**2. Theory**

32 Photonic Crystals – Innovative Systems, Lasers and Waveguides

During recent years different approaches have been done by scientists for generating of slow light. Electromagnetically induced transparency (EIT) is the first technique for achieving group delay and slow light in vapor mediums [5-9]. The group delay of incident pulses in an EIT system was studied first by Kasapi *etal* in lead vapor. Also, in 1999 Hau

As a reason of complicated situation for slow light generation in EIT systems, it is important for real applications that these phenomena could be occurred in room temperature and in

Therefore, Spectral holes due to coherence population oscillation (CPO) in room temperature and in solids, can be considered as one of the interesting methods for slow light generation [11]. Slow light generation based on CPO has been studied in various material systems. So, ultraslow light, 57.5m/s, in ruby crystal has been studied by Boyd's group at

On the other hand, during recent years a new kind of optical waveguides have been considerable attracted both theoretical and experimental attention due to their intense applications not only in data transferring but also in optical data processing [13]. Coupled cavity optical waveguides (CCWs) can be considered as the latest proposal mechanism for optical waveguiding which have been introduced by Yariv *etal* at Caltech [14]. This new kind of waveguides is based on the periodic dielectric structures as photonic crystals, which is separated by the high quality factor cavities that are coupled to each of the nearest neighbor in multiple spatial dimensions [15]. Due to existence of the cavities along the structure and as a result of the overlapping of the evanescent fields, light can be propagated through the CCWs [16]. Actually, it was investigated theoretically and experimentally that CCWs exhibit more advantages over conventional optical waveguides. One of the most important features of CCW is that due to strong optical confinement in defect medium, and high slope of transmission at resonance wavelengths, group velocity at the edge of each resonance modes can be reduced considerably [17]. Slow light generation in CCWs offers several practical advantageous like, design freedom, direct integration with other optoelectronics devices and ability to slow down light in a desired region of wavelengths at

On the other hand, it should be noticed that the CCWs structures for stopping light suffer a fundamental trade off between the transmission and the optical delay bandwidth [19,20]. Therefore, in the field of slow light technology, delay-bandwidth product is an important

Furthermore, from system points of view, in future photonic circuits, adjustability of the optical properties of components is a great and important bottleneck which many scientists have been proposed special methods. In CCWs, the optical properties such as group velocity, dispersion and its higher order can be modulated through different mechanism such as electro optic effect, free-carrier injection and thermo optic effect [21]. As a consequence of necessity for all optical networks in future, dynamic all optical processing, controlling light by light, can be considered as one of the crucial bottlenecks for future all

s delay in an EIT system which consists of condensed cloud of sodium

*etal* showed 7.05

Rochester [12].

room temperature [18].

optical systems [22].

parameter which should be considered greatly.

atoms [10].

compact and solid materials.

1D-NCCW is formed by placing optical resonator in a linear array, to guide light through whole of the structure by photon hopping between adjacent resonators. As a result of overlapping the evanescent field in the defect medium, cavity zone, electric field enhancement in this region can be obtained. Tight binding (TB) approximation (like using it in solid state physics) is used to describe the mechanism of waveguiding of the structure.

The basic structure for 1D-NCCW is as:
