G. Seifert

Ultrashort laser pulse induced deformation of silver nanoparticles in glass

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.

Vibrational and orientational relaxation of monomeric water molecules in liquids

A polarization resolved infrared double resonance experiment with picosecond pulses has been used to measure the vibrational and orientational relaxation times of monomeric water and heavy water molecules in different organic solvents after excitation of the antisymmetric ν3 vibration. In all investigated systems a fast (but definitely time resolved) equilibration among the OH (OD) stretch vibrations was found, followed by a rather slow decay of this ensemble. Furthermore hints to a medium lived intermediate state, most likely the bending vibration, are reported. The principal relaxation scheme is very similar to that of water vapor. Comparing H2O and D2O in different solvents significant differences were found, which can be understood at least semiquantitatively. The orientational relaxation times of H2O and D2O differ by a factor of 2, which can be explained by the influence of weak hydrogen bonds of different strength for H2O and D2O on the orientational relaxation. Additionally interesting aspects of ...

Direct observation of rotational relaxation times by time-resolved infrared spectroscopy

Abstract Using picosecond pulses tuned to vibrational absorption bands, time-resolved polarization spectroscopy is extended to the infrared and to the study of the orientational motion of small molecules in condensed matter. Experimental data are presented for CHBr3 in different solvents.

Poling-assisted bleaching of metal-doped nanocomposite glass

Thermal poling of soda-lime glass which was doped with spherical or ellipsoidal silver nanoparticles has revealed what we believe to be a phenomenon of general interest in the physics of nanocomposite materials: The field-assisted dissolution of metal nanoparticles embedded in glass. Macroscopically, this phenomenon manifested itself as poling-assisted bleaching of the glass in the sense that the glass became more (or even completely) transparent under the anode. The phenomenon is physically interpreted in terms of the ionization of metal nanoclusters followed by the removal of ions from the clusters and their drift in the depth, under the action of the extremely high electric field which is created underneath the anodic surface during poling. The underlying physical mechanism is expected to offer unique opportunities for the control of structural and optical properties of nanocomposite glasses.

Vibrational and reorientational dynamics of water molecules in liquid matrices

Abstract Polarization-resolved double-resonance spectroscopy with tunable picosecond pulses in the infrared is used to study the vibrational and reorientational relaxation of monomeric water molecules in various organic solvents. After excitation of the antisymmetric OH stretching vibration ν 3 a rapid population redistribution with the neighboring stretching vibration ν 1 is observed, followed by a much slower and strongly solvent-dependent population decay of this vibrational ensemble. The findings are explained by energy transfer to the overtone 2ν 2 of the bending mode and the influence of weak hydrogen bonds between water and solvent molecules. The latter govern the energy difference between the initial (ν 1 ) and final (2ν 2 ) state of the relaxation process. A theoretical discussion of polarization effects is presented taking into account the varying directions of the transition dipole moments of different modes in the molecular coordinate frame. It is shown that measurements of the induced dichroism facilitate the interpretation of our transient infrared spectra and allow the determination of reorientational relaxation times. The reorientational time constants also strongly depend on the weak hydrogen bonds between solute and solvent molecules.

Second-harmonic generation from ellipsoidal silver nanoparticles embedded in silica glass

Second-harmonic generation of uniformly oriented, ellipsoidal silver nanoparticles in a glass matrix was observed and investigated as a function of incidence angle, light polarization, and spatial arrangement of the particles. The results can be explained by the symmetry of the spatial nanoparticle arrangement and by resonance enhancement that is due to the localized surface plasmons of the particles. Second-harmonic enhancement is observed only in sufficiently thin layers (deltakl < pi); on a sample with two separate layers, strong modulation owing to quasi-phase matching is obtained.

In situ surface-enhanced Raman spectroscopy of monodisperse silver nanowire arrays

Highly ordered two-dimensional arrays of monodisperse coinage metal nanowires embedded in an alumina matrix have been prepared. When light is propagating in the direction of the long axis of the nanowire, plasmon-enhanced absorption and light guidance of the nanowire were observed by optical microspectroscopy and scanning near-field optical spectroscopy and compared to Mie scattering theory. By selectively dissolving the matrix at a constant etching rate, we detected in situ and ex situ the surface-enhanced Raman scattering (SERS) of organic dyes. In contrast to earlier publications, we find that the SERS signal is linearly proportional to the free-surface area of the nanowires that are in contact with the dye. We cannot detect any change in the enhancement factor due to the releasing of the nanowires from the host structure.

Time-resolved observation of intermolecular vibrational energy transfer in liquid bromoform

Efficient intermolecular energy transfer between the C–Br stretching modes ν2 and ν5 of bromoform molecules in the liquid phase has been observed directly in time-resolved experiments applying vibrational pump–probe spectroscopy with picosecond laser pulses. An analysis of results on CHBr3, CDBr3, and an isotopic mixture of both yields a typical time constant of 25±15 ps for this rather efficient, near-resonant intermolecular relaxation channel. Additional new details about the intramolecular pathways of vibrational energy relaxation have been determined for the pure substances.

Optical three-dimensional shape analysis of metallic nanoparticles after laser-induced deformation.

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.

Surface-enhanced Raman spectroscopy employing monodisperse nickel nanowire arrays

We have prepared two-dimensional arrays of hexagonally arranged, monodisperse nickel nanowires embedded in an alumina template. The degree of template filling is nearly 100% using an improved electrochemical deposition technique. Optical transmission measurements in the direction parallel to the long axis of the nickel nanowires show a plasmon-enhanced absorption around 400nm. We observe for typically surface-enhanced Raman spectroscopy (SERS) inactive metals like nickel a strong, but locally strongly inhomogeneous SERS signal during in situ Raman microspectroscopy. Supported by our numerical modeling, we suggest that significant SERS enhancement factors are possible only when nanowires in bundles are touching each other.