Faraday rotation up to 19∘ 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.

Faraday rotation is a well-known magneto-optical phenomenon: polarization plane of the light wave is rotated upon transmission through a magnetized medium. It is believed that most efficient Faraday rotators are transparent magnetic materials, such as rare-earth doped glasses and garnets, as well as diluted magnetic semiconductors, where magnetic ions create strong magnetization. In contrast, in conventional non-magnetic semiconductors the rotation due to magnetization of electron gas in the presence of equivalent magnetic field is much smaller. We show, that using optical orientation of electron gas in GaAs confined in a microcavity (Q=50000) it is possible to reach the rotation angles larger than 10° at only 1% spin polarization of the electron gas and in the absence of magnetic field ( Fig. 1 ). This strong rotation exceeds by orders of magnitude the rotation ever measured in bulk n-GaAs samples of similar concentration () [1] [2] . We deduce from these experiments the Verdet constant associated with the electron spin polarization density. This also allows for the direct detection of the extremely slow electron spin polarization decay (~250 ns) in the absence of the pump ( Fig. 2 ). It suggests that spin relaxation time in the dark can be much longer than the relaxation time extracted from Hanle depolarization in either photoluminescence [3] or Faraday rotation ( Fig. 1 ) experiments.

We theoretically describe the spin-excitation spectrum of a two-dimensional electron gas embedded in a quantum well with localized magnetic impurities. Compared to the previous work published in [ Phys. Rev. B 70 45205 (2004)] and [ Phys. Rev. B 79 045306 (2009)], we introduce equations that allow us to consider the interplay between the Coulomb interaction of delocalized electrons and the sd exchange coupling between electrons and magnetic impurities. Strong qualitative changes are found: mixed waves propagate below the single-particle continuum, an anticrossing gap is open at a specific wave vector, and the kinetic damping due to the electron motion [ Phys. Rev. B 81 100403 (2010)] strongly influences the coupling strength between electrons and impurities spins.

We report on a pronounced nonlinear optical effect in a GaAs microcavity operating in the strong-coupling regime: cavity transmission in circular polarization is found to be much stronger than in linear polarization. This behavior has its origin in the spin-dependent repulsive interaction between exciton polaritons. Quantitative analysis allows estimating the strength of interaction between polaritons with parallel spins. The observed effect shows the potentiality of microcavities for circular polarization sensing essential for transmission of polarization encoded signals.

Time-resolved Kerr rotation experiments show that two kinds of spin modes exist in diluted magnetic semiconductor quantum wells: (i) strongly coupled electron-magnetic ion spin excitations and (ii) excitations of magnetic ion spin subsystem decoupled from electron spins. The coexistence of these two kinds of spin precession modes cannot be understood in terms of average spins but requires a description, which goes beyond the mean-field approximation.