Giant photoinduced Faraday rotation due to the spin-polarized electron gas in an n-GaAs microcavity

Faraday rotation up to 19 ^â̂̃ in the absence of an external magnetic field is demonstrated in an n-type bulk GaAs microcavity under circularly polarized optical excitation. This strong effect is achieved because (i) the spin-polarized electron gas is an efficient Faraday rotator and (ii) the light wave makes multiple round trips in the cavity. We introduce a concept of Faraday rotation cross section as a proportionality coefficient between the rotation angle, electron spin density and optical path and calculate this cross section for our system. From independent measurements of photoinduced Faraday rotation and electron spin polarization we obtain quantitatively the cross section of the Faraday rotation induced by free electron spin polarization σFexp=-(2.5±0.6)×10 ^-15 rad×cm2 for photon energy 18 meV below the band gap of GaAs, and electron concentration 2×1016 cm ^-3. It appears to exceed the theoretical value σFth=-0.7×10 ^-15 rad×cm2, calculated without fitting parameters. We also demonstrate the proof-of-principle of a fast optically controlled Faraday rotator.