LASER-INDUCED PERIODIC STRUCTURES IN POROUS SILICON

Permanent relief holographic gratings with variable modulation depth h ave been induced in porous Si films using the interference of two beam s obtained by a frequency-doubled Nd:YAG laser (lambda = 530 nm) and a conventional optical setup. Consequently, the gratings have been conf ined in the porous Si film and the diffraction has been observed by re flection. For an angle of 6 degrees between the interfering beams and for diffracted light at 530 and 633 nm, the Raman-Nath condition is fu lfilled and we have observed up to 11 diffraction orders (large phase modulation). We assume that the periodic structures are obtained by th ermal effects. which appear in porous Si film from very low laser beam fluences.