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Topological Light at Structured Surfaces

Periodic Reporting for period 4 - TOPOLOGICAL (Topological Light at Structured Surfaces)

Reporting period: 2020-06-01 to 2020-11-30

The overall objective of this project is to design and engineering novel types of artificial structures - so called metamaterials, for controlling the propagation of light in an unconventional way, by incorporating topological physics into the metamaterial design. By using metamaterials and metasurfaces as platform, this proposal focuses on the novel topological physics and applications introduced by Berry phase. The flexibility in engineering the artificial ‘atoms’ and ‘molecules’ of metamaterials provides unlimited possibilities to create new structural effect where symmetry (or symmetry breaking) and topology play critical roles. We are particularly interested in the role Berry phase plays in various nontrivial surface optical effects, including topological surface states and spin Hall effect of light. The investigation of the scattering immune surface states in a topological metamaterial, i.e. an effective medium approach, acts to unify the spin Hall effect of light with the more unconventional scheme of topological orders and protected surface states. We will further exploit Berry phase in the nonlinear regime, in particular harmonic generations, to control the nonlinear polarizations to an unprecedented level. Hence our study on Berry phase in the nonlinear regime will point to a new research direction on nonlinearity coefficient engineering, which will have important impact in the area of nonlinear optics. The proposal also investigate into practical applications brought by a novel type of geometrical metasurfaces, where the phase and hence the wavefront are finely controlled by the Berry phase in a highly robust manner. The proposal involves the development of innovative synthesis technologies, theoretical analysis, numerical simulations, experimental characterizations, and device development. The new symmetry and topological effects in this research will greatly impact a number of disciplines including material science, condensed matter physics and photonics. The findings of this project may be applied in various applications such as chemical and bio sensing, medical imaging, optical communications.
TOPOLOGICAL has had a very successful start and is progressing more than planned. Before giving a more detailed overview I briefly summarize the major achievements to date. Staffing and training: I recruited four highly talented post-doctoral research assistants (Dr. Hongchao Liu, Dr. Wenlong Gao, Dr. Changxu Liu, Dr. Peng Mao). They have made tremendous contribution to working packages 1, 2, 3 and 4. Work Packages: Significant progress has been made in all working packages. Regarding work package 1, we have designed realistic topological metamaterials exhibiting type II weyl degeneracies. We have also theoretically proposed Weyl degeneracies could exist in magnetized plasma. Concerning work package 2, we have demonstrated dynamically tunable metasurface holograms, spin and wavelength multiplexed nonlinear holography. Regarding work package 3, we have developed large area fabrication of metasurfaces using microsphere lithography and metal cluster deposition. We have also demonstrated 3D fabrication of nanostructures. Finally, regarding work package 4, we have successfully measured Fermi arc like surface states on the surface of topological metamaterials at the microwave frequencies. We have demonstrated ideal Weyl metamaterials, and the associated chiral zero mode when a gauge field was applied. We have demonstrated non-abelian topological nodal links and non-abelian topological charges. We have demonstrated linked Weyl surfaces and Weyl arcs in synthetic 5D momentum space. Dissemination: The results produced during the period led to 76 journal publications including 3 in Science, 1 in Nature, 9 in Nature Communications, 9 in Physical Review Letters, 6 in Advanced Materials, 7 in Nano Letters, 5 in Light: Science & Applications, 1 in Nature Reviews Materials and 6 in Science Advances. In addition, we have 6 papers under review and many in preparation. The results of the project have been presented at many international conferences and workshop, as well as departmental talks by the PI.
During the project, we have pioneered the realization of topological photonics using the effective medium, i.e. metamaterial approach. In comparison to the commonly studied photonic crystal approach, our topological metamaterials have unit cells much smaller than the wavelength of light, therefore the topologically protected surface wave supported by the topological metamaterials can be more tightly confined to its interface. The topological metamaterials can be potentially used as the building blocks for realizing three dimensional photonic integrated circuits, where light signals can flow freely not only in the in-plane directions, but also vertically into other optical circuit layers without experiencing scattering. This will have a fundamental impact on industrial sectors such as optical communications, optical information processing etc.