Emission by a faster-than-light moving source

Summary form only given. Superluminal sources (SLS) are sources whose propagation velocity can be designed to exceed the speed of light in medium or in vacuum. Examples of SLS are oblique intersection of a pulse of light and a screen, moving foci produced during self-focusing of short pulses: and polarization/excitation of medium. The motion of SLS does not violate causality since they do not transfer information or energy. SLS may find applications in generation of Cerenkov radiation in vacuum or medium with refraction index n<1, X-ray laser pumping, electron acceleration using inverse Cerenkov effect, and in group velocity matching in Raman, harmonic, and parametric generation and pump-probe experiments with ultrashort pulses. We report on Cerenkov-type Raman emission by an axicon-generated SLS. We show that by adjusting the SLS velocity the group velocity dispersion (GVD) between the pump and Stokes pulses can be compensated, leading to a reduced threshold and an efficient stimulated Raman emission for ultrashort pulses. Moreover, self-phase modulation (SPM) of the pump, which limits Raman conversion of intense pulses, is 2-3 orders of magnitude smaller for the axicon focusing as compared to SPM while focusing by a lens.