Dynamics of optical nonlinearity of Ge nanocrystals in a silica matrix

The optical nonlinearity and excited carrier lifetime in Ge nanocrystals (nc-Ge) embedded in a silica matrix have been investigated by means of single beam z scan and pump-probe techniques with laser pulse duration of 35 ps and 532 nm wavelength. The nc-Ge samples were prepared using magnetron cosputtering and postgrowth annealing at 800 °C. The nonlinear absorption coefficient α2 and refractive index n2 were found to range between 190 and 760 cm/GW, and 0.0026 and 0.0082 cm2/GW, respectively, and be proportional to the Ge concentration in the film. The confined excited carriers were found to depopulate with a lifetime of ∼70 ps. The nonlinearity in Ge nanocrystals is deduced to originate mainly from excited carrier absorption, with two-photon absorption providing a small contribution.