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We have used pulsed tunable infrared laser irradiation to modify the optical and physical properties of metal
nanoparticles in a SiO2 substrate. The nanoparticles were fabricated by implanting high-energy Au+ or low-energy
Ag+ ions at a dose of 6.1016 ions/cm2. The substrate temperature was held at 400 oC during implantation. The depth of the nanoparticles was well within the 1/e absorption length of the SiO2 substrate at our primary
laser wavelength of 8 &mgr;m. The infrared laser beam generated by a picosecond free electron laser (FEL) was
scanned across the implanted surface at various fluences. The optical absorption spectra of the gold implanted
sample show that the absorption maximum at 520 nm, which is related to the presence of gold colloids, increases
with laser fluence. On the other hand, the absorption maximum at 415 nm in the spectra of the silver- implanted
sample decreases with increasing laser fluence and shifts to slightly lower wavelengths. In both cases a visible
change in the color of the sample is observed, a clear indication of changes in the size distribution of the
nanoparticles. Previous experiments used nanosecond excimer lasers that directly interact with the nanoparticles
to modify their size and size distribution in different matrices. Our successful modifcation of the nanoparticles
by excitation of the matrix vibrational modes, rather then melting of the nanoparticles, shows another possible
approach to the processing of nanocomposite optical materials.
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