Abstract

X waves are weakly localized wavepackets that can propagate without experiencing the beam spreading caused dispersive and diffractive phenomena. Recently it has been demonstrated that envelope X waves, wave packet solutions of wave equations of the Schrodinger-type, have a key role in the nonlinear regime. In fact nonlinear X waves have been invoked as a paradigm that allows to explain different experimental and numerical results in both quadratic and Kerr media[l]. Among new phenomena, we consider here the possibility that X waves induce self-transparency in a bulk medium with one-dimensional periodicity (Bragg grating) and Kerr response. Self-transparent solitary pulses, known as gap solitons because they can propagate at frequencies in the linear stop-band (gap), are known and have been observed in different systems such as fiber Bragg gratings, array of coupled waveguides, or Bose condensed gases with optical lattice and transverse trap. However, both ID or multi-D (bullets) gap solitons can be observed only above the Bragg frequency where the effective dispersion is anomalous. Viceversa, we consider the case of carrier frequencies close to lower edge of the gap, where the dispersion is normal and gap solitons are unstable. In this case X envelope of Bloch waves happen to be natural spatio-temporal eigenmodes of the dynamics in the presence of free diffraction [2]. Here we solve the problem of their excitation showing that they can be excited spontaneously from a gaussian (beam and pulse) impinging on the grating, allowing for self-transparency and a new form of slow-light propagation.

© 2007 IEEE