**2.2 Techniques to achieve slow light**

In Fig. 2 we present a classification of the different approaches to slow light.

Fig. 2. Classification of the proposed approaches to slow light

How to control something that has no mass and no electric charge? These fundamental properties make the task of stopping light very challenging. However, far from being intimidated, science community has dedicated a great number of research efforts to 'trap' light. More specifically, these efforts try, not to store the photons, but the information that they carry.

In Fig. 2 slow light techniques have been separated into two subclasses: those based in modifying the waveguide dispersion by using engineered structures, such as gratings, photonic crystals or micro-ring resonators; and those based upon the change of material dispersion to large and positive values involving the use and modification of properties inherent to the material, e.g. the transmission spectrum or the scattering effects.
