Physical and optical characterisation of Ge-implanted silica

Ge nanocrystals formed in silica by implantation with 1.0 MeV Ge ions and subsequent annealing at 1100°C were characterised by transmission electron microscopy and Raman spectroscopy. The nanocrystals were found to be approximately spherical in shape and to have a structure consistent with that of bulk Ge. The average size of the crystallites increased with increasing fluence and for a fluence of 1 × 10¹⁷ Ge cm^-2 the size varied from 2.5 to 12 nm. The nonlinear optical response of the material was measured at a wavelength of 800 nm using degenerative four wave mixing and z-scan techniques. The former provided information about the magnitude and temporal response of the nonlinearity whilst the latter provided information about the operative mechanism. The magnitude of the nonlinear refractive index, |n₂|, was shown to be more than three-orders of magnitude larger than that of pure silica and to have a relaxation time of the order of picoseconds. The mechanism causing this nonlinear response is shown to be absorptive and to increase with increasing implant fluence as a consequence.