Tilted pulses and X(2) cascading: effects on transient compression and temporal-soliton formation

In X(2) cascading, two relevant phenomena are expected in case of suitable group-velocity mismatch (GVM) and group-velocity dispersion (GVD) material parameters: (i) if the GVM dominates and the low-frequency pulses run with opposite velocities respect to the high-frequency one, then interaction could lead to the so called “non-linear transient pulse compression” or to the elastic scattering of non-trapped soliton pulses, (ii) If the GVD dominates and has the same sign for all three waves, then mutually-trapped dispersion-free bright solitons (or quasi-solitons) are attained, due to the interplay between GVD and nonlinearity. Unfortunately, such operating conditions are hardly achievable in real experiments. Non-linear pulse compression was demonstrated only for very specific crystals and wavelengths; regarding the GVD solitons, they are commonly considered as not obtainable due to the low intrinsic dispersion of available X(2) materials, which makes the GVM split the interacting pulses before the trapping mechanism sets in. For a review of the argument, see ref. [1] and references therein.