基于压电压磁超晶格的光子拓扑绝缘体研究

The topology of electronic systems has recently attracted much attention in condensed matter physics research because of its unique properties including robust unidirectional chiral edge states in quantum Hall system and time-reversal symmetry protected edge states in quantum spin Hall (QSH) system. The discovery of QSH system has led to a new state of matter, i.e. topological insulators, in which robust non-zero spin current together with zero net electric current is supported by the helical spin gapless edge states resulted from the strong spin-orbit coupling. Motivated by these novel physics, researchers have begun to explore non-trivial photonic topological states due to the inherent similarity between electrons and photons. Recent study presented an approach of achieving photonic topological insulator in magneto-electric coupled metamaterials. Two gapless hybrid states are constructed, constituting Kramers partners of the photonic states, analogous to spin-up and spin-down states in electronic systems. However, photon is Bose which is different from electron as Fermion. We found that some time-reversal impurities, such as chiral or pure dielectric impurities, would break the backscattering immune property. Thus, the symmetry, which is responsible for the protection of the robustness of photonic topological states, is still unclear and remains to be investigated. We propose a photonic topological insulator in a two-dimensional photonic crystal composed of piezoelectric and piezomagnetic superlattices. The electromagnetic (EM) waves can couple with the vibration of this piezoelectric and piezomagnetic superlattices to form phononic polaritons. The electric and magnetic fields of EM waves can transfer to each other via such phononic polaritons analogous to the spin and orbit coupling of electrons. We plan to study the general symmetry protected mechanism of photonic topological insulator. Our proposal may provide a viable way to exploit the photonic topological property, and also can be leveraged to develop a photonic platform to mimic the spin properties of electrons.

物质量子态拓扑性质的研究将凝聚态原子物理带入了一个新的阶段，拓扑描述和拓扑分类，如电子系统中整数量子霍尔效应和拓扑绝缘体。在光学方面，人工电磁结构的研究常常借鉴于电子系统，寻找原子物理的光学对应。研究表明磁电耦合的光子晶体中的无能隙线性偏振边界态可类比拓扑绝缘体。但是，作为波色子的光子和作为费米子的电子的时间反演算符并不相同，我们发现即使是符合波色子时间反演对称的杂质也会破坏其拓扑保护性质。本项目拟借助电磁波在压电压磁超晶格中所形成的声子极化激元，分析其晶格振动带来的电磁能量转换实现自旋轨道耦合，并引入宇称－时间反演对称性和本征拓扑序的分析，研究可类比电子系统的光子宏观类量子效应和其拓扑保护机制探究人工超构材料能带的拓扑分类。发现其中可能存在的诸如光自旋量子霍尔效应，光的拓扑绝缘体，甚至声的拓扑绝缘体，并探索光二极管，光晶体管和光隔离器等原理型光子逻辑器件的开发。

本项目主要关注可类比电子系统中的光/声拓扑态问题。经过三年的研究，主要取得以下进展：.1、基于压电/压磁超晶格光子晶体中的声子极化激元，提出了一种时间反演对称性破缺的光拓扑绝缘体，指出实现玻色子拓扑绝缘体的关键是人工构造赝时间反演对称性。超晶格的晶格振动可与入射电磁波耦合形成极化激元。且具有偏振依赖性：左旋光和右旋光与之耦合所形成的极化激元具有大小相同但符号相反的耦合系数，即偏振-轨道耦合，经历相反的等效规范场，从而实现光拓扑绝缘体。更为重要的是，其中的磁电耦合使得左、右旋光满足赝时间反演算符Tp（Tp2 = -1），确保了Kramers简并。理论和模拟分析证明：光拓扑绝缘体的拓扑性质受人工构造的赝时间反演对称性Tp保护，而不是通常认为的玻色子时间反演对称性Tb。进一步研究发现：玻色子时间反演对称对于设计和构造光拓扑绝缘体既不充分也不必要。时间反演对称与否只反映系统是否需要外加“磁场”或存在“内在磁化”，若解除这一条件，则可获得更多自由度，构造更易于实现的材料以调控拓扑光子态。.2、首次成功研制声拓扑绝缘体并实验验证了声量子自旋霍尔效应和背散射免疫的声“鲁棒”传输。我们基于石墨烯晶格的对称性在布里渊区中心处构造了p带和d带偶然简并的双重狄拉克锥，在晶格对称性保护下，利用该点处p带和d带声布洛赫态之间的杂化，构造了纵声波的两种赝自旋态，通过赝自旋和轨道的相互作用实现了p-d能带反转，进而构造声拓扑绝缘体的新机制。实验证明，在拓扑波导中加入空腔、无序和弯曲等缺陷，声波拓扑边界态均可无背散射的通过，具有鲁棒性，而常规界面波导中声波则被强烈的反射。同时，申请人团队巧妙地构造了一种“x”型的分路器模型，验证了声自旋量子霍尔效应和声拓扑边界态的手征螺旋传播。. 3、实验实现了时间-宇称同时破缺的电磁波单向负折射效应。.本项目总共发表项目相关论文8篇，他引100余次，相关成果被国内外多家科技媒体报道，取得了一定的国际影响。