Coherent phonon-polaritons and subluminal Cherenkov radiation

We use an optical method based on sub-picosecond pulses moving in a nonlinear medium to generate relativistic dipoles that emit Cherenkov phonon-polariton radiation. Experiments on ZnTe and ZnSe reveal emission at subluminal speeds involving frequencies below those of the corresponding infrared-active phonon. The generation of Cherenkov radiation and impulsive stimulated Raman scattering by polaritons are the same physical phenomenon. (C) 2002 Elsevier Science B.V. All rights reserved.