Dynamics of optical nonlinearity of Ge nanocrystals in a silica matrix

The optical nonlinearity and excited carrier lifetime in Ge nanocrystals (n c-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 a nd 532 nm wavelength. The nc-Ge samples were prepared using magnetron cospu ttering and postgrowth annealing at 800 degreesC. The nonlinear absorption coefficient alpha (2) and refractive index n(2) were found to range between 190 and 760 cm/GW, and 0.0026 and 0.0082 cm(2)/GW, respectively, and be pr oportional to the Ge concentration in the film. The confined excited carrie rs were found to depopulate with a lifetime of similar to 70 ps. The nonlin earity in Ge nanocrystals is deduced to originate mainly from excited carri er absorption, with two-photon absorption providing a small contribution. ( C) 2000 American Institute of Physics. [S0003-6951(00)05249-9].