Ideal Phase-Free Wave Propagation in Air Channel

Despite many intriguing approaches to realize zero-index media by metamaterial, a more common and attractive way to observe phase-invariant phenomenon at finite frequency is actually by the photonic waveguide mode at the cutoff frequency, which can reside in air channel surrounded by photonic band gap media. However, the group velocity of the gap-confined waveguide mode goes to zero (∂ω/∂k = 0) for k = 0 at the cutoff frequency, making it extremely difficult to excite and transport information by the phase advance-free propagation. In this work, we propose a dielectric photonic crystal platform for realizing an ideal air-concentrated phase-free waveguide. The waveguide is designed as a sandwiched structure like a photonic crystal (PC) line defect (e.g., PC-air-PC), which supports a pair of bound states in the continuum (BICs) at the PC-air domain, as well as multiple orders of gap-confined waveguide modes (WMs) inside the air channel. It is shown that the two BIC field distributions are allowed to be of symmetric and antisymmetric characteristics and are respectively coupled to the first- and second-order waveguide mode around k = 0. The interaction between these modes results in a linear band crossing (Δω ∼ k) at finite frequency, leading to the ideal phase-free wave propagation. We numerically and experimentally show the ideal phase-free propagation of the first- and second-order waveguide modes in the air channel. Our work provides an alternative way toward ideal phase-free waveguide, and the results may be of potential application for on-chip communication, qubit entanglement, precise signal processing, etc.