We review recent experiments on spin excitation and manipulation in the ferromagnetic semiconductor GaMnAs. Spin
dynamics in GaMnAs have been studied by two complementary approaches - by frequency-domain techniques, such as
Brillouin light scattering (BLS) and ferromagnetic resonance (FMR); and by optical real-time techniques, such as
ultrafast pump-probe magneto-optical spectroscopy. Using BLS and FMR, magnon frequencies (or resonance fields),
were investigated as a function of Mn concentration, temperature and direction of magnetization, leading to information
on magnetic anisotropy. Time-resolved magneto-optical Kerr effect, on the other hand, was used to study photo-induced
femtosecond magnetization rotation, ultrafast optical demagnetization, and collective magnetization precession.
Optically-induced transient changes in magnetization of GaMnAs produced by femtosecond laser pulses are analyzed
and discussed in terms of the Landau-Lifshitz-Gilbert model. Finally, for completeness, we also discuss carrier-mediated
nonthermal and thermal (lattice-heating) contributions to spin dynamics.
We report comprehensive temperature and photoexcitation intensity dependent studies of the photoinduced
magnetization precession in Ga<sub>1-x</sub>Mn<sub>x</sub>As (<i>x</i> = 0.035) by time-resolved Kerr rotation measurements. We observe coherent
oscillations of local Mn spins triggered by an ultrafast photo-induced reorientation of the easy axis due to changes in the
magnetic anisotropy. The amplitude saturation of these oscillations above certain pump intensity is indicative of
stabilization of the magnetic easy axis orientation on temperatures above ~Tc/2. We find that the observed magnetization
precession damping (Gilbert damping) is strongly dependent on pump laser intensity, but largely independent of ambient
Femtosecond optical reflectivity measurements of La<sub>2-x</sub>Sr<sub>x</sub>CuO<sub>4</sub>, La<sub>2</sub>CuO<sub>4+y</sub>, Bi<sub>2</sub>Sr<sub>2</sub>CuO<sub>6+z</sub> and Bi<sub>2</sub>Sr<sub>2</sub>CaCu<sub>2</sub>O<sub>8+δ</sub> thin films and single crystal samples indicate qualitative changes with fluence. At the lowest fluencies, there is a power law divergence in the relaxation time. The divergence has an onset temperature of 55±15K, independent of whether the sample is in the superconducting or normal states. At slightly higher fluencies, still perturbative, the additional response does not exhibit this power law divergence. At quite high fluencies- no longer perturbative- the metallic samples exhibit oscillations in the reflectivity amplitude. The period of these oscillations varies with the probe wavelength but not with the pump wavelength. The oscillations exhibit a decay time as long as 10 nsec.
We present fabrication and optical time-resolved photoresponse characterization of MgB<sub>2</sub> superconducting thin films. The films were prepared on crystalline and flexible plastic substrates by vacuum co-deposition of B and Mg precursors and high-temperature annealing in an Ar or vacuum atmosphere. The post-annealed films exhibited very smooth surfaces and amorphous structures with nanocrystal inclusions. The best films exhibited the critical temperature Tc of up to 38 K, the transition width of 1 K, and the current density j<sub>c</sub> at 4.2 K of about 10<sup>6</sup> A/cm<sup>2</sup>. In our pump-probe photoresponse experiments, we used 100-fs-wide optical pulses generated by a Ti:Sapphire laser. The pump and the probe beams had 800-nm wavelength and the measurements were performed in the temperature range from 3.5 K to room temperature. The transient reflectivity change (ΔR/R) signals exhibited around 300-fs (10%-90%) risetime. At room temperature and far above Tc, (ΔR/R) the transient reflectivity change was characterized by a ~160-fs, single-exponential decay, interpreted as the electron-Debye-phonon interaction time. Below 60 K and in the superconducting state, the ΔR/R photoresponse was biexponential, with the initial femtosecond decay followed by a much slower, several-ps-long relaxation. We associate the latter slow relaxation with the electron-phonon interaction related to the Cooper pair recombination dynamics. The existence of this signal above the nominal T<sub>c</sub> of our films, we tentatively interpret as the presence of superconducting fluctuations in our MgB<sub>2</sub> films. Our work gives the first insight into the carrier dynamics in MgB<sub>2</sub> by time-resolved experimental studies of the Cooper pair breaking and thermalization mechanisms for the films perturbed by femtosecond optical excitations.