  In experiment and theory, we resolve the mechanism of ultrafast optical
magnon excitation in antiferromagnetic NiO. We employ time-resolved optical
two-color pump-probe measurements to study the coherent non-thermal spin
dynamics. Optical pumping and probing with linearly and circularly polarized
light along the optic axis of the NiO crystal scrutinizes the mechanism behind
the ultrafast optical magnon excitation. A phenomenological symmetry-based
theory links these experimental results to expressions for the optically
induced magnetization via the inverse Faraday effect and the inverse
Cotton-Mouton effect. We obtain striking agreement between experiment and
theory that, furthermore, allows us to extract information about the spin
domain distribution. We also find that in NiO the energy transfer into the
magnon mode via the inverse Cotton-Mouton effect is about three orders of
magnitude more efficient than via the inverse Faraday effect.
