  Recently, the fundamental and nanoscale understanding of complex phenomena in
materials research and the life sciences, witnessed considerable progress.
However, elucidating the underlying mechanisms, governed by entangled degrees
of freedom such as lattice, spin, orbit, and charge for solids or conformation,
electric potentials, and ligands for proteins, has remained challenging.
Techniques that allow for distinguishing between different contributions to
these processes are hence urgently required. In this paper we demonstrate the
application of scattering-type scanning near-field optical microscopy (s-SNOM)
as a novel type of nano-probe for tracking transient states of matter. We
introduce a sideband-demodulation technique that allows for probing exclusively
the stimuli-induced change of near-field optical properties. We exemplify this
development by inspecting the decay of an electron-hole plasma generated in
SiGe thin films through near-infrared laser pulses. Our approach can
universally be applied to optically track ultrafast/-slow processes over the
whole spectral range from UV to THz frequencies.
