Violation of Raman selection rules induced by two femtosecond laser pulses inKTaO3

We have found that the crystalline symmetry of $\mathrm{K}\mathrm{Ta}{\mathrm{O}}_3$ is temporally changed by two incident femtosecond laser pulses having near-infrared frequencies. Raman-active two-phonons were excited coherently and resonantly by the frequency difference of the pulses. As a result, multi-step Coherent Anti-Stokes Raman Scattering (CARS) signals were observed; the frequency spacing of the signals corresponds to the frequency of originally Raman-inactive single-phonons at the Brillouin zone (BZ) edge. A model of BZ folding due to coherent accumulation of the phonons is presented to explain all aspects of this new phenomenon.