Broadband coherent radiation based on peculiar multiple Raman scattering by laser-induced phonon gratings in TiO2

We have found in TiO2 broadband coherent anti-Stokes Raman scatterings in unusual origin by exciting a pair of optical phonons with the use of crossed beams of two-color subpicosecond pulses. Angle-resolved spectroscopy reveals that the spectral lines up to the 22nd-order with nearly equal scattering-angle separation appear with their intensities of the same order; the conventional phase matching is not satisfied. Formation of a standing-wave type phonon grating is responsible for the signals. Application of this phenomenon to developing ultrafast optical pulses is discussed.