Fund Project:Project supported by the National Natural Science Foundation of China (Grant No. 11674168)
Received Date:29 May 2019
Accepted Date:15 July 2019
Available Online:01 November 2019
Published Online:05 November 2019
Abstract:In this paper, we study the second harmonic generation (SHG) from the stero-stacked meta-molecules consisting of two vertically stacked split ring resonators (SRRs) that resonate at the fundamental wavelength. When pumped by the linearly polarized incident wave with the electric field direction along one of the SRRs’ arms, the meta-molecules emit the SHG that can have two non-zero orthogonal electric field components, provided that the top SRR and the bottom SRR are not arranged in mutually parallel or anti-parallel manner. Due to the strong coupling between the two SRRs, the plasmonic properties of the stero-stacked meta-molecules could be tuned by varying the twist angle between the two SRRs. In this process, we demonstrate that the amplitudes of the two orthogonal SHG field components, and the phase difference between these two components can be varied with changing the twist angle between two SRRs. Based on the concept of the light polarization, different polarization states can be achieved by changing the differences in phase and amplitude between the orthogonal field components. Therefore, the twist angle dependent amplitudes of and phase difference between two orthogonal SHG field components can be used to manipulate the polarization states of the emitted SHG. For the stero-stacked meta-molecules with a fixed twist angle of 60°, elliptically, near-circularly andnear-linearly polarized SHG emission can be obtained at different fundamental wavelengths. In addition, for the fundamental wave with a fixed wavelength of 1500 nm, the stero-stacked meta-molecules with different twist angles are demonstrated to be able to emit SHG with elliptical andnear-linear polarization states. Keywords:split ring resonator/ second harmonic generation/ polarization control
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3.非线性表征及偏振调控的实现在保持上层SRR开口取向以及入射波电场方向不变的前提下, 进一步计算了上下层SRR之间相对开口朝向夹角φ从0°变化为180°时的超构分子的吸收谱的变化, 如图2(a)所示. 可以看出, 在每个开口朝向夹角的吸收谱中都展现出了类似图1(b)中的对称和反对称耦合模式所对应的两个吸收峰的现象. 并且, 随着开口朝向夹角从0°增加到60°, 两个吸收峰之间的距离逐渐变小, 在60°左右时达到最小, 继续增加开口朝向夹角从70°增加到180°, 两个吸收峰之间的距离又开始逐渐变大. 这是由于随着开口朝向夹角的变化, 双层开口环的电耦合和磁耦合共同作用导致耦合强度的变化所致. 基于流体力学的模型[26], 我们在FEM仿真软件COMSOL Multiphysics中将基波处SRR表面附近的电场转化为二阶非线性表面电流, 并以此作为产生SHG的源, 计算了不同开口朝向夹角下的SHG产生强度随基波波长的变化, 如图2(b)所示. 可以看出, 与超构分子线性吸收谱峰值之间距离的变化趋势相对应, SHG强度的峰值也随角度发生了先靠近后分开的情况, 这体现了基波处模式的耦合效应对于SHG具有显著的调控. 图 2 双层相对角度改变时, (a)吸收谱和(b) SHG强度的变化, 黑色划线代表每条线左右峰值的连线 Figure2. (a) Absorption spectrum and (b) the SHG intensity with the relative angle of the two layers changing. The black dash line represents the line connecting the left and right peaks of each line.
我们知道单个SRR结构在入射基波偏振沿着其底边方向时, 所产生的SHG也是一种线偏振, 并且电场振动方向与SRR底边相垂直, 即沿着SRR臂的方向. 而在由两个SRR组成的超构分子中, 我们已经在图1中展示了即使下层SRR无法被入射基波直接激发的情况下, 也可以通过先激发上层SRR的磁共振模式, 继而通过模式耦合作用从而将下层SRR的磁共振模式激发出来. 由于SRR中SHG辐射的电场振动方向是与其臂的方向平行的, 因此在我们研究的超构分子中, 上层SRR将提供SHG的y分量, 而调节下层SRR的开口方向, 使得上下两个SRR的开口朝向夹角不为0°或180°时, 下层SRR产生的SHG就会具有x分量, 这就为调控SHG的偏振态提供了一个可能的途径. 我们选取相对角度从30°到150°每隔30°变化的5个参数, 分别给出了这五种结构下的远场SHG强度、SHG辐射的y和x分量的振幅比Ey/Ex和相位差δ, 如图3(a)—(c)所示. 可以看到, 由于下层SRR的开口偏离了y轴, 引入了SHG在x方向的电场分量, 因而SHG在两正交方向上的电场不再以y分量为主导, 振幅比Ey/Ex总体在1左右变化. 与此同时, SHG辐射的电场两正交分量之间的相位差δ在所选波段内都有比较大的变化. 图 3 双层相对角度为30°—150°时的远场SHG强度 (a)、SHG辐射的y和x分量的振幅比Ey/Ex (b)和相位差δ (c)随着波长的变化; 双层相对角度为60°时的远场SHG强度 (d)、SHG辐射的y和x分量的振幅比Ey/Ex (e)和相位差δ (f)随着波长的变化; 其中阴影区域表示SHG效率较大的一段波长区域 Figure3. When the relative angle of the two layers changes from 30° to 150°, (a) SHG intensity, (b) amplitude ratio of SHG in the y and x directions and (c) the phase difference of SHG in the y and x directions as a function of wavelength; when the relative angle of the two layers is 60°, (d) SHG intensity, (e) amplitude ratio of SHG in the y and x directions and (f) the phase difference of SHG in the y and x directions as a function of wavelength. The shaded area indicates a wavelength region where the SHG efficiency is relatively large.
其中Ey和Ex分别是SHG电场的y和x分量, δ就是两个分量之间的相位差. 如前面所述, 在接近吸收峰时, 两个分量之间的相位差δ变化比较大. 我们在SHG产生效率较高的波段选择了6个特殊的波长1410, 1430, 1460, 1480, 1510以及1550 nm, 使得δ近似以90°的步长变化, 并分别求出这些波长处SHG的两个正交电场分量, 并和δ一起代入(1)式. 通过导入数值计算软件Matlab, 得到了SHG的偏振态对应的电场矢量轨迹, 结果如图4所示. 可以看出, 随着基波波长的改变, 激发出的SHG椭圆偏振态的长轴发生了旋转, SHG在远场出现了椭圆偏振、近线偏振和近似圆偏振, 实现了SHG偏振态的变化, 同时SHG的产生效率又保持在较高的水平. 图 4 双层相对角度为60°、不同波长的基波入射时, 远场SHG偏振态的变化 Figure4. Polarization of the far-field SHG changes when the relative angle of the two layers is 60°, and the the fundamental wave of different wavelengths is incident.
另外, 非线性偏振转换器件的工作环境有时需要固定基波波长就可以产生不同偏振态的非线性信号. 于是又按照图3选取了固定的基波波长1500 nm, 根据此波长处不同角度时的SHG强度和振幅比, 求出了不同角度下SHG的两个正交电场分量Ey和Ex, 并和相应的相位差δ一起代入(1)式. 再导入数值计算软件Matlab, 得到了固定波长下, 远场SHG偏振态随相对角度的变化, 结果如图5所示. 可以看出, 随着双层SRR相对角度的变化, 固定的基波激发出的SHG椭圆偏振态的长轴发生了旋转, 展示了近线偏振和椭圆偏振态的变化. 图 5 基波波长为1500 nm入射时, 不同相对角度的结构的远场SHG偏振态的变化 Figure5. When the incidentwavelengthis 1500 nm, the polarization of the far-field SHG changes with the relative angle of the two layers