  A charge excitation in a two-dimensional Mott insulator is strongly coupled
with the surrounding spins, which is observed as magnetic-polaron formations of
doped carriers and a magnon sideband in the Mott-gap transition spectrum.
However, the dynamics related to the spin sector are difficult to measure.
Here, we show that pump-probe reflection spectroscopy with 7-fs laser pulses
can detect the optically induced spin dynamics in Nd$_2$CuO$_4$, a cuprate Mott
insulator. The bleaching signal at the Mott-gap transition is enhanced at
$\sim$18 fs, which corresponds to the spin-relaxation time in magnetic-polaron
formations and is characterized by the exchange interaction. More importantly,
ultrafast coherent oscillations appear in the time evolutions of the
reflectivity changes, and their frequencies (1400-2700 cm$^{-1}$) are equal to
the probe energy measured from the Mott-gap transition peak. These oscillations
originate from interferences between charge excitations with two magnons and
provide direct evidence for charge-spin coupling.
