Figure 1

(a) Photo of the photonic crystal (before metallization) which is a simple cubic lattice structure with lattice constant, \({\text{a}} = 580\;\upmu {\text{m}}\), and the width of the square rod, \({\text{d}} = 130\;\upmu {\text{m}}\). The picture of the unit cell is shown in (d). The sample is about 30 mm × 30 mm in transverse size and about 5 mm (9 unit cells) in thickness. (b) Band structure along high symmetric paths in the Brillouin zone, where the 3rd and 4th bands are degenerate at 0.341THz. The positions of high symmetric points in the Brillouin zone are labeled in (c). (c) Three-dimensional view of band 3 and 4 in the first Brillouin zone around 0.341THz, where the degeneracy presents the nodal chain geometry. (d) The arrow plots of the electric field eigenmodes of band 3 and 4 at the degenerate point along XM path, as labeled by orange dot in (b).