講演情報
[22a-A308-6]Design of post-based triangular-lattice topological photonic crystal with broken time-reversal symmetry
〇(D)Guangtai Lu1, Siyuan Gao1, Yasutomo Ota2, Satoshi Iwamoto1 (1.Univ. Tokyo, 2.Keio Univ.)
キーワード：
photonic crystal、topological photonic crystal
The realization of topological photonic crystals (PhCs) with broken time-reversal symmetry is one of the most attractive topics in topological photonics due to their ability to support chiral edge states and to resultant topologically protected unidirectional propagation. However, studies on such systems in the optical regime have been limited: they are based on square-lattice PhCs or on hole-based triangular-lattice PhCs. Here, we propose a design of triangular-lattice topological PhC with broken time-reversal symmetry using a post-based hybrid structure, which can have a topologically nontrivial band gap in TE-like modes.
Our structure consists of silicon (n=3.4) posts with a thickness of 250 nm bonded to a thin film yttrium iron garnet (YIG, permittivity tensor [4.84, 0.002i, 0; -0.002i, 4.84, 0; 0, 0, 4.84]), with a thickness of 50nm. We arrayed triangle-shaped posts (side length: 700nm) with a small cylinder (radius: 40nm) at each corner in the triangular lattice (lattice constant: 1125nm). Applying an external magnetic field perpendicular to the structure lifts the degeneracy between the 3rd and 4th bands at G point and opens a ~85-pm-wide gap in the near infrared. Without the small cylinders, no bandgap is obtained because the bulk modes at around M point overlap with the gap around G point. The distributions of Berry curvature and the Chern numbers for the bands below the gap are calculated. The total Chern number below the bandgap is -1, indicating the gap is a topologically non-trivial complete bandgap. We expect the proposed structure can be fabricated by using the technology of making a thin YIG film.
Our structure consists of silicon (n=3.4) posts with a thickness of 250 nm bonded to a thin film yttrium iron garnet (YIG, permittivity tensor [4.84, 0.002i, 0; -0.002i, 4.84, 0; 0, 0, 4.84]), with a thickness of 50nm. We arrayed triangle-shaped posts (side length: 700nm) with a small cylinder (radius: 40nm) at each corner in the triangular lattice (lattice constant: 1125nm). Applying an external magnetic field perpendicular to the structure lifts the degeneracy between the 3rd and 4th bands at G point and opens a ~85-pm-wide gap in the near infrared. Without the small cylinders, no bandgap is obtained because the bulk modes at around M point overlap with the gap around G point. The distributions of Berry curvature and the Chern numbers for the bands below the gap are calculated. The total Chern number below the bandgap is -1, indicating the gap is a topologically non-trivial complete bandgap. We expect the proposed structure can be fabricated by using the technology of making a thin YIG film.