导　　师： 卢克清

学科专业： 070207

授予学位： 硕士

作　　者： ;

机构地区： 中国科学院研究生院

摘　　要： 本文主要对光折变空间光孤子的时间行为进行了研究。从光折变效应的物理过程出发,阐述了光折变空间光孤子的形成机理,引入了带输运模型,介绍了描述光折变空间光孤子形成过程的基本方程,并分别推导了开路系统和闭路系统中与时间有关的空间电荷场的表达式。 研究了开路低振幅系统中一维光伏空间光孤子的时间行为,并用数值方法模拟了孤子的传输过程以及强度的半峰值宽度随时间的演化过程。发现孤子的宽度随时间单调递减至一个最小值。而且,对于不同的峰值强度与暗辐照比值,孤子达到稳态时所需要的时间接近于一个确定值,它的大小等于无照明场时晶体的介电弛豫时间。当时间趋于无穷时,得到了暗孤子和亮孤子的解析解。 通过数值模拟的方法研究了开路亮光伏空间光孤子随时间的演化过程。当孤子峰值强度与暗辐照的比值较小时,孤子的宽度随时间单调递减直至稳态形成;当孤子峰值强度与暗辐照的比较大时,孤子的宽度先是随时间递减至一个最小值然后又开始增加,而且稳态孤子的形成时间与强度比值无关。并以稳态孤子作为输入,研究了稳态孤子在钛酸钡晶体中的传输过程,观察到稳态孤子的传输过程中存在强度的准周期性变化和弯曲现象。研究表明,随着光伏电场的增强,晶体内的局域非线性效应也得到加强,具体表现为孤子的宽度变得更窄,自聚焦的准周期变得更短。此外,还讨论了时间和温度对光束在晶体里传播时弯曲效应的影响。 从与时间有关的非线性波动方程出发,研究了闭路光伏介质中的准稳态孤子和稳态孤子的性质。结果表明,开路准稳态孤子和开路稳态孤子的形成时间随着孤子峰值强度与暗辐照比值的增加而减小。而且准稳态孤子只有在孤子峰值强度较大时才存在。对于闭路系统,当孤子峰值强度较小时,无论电流密度取何值,都只产生稳态孤子,且改变电流密度值并不影响稳态孤子的形成时间。当孤子峰值强度较大时,改变电流密度,孤子的峰值宽度将呈现出三类不同的演化过程。第一类过程,电流密度取值较小时,稳态暗孤子的形成时间随着电流密度的增加而增加,但准稳态孤子的形成时间与电流密度无关;第二类过程,电流密度取值稍大时,稳态孤子的形成时间都随着电流密度的增大而减小,准稳态孤子的形成时间都随着电流密度的增大而增大;第三类过程,电流密度更大时,改变电流密度只能产生稳态暗孤子,而且稳态孤子的形成时间随着电流密度的增加而增大。 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 inτ, whereτis 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 inτand then increases withτ, 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 J/_0 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 J/_0 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 J/_0 whereas the formation time of quasi-steady-state dark solitons is independent of J/_0. The second regime shows that the formation time of steady-state dark solitons decreases with an increase in J/_0 and the formation time of quasi-steady-state dark solitons increases with J/_0. The third regime show that changing J/_0 enables only steady-state dark solitons in the time-dependent nonlinear wave equation, of which the formation time increases with J/_0.

关 键 词： 光折变晶体 空间光孤子 光伏效应 时间行为

分 类 号： [O437]

领　　域： [机械工程] [理学] [理学]