Figure 1.
From: Fiber guiding at the Dirac frequency beyond photonic bandgaps
Cross-section and band structures of photonic crystal fiber. (a) A typical cross-section of photonic crystal fibers, where the lattice constant is a=2.21 μm, the hole radius is ra=0.47a, and the central hollow defect radius is R=1.9a. The fiber is made of either silica glass (εr=1.452), germania glass (εr=1.592), or SF6 glass (εr=1.82). The inset in a is the first Brillouin zone (shaded light gray) of the reciprocal lattice for the triangular lattice of air holes, revealing the high-symmetry points Γ, M, K at the corners of the irreducible Brillouin zone (shaded dark gray). (b–d) Band structures of the fiber cladding made of germania glass, at kza/2π=0.001 (b), kza/2π=1.581 (c), and kza/2π=1.910 (d). The four lowest order Dirac points are indicated by red arrows. A well-isolated, wide-open Dirac spectrum is shown in d in the band structure for kza/2π=1.910. The inset to d shows an enlarged three-dimensional view around the second Dirac point, where the two bands touch as a pair of linear Dirac cones at the six Brillouin zone corners.
