Abstract

Multicolor soliton formation mediated by cascading of quadratic nonlinearities has been demonstrated in a variety of geometries. Solitons exist in particular in the process of second-harmonic generation (SHG) that is addressed here, where solitons exist above a threshold light intensity for all values of the fundamental (FF) and second-harmonic (SH) waves. Rigorously speaking, spatial solitons are non-diffracting, self-trapped beams formed with continuous wave light signals. However, because of the high peak-powers, at the GW/cm² scale, required to form quadratic solitons with feasible crystal lengths in the existing suitable materials that exhibit large nonlinearity and low-losses at the relevant wavelength bands, in practice spatial solitons are generated using pulsed laser light. Actually, to avoid crystal damage and thermal effects, mode-locked laser sources delivering picosecond, and even subpicosecond pulses are required. Also, potential applications of the multicolor solitons to ultrafast photonic devices involve spatial soliton formation with subpicosecond pulses. Nevertheless, with too short pulses temporal effects may induce large departures from the ideal continuous wave soliton features, and even prevent soliton formation. Existing materials with large quadratic nonlinearities and low absorption at optical wavelengths typically exhibit small group-velocity-dispersion (GVD) at their transparency bands. Thus, GVD is expected to impact the light evolution only in the femtosecond regime. However, quadratic solitons are intrinsically multiple-frequency entities. Therefore, the main limitation for spatial soliton formation is expected to come from the group-velocity-mismatch (GVM) experienced by the multiple-frequency pulses that are to overlap to form the spatial solitons. Here we study the impact of GVD and GVM to the generation of spatial solitons under conditions of SHG. The conclusions are relevant to multicolor soliton formation in all parametric processes.

© 2001 EPS