  Electromagnetic radiation in the mid- to far-infrared spectral range
represents an indispensable tool for the study of numerous types of collective
excitations in solids and molecules. Short and intense pulses in this THz
spectral range are, however, difficult to obtain. While wide
wavelength-tunability is easily provided by free-electron lasers, the energies
of individual pulses are relatively moderate, on the order of microjoules. Here
we demonstrate a setup that uses cavity-dumping of a free-electron laser to
provide single, picosecond-long pulses in the mid- to far-infrared frequency
range. The duration of the Fourier-limited pulses can be varied by cavity
detuning, and their energy was shown to exceed 100 {\mu}J. Using the
aforementioned infrared pulse as a pump, we have realized a two-color
pump-probe setup facilitating single-shot time-resolved imaging of
magnetization dynamics. We demonstrate the capabilities of the setup first on
thermally-induced demagnetization and magnetic switching of a GdFeCo thin film
and second by showing a single-shot time-resolved detection of resonant
phononic switching of the magnetization in a magnetic garnet.
