Cascade quadratic nonlinearity and its application to ultrafast nonlinear optics

This thesis presents an experimental and numerical investigation of cascade nonlinearity and its application to ultrafast nonlinear optics. The primary emphasis of this work has been on the investigation of femtosecond nonlinear wave propagation with cascade quadratic nonlinearity. A series of experiments were performed to demonstrate some important applications of cascade nonlinearity such as pulse compression, nonlinear polarization rotation, mode-locking, and spatio-temporal soliton formation. Extensive numerical simulations were also performed to better understand the initial experimental observations, as well as to guide deeper experimental investigations.;As the first applications of cascade nonlinearity with femtosecond laser pulses, intense 120 fs pulses were efficiently compressed to ∼30 fs, and a solid-state laser was mode-locked by the use of negative nonlinear phase shifts generated in cascade quadratic processes. An ultrafast amplitude modulator based on intensity-dependent polarization evolution with cascade nonlinearity is demonstrated, and it can be potentially used for mode-locking of femtosecond fiber lasers.;Multi-dimensional spatial-temporal solitons (STS), were observed in quadratic nonlinear media. This is the first experimental observation of STS which have been theoretically predicted before by other workers. Systematical study of STS's are presented. Experimental observations are compared with numerical simulations to provide better understanding of the formation and propagation of STS. Furthermore, the transverse instability (TI) induced breakup of 2D STS was observed. A simple analysis, together with a series of numerical simulations, was given to quantitatively explain the experimentally observed super-linear dependence of the spatial filament frequency on incident intensity. The TI induced breakup of 2-D STS may provide a route to the generation of 3-D STS, light bullets traveling in time and space with fully confined and stable spatial-temporal profile. These light bullets may be potential carriers for future all-optical signal processing. As an interesting application, an ultra-fast and -gate based on the non-collinear generation of 2D STS was experimental demonstrated and presented.