Porous aluminum oxide (Al2O3), hereinafter referred to as porous alumina, is a transparent material with electrically insulating properties. Porous alumina, whose structure can be ideally schematized as a lattice of parallel pores in an alumina matrix, is an example of two-dimensional photonic crystal, periodical on two of its axes and homogenous on the third one. The periodicity of such structure, and thus the alternation of means with different dielectric constant, enables to determine a photonic band gap and as a result to prevent light propagation in given directions with specific energies. In particular, by controlling the size and spacing between alumina pores a band gap in the visible spectrum can be determined, with consequent iridescence effects due to reflection in the plane of incident light.
The present Applicant has previously suggested to exploit the properties of two-dimensional photonic crystal of porous alumina for reducing the emission lobe of a light source and the focalization of the light bundle as a function of period size.
To this purpose document EP-A-1 385 041 describes a light emitting device of the backlight type having a transparent substrate, to one of whose surfaces means for generating an electromagnetic radiation are associated, in which a porous alumina layer operate to inhibit propagation of the electromagnetic radiation in the directions parallel to substrate plane, thus improving the efficiency of light extraction from said substrate and increasing the directionality of emitted light. In the various possible implementations described in the above document, the means for generating the electromagnetic radiation comprise a layer of electroluminescent material to be excited by a first electrode, consisting of a metal layer, and a second electrode, consisting of a ITO film (Indium Tin Oxide), or possibly by a percolated metal layer or by a mesoporous oxide.
A light emitting device based on the use of porous alumina is also described in the article “Porous alumina based cathode for organic light-emitting device”, in Proceedings of SPIE—The International Society for Optical Engineering, vol. 4105, 31.07.00, pages 405-412.
The device described in the above article has an alumina templating element filled up with lumino-phosphors excited by field effect, in which one of the electrodes of the device consists of an aluminum film underlying alumina. The luminescent molecules are adsorbed on the walls of alumina pores, so as to be excited thanks to the strong electric fields applied to the electrodes. In order to obtain the field effect required to enable the excitation of the luminescent molecules, the thickness of a barrier layer of alumina has to be reduced. The device has to be supplied with high voltages, required to extract sufficiently energetic electrons and to accelerate them from one electrode to the other.