E*PCOS05 Phase change materials and superrens B.Hyot, S.Gidon, M.-F.Armand, L.Poupinet, CEA/Leti J.Pichon, R.Anciant, J.M.Bruneau, MPO G.Pilard, H.Richter, Thomson Keywords: Superresolution, phase change, ROM, R INTRODUCTION In a never ending quest for a higher data storage capacity, several different solutions are evaluated. As a straightforward conceptual evolution of CD, DVD and BD, it should be proposed to reduce the wavelength and to increase the numerical aperture. The wavelength reduction is limited to a small amount compared to the BD wavelength (405 nm) since deep UV laser diodes are still at the research level. To increase a numerical aperture of 0.85 (BD) at a significantly higher level implies the use of near-field optics. Optical near field generation can be obtained by several means: metallic coated cone shaped fibers, metallic diffusing tips, VCSEL with holes, Solid Immersion Lens, …. All those techniques require to control very precisely the distance between the near field source and the recording area. Despite impressive progress on servo, regarding SIL for example, this requirement might compromise the removability and/or the handling of the optical disc. Those two points are key properties of optical discs since manufacturer are targeting content distribution and personal archiving. Hence superresolution [1,2,3,4] appears to be an appealing solution since the near field is generated inside the disc thin film stack and the distance between the source and the recording area is fixed by a constant physical layer thickness. Huge progresses have been made recently on recordable superrens disc, however some progresses have still to be made on ROM disc and the physical understanding of the materials behaviour still seems open to discussion. It is our objective here to propose an analysis of superrens based on optical and electronic properties of phase change materials and to see how this analysis has allowed us to find new structures of superrens discs. PHENOMENOLOGICAL ANALYSIS When a photon is incident on a semiconductor material it can excite an electron from the valence band to the conduction band if the energy of the photon is higher than the gap of the semiconductor material. Those free electrons are generated with a rate G (in s -1 ). They can come back to the valence band either by collision with the lattice, inducing an increase of the material temperature, or by emission of another photon. This recombination rate (in s -1 ) is named R. If G is much higher than R, the conduction band has always a certain number of free excited electrons. In this case the optical properties of the material are changed from a semiconducting behaviour to a more metallic one. Then the superrens effect is no more obtained by making a small aperture in the mask layer but by creating a brighter region smaller than the beam size in this layer.