Characterization of optical properties of Porous Silicon using Photoacoustic Technique S. Abdallah, T. A. El-Brolossy, S. Negm and H. Talaat Eur. Phys. J. Special Topics, 153 (2008) 199-202 Published online: 12 March 2008 DOI: 10.1140/epjst/e2008-00427-3