Harnessing the Frozen-mode in Coupled Silicon Ridge Waveguides for True Time Delay Applications

Summary form only given. Periodic material arrangements and sophisticated metamaterial structures have enabled novel ways of manipulating light by engineering their dispersion to achieve unconventional functionality. Extraordinarily-slow or frozen-modes stemming from mode degeneracy in anisotropic magnetic photonic composites are among the most exotic manifestations of dispersion engineering. In particular, the frozen modes are only enabled by the simultaneous breakdown of time and space inversion symmetries in bulk anisotropic metamaterials. As such, they must include biased magnetic materials in their construction [1]. However, the same frozen-mode phenomena can be more readily achieved in all-dielectric light waveguides by introducing more degrees of freedom using multiple coupled sections. With more-than-2 branches in the dispersion diagram afforded by the multiple transmission lines, it is hence possible to introduce 3-way coupling and tune the dispersion to exhibit a maximally-flat behavior within the propagation band. Moreover, due to mode degeneracy at the so-called stationary inflection point (SIP), coupling into the frozen-mode becomes readily achievable. Such modes have been studied in [2] using 3-way coupled periodic waveguides and coupled resonator optical waveguides in [3]. In this paper, we propose a simpler structure consisting of straight Silicon ridge waveguides with periodic gaps to achieve the SIP dispersion leading to the frozen-mode. The structure consists of three ridge structures in close proximity. Periodic gaps are introduced in the outer guides to create a band gap. An advantage of the gaps in the Silicon ridge waveguide is the improved coupling between the structures due to the gap discontinuity. This linear structure is also easier to fabricate and more resilient to fabrication uncertainties as compared to the previously studied structures. A carefully-tuned structure results in the SIP behavior at 190.38 THz (1575.8 nm wavelength) and can be excited through conventional fiber coupling. Subsequently, we study a finite length section of this 3 way coupled guide to verify its extraordinary group delay performance. Tuning control can also be incorporated into the design to create an integrated-photonic true-time-delay engine. Potential applications for such a compact device for communications and sensing will be discussed at the conference.