The well known information society, in which we are nowadays involved, has been possible due to the technological revolution derived from the development of microelectronics during half a century. Since the invention of the transistor as a basic component, considerable efforts have been performed towards miniaturization. The number of components that can be allocated in a single chip has been doubled every 18 months as it was predicted during the 70’s by  G. Moore. Nowadays, technology has arrived to a frontier, with patterns of the order of few nanometres, where severe problems have risen leading to a slow down in the evolution.
In order to overcome these problems risen in microelectronics the use of the photon has been proposed to continue the technological development. Discoverings in this direction may lead to big benefits in the field of optical networks providing new optical functionalities that may substitute the bottlenecks of the optoelectronic components. Additionally, other research fields, like optical computing or optical sensors, would benefit from it. A new wide and complex field appears, known as Nanophotonics.
In this Thesis planar photonic crystals are studied as a candidate technology in the field of Nanophotonics. More precisely, the implementation of a directional coupler in photonic crystals is considered. This is a basic device used in a wide range of applications like power splitters, multiplexers and demultiplexers, Mach-Zehnder Interferometers or even in switches. The thesis tackles subjects ranging from the modelling of photonic crystal structures and the theoretical design of the directional coupler for several applications  to the experimental demonstration of the designs, which are performed at optical frequencies and, in scaled models, at microwave frequencies.
Photonic crystals have the advantage of allowing big integration levels with a high control of light. It is obtained, theoretically, coupling lengths of the order of 1µm, smaller than that of other integrated technologies with high contrast. It is also studied in the thesis, the implementation of a switch. Taking profit of the high confinement capabilities and the slow group velocities of the photonic crystal structures, the implementation of small structures with low power consumption is achieved.
The main drawbacks shown by photonic crystals are the fabrication tolerances, that affect the performance, and the difficulties in the insertion and extraction of light in the structures. The aim of the thesis, with the results obtained, is to motivate further research in the field of photonic crystals.