Pulsed x-ray scattering is used to examine the lattice dynamics in gold nanoparticles in water following excitation with intense femtosecond laser pulses. At lower excitation power the initial lattice heating is followed by cooling on the nanosecond time scale. The decay can be described by solving the heat transfer equations including both the bulk conductivity in water and a finite thermal boundary resistance at the particle-water interface. The lattice expansion rises linearly with excitation power, up to an excitation power corresponding to a lattice temperature increase of 529 K. At higher temperatures the lattice shows a loss of long-range order due to pre-melting of the particles. At the bulk melting temperature, complete melting occurs within the first 100 ps after laser excitation.

Short-pulse laser ablation is promising owing to the low threshold for material removal from surfaces. In the laser-ablation process, solid material transforms into a volatile phase initiated by a rapid deposition of energy. Explosive boiling can be one of the mechanisms in which matter is heated close to the critical point. Other pathways of non-thermal excitation will be present for very short laser pulses¹. Here we observe a different channel of ablation from gold nanoparticles, which takes place below the particle melting point. This process is induced by the optical near-field, a subwavelength field enhancement close to curved surfaces, in particular. Using picosecond X-ray scattering, we can track the temporal and energetic structural dynamics during material ejection from the nanoparticles. This effect will limit any high-power laser manipulation of nanostructured surfaces, such as surface-enhanced Raman measurements² or plasmonics with femtosecond pulses.

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.

We investigated nanoparticle formation after pulsed laser ablation in liquid using time-resolved small angle x-ray scattering. Laser ablation of a gold target in water induces a cavitation bubble in which two different particle species could be identified at maximum bubble extension: (i) primary particles of about 8–10 nm diameter, which show a smooth concentration gradient starting from the target and can also be found outside the cavitation bubble in the free liquid and (ii) secondary particles in the range of 45 nm diameter which have highest concentration in the upper part of the cavitation bubble but do not penetrate into the liquid.

The formation of nanoparticles within the laser-induced cavitation bubble is studied in situ using small angle X-ray scattering with high spatiotemporal resolution. Directly after laser ablation, two different particle fractions consisting of compact primary particles of 8–10 nm size and agglomerates of 40–60 nm size are formed. The abundance of these species is strongly influenced by the dynamics of the oscillating cavitation bubble. Primary particle mass is most abundant during maximal expansion of the first bubble and reappears a little weaker in the rebound. In contrast to this, the mass abundance of agglomerates is relatively low in the first bubble but strongly increases during first bubble collapse and following rebound. Although most of the ablated material is trapped inside the bubble and follows its oscillation, a minor fraction of both species could be detected outside the cavitation bubble even before its final collapse.