We develop a theory of ultrafast light-induced magnetization dynamics in ferromagnetic semiconductors. We demonstrate magnetization control during femtosecond time scales via the interplay between nonlinear circularly polarized optical excitation, hole-spin damping, polarization dephasing, and Mn-hole-spin interactions. Our results show magnetization relaxation and precession for the duration of the optical pulse governed by the nonlinear optical polarizations and populations.

The magnetic compound Ba2CuGe2O7 has recently been shown to be an essentially two-dimensional spiral antiferromagnet that exhibits an incommensurate-to-commensurate phase transition when a magnetic field applied along the c-axis exceeds a certain critical value Hc. The T = 0 dynamics is described here in terms of a continuum field theory in the form of a nonlinear σ model. We are thus in a position to carry out a complete calculation of the low-energy magnon spectrum for any strength of the applied field throughout the phase transition. In particular, our spin-wave analysis reveals field-induced instabilities at two distinct critical fields H1 and H2 such that H1 < Hc < H2. Hence we predict the existence of an intermediate phase whose detailed nature is also studied to some extent in the present paper.

We study the microscopic processes that determine the dynamics of polaritons in strongly-coupled organicsemiconductor microcavities. Using a quantum kinetic theory, we show that resonant couplings and interactions between cavity photons, excitons, and localized molecular vibrations strongly affect the polariton resonance line shapes in J-aggregate microcavities. We use our many-body calculation to reproduce the measured polariton emission at exciton-photon resonance which is determined as a function of the Rabi-splitting energy.