Studies On Temporal Development Of Photorefractive Spatial Solitons

The temporal behavior of photorefractive spatial solitons is studied in this thesis. Based on the physics process of photorefractive effect, the formation mechanism of photorefractive spatial solitons is depicted, band transport model is introduced, the set of equations for explaining the formation of photorefractive solitons is shown and time-dependent wave propagation equation of open-circuit regime and closed-circuit regime is deduced, respectively.The propagation properties and the evolution of intensity width under open-circuit conditions in low-amplitude regime for one-dimensional photovoltaic spatial solitons are analyzed. Our investigation indicates that a broad soliton is generated at the beginning, and as time evolving, the intensity width of photovoltaic spatial solitons decreases monotonously to a minimum value toward steady state. The time of the whole process requiring is close to a fixed value equals to the dielectric relaxation time in the absence of an illuminating field for different intensity ratios of soliton, which are the ratio between the peak intensity of the soliton and dark irradiance. The analytical solutions of both dark and bright solitons are exhibited as time extending infinitely. Relevant examples are visualized through numerical methods.The temporal behavior of open-circuit bright photovoltaic spatial solitons is investigated by using numerical techniques. We show that when the intensity ratio of the soliton is smaller, the quasi-steady-state soliton width decreases monotonously with an increase inÏ„, whereÏ„is the parameter correlated with the time, that when the intensity ratio of the soliton is bigger, the quasi-steady-state soliton width decreases with an increase inÏ„and then increases withÏ„, and that the formation time of the steady-state solitons is not correlated with the intensity ratio of the soliton. We find that the local nonlinear effect increases with the photovoltaic field, which behaves as that the width of soliton beams is smaller and the self-focusing quasi-period is shorter. On the other hand, we also discuss that both the time and the temperature have an effect on the beam bending.Beginning with the time-dependent nonlinear wave propagation equation in closed-circuit photovoltaic media, quasi-steady-state and steady-state spatial solitons are exhibited. The formation time of open-circuit quasi-steady-state and open-circuit steady-state dark solitons decreases with an increase in the intensity ratio between the soliton peak intensity and the dark irradiance. And quasi-steady-state solitons only can exist when the intensity ratio is big. In closed-circuit system, when the peak intensity of solitons is small, only steady-state solitons can show up, changing the electric current density J₀ dose not generate quasi-steady-state dark solitons and affects the formation time of steady-state dark solitons and when the peak intensity of solitons is big, quasi-steady-state dark solitons can also be formed, changing J₀ gives rises to three different time evolution regimes of the full width half maximum of the soliton's intensity. The first regime shows that the formation time of steady-state dark solitons increases with J₀ whereas the formation time of quasi-steady-state dark solitons is independent of J₀. The second regime shows that the formation time of steady-state dark solitons decreases with an increase in J₀ and the formation time of quasi-steady-state dark solitons increases with J₀. The third regime show that changing J₀ enables only steady-state dark solitons in the time-dependent nonlinear wave equation, of which the formation time increases with J₀.