**2.2.2 Engineered structures for slow-light generation**

Differing from the previously explained material-based methods, the following slow-light mechanisms use strong spatial resonances between electromagnetic waves travelling along special structures. The structure-based slow-light approaches can be materialized in various types of arrangements, such as Fabry-Perot resonators, cascaded fiber Bragg gratings (FBGs), photonic crystals defects, or ring resonators. In general, these approaches outperform material-based ones for high-bandwidth signals, due to the fact that material resonances have generally a narrow linewidth and are more limited by dispersion effects (Melloni et al. 2010).

The election of a slow-light technique strongly depends on the targeted application. Photonic crystals constitute, to our understanding, the most promising approach in order to build devices for applications as the ones outlined bellow: small operating power (dozens of μW per cell), compact footprint (cell size around 10μm2), and fast access to information (tens to hundreds of picoseconds). Contributing to its practicality are also the facts that they can be operated at room temperature offering wide bandwidth. Finally, their fabrication processes can be made compatible with CMOS technology, enabling the use of silicon industry mass manufacturing facilities, and thus reducing mass fabrication costs and simplifying its integration with electronic circuitry.
