Nonlinear optical properties of laser ablated silicon nanostructures

The role of crystallite symmetry on the nonlinear optical properties of laser ablated silicon films is discussed in this article. Laser ablated Si films exhibit a nonlinear refractive index change which may be as high as Δn=-0.5 at wavelength of λ=532 nm for films with an average thickness of 200 nm. However, the origin of this nonlinearity is not known. These films consist of large droplets comprised of hexagonal wurtzite symmetry crystallites and nanoclusters interspersed between them. Using Raman spectroscopy and linear and nonlinear optical measurements, we monitored the crystallographic symmetry of these droplets when annealed under various conditions. Based on the results we attribute the large nonlinear refraction coefficient to the hexagonal wurtzite symmetry of the crystallites, hence raising the possibility for developing very efficient nonlinear optical devices.