Search

We investigate the possibility of preparation of laser-induced dichroism in composite glasses with a high concentration of silver nanoparticles. A detailed analysis based on the Maxwell-Garnett theory and experimental results shows that particles at different volume fractions react differently to the same laser irradiation parameters. Based on these findings, we demonstrate that a well-defined sequence of multiple irradiation and intermediate annealing can maximize the particles' aspect ratio and avoid unwanted partial destruction. The proposed irradiation technique permits production of micropolarizing structures with high polarization contrast in the visible and near-IR regions.

We prove by help of optical microspectroscopy that spherical Ag nanoparticles embedded in glass can be transformed to oblate (disklike) shapes by irradiation with circularly polarized femtosecond laser pulses. The symmetry axis (short axis) of the oblate spheroids produced by this method is oriented along the laser propagation direction. This observation also confirms the assumption that the mechanism of shape transformation starts with directed photoionization of nanoparticles induced by laser irradiation in combination with the electric field enhancement at the particle-glass interface.

We have studied the influence of irradiation parameters on the laser-induced shape transformation of Ag nanoparticles in glass by fs pulses. The amount of dichroism which can be achieved by this technique is related to the aspect ratio of the prepared spheroidal particles. Looking for maximal dichroism, it was found that 'off-resonant' irradiation with wavelengths considerably larger than the surface plasmon (SP) resonance at 413 nm can produce a larger spectral gap between orthogonally polarized SP bands of the reshaped nanoparticles than resonant excitation. It is also shown that subsequent irradiation with successively larger wavelengths may increase the spectral gap even further. A sequence of only two steps of irradiation at 535 and 670 nm already caused a shape transformation of initially spherical Ag nanoparticles to spheroids with an aspect ratio of (a/c)>3.

The 3D shape of Ag nanoparticles in glass irradiated by fs laser pulses is investigated by optical spectroscopy. It is shown that in general spheroids are produced with their symmetry axis oriented along the direction of the linear laser polarization. Depending on the actual irradiation conditions, oblate or prolate spheroids are obtained. The halo of small Ag clusters and Ag ions around the reshaped particles causes a redshift of the surface plasmon resonances via refractive index increase.

The interaction of C60 fullerenes with 765-797 nm laser pulses as short as 9 fs at intensities of up to 3.7 x 10(14) W cm(-2) is investigated with photoion spectroscopy. The excitation time thus addressed lies well below the characteristic time scales for electron-electron and electron-phonon couplings. Thus, energy deposition into the system is separated from energy redistribution among the various electronic and nuclear degrees of freedom. Insight into fundamental photoinduced processes such as ionization and fragmentation is obtained from the analysis of the resulting mass spectra as a function of pulse duration, laser intensity, and time delay between pump and probe pulses, the latter revealing a memory effect for storing electronic energy in the system with a relaxation time of about 50 fs. Saturation intensities and relative abundances of (multiply charged) parent and fragment ions (C60(q+), q=1-6) are fingerprints for the ionization and fragmentation mechanisms. The observations indicate that for final charge states q1 the well known C60 giant plasmon resonance is involved in creating ions and a significant amount of large fragments even with 9 fs pulses through a nonadiabatic multielectron dynamics. In contrast, for energetic reasons singly charged ions are generated by an essentially adiabatic single active electron mechanism and negligible fragmentation is found when 9 fs pulses are used. These findings promise to unravel a long standing puzzle in understanding C60 mass spectra generated by intense femtosecond laser pulses.

A system for the course control and spatial stabilization of the output beam of a femtosecond laser system is realized. An accuracy of 0.5 microrad is achieved long term in a laboratory environment for a beam with an initially daily deviation of more than 40 microrad. The application and importance of the method is illustrated for laser pulse shortening in a hollow waveguide resulting in a stable output power and an 8 fs pulse duration for more than 10 h operation time.

In recent years, the fabrication, structural modification and characterisation of nanostructured materials have gained a great deal of attention due to their distinctive properties and potential use in technological applications. Glass containing silver (Ag) nanoparticles (NP), or Metal-Glass Nanocomposite (MGN), is routinely used as high-contrast optical polarisers [1], and has been recently shown as a promising medium for high-volume optical storage of information [2]. Strong second and third harmonic generation has been demonstrated when the MGN containing mechanically reshaped NPs was subjected to fs pulse excitation [3], thus showing potential as an effective nonlinear optical medium for photonics nanodevice applications.