Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 11674168)
Received Date:30 August 2020
Accepted Date:28 October 2020
Available Online:21 February 2021
Published Online:05 March 2021
Abstract: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 ${a_x}$ is much larger than ${a_y}$, 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 ${a_y}$ is much larger than${a_x}$, 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. Keywords:strong coupling/ gold split-ring resonators/ diffraction mode/ second harmonic
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3.结果与讨论首先, 我们计算了两种周期阵列结构的透射谱, 如图2所示, 这两个周期结构的${a_y} = 400\;{\rm{nm}}$, ${a_x}$分别等于400 nm和1200 nm, 从图中可以发现, 两个透射谱都有一个宽带透射谷(Dip1), 该位置是开口环谐振器的磁共振模式, 位置几乎不随周期的改变而改变, 图1中插图表示Dip1位置x-y截面的磁场分布图和电流分布情况, 红色箭头代表电流, 从插图中的环形电流分布图也可以看出, 该位置是SRRs被激发的磁共振模式. 但在${a_x} = 1200 \;{\rm{nm}}$的透射谱中还存在一个窄带透射谷(Dip2), 该位置是由周期结构引入的衍射模式, 之后我们会对Dip1和Dip2的位置随周期的变化规律进行详细地分析. 图 2${a_y} = 400\;{\rm{nm}}$固定不变, ${a_x} = 1200\;{\rm{nm}}$ (黑线)和${a_x} = 400\;{\rm{nm}}$ (红线)两种不同周期阵列结构的透射谱, 插图表示宽带透射谷(Dip1)位置x-y截面的磁场电流分布图 Figure2. The transmission spectrum of two different periods along the x axis, ${a_x} = 1200\;{\rm{nm}}$ (black line) and ${a_x} = 400 \;{\rm{nm}}$ (red line). The insert shows the distribution of magnetic field and current in x-y section at the position of Dip1.
为探索衍射模式和磁共振模式发生强耦合所需要满足的条件, 我们分别研究了只改变x方向周期${a_x}$和只改变y方向周期${a_y}$两种情况下的耦合过程. 如图3所示, 图3(a) 和图3(b) 表示保持${a_y} = 400\;{\rm{nm}}$固定不变, 只改变${a_x}$时的透射谱和两透射谷位置随周期的变化规律. 图3(c) 和图3(d) 分别与图3(a) 和图3(b) 相对应, 区别在于${a_x} = 400\;{\rm{nm}}$固定不变, 而${a_y}$从1200 nm变化到1500 nm, 图3(a),(c)展示了金开口环谐振器阵列结构的透射谱, 可以观察到每一个透射谱都有两个透射谷:一个是窄带, 一个是宽带. 图3(b) 和图3(d)中空心圆圈代表了这两个透射谷位置随周期的变化规律; 黑色实线代表单个金开口环谐振器的磁共振, 磁共振的位置由金属材料特性和开口环谐振器的几何参数决定, 但不受阵列周期的影响; 蓝色实线代表介质环境中衍射模式随周期移动的曲线图; 两条红色曲线代表拟合的混合模式态—高能态和低能态, 该混合模式态由金属开口环谐振器激发的LSPs共振和周期结构Wood异常引入的衍射模式耦合形成, 二者能量可以通过耦合共振模型来计算[30]: 图 3${a_y} = 400\;{\rm{nm}}$, ${a_x} = 1200$—1550 nm (间隔50 nm) 时的 (a) 线性透射谱及(b) 透射谱中两透射谷随周期的变化; ${a_x} = 400\;{\rm{nm}}$, ${a_y} = 1200$—1500 nm (间隔50 nm)时的 (c) 线性透射谱及(d) 透射谱中两透射谷随周期的变化 Figure3. (a) Linear transmission spectrum and (b) the positions of two dips in transmission spectrum change with the period along the x axis, ${a_y} = 400\;{\rm{nm}}$, ${a_x} = 1200\!-\!1550 $ nm (interval 50 nm); (c) linear transmission spectrum and (d) the positions of two dips in transmission spectrum change with the period along the y axis, ${a_x} = 400\;{\rm{nm}}$, ${a_y} = 1200\!-\! 1500$ nm (50 nm interval).