Yandong Li

Yandong Li in the lab
  • Hometown: Nanjing, China
  • Applied and Engineering Physics

Yandong Li is a Ph.D. student in Prof. Gennady Shvets’ group who focuses on topological photonics, a new field that is inspired by condensed-matter physics and explores how to robustly control the behavior of light. This field is considered by many to still be fundamental research: currently in the early, theoretical stage and just starting to be applicable to real-life scenarios. In the recent decade, researchers in physics, engineering, material science, etc., have shown how topological insulators for photons— analogous to those for electrons—- can be created. “One of the main themes of my Ph.D. study is about pushing the field of topological photonics one step further toward application,” says Li.

Li has designed several topological photonic structures, including a topological cavity that was published in Physical Review Letters1. This study introduced a novel way of achieving optical energy confinement: by delaying the reflection at a mirror surface. Another design, published in and featured on the cover of Applied Physics Letters2 (Volume 120, Issue 22), demonstrated a practical and convenient way of selectively exciting a specific topological mode. Because of the peculiar electromagnetic field distribution of topological modes, previous research often used a phased array to selectively excite one of them. Li’s experiment used only one linearly polarized dipole-like source. Most of Li’s designs can be realized with silicon photonics and integrated into photonic circuits, which may lead to more compact devices for photonic information processing and quantum computing.

Li describes the procedure of designing a topological photonic component as having three steps: the first step is to understand the physics. Topology in condensed-matter physics is about how the properties of a system withstand adiabatic changes. After understanding the physics, the next step is to design a photonic structure that realizes the physics and, more importantly, has potential applications. This step is where problems usually arise because, in some cases, such a design is too complicated to be carried out in a university laboratory. The final step is to perform an experiment to demonstrate and test the design, using the microwave/RF and optical equipment in Prof. Shvets’ lab.

 “To push topological photonics toward application, I examined several realistic problems in the course of my Ph.D.,” said Li. “For example, I analyzed to what extent topological properties can be preserved in a finite-sized photonic structure, and to what extent the functionalities of a topological photonic component can be tuned.”
In recent months, his research has extended to processing and programming light, using topological photonic logic gates. Concurrent with his Ph.D., he has pursued a minor in computer science. Believing in the great power of large-scale matrix computation, physics-informed knowledge discovery, and optimization, Li wishes to pursue a postdoctoral study where he may contribute his knowledge in photonics and scientific computing to discovering novel phenomena and designing new photonic devices.

Li greatly enjoys the collaborative environment in Prof. Shvets’ group. He believes that the students’ in-depth understanding of physics, the advisor’s guidance, and the inter- and intra-group collaborations are key factors for making progress in research. He added, “the discussions in group meetings, professor’s advice, and the after-meeting communications between the students have always been inspiring me and helping me throughout my Ph.D. research.”

Image featured on the cover of Applied Physics Letters and described in the caption.
This figure, featured on the cover of Applied Physics Letters, diagrams how topological photonics provide an alternative way of thinking about propagation robustness, reflection suppression, and other empirically useful phenomena that can be engineered through preserving binary DoFs. For example, a domain wall between two photonic crystals with different topological indices can support topologically robust edge or kink (TREK) states. These TREK states are associated with conserved DoFs and do not suffer from back-scattering. The figure shows the photonic structure for valley-selective splitting of the TREK states. Key—Blue (red) circles: spin photonic crystals with positive (negative) spin indices; Yellow tripods: valley photonic crystals with reconfigurable orientations. The valley-selective emitter is embedded inside the topological multimode waveguide. Port 1 and Port 2: receiving ports; Orange (cyan) arrows: Ψ↑K' (Ψ↑K) TREK states.

1Topology-Controlled Photonic Cavity Based on the Near-Conservation of the Valley Degree of Freedom. Yandong Li, Yang Yu, Fengyu Liu, Baile Zhang, and Gennady Shvets. Physical Review Letters (Editor’s Suggestion) 125, 213902 (2020)
2 Mode-selective single-dipole excitation and controlled routing of guided waves in a multi-mode topological waveguide. Yandong Li, Yang Yu, Kueifu Lai, Yuchen Han, Fei Gao, Baile Zhang, and Gennady Shvets. Applied Physics Letters. (Featured as cover page) 120, 221702 (2022)


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