Laser induced periodic structures in porous silicon

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