拓撲光子晶體及微環共振器之光學特性

Abstract

本研究主要探討一維共軛拓撲光子晶體及微環共振器之光學特性，提出共軛拓撲界面態的新物理概念，並研究其界面反射相位、Zak相位及光子能帶理論。研究結果發現，光在共軛拓撲光子晶體結構中傳輸時，與傳統拓撲光子晶體不同，其具有更優異的拓撲保護特性及強烈的界面共振現象，不受製程缺陷及雜質等影響，可達到穩健地高品質及完美光傳輸性能，即使品質因子隨著光子晶體的週期層數增加，這些共振峰值仍然是完全穿透，此優異的光學特性可應用於先進光學濾波器、感測器、雷射共振光源及光通訊元件。此外，本研究還討論將單層石墨烯嵌入在非對稱共軛拓撲光子晶體的界面中，發現由於共軛拓撲界面態的激發，會產生強烈的石墨烯光吸收現象，此吸收光譜強烈地依賴石墨烯的費米能階和光子晶體的週期數，並透過調整石墨烯的費米能階，可迅速地控制光的吸收、反射及穿透等光學特性，此效應的發現可大幅提升光學感測器、光吸收器及光開關切換之效能。最後，本文還理論驗證微環共振器亦存在共軛拓撲界面態，並在環徑填充因子互為共軛條件況下，具有超高品質因子及完美穿透之光學特性，與層狀拓譜光子晶體相較其光學性質更優異，這些優良的光學特性可大幅改善傳統光子晶體中，由於高質因子增加所導致光穿透降低的問題。

Optical properties of conjugated topological photonic crystals (CTPC) and microring resonators are the subjects for study in this dissertation. A novel concept of conjugated topological interface states (CTIS) is first proposed. The effect of its topological phenomenon on high quality factor (High-Q) and absorption enhancement is also investigated. The interface reflection phase, Zak phase and photonic band-gap are developed to verify the existence of CTIS. The study results show that the robust high- quality factor with perfect transmission through one-dimensional CTPC is obtained. It is also found that even though the quality factor of resonances increases as the periodic number of multilayers increases, these resonances are still perfect transmission. Moreover, a monolayer graphene embedded in the interface of asymmetric conjugated topological photonic crystals (ACTPC) is studied. A strong absorption phenomenon occurs by the excitation of CTIS. It is found that the absorption spectra are intensively dependent on the Fermi energy of graphene and the periodic number of the ATPC. Therefore, the absorption can be rapidly switched in a slight variation of chemical potential, which is modulated by the applied gate voltage on graphene. The result shows great promise for applications in advanced optical filters, sensors and absorbers. Finally, optical properties of topological microring resonators are also discussed. The existence of topological interface states in the microring resonators is first theoretically demonstrated. It is found that an ultra-high-Q of more than 1012 with complete transmission is obtained by microring resonators from the excitation of CTIS. Therefore, the problem of transmission decreases resulting from high quality factor increases in the traditional photonic crystals is significantly improved by this approach. These extraordinary optical properties of CTIS contribute to the understanding of topological microring resonators and open up potential applications in topological devices.