Preparing large-aspect-ratio prolate metal nanoparticles in glass by simultaneous femtosecond multicolor irradiation.

By irradiating spherical metal nanoparticles embedded in glass with several hundred ultrashort laser pulses at peak intensities of 0.2-1.5 TWcm(2), dichroic microstructures can be written in these nanocomposite materials. The underlying mechanism is transformation of the nanoparticles to prolate shapes. Using a single wavelength, the maximum aspect ratio achievable with this process is limited by partial destruction of particles. Here we show that this limitation can be overcome by simultaneous irradiation with different wavelengths. In particular, adding a relatively weak intensity at 800 nm to the main irradiation at 532 nm increases the maximum aspect ratio of Ag nanoparticles and the resulting separation between polarized surface plasmon bands dramatically. These effects are explained by the efficiency of electric field enhancement in the vicinity of nanoparticles, which influences the directed photoionization needed for particle shape transformation.

Nonlinear refractive index of porcine cornea studied by z-scan and self-focusing during femtosecond laser processing.

We have investigated the nonlinear refractive index of ex-vivo pig cornea by a combined approach using the standard z-scan technique on extracted corneas or corneal slices, as well as studying the deviations caused by self-focusing during femtosecond laser processing of the pig eyes. The experiments yield consistently an upper limit of 1.2 MW for the critical power of self-focusing in porcine cornea, and a value of 2 x 10(-19) m(2)/W for its nonlinear refractive index. We also demonstrate that due to this nonlinear refraction the cutting depth of typical fs-laser surgery processing in cornea may depend considerably, albeit in a well controllable way, on the laser parameters.

Solvent dependence of OH bend vibrational relaxation of monomeric water molecules in liquids.

The vibrational relaxation rates of the OH bending mode of monomeric H(2)O molecules diluted in various liquid halogenated methane and ethane derivates have been determined by a picosecond infrared pump-probe study. Relaxation time constants between 4.8 and 40.5 ps have been obtained. The discussion of the general solvent dependence suggests that in all cases the solvent fundamental with the smallest energy mismatch is favorably populated by this intermolecular energy transfer process.

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.

Comprehensive investigation of vibrational relaxation of non-hydrogen-bonded water molecules in liquids.

The vibrational dynamics of isolated water molecules dissolved in the nonpolar organic liquids 1,2-dichloroethane (C(2)H(4)Cl(2)) and d-chloroform (CDCl(3)) have been studied using an IR pump-probe experiment with approximately 2 ps time resolution. Analyzing transient, time, and spectrally resolved data in both the OH bending and the OH stretching region, the anharmonic constants of the bending overtone (v=2) and the bend-stretch combination modes were obtained. Based on this knowledge, the relaxation pathways of single water molecules were disentangled comprehensively, proving that the vibrational energy of H(2)O molecules is relaxing following the scheme OH stretch-->OH bend overtone-->OH bend-->ground state. A lifetime of 4.8+/-0.4 ps is determined for the OH bending mode of H(2)O in 1,2-dichloroethane. For H(2)O in CDCl(3) a numerical analysis based on rate equations suggests a bending overtone lifetime of tau(020)=13+/-5 ps. The work also shows that full 2-dimensional (pump-probe) spectral resolution with access to all vibrational modes of a molecule is required for the comprehensive analysis of vibrational energy relaxation in liquids.

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.

Size dependent ultrafast cooling of water droplets in microemulsions by picosecond infrared spectroscopy.

The ultrafast thermal relaxation of reversed micelles in n-octane/AOT/water (where AOT denotes sodium di-2-ethylhexyl sulfosuccinate) microemulsions was investigated by time-resolved infrared pump-probe spectroscopy. This picosecond cooling process can be described in terms of heat diffusion, demonstrating a new method to determine the nanometer radii of the water droplets. The reverse micelles are stable against transient temperatures far above the equilibrium stability range. The amphiphilic interface layer (AOT) seems to provide an efficient heat contact between the water and the nonpolar solvent.

The examination of skin lipid model systems stressed by ultraviolet irradiation in the presence of transition metal ions.

In this study, we investigated the effects of ultraviolet (UV) radiation on lipid peroxidation in the presence of ionised iron as a transition metal. Fatty acids as important intercellular stratum corneum lipids and liposomes were used to model skin lipid systems for our experiments. A UV-A laser and a broad spectrum UV lamp were used to create high-level radiation. UV-related damage was quantified by the thiobarbituric acid assay detecting malondialdehyde. Electrospray mass spectrometry was used to characterise peroxidation products following UV exposure. We have shown that hydro- and endoperoxides are long stable intermediates deriving from lipid peroxidation. The incorporation of unsaturated fatty acids into phospholipid liposomes increased the average liposomal diameter and enhanced sensitivity to UV radiation. By comparing our data from laser induced monochromatic UV-A radiation and broad-spectrum UV irradiation, we have demonstrated that UV-A radiation can also induce lipid peroxidation in lipid model systems.

Broadly tunable femtosecond pulses generated by optical parametric oscillation.

A singly resonant optical parametric oscillator is demonstrated that uses a beta-barium borate crystal and is synchronously pumped by microsecond pulse trains of a Nd:glass laser (0.8 psec, 527 nm). Broadly tunable pulses are generated in the wavelength range 0.7-1.8 microm with a duration of 160-260 fsec and an energy conversion of 3%. Steep pulse wings are observed, with a 1/e decay time of approximately 90 fsec. Under special conditions, parametric pulses as short as 65 +/- 7 fsec are obtained.

Picosecond coherent anti-Stokes Raman spectroscopy of molecules in free jet expansions.

By using time-resolved coherent Raman scattering, closely spaced vibrational resonances are simultaneously excited and subsequently monitored over long, nanosecond time intervals. Coherent superposition of molecular transitions leads to dramatic oscillations of the picosecond anti-Stokes scattering signals. Experimental data are presented for the nu(1) mode of CH(4) in supersonic expansions at low rotational temperatures (T(ROT) >/= 25). Our timedomain data are in good agreement with spectroscopic results. The corresponding frequency resolution of less, similar3 x10(-3) cm(-1) suggests that this method of Fourier-transform Raman spectroscopy may prove useful in high-resolution molecular spectroscopy.