Ultrafast Dynamics of Femtosecond Laser-Induced Shape Transformation of Silver Nanoparticles Embedded in Glass

Abstract

Metallic nanoparticles in a dielectric matrix can be transformed from spherical to nonspherical shapes by irradiation with intense femtosecond laser pulses. In particular, it has been previously shown that several hundred linearly polarized pulses can successively transform initially spherical Ag nanoparticles in glass into prolate spheroids with symmetry axes along the laser polarization. Using pulses of ∼150 fs duration at a wavelength of 400 nm, which is close to the surface plasmon resonance of the nanoparticles, this process works well for peak intensities in the range of ∼0.5 to 1.5 TW/cm2. We have now studied the ultrafast dynamics of this reshaping process in situ for each individual laser shot by help of an optimized femtosecond pump–supercontinuum probe setup. This unique setup allows us to trace the transient spectral changes up to ∼1 ns after each pulse as well as the step-by-step evolution of the persisting changes of the extinction spectrum in the focal volume. The results of our investigation provide experimental evidence that the directed emission of electrons from the nanoparticles and their trapping in the glass matrix within the first few picoseconds after the 400 nm pump pulse are the key to explain the anisotropy of the reshaping process occurring on considerably slower time scales. In addition, the very prominent role of the first of several hundred laser shots is discussed.