基於相變化材料之主動調變式相位梯度超穎界面

Active gradient-phase metasurface based on phase-change material Ge2Sb2Te5

陳婕(Jie Chen)

指導教授 ： 蔡定平

國立臺灣大學/理學院/物理研究所/碩士(2015年)

https://doi.org/10.6342/NTU.2015.02813

摘要

超穎材料因為本身所特有的自然界中不存在的光學性質而備受關注，被運用在了隱形材料、負折射材料等研究中。超穎界面是其中一個重要的分支：它將超穎材料與超穎器件連接在一起，衍生出了平面透鏡、超穎全像片以及超穎光學元件等研究。在最近的一些研究中，科學家們運用相變化材料實現了對超穎材料異常光學現象的主動控制。在眾多的相變化材料中，鍺銻碲合金 ( Ge2Sb2Te5 ) 由於其良好的穩定性、極短的轉態時間（飛秒量級）以及不同相態之間光學常數、電阻值的明顯差異，被運用在了高密度的光存儲、相變化記憶體以及奈米微影技術中。
本論文運用有限元(Finite Element Method, FEM)以及時域有限差分法(Finite-Difference Time-Domain, FDTD)的模擬方法，對相變化材料鍺銻碲合金運用到主動式相位梯度超穎界面進行研究。我們在光通訊波段（1550 奈米）設計了一種主動式調變的具有異常反射現象的超穎界面。通過對不同結構尺寸的鍺銻碲合金的組合以及雷射對特定位置的鍺銻碲合金結構的相態控制，實現不同角度的反射現象：當所有結構都為晶態時，反射現象為正常反射現象，符合傳統的光學定律；當部分結構為晶態，部分結構為非晶態時，反射光會沿著偏離傳統的反射光線19度或是40度的角度方向出射。產生異常反射現象的原因是由於電磁波對不同尺寸、不同相態的結構有不同的相位延遲。對特定的結構進行排列，可以藉由人工的方式控制波前，從而實現異常反射的現象。利用雷射控制特定位置的鍺銻碲合金奈米結構的晶態發生改變，等效於改變超穎界面的排列方式。這可以使得相位調控單元的週期發生變化，從而改變異常反射的角度。
與傳統貴重金屬構成的超穎界面相比，引入了材料的相態作為變數，我們只需要兩種尺寸就能達到三階的相位調製效果。依賴相變化材料的相態變化，我們可以通過改變相位調控單元週期實現主動式的調變效果：實現一個可以在不同反射角之間切換的相位梯度超穎界面。這克服了傳統超穎界面難以實現主動變化的缺點，未來可以運用在光通訊波段的主動式奈米元件中。

關鍵字：超穎界面 相變化材料 主動調製

關鍵字

超穎界面 ； 相變化材料 ； 主動調製

並列摘要

Metamaterial is a hot topic in these years which is applied to the researches of clocking, negative refraction, relying on its non-existing proprieties in nature. Metasurface is an important part connecting metamaterial with metadevice, contributing to a great many of applications, such as flat lens, metahologram and some meta-optical-component. Recently, Phase-change materials are applied to active metamaterial due to their distinctions in optical constant between crystal state and amorphous state. Ge2Sb2Te5 (GST) alloy is widely used in optical data storage, phase change memory and nanolithography owing to its nature of stability, quick response (femtosecond order) to external stimuli and dramatical difference in optical constant and electrical resistance between two phases.
We present an active gradient-phase metasurface design based on algorithms of Finite Element Method (FEM) and Finite-Difference Time-Domain (FDTD). An active gradient-phase metasurface made of phase-change material exhibiting normal or abnormal reflection working at communication frequency (around 1550 nm) is achieved. When GST alloy nanostructures are all in crystal state, there is a normal reflective phenomenon following traditional laws. Under an external stimuli, an anomalous reflected beam can be detected in the angle of 19 or 40 degree off the traditional light path. The difference of the geometry and phase-state leads to a distinct phase delay. With a specific arrangement, the wavefront can be reshaped and the metasurfce give rise to an abnormal reflected phenomena. The phase-state of GST rods can be changed to attain a period variation of phase modulation by fs-laser process. In a word, there is a three-level phase modulation when elements are partly in amorphous and partly in crystal state.
Here, we achieve a three-level phase modulation only relying on two different geometry by introducing phase-state of PCMs. Meanwhile, an active phase-gradient metasurface is realized to improve tunability of metamaterial which is potential for active metadevice in optical communication process.

Key words: Metasurface, Phase-change materials, Active modulation

並列關鍵字

Metasurface, ； Phase-change materials, ； Active modulation

參考文獻

[29] L. Huang, X. Chen, H. Mühlenbernd, H. Zhang, S. Chen, B. Bai, Q. Tan, G. Jin, K. Cheah, C. Qiu, J. Li, T. Zentgraf and S. Zhang, “Three-dimensional optical holography using a plasmonic metasurface,” Nature communications 4, 3808 (2013).

[31] W. T. Chen, K. Y. Yang, C. M. Wang, Y. W. Huang, G. Sun, I. D. Chiang, C. Y. Liao, W. L. Hsu, H. T. Lin, S. Sun, L. Zhou, A. Q. Liu and D. P. Tsai, “High-efficiency broadband meta-hologram with polarization-controlled dual images,” Nano letters 14 225-230 (2013).

[1] V. G. Veselago, “The electrodynamics of substances with simultaneously negative values of ϵ and μ,” Soviet physics uspekhi 10, 509-5141968 (1968).

[2] J. B. Pendry, A. J. Holden, W. J. Stewart and I. Youngs. “Extremely low frequency plasmons in metallic mesostructures,” Physical review letters 76, 4773 (1996).

[3] J. B. Pendry, A. J. Holden, D. J. Robbins and W. J. Stewart. “Magnetism from conductors and enhanced nonlinear phenomena,” Microwave Theory and Techniques, IEEE Transactions on 47, 2075-2084 (1991).