Quantitative Measurements of Giant and Quantized Microwave Faraday Rotation

We report {\it quantitative} microwave Faraday rotation measurements conducted with a high-mobility two-dimensional electron gas (2DEG) in a GaAs/AlGaAs semiconductor heterostructure. In a magnetic field, the Hall effect and the Faraday effect arise from the action of Lorentz force on electrons in the 2DEG. As with the Hall effect, a classical Faraday effect is observed at low magnetic field as well as a quantized Faraday effect at high magnetic field. The high electron mobility of the 2DEG enables a giant single-pass Faraday rotation of $\theta_F^{max} \simeq 45^\circ$ $(\simeq0.8$~rad) to be achieved at a modest magnetic field of $B \simeq 100$~mT. In the quantum regime, we find that the Faraday rotation $\theta_F$ is quantized in units of $\alpha^*= 2.80(4)\alpha$, where $\alpha\simeq 1/137$ is the fine structure constant. The enhancement in rotation quantum $\alpha^* > \alpha$ is attributed to electromagnetic confinement within a waveguide structure.