One major challenge in the next generation THz (1012) technology is to develop highly efficient, ultra-broadband and low-cost terahertz emitters with a gapless spectrum. Up-to-date, most broadband and table-top THz emitters are based on the femtosecond laser excitations, taking advantage exclusively of the charge property of the electron. Here, we introduce two novel types of broadband spin-based THz emitters composed of the ferromagnetic metallic heterostructures [1-4], e.g. (Co, Fe)/Pt and Fe/Ag/Bi. We have carried out detailed thickness-dependent experiments in these samples. Such investigations not only enable us to clarify the intrinsic mechanisms behind the THz radiation - the inverse spin Hall effect (ISHE) and the inverse Rashba-Edelstein effect (IREE), but also help to determine the key parameters to optimize the THz emission. The emitted THz wave, with its phase and polarization easily manipulated by changing the film stacking order and the magnetization direction, has an ultra-broadband width (~0.1-20 THz) and strong amplitude (comparable to the conventional nonlinear crystals). We also demonstrate that the THz radiation arising from both the ISHE and IREE can be selectively superimposed with each other.
 Seifert, T. et al. Efficient metallic spintronic emitters of ultrabroadband terahertz radiation. Nat. Photon. 10, 483 (2016).
 Yang, D. et al. Powerful and Tunable THz Emitters Based on the Fe/Pt Magnetic Heterostructure. Adv. Opt. Mater. 4, 1944 (2016).
 Wu, Y. et al. High-performance THz emitters based on ferromagnetic/nonmagnetic heterostructures. Adv. Mater. 29, 1603031 (2017).
 Zhou. C et al. Broadband terahertz generation via the interface inverse Rashba-Edelstein effect (submitted).
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