金属微纳结构中谐波产生效率提高和谐波位相调控的研究

Nonlinear plasmonics has been one of today’s hottest research subjects in the field of nano-photonics. Compared to the conventional nonlinear optics, nonlinear plasmonics is advantageous in that the elaborate design of the metallic micro/nano-structures can enable highly efficient nonlinear-optical processes to be occurred in the subwavelength scale, enable operation wavelengths to be widely tuned, and also enable the manipulation and control of the phase of the local effective nonlinear polarizability. In this project, we mainly focus on several new scientific problems emerging in the field of nonlinear plasmonics. We plan to develop new methods for further enhancing the local fields and study the efficiency enhancement of harmonic generation associated with the enhanced fields. We will try to find a solution to break the quantum limit for plasmonic field enhancement established by the quantum tunneling, which could lead to the greatly enhanced local fields and efficiency of harmonic generation in the dipolar nano-antennas with sub-nanometer gaps. We will study the effect of the excitations of cavity plasmon resonances on the control of spatial distributions of third-harmonic generation from individual plasmonic cavities, find the best condition of double cavity plasmon resonances under which the maximum third-harmonic generation efficiency could be obtained, and realize the control of third-harmonic diffraction in the array of plasmonic cavities. Furthermore, we will also develop new mechanisms that can provide the continuous phase control over the local effective nonlinear polarizability of each element in a nonlinear metamaterials. Based on these mechanisms, we will design novel metamaterials with homogeneous linear optical properties but spatially varying effective nonlinear polarizability with continuously controllable phase for the manipulations of wave-front and polarization state of the harmonic generation. We hope the implementation of this project can pave the way for efficiency enhancement and phase control of harmonic generation from metallic micro/nano-structures.

非线性等离激元光学是当今纳米光子学的研究热点，其相对于传统非线性光学的巨大优势在于通过金属微纳结构的精心设计可以在亚波长尺度获得高效的非线性光学过程，可以实现工作波长的调谐以及实现局部有效非线性极化相位的调控和操纵等。本项目主要围绕非线性等离激元光学中的一些新的科学问题，研究局域电场进一步增强的新方法及其在谐波效率增强上的应用，研究突破量子隧穿效应带来的等离激元局域场强的量子极限值，获得亚纳米间隙光学偶极天线中局域场以及谐波产生效率的极大增强，研究等离激元腔模激发对三次谐波辐射的调控特性，研究最佳双腔模共振条件以获得最大化的谐波产生效率，研究金属微腔阵列中的三次谐波衍射调控，发展新型非线性超构材料，研究对其结构单元的非线性极化相位调控的新机制及其在操控谐波辐射波前、偏振上的应用，为后续应用研究提供可靠的科学依据和技术支撑。

精心设计金属微纳结构，可以在亚波长尺度获得高效的非线性光学过程，有效调谐非线性光学过程的工作波长，获得全新的相位匹配机制，实现局部有效非线性极化相位的调控和操纵等，是非线性等离激元光学相对于传统非线性光学的一个巨大优势，因而受到国内外的极大关注。我们在本项目中主要研究了非线性等离激元光学中的二次和三次谐波产生效率提高和相位调控，取得的主要研究成果及其科学意义包括：1）基于介质加载的方法，设计了新型介质-金属等离激元纳米光学天线，实现了局域电场热点从金属到介质表面的转移，并获得了更大的局域电场强度，为进一步提高表面增强相关的线性和非线性光谱信号强度提供了一种新途径；2）设计了一种双曲超构材料微腔，可以在微腔核心区域获得均匀分布并且极大增强的局域电场，使得处于双曲超构材料微腔中心的非线性纳米晶二次谐波的产生效率比单层的金属包裹纳米晶的效率提高两个数量级，对于非线性谐波激发效率的增强有着重要的应用；3）调节金开口环谐振器阵列的周期大小，引入周期衍射模式与开口环谐振器的磁共振模式发生强耦合作用，使得在谐振器数密度降低时二次谐波强度反而可以获得2倍以上的增强，为金属超表面二次谐波产生效率的提高提供了一种新的可能途径。4）设计并制备了由开口相反的以及不同几何尺寸的金属开口环谐振器构成的非线性超构表面，实现了二次谐波局部相位的连续调控，实验观测到与理论预测吻合的二次谐波光束的弯折以及聚焦功能，在此基础上，利用金属开口环谐振器及其互补结构所产生的二次谐波偏振方向具有正交特性，实现了集合非线性谐波产生、分束和偏振的任意调控为一体的非线性集成化功能器件。