Topological Valley Photonics: Physics and Device Applications

By Haoran Xue, Yihao Yang, Baile Zhang. Advanced Photonics Research, 2021

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Since the pioneering works of Yablonovitch and John in 1987, it has been recognized that periodic photonic structures have the remarkable capability of manipulating the flow of light in much the same way, as semiconductors steer the passage of electrons in an electric current. As this idea was inspired by the concept of energy bandgap in semiconductors, Yablonovitch and John decided to name these periodic photonic structures as photonic crystals. Extending their insight, studies in photonic crystals in the past three decades have remarkably changed the physics and applications in photonic communication and information processing with various photonic-crystal devices. Traditional physics and device applications of photonic crystals are based on utilizing the limited photonic degrees of freedom (DOFs), such as frequency, polarization, and phase. In fact, electrons can also have additional DOFs, such as spin and valley. The manipulation of spin DOF of electrons, rather than the charge current, for information storage, transmission, and processing, has led to the blooming field of spintronics. A strong spin– orbital coupling, as in the quantum spin Hall (QSH) effect, can lead to a class of exotic materials called topological insulators, which are some narrow-gap semiconductors that are insulating in the bulk but conduct electricity along the surfaces via a class of topological edge states. These exciting discoveries in semiconductor electronics have also started a new chapter in photonics by stimulating the emerging field of topological photonics. In particular, many “photonic topological insulators” have been proposed and demonstrated as analogues of topological insulators, where the electronic spin DOF can be simulated by constructing photonic pseudospin. For example, in photonic topological insulators made up of bianisotropic metamaterials, photonic pseudospins can be constructed from superposition of different polarizations of light, and bianisotropy can play the role of spin–orbital coupling. A valley, which generically emerges at high-symmetry points of the Brillouin zone, refers to a local minimum in the conduction band or a local maximum in the valence band. In addition to charge and spin, an electron in a lattice is also endowed with a valley DOF, which specifies the valley that the electron occupies. As shown in Figure 1a–c, a 2D honeycomb lattice with broken inversion symmetry (I) contains K and K 0 valleys whose lowenergy excitations carry an angular momentum that produces a spin-like magnetic moment, thus the name of “valley pseudospin.” The possibility of using the valley DOF to store and carry information (similar to spin in spintronics) has led to conceptual electronic applications known as valleytronics. The recent explosion in valleytronics has been largely enabled by the invention of graphene and, subsequently, many other 2D materials. Dr. H. Xue, Dr. Y. Yang, Dr. B. Zhang Division of Physics and Applied Physics School of Physical and Mathematical Sciences Nanyang Technological University Singapore 637371, Singapore E-mail: haoran001@e.ntu.edu.sg; blzhang@ntu.edu.sg Dr. Y. Yang Interdisciplinary Center for Quantum Information State Key Laboratory of Modern Optical Instrumentation ZJU-Hangzhou Global Science and Technology Innovation Center Zhejiang University Hangzhou 310027, China E-mail: yangyihao@zju.edu.cn Dr. B. Zhang Centre for Disruptive Photonic Technologies Nanyang Technological University Singapore 637371, Singapore

Subfields: Artificial Intelligence, Atomic and Molecular Physics, and Optics

View Paper: https://www.semanticscholar.org/paper/47cd1fce2c5cf70960e9c0e29e9335b2383a317c