved using a terbium gallium garnet (TGG) crystal in a solenoid with an average axial magnetic field of 0.9 T. A green (l = 532 nm) linearly polarized laser probed the magnetic response of the sample. The applied magnetic field was oscillated at 783 Hz and a lock-in amplifier was used to acquire data at that frequency. Multiple measurements of the systems' noise were performed with controlled changes in laser intensity. Tests have shown that the optical bridge attains a shot noise limited measurement with three orders of magnitude more photons than the Sagnac interferometer. While both configurations are shown to be capable of performing within the shot noise range, the reduced intensity required in the Sagnac negatively impacts the signal-to-noise ratio. Hence, for the purposes of strictly measuring rotations in the polarization of light, the optical bridge is the preferred arrangement. However, this does not undervalue the Sagnac's capacity to perform well in other areas. For instance, they are uniquely able to isolate rotations arising from phenomena with particular time and parity symmetries.