Irradiating intense femtosecond laser pulses on a glass sample containing silver nanoparticles results in permanent sample color changes if the laser wavelength is in the region of the particles’ surface plasmon resonance. In particular, even a single pulse of appropriate intensity can modify the initially isotropic extinction of glass containing spherical particles to a dichroic sample behavior in the irradiated area. This observation is interpreted as ultrafast particle deformation induced by the laser pulse creating nonspherical particles of uniform orientation.

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Glass containing spherical silver nanoparticles has been irradiated with single, intense, ultrashort laser pulses, with a wavelength corresponding to the surface plasmon resonance of the particles. After irradiation with linear polarization, dichroism is observed. Transmission electron microscopy studies reveal that these spectral changes are caused by deformation of the particles to anisotropic (in the TEM projection approximately ellipsoidal) shapes with an additional halo of small silver particles around the central one. The deformed particles are uniformly oriented with their longer axes perpendicular to the laser polarization. Using laser pulses with circular polarization, again a halo is formed around the particles, but the central particles remain spherical, and no dichroism was observed in the optical spectra.

The lattice expansion and relaxation of noble-metal nanoparticles heated by intense femtosecond laser pulses are measured by pump-probe time-resolved X-ray scattering. Following the laser pulse, shape and angular shift of the (111) Bragg reflection from crystalline silver and gold particles with diameters from 20 to 100 nm are resolved stroboscopically using 100 ps X-ray pulses from a synchrotron. We observe a transient lattice expansion that corresponds to a laser-induced temperature rise of up to 200 K, and a subsequent lattice relaxation. The relaxation occurs within several hundred picoseconds for embedded silver particles, and several nanoseconds for supported free gold particles. The relaxation time shows a strong dependence on particle size. The relaxation rate appears to be limited by the thermal coupling of the particles to the matrix and substrate, respectively, rather than by bulk thermal diffusion. Furthermore, X-ray diffraction can resolve the internal strain state of the nanoparticles to separate non-thermal from thermal motion of the lattice.

Persistent form modifications of silver nanoparticles in glass and the resulting dichroitic color changes induced by irradiation of a single intense femtosecond laser pulse were studied by means of high-resolution electron microscopy and spatially resolved optical transmission spectroscopy. Electron microscopy reveals a variety of morphological changes of the silver particles, in particular irregularly shaped particles surrounded by halolike structures consisting of very small silver clusters. The spectral analysis suggests that at lower intensities preferably desorption of silver into the glass matrix occurs producing isotropic spectral changes, while at higher intensities anisotropic spectra caused by particle deformation are observed. An analysis of the effects produced by different laser wavelengths allows one to correlate these findings with the variations of particle sizes depending on their penetration depth.