Chiral Topological Surface States on a Finite Square Photonic Crystal Bounded by Air

Anna C. Tasolamprou, Maria Kafesaki, Costas M. Soukoulis, Eleftherios N. Economou, and Thomas Koschny
Phys. Rev. Applied 16, 044011 – Published 11 October 2021
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Abstract

Chiral, topologically protected, photonic surface states can be found at the boundary between gyrotropic photonic crystals where a changing magnetic field induces different topology across the interface. Typically, photonic crystals with either a suitable band structure on both sides of the interface to provide a band gap and evanescent decay of the surface states away from the interface, or an outer layer with engineered material properties is required. In this paper, we show the emergence of topological, unidirectional surface states at the termination of finite gyrotropic photonic crystals with a simple square lattice and C4 rotational symmetry bounded by a vacuum, eliminating the need for an outside layer to enable chiral surface modes. We start from an infinite, time-reversal-symmetry-breaking photonic crystal with a band gap associated with bands with nonzero Chern numbers, different from all-zero Chern numbers in air. We then modify the photonic crystal to move this band gap below the light line, while maintaining the Chern-number discontinuities. Band-structure calculations for a supercell approximating a photonic crystal finite in the direction normal to the surface demonstrate the existence, dispersion, and chirality of the surface mode. Extensive direct scattering calculations for a point source and spatial Fourier analysis further reveal a unidirectional free-space topological surface state, which propagates counterclockwise around the surface of a finite photonic crystal, providing a nearly foolproof way to cross-check the surface-mode band structure unaffected by backscattering from local defects. Additionally, scattering simulations allow an independent characterization of the state dispersion and unveil the robustness of the topological plasmonic mode propagation around the 90 bends of the structure, being due to only radiation leakage. In contrast to buried topological surface states, the observed surface modes at the photonic crystal–air interface have the advantage of being accessible to the outside world, allowing one to take advantage of the defect-tolerant backscattering-free surface modes to engineer emission from photonic crystal surfaces into arbitrary free-space beam shapes and directions.

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  • Received 23 December 2020
  • Revised 30 July 2021
  • Accepted 16 September 2021

DOI:https://doi.org/10.1103/PhysRevApplied.16.044011

© 2021 American Physical Society

Physics Subject Headings (PhySH)

General Physics

Authors & Affiliations

Anna C. Tasolamprou1,*, Maria Kafesaki1,2, Costas M. Soukoulis1,3, Eleftherios N. Economou1,4, and Thomas Koschny3

  • 1Institute of Electronic Structure and Laser, Foundation for Research and Technology Hellas, 100 Nikolaou Plastira Street, 700 13 Heraklion, Greece
  • 2Department of Materials Science and Technology, University of Crete, 700 13 Heraklion, Greece
  • 3Ames Laboratory and Department of Physics and Astronomy, Iowa State University, Ames, Iowa 50011, USA
  • 4Department of Physics, University of Crete, 700 13 Heraklion, Greece

  • *atasolam@iesl.forth.gr

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Vol. 16, Iss. 4 — October 2021

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