Formation and third-order optical nonlinearities of fractal Ge nanocrystals embedded in Au matrix

Nonequilibrium growth processes of low-dimensional materials have attracted considerable attention, which controlled micro/nanostructures, properties, and various performances of materials. Semiconductor Ge in contact with some metals, e.g., Au, Pd, and Al, etc., is a class of distinctive materials with noninteger dimensions that differ from integer dimensional materials such as nanoparticles, nanowires, nanorods, nanotubes, nanoribbons, and thin films. Here, Au/Ge bilayer films with interesting fractal Ge nanocrystals were successfully prepared by high-vacuum thermal evaporation techniques. It was found that Ge nanocrystals embedded in Au matrix showed fascinating fractal morphologies with average size of fractal clusters at 550 nm and fractal dimension at 1.756 when the films were annealed at 150 C for 30 min. Third-order optical nonlinearities of the annealed Au/Ge bilayer films were investigated in detail by Z-scan technique using a femtosecond laser. Experimental results indicated that the nonlinear absorption coefficient and refractive index of the fractal Ge nanocrystals embedded in Au matrix were in the ranges 4.2 to 4.7 × 10^-7 cm/W and 5.2 to 5.6 × 10^-12 cm ²/W, respectively, when the input irradiance (I_p) ranged from 0.58 to 1.65 GW/cm². This nonlinear optical material may be tailor-made for a large number of applications such as high-speed microelectronics and infrared optical micro/nanodevices. © 2013 American Chemical Society.