ISSN: 2277-9655 [Mohammed* et al., 6(3): March, 2017] Impact Factor: 4.116 IC™ Value: 3.00 CODEN: IJESS7 http: // www.ijesrt.com © International Journal of Engineering Sciences & Research Technology [220] IJESRT INTERNATIONAL JOURNAL OF ENGINEERING SCIENCES & RESEARCH TECHNOLOGY NONLINEAR REFRACTIVE INDEX OF SILICON NANOSTRUCTURE PRODUCED BY PHOTO-ELECTROCHEMICAL ETCHING Mohammed Salman Mohammed*, Luma Kadhim Rasheed * Applied Sciences Department, University of Technology, Baghdad, IRAQ DOI: 10.5281/zenodo.400834 ABSTRACT Self-phase modulated optical fringe pattern are used to study the nonlinear optical response of nanocrystalline silicon produced by photoelectron chemical etching. Irradiation time-dependent changes in the refractive index are calculated for various sizes of nanocrystallites. Fabrication of porous silicon contains silicon nano- structures has been carried out via process PECE on n-type Si wafer with >100< orientation and electrolyte solution contain Hydrofluoric acid HF concentrations of (25% HF), various laser wavelengths (532nmand442nm). The observed morphological changes using scanning electron microscopy reveals formation of silicon nanostructure. There is a strong correlation between the nonlinear optical phenomenon of silicon nanocrystallites and the nanocrystallites size distribution. KEYWORDS: Porous Silicon, Nonlinear Phenomena, Refractive Index, Self-phase Modulation. INTRODUCTION Nonlinear optical properties of the nanoscale semiconductors have been a topic of fundamental interest and potential applications in switching devices [1-5]. An enhancement in the third order susceptibility and the change in the refractive index due to quantum confinement effect have been reported for laser ablated Si nanoclusters [6,7] and free-standing porous silicon materials[8] and silicon nanocrystals grown by plasma-enhanced chemical vapor deposition [9]. Many authors [10-13] have utilized the Z- photon has energy below the absorption edge of the nanoparticles, i.e. optical nonlinearity results from the anharmonic motion of bound electrons. Whereas other groups [14-17] have also utilized the self-phase modulation (SPM) of continuous wave (CW) laser beams to generate optical fringes in a nonlinear medium. The optical properties such as nonlinearity associated with changes in refractive index are significantly affected by the presence of nanocrystallites and size distribution of nanocrystallites [8,18]. This can lead to well- known phenomena of self- focusing and self-phase modulation [19]. Cotter et al [13] have reported the third-order optical nonlinearity of nanometer-size semiconductors caused by electronic quantum confinement. Moreover, Koker and Kolasinski observed optical fringes during the laser-induced etching (LIE) process and attributed the fringe patterns formation to the optical interference and the Fresnel diffraction of the light reflected from the porous layer [20]. When an intense laser beam having a Gaussian profile, is incident on a medium containing nanocrystallites of Si, the refractive index of such material is altered by the intensity of the laser beam. As the nanocrystallites of silicon interact with spatially varying laser intensity, the refractive index of the medium changes thus the beam propagates in varied optical paths and hence spatial phase variation occurs. This leads to establish a visible optical fringe pattern in the transverse plane and the phenomenon is known as spatial self-phase modulation [21]. The far-field diffraction intensity distribution of the optical generated fringe pattern is given by the relation [17].