基于开口环阵列结构的表面晶格共振产生及二次谐波增强

Conditions for surface lattice resonances and enhancement of second harmonic generation based on split-ring resonators

理论研究了二维周期排列的金开口环谐振器的磁共振模式与周期阵列的衍射模式发生强耦合所需满足的条件及其对二次谐波产生效率的影响.通过控制阵列结构在x和y方向的周期大小,使得衍射模式只在其中一个方向产生,当衍射模式的电场方向与入射光电场偏振方向一致时,衍射模式才会与开口环谐振器的磁共振模式发生强耦合作用,产生表面晶格共振进而实现近场场增强.在此基础上,进一步计算了金开口环谐振器阵列的二次谐波产生效率,随着阵列周期逐渐增大,即开口环谐振器的数密度减小,二次谐波强度呈现先增加后降低的趋势,当开口环谐振器数密度降为原来的1/4左右时,二次谐波强度可以增强2倍以上.本文的研究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.

In this paper, we theoretically study the condition for the strong coupling between magnetic resonance mode of the two-dimensional periodically arranged gold split-ring resonators and the diffraction mode of the periodic array and its influence on the second harmonic generation efficiency. By controlling the size of the period of the array structure in the x-axis and y-axis, the diffraction mode is excited near the magnetic resonance provided by the gold split-ring resonator, solely in one of the directions. In both cases, the diffraction mode and the magnetic resonance coincide in the linear resonance spectrum, but by analyzing the electric field distribution at the position of the diffraction mode, it can be found that when ax is much larger than ay, the electric field direction of the diffraction mode is perpendicular to the polarization direction of the incident light,and no strong coupling occurs. Therefore, the dilution effect is dominant, and the second harmonic intensity gradually decreases with the increase of the period. When ay is much larger than ax, the electric field direction of the diffraction mode is the same as the polarization direction of the incident light. At this time, the diffraction mode and the magnetic resonance mode are strongly coupled. As the period increases, the second harmonic intensity first increases and then decreases. The increase is due to the dominant mode coupling and the decrease is due to the dominant dilution effect. When the number density of split-ring resonators is reduced to about 1/4 of the original one, the second harmonic intensity can be increased by more than twice. From this,we find that the strong coupling between diffraction mode and magnetic resonance can occur when the electric field direction of the diffraction mode is consistent with the polarization direction of incident light, thus generating the surface lattice resonance to achieve near-field enhancement. In short, the rectangular periodic structure is used to distinguish the field enhancement effects in different directions, and the second harmonic enhancement can still be achieved when the number density of split-ring resonators is reduced, which relaxes the requirements for processing technology. This research provides a new possible way to improve the second harmonic generation efficiency based on metal metasurfaces.