基于结构光场非线性拉曼纳斯效应的新型大角度可调谐光束扫描技术

The current manipulation means of the beam scanning technique were divided into mechanical, acousto-optical, and electro-optical types. However, the drawbacks such as low rotation speed and the need of external electric field supply restricted their applications and development in the miniaturization, integration and speed of devices. Based on the nonlinear Raman-Nath effect caused by the interaction between the structural beam whose phase distribution of wave front was specially designed with nonlinear medium without any domains inversions, a new scheme for deflection of beams by means of the nonlinearity was introduced here. We built the structured beams via a spatial light modulator, and studied physical principle of nonlinear Raman-Nath diffraction caused by the structural light field whose phase distributions were periodic, quasi-periodic, aperiodic, and hybrid structure respectively in 1D or 2D cases. Moreover, we also investigated in detail the relationship between the spatial emission angles of second harmonic generation and the parameters such as periodic length, the coefficient of quasi-period, and the degree of randomness. Meanwhile, the propagated and diffracted process of aperiodic structural beam whose phase distribution was depended on greatly the self-adjustment algorithm in a nonlinear photonic lattice was studied as well. The.tunable and flexible phase distribution expanded the applied range of beam scanning, which had potential applications in free-space communication, laser radar, and astronomical measurement. Finally, the most remarkable features including no external electric field supply, large angle tunability, structural variation in real-time were contributed to the development of the beam scanning technique.

光束扫描技术，目前实现手段有机械式、声光控制、电光控制型。但这些手段转速慢和需要外接电源的缺点制约了其在器件小型化、集成化、高速化的应用发展。本项目提出基于波前相位空间分布经过特殊设计的结构光场与无任何畴反转非线性晶体相互作用产生非线性拉曼纳斯衍射效应，以非线性手段实现激光束偏转。我们通过空间光调制器产生结构光场，分析一维和二维条件下当波前分别为周期性、准周期性、非周期性和混合结构光场的非线性拉曼纳斯衍射效应的物理原理以及结构的周期长度、准周期系数、随机度这些参数对二次谐波空间衍射角度分布的影响关系，分析了基于自调整算法设计的非周期相位分布的结构光场在非线性晶体内传播演化。光场相位空间结构的可调谐性和灵活性扩大了光束扫描技术的应用范围，在空间光通信、激光雷达、天文测量等领域具有应用前景。所以，本方案最重要三个特点：无外接电场、大角度调谐、结构实时变化有助于光束扫描技术的发展。

光束扫描技术，常见控制方式有机械式、声光控制、电光控制型。这些方式因转速慢和需要外接源的缺点制约了其在器件小型化、集成化、高速化的应用发展。因此，有研究者提出可以利用光栅衍射实现光束扫描，但该方式因固定的光栅常数而无法实时且任意调节衍射角度。为解决这一问题，本项目实验研究了一种基于空间光调制器（SLM）调制光束的波前相位，当这种被调制的结构光场与无任何畴翻转的二阶非线性晶体相互作用后，其二次谐波会在不用角度下衍射的空间扫描技术。SLM上加载的相位结构可以实时改变，所以本方法可实现光束扫描的任意性和实时性。方案上，我们分别从一维和二维方向上设计了周期性、混合周期、非周期性的结构光场，研究发现这些结构的非线性拉曼纳斯衍射光斑均呈一系列对称性分布，这要归因于相位结构所提供的倒格矢均为对称性分布，即使非周期也不例外。但是，不同类型相位调制的衍射角度各不相同，主要与周期长度、随机度、结构傅里叶系数有关。进一步研究发现，当波前调制设计为螺旋型时，利用双折射匹配，入射的高斯型基频光场的二次谐波转变为圆环型涡旋光场。其强度分布依赖基频的拓扑荷数和倾斜因子。倍频光拓扑荷数是基频拓扑荷数的两倍。而且，当基频倾斜因子不等于1时，其涡旋光场从圆环型蜕化为椭圆型。.除此以外，本项目还研究了古斯－汉欣（GH）位移增强和全光和电光光学双稳态关系。首先，我们在双面金属包覆波导上覆盖另一个波导，这两个不同Q值的波导所激发的导模相互耦合引发Fano共振，从而引起反射光的相位剧烈变化，因此产生高反射率的GH位移明显增强，结果显示GH位移可正可负，均达亚毫米量级。其次，通过改进传统的双面金属包覆波导结构，即将非线性Kerr材料或电光材料放入导波层中，这两种材料的折射率和厚度对入射光强和外载电压反应非常灵敏，这些变化都会引起超高阶导模移动，最后改变反射率和反射光强。当入射角选择在一些特殊角时，会存在全光或电光的双稳态关系。