**2.3 Practical applications of slow light**

The very first consequence of slow light is light pulse delaying. This fact unleashes in itself a series of valuable applications as optical buffers for optical packet switching (OPS) (Blanco et al. 2009a; Tucker, 2009), tuneable delay-lines for optical synchronization and correlation (Willner et al., 2009), or photonic true-time delay beamforming for phased array antennas, radio-over-fiber and analog-to-digital conversion (Capmany & Novak, 2007).

Fig. 3. Schematics of a proposed optical packet switching router design, including optical buffers and optical labels; a photonic switch fabric is represented by X (Blanco et al., 2009b)

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

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

The very first consequence of slow light is light pulse delaying. This fact unleashes in itself a series of valuable applications as optical buffers for optical packet switching (OPS) (Blanco et al. 2009a; Tucker, 2009), tuneable delay-lines for optical synchronization and correlation (Willner et al., 2009), or photonic true-time delay beamforming for phased array antennas,

Fig. 3. Schematics of a proposed optical packet switching router design, including optical buffers and optical labels; a photonic switch fabric is represented by X (Blanco et al., 2009b)

radio-over-fiber and analog-to-digital conversion (Capmany & Novak, 2007).

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

simplifying its integration with electronic circuitry.

**2.3 Practical applications of slow light**

(Melloni et al. 2010).

To illustrate the use of optical buffers, Fig.3 shows the schematics of a proposed design for an OPS core router. Optical packets are wavelength-demultiplexed and immediately tapped: a copy of the packet is passed to the control subsystem while the other copy must remain "stored" in the optical domain. This latter copy must be released as soon as control decisions are made to ensure efficiency. Once the packets have been optically switched to the appropriate output, optical buffers are needed to resolve possibly arising collisions.

Not so obvious applications arise from the slow-light-based enhancement of light-matter interaction. Two facts explain this effect: on the one hand, slowly travelling photons are more likely to interact with the surrounding matter simply because it takes longer for them to go through it; on the other hand, at the very moment a light pulse enters a slow light device, its leading edge starts propagating at extraordinarily low speeds while the rest of the pulse is still propagating at normal velocities. This fact generates an accordion effect that implies spatial pulse compression and consequently an increase of local energy density and nonlinear effects. In Fig. 4 one of our simulations serves to illustrate the pulse compression and energy density increase suffered by a gaussian pulse entering in a photonic crystal slow light region from a conventional ridge waveguide.

Optical sensing, especially biosensing, is one of the application fields most benefitted from the strengthening of light-matter interaction, since its principle relies on identifying substance changes on the basis of light-matter relations. Higher standards of sensitivity and resolution are enabled by the use of slow light in sensing devices (Biallo et al., 2007; Pedersen et al., 2008). Fig. 5 illustrates the concept of optical biosensing using a slow-light photonic crystal waveguide to improve sensitivity to the biomarkers bounded to its surface.

Distributed Raman amplification is one of the nonlinear systems benefiting from slow light, with an expected efficiency improvement by a factor of 66.000 (McMillan et al., 2006). In quantum optics, as another example, slowing light down provides a mean to achieve sufficiently long storage time of quantum states to enable quantum operations (Dutton et al. 2004). Finally, it is worth to mention the role that slow light may play at improving the conversion efficiency of next generation solar cells. It is well known that recent thin film and organic photovoltaic (PV) technologies lack of a sufficient solar light-to-electrical energy conversion efficiency. By making photons travel slower within the active zone of the PV cell, up to a 50% increase in photon absorption (El Daif et al., 2010).

Fig. 4. Slow light photonic crystal waveguide (bottom inset), and transverse magnetic field component of the optical wave (top inset) to illustrate pulse compression

Photonic Band Gap Engineered Materials for Controlling the Group Velocity of Light 61

Fig. 6. Examples of one-, two- and three-dimensional photonic crystals.

resultant master equation is the following:

indispensable to understand the basic principles of photonic crystals, are summarized. The

*r c*

Given a known spatial dielectric constant arrangement, *ε(r)*, one can find the magnetic field spatial profile of the modes allowed by the structure, *H(r)*, and their corresponding

> ( ( ) ( )) 0

frequencies, ω, by solving Eq. 4 subject to the following transversality requirements:

Subsequently one can obtain the electric field by using the following expression:

<sup>2</sup> <sup>1</sup> () () ( ) *Hr Hr*

(4)

*H r*() 0 (5)

*rEr* (6)

1D

2D

3D

Fig. 5. Illustration of an optical biosensor based on a slow light photonic crystal waveguide.
