Vassilios Kotaidis

Excitation of nanoscale vapor bubbles at the surface of gold nanoparticles in water

Intense nonequilibrium femtosecond laser excitation of gold nanoparticles in water leads to a transient heating of the nanoparticles, which decays via heat transfer to the water phase. It is shown that the water temperature rises to near the critical temperature and the water undergoes an explosive evaporation in the subnanosecond range. The formation of vapor bubbles shows a threshold dependence on laser fluence. The nascent nanoscale vapor bubbles change the heat dissipation drastically. The nanoscale structure is resolved directly with a combination of x-ray scattering methods sensitive to the particle lattice expansion and the change in the water structure factor.

Cavitation dynamics on the nanoscale

The ultrafast excitation of gold nanoparticle sols causes a strong nonequilibrium heating of the particle lattice and subsequently of the water shell close to the particle surface. Above a threshold in laser fluence, which is defined by the onset of homogeneous nucleation, nanoscale vapor bubbles develop around the particles, expand and collapse again within the first nanosecond after excitation. We show the existence of cavitation on the nanometer and subnanosecond time scale, described within the framework of continuum thermodynamics.

Small-angle pump–probe studies of photoexcited nanoparticles

X-ray scattering experiments on femtosecond laser-excited gold nanoparticle suspensions are presented. It is shown that the time-resolved pump-probe technique using the X-ray pulse structure at synchrotron sources is capable of resolving structural dynamics on the nanometer scale to high precision. The estimation of X-ray flux density allows the projection of experiments on an X-ray free-electron laser probing single nanoparticles in a one-shot exposure.

Dynamics of the laser-induced ferroelectric excitation in BaTiO3 studied by x-ray diffraction

Ultrafast time resolved x-ray powder scattering is used to reveal the structural dynamics in BaTiO3 powder excited by femtosecond laser pulses. The lattice excitation close to the Curie point of BaTiO3 is analyzed by refining the powder pattern (Rietveld method) to obtain the lattice state and the atomic positions. It is found that the tetragonal distortion of the ferroelectric phase is diminished within the 100ps time resolution of the experiment. The unit cell polarization, however, is not affected by this excitation, evidenced by the preserved Ti and oxygen displacement. The change in splitting relaxes on the nanosecond time scale.

Light-induced structural phase behaviour of metal nanoparticle materials

We have investigated the structural dynamics of gold nanoparticles induced by femtosecond light excitation. Structure evolution in both embedded particles (glass matrix or liquid water suspension) and quasi-free particles adsorbed on a solid surface is analyzed. By use of stroboscopic laser pump- x-ray probe techniques the structural relaxations have been resolved on the 100 ps time scale at the European Synchrotron Radiation Facility. Several methods including powder scattering, liquid scattering and small angle scattering serve to resolve microscopic and mesoscopic length scales of the composite system. The thermal response includes the heating, lattice melting, explosive solvent evaporation and solvent cooling subsequent to the laser flash excitation. Nonthermal effects are observed with femtosecond excitation. They are attributed to ablation from the particle and particle explosion at strong nonequilibrium conditions. The observations can form a complete picture of the energy dissipation and phase transitions involved in nanoscale composites.

High-speed asynchronous optical sampling for high-sensitivity detection of coherent phonons

A new optical pump-probe technique is implemented for the investigation of coherent acoustic phonon dynamics in the GHz to THz frequency range which is based on two asynchronously linked femtosecond lasers. Asynchronous optical sampling (ASOPS) provides the performance of on all-optical oscilloscope and allows us to record optically induced lattice dynamics over nanosecond times with femtosecond resolution at scan rates of 10 kHz without any moving part in the set-up. Within 1 minute of data acquisition time signal-to-noise ratios better than 107 are achieved. We present examples of the high-sensitivity detection of coherent phonons in superlattices and of the coherent acoustic vibration of metallic nanoparticles.

Structural kinetics in protein-coated gold nanoparticles probed by time-resolved x-ray scattering

Laser-excited gold nanoparticles in aqueous suspension act as nanoscale heat sources on a short time to the surrounding. By employing pulsed small-angle x-ray scattering the structural kinetics of an adsorbed protein layer is temporally resolved. The scattering data reveals a layer expulsion from the surface.

Dynamics of laser-excited nanoparticles

By the use of stroboscopic laser pump - x-ray probe techniques and x-ray scanning techniques the structural relaxations of gold nanoparticles have been resolved on the 50 ps time scale. The structural dynamics are addressed by several methods including power scattering and small angle scattering (SAXS) to resolve microscopic and mesoscopic length scales of the composite system. The laser power is a direct measure of the dissipated heat. Thus the caloric reaction and melting transition can be monitored as function of temperature, particle size and time. Nonlinear effects are observed with femtosecond excitation, attributed to ablation. While the phenomenology for nanoparticle suspensions and surface supported monolayers display similar energetics, structure formation processes are strongly altered on the surface due to interparticle interactions.