Coherent polaritons and Cerenkov radiation at subluminal speeds

Summary form only given. As shown by Auston and co-workers, an ultrafast pulse moving in an electro-optic medium produces Cerenkov radiation (CR) if its group velocity c/sub g/, is greater than the phase velocity of light at infrared frequencies. The pulse generates a low-frequency polarization as it interacts with itself through a /spl chi//sup (2)/-process and, in so doing, it behaves as an optical analog of a relativistic dipole. Here, we demonstrate new and unexpected features of CR resulting from optical dispersion. Contrary to the common understanding, we identify conditions for which CR is emitted at subluminal but not at superluminal speeds, and we verify this prediction in pump-probe experiments on ZnSe. We also present calculations for a point source revealing that an arbitrary cone angle is actually associated with two velocities, one above and one below a certain speed threshold. Finally, we argue that CR due to optical nonlinearities and phase-matched excitation of phonon polaritons by ultrafast pulses are the same physical phenomenon.