Multistep Anti-Stokes Raman Scattering by Coherent Gratings of Brillouin Zone Edge Phonons

We have observed the originally Raman-inactive phonon at the Brillouin zone (BZ) edge as multistep coherent anti-Stokes Raman scattering signals in a series of directions in which the phase-matching conditions are apparently broken under resonant pumping of the overtone of these phonons. The two anomalous behaviors of (1) breaking the Raman-selection rule and (2) the phase-matching-free signals, have been theoretically resolved by introducing a model of folding the BZ edge onto the Γ-point and the periodic modulation of the dielectric function with the wavevector of the condensed phonons. This model is shown to make the observed results well understood.