  Plane inertial waves are generated using a wavemaker, made of oscillating
stacked plates, in a rotating water tank. Using particle image velocimetry, we
observe that, after a transient, the primary plane wave is subject to a
subharmonic instability and excites two secondary plane waves. The measured
frequencies and wavevectors of these secondary waves are in quantitative
agreement with the predictions of the triadic resonance mechanism. The
secondary wavevectors are found systematically more normal to the rotation axis
than the primary wavevector: this feature illustrates the basic mechanism at
the origin of the energy transfers towards slow, quasi two-dimensional, motions
in rotating turbulence.
