One of the most important optical properties of photonic crystals is that the waveguide dispersion relations can be tailored and allow for many non-conventional applications such as guiding and processing of the light signal. On the other hand, a variety of physical phenomena make liquid crystals (LC's) one of the most interesting subject of modern fundamental science. Moreover, in the last years, it has been proved that in order to obtain active tuning of the photonic crystals device a very promising approach can be achieved by infiltrating photonic structure with liquid crystals. On this line of argument, in this paper, the design of an electro-optical switch based on 2D silicon photonic band-gap structure and using liquid crystals as active medium is presented. We consider a T-junction PhC diplexer in two dimensional photonic crystals composed of silicon rods with square lattices with nematic liquid crystals as …