We address the existence and properties of three-dimensional solitons localized in space and time, or light bullets, in bulk dispersive quadratic nonlinear media under conditions for Type I second-harmonic generation, with emphasis on the general case of different group-velocity dispersions at the fundamental and second-harmonic frequencies. We find families of three-dimensional light bullets and study their features in detail, including their stability on propagation. Most of the three-dimensional spatio-temporal solitons are shown to be stable.

Peer Reviewed