of the probe polarization as a function of the pump-probe delay. The
data are normalized on the pump fluence, which was approximately
12 mJ/cm². The central photon energies of the pump and probe pulses
were 1.5 and 1.9 eV, respectively. The temperature was 10 K.
The absorption coefficient corresponding to the pump photon energy
is shown. In the inset we report the temperature
dependence of the incoherent dynamics, measured with pump and
probe photon energies set to 1.5 eV. The dashed line is a guide to
the eye. (b) Diagram showing the energy flow to the spin system in
the case of excitation resonant with an absorption band.
The femtosecond duration of modern laser sources allows to induce and detect spin and charge dynamics by means of time-resolved methods, whose resolution is on the same timescale of the fundamental (electronic, phononic and magnetic) interactions in solid state materials. The measurement of transient optical properties (i.e. reflectivity and transmissivity) provides access to the charge dynamics, while the temporal evolution of the spin degree of freedom can be monitored relying on magneto-optical effects, expressed by modifications of the polarization of light.
Such techniques are the workhorses of the research field of ultrafast magnetism and they allowed pioneer investigations and breakthroughs.
It has been previously shown that ultrashort laser pulses can excite, manipulate and detect low-energy coherent spin waves (or magnons) at the center of the Brillouin zone of dielectric antiferromagnets. The excitation takes place via the mechanism of impulsive stimulated Raman (i.e. inelastic) scattering, even in the absence of direct heating of the lattice and electrons (PRB 89, 060405(R) (2014)). Subsequently, the excitation and coherent manipulation of pairs of high-energy magnons at the edges of the Brillouin zone (Nat. Comm. 7, 10645 (2016)), via the same mechanism, has been demonstrated. With respect to the intrinsic energy of these magnetic excitations, our experiment established the manipulation of coherent spin dynamics at a frequency of 22 THz. The spin dynamics induced by high-energy magnons is a purely quantum mechanical phenomenon (PRB 100, 024428 (2019)). While the understanding of the aforementioned effects is one of our major research efforts, we strive also to expand our experimental capabilities. In particular we aim at developing a resonant excitation of magnon modes and a detection of the ultrafast spin dynamics with combined time- and space-resolution.
