  We experimentally investigate the parametric down-conversion process in a
nonlinear bulk crystal, driven by two non-collinear pump modes. The experiment
shows the emergence of bright hot-spots in modes shared by the two pumps, in
analogy with the phenomenology recently observed in 2D nonlinear photonic
crystals. By exploiting the spatial walk-off between the two extraordinary pump
modes we have been able to recreate a peculiar resonance condition, reported by
a local enhancement of the parametric gain, which corresponds to a transition
from a three-mode to a four-mode coupling. From a quantum point of view this
opens the way to the generation of multimode entangled states of light such as
tripartite or quadripartite states, in simple bulk nonlinear sources.
