On the optical characteristics of the conical emission

An intense laser beam blue detuned with respect to an atomic transition produces a forward conical emission. We demonstrate new experimental evidence that in sodium vapor this emission is generated at the surface of self-trapped filaments. This includes comparison of the polarization of the incident and emitted light as well as comparing the output pattern obtained by cylindrical and spherical focusing modes. As a surface phenomenon, the conical emission shares its characteristics with Cherenkov-type processes.