Cherenkov radiation at speeds below the light threshold: Phonon-assisted phase matching

Charged particles traveling through matter at speeds larger than the phase velocity of Light in the medium emit Cherenkov radiation. Calculations reve al that a given angle of the radiation conical wavefront is associated with two velocities, one above and one below a certain speed threshold. Emissio n at subluminal but not superluminal speeds is predicted and verified exper imentally for relativistic dipoles generated with an optical method based o n subpicosecond pulses moving in a nonlinear medium. The dipolar Cherenkov field, in the range of infrared-active phonons, is identical to that of pho non polaritons produced by impulsive Laser excitation.