**1. Introduction**

Optical components that permit the miniaturization of photonic integrated circuits to a scale comparable to the wavelength of light are good candidates for future optical network and optical computing. All-optical communication is one of the solution for the electronic bottleneck viz speed and size, thanks to their ability to process the information at the speed of light. Optical logic gates are the fundamental components in optical digital information processing. In recent years, researchers have demonstrated all-optical logic gates using different schemes based on nonlinear effects in optical fibers (Ahn et al., 1997; Bogoni et al., 2005; Menezes et al., 2007), in semiconductor devices (Kyoung Sun Choi et al., 2010; Kim et al., 2002; Zhihong Li & Guifang Li 2006; Dorren et al., 2004; Stubkjaer, 2000) and in waveguides (Tetsuro Yabu et al., 2002; Yaw-Dong Wu, 2005; Yaw-Dong Wu et al., 2008). But most of the reported works suffer from certain fundamental limitations including big size, low speed and difficult to perform chip-scale integration.

Nowadays, photonic crystals (PhC) draw significant attention as a platform on which to build devices with dimensions in the order of wavelengths of light for future photonic integrated circuits. They are having some unique properties such as compactness, high speed, low power consumption, better confinement which make them promising candidate in photonic integrated circuits (Yablonovitch, 2003; Cuesta-Soto et al., 2004). Logic functions based on photonic crystal can be realized by nonlinear effect (Notomi et al., 2007), ring resonator (Andalib & Granpayeh, 2009), and multimode interference (Hong-Seung Kim et al 2010). They require significant amount of power, nonlinear material, long interaction length and two different wavelengths for probing and input signals. One of the effects of complex spatial dispersion property in PhC namely self collimation provides a mechanism to employ optical switching and logic functions (Zhang et al., 2007; Susan et al., 2010).
