[论文解读] Special polarization characteristic features of a three-dimensional terahertz photonic crystal with a silicon inverse diamond structure

Chikara Sakurai|arXiv (Cornell University)|Mar 24, 2017

Photonic Crystals and Applications被引用 3

一句话总结

本研究调查了在约0.4 THz处具有完整光子带隙的三维硅反尖晶石光子晶体中的偏振各向异性。利用太赫兹时域光谱法，实验表明在0.38 THz处，入射S偏振波（取向I(1,0)或I(0,−1)）几乎完全转换为P偏振反射波，而I(1,1)和I(1,−1)取向则保持偏振状态，表明在带隙内具有强烈的偏振依赖响应。

ABSTRACT

The band structure of an Si inverse diamond structure whose lattice point shape was vacant regular octahedrons, was calculated using plane wave expansion method. A complete photonic band gap was theoretically confirmed at around 0.4 THz. It is said that three-dimensional photonic crystals have no polarization anisotropy in photonic band gap (stop gap, stop band) of high symmetry points in normal incidence. However, it was experimentally confirmed that the polarization orientation of a reflected wave was different from that of an incident wave, [I$(X,Y)$], where $(X,Y)$ is the coordinate system fixed in the photonic crystal. It was studied on a plane (001) at around X point's photonic band gap (0.36 $-$ 0.44 THz) for incident wave direction [001] by rotating a sample in the plane (001), relatively. The polarization orientation of the reflected wave was parallel to that of the incident wave when that of the incident wave was I(1, 1) or I(1, $-$1). In contrast, the former was perpendicular to the latter when that of the incident wave was I(1, 0) or I(0, $-$1) at around 0.38 THz. As far as the photonic crystal in this work is concerned, method of resolution and synthesis of the incident polarization vector is not able to apply to the analyses of rotation of the measured reflected spectra in appearance.

研究动机与目标

• 研究具有完整光子带隙的三维太赫兹光子晶体中的偏振各向异性。
• 确定在光子带隙区域内，反射波的偏振取向是否与入射波不同。
• 分析在(001)平面上不同入射偏振态（I(1,1)、I(1,0)、I(1,−1)、I(0,−1)）下反射波的行为。
• 评估标准偏振矢量分解与合成方法在解释观测到的光谱旋转时的局限性。

提出的方法

• 使用平面波展开法计算晶格常数为300 µm、八面体空位为150 µm的Si反尖晶石结构的光子能带结构。
• 制备了48层硅样品，通过周期性(001)面图案化形成在约0.4 THz处具有完整带隙的三维光子晶体。
• 采用太赫兹时域光谱法（THz-TDS），以7°入射角测量在受控入射偏振态下的反射光谱。
• 使用1/2波片选择性地透射和分析反射波的S和P偏振分量，从而分离偏振贡献。
• 使用金膜参考进行归一化反射率光谱，并应用光谱分解以在反射信号中分离S-p和P-p分量。
• 通过比较有无波片时的反射率光谱进行光谱分析，以提取偏振依赖的响应特性。

实验结果

研究问题

• RQ1 在约0.4 THz处具有完整光子带隙的三维Si反尖晶石光子晶体中，反射波是否表现出偏振各向异性？
• RQ2 反射波的偏振取向如何依赖于(001)平面上入射波的偏振方向？
• RQ3 为何对于某些入射偏振态（如I(1,0)）尽管晶体具有高对称性，反射波的偏振仍发生90°旋转？
• RQ4 标准偏振矢量分解与合成技术能否解释观测到的反射波光谱旋转？
• RQ5 在约0.38 THz处对I(1,0)和I(0,−1)入射波观察到强烈偏振转换的物理起源是什么？

主要发现

• 在具有空置规则八面体晶格点的Si反尖晶石结构中，确认了约0.4 THz处的完整光子带隙（CPB）。
• 对于入射偏振态I(1,1)和I(1,−1)，反射波保持与入射波平行的偏振取向，表明无旋转。
• 对于入射偏振态I(1,0)和I(0,−1)，反射波表现出90°偏振旋转，且在0.38 THz处S-p入射波几乎完全转换为P-p反射波。
• 在BGX（0.36–0.44 THz）带隙内的0.38 THz处，I(1,0)的反射波主要为P偏振，表明存在强烈的偏振转换。
• 观测到的偏振旋转无法用标准偏振矢量分解与合成方法解释，表明存在非平凡的内在机制。
• 光谱分解证实，I(1,0)的反射信号中包含显著的P-p分量，而I(1,1)则主要由S-p分量主导，验证了偏振依赖响应。