Silicon-based Photonic Integrated Circuits (PICs) are of great interest for the photonics community. Slow-light devices institute an important building block for such circuits, allowing the enhancement of light-matter interaction for a traveling wave. Typical examples are Mach-Zehnder interferometers used in electro-optical modulators. Various ways of realizing such devices have been studied, with different levels of complexity and performance.
In this work, we focus on the slow-light effect arising in 1D periodically patterned waveguides near the edge of the band-gap region, where the dispersion becomes naturally flat (Figs. 1, 2). The periodic pattern is inserted in a conventional silicon ridge waveguide (400 x 310 nm Si on SiO2) by a simple modulation of the width. The advantages of this approach are the ease of fabrication and the low losses involved, which is crucial for prospective applications in PICs.
We present a theoretical analysis of the band dispersion and slow light effect in such grating waveguides [1]. By a combination of numerical simulations by the aperiodic Fourier-modal method and perturbation theory, we are able to explore a broad space of parameters and to identify those that maximize the slow light bandwidth. We find that the slow-light bandwidth improves when increasing the modulation width and decreasing the thickness of silicon in the cladding region, reaching the optimal solution when the modulation is complete and the waveguide reduces to a lattice of trenches (Fig. 3). The fully modulated structure is, however, not convenient when considering insertion losses, which should be minimized by including an adiabatic taper that cannot be realized for the lattice of trenches. A viable solution is to use a deep grating, i.e., a structure in which the internal width is small but not zero. We show that this configuration is able to maintain performances neat the optimal ones, while being compatible with insertion of a taper. Finally, we demonstrate a slow-light design with a group index ng>10 over a bandwidth of about 10 nm, which can be connected by a low-loss adiabatic taper to a standard silicon rib waveguide (Fig. 4).
[1] M. Passoni et al, Optics Express 26, 8470 (2018).
