Andrea Cavalleri

Abstract:

We have investigated the dynamics of femtosecond laser-induced ablation from the surface of various materials. Combining pump-probe techniques with optical microscopy we have monitored the structural modifications of the irradiated surfaces both in space and time. In the fluence regime below the threshold for plasma formation ablation is caused by the hydrodynamic expansion of laser-heated material. Upon expansion into vacuum each of the investigated materials (Si, GaAs, Al, Au, Mg, Hg) evolves from the initial metallic state into an optically transparent phase with high index of refraction. (C) 1998 Elsevier Science B.V.

Abstract:

The electron dynamics in metallic nanoparticles subjected to fs pulses shows distinctive features as a function of size. By reducing the size from approximate to 20 down to approximate to 1 nm, we progressively go: (i) first below the light penetration depth (approximate to 13-14 nm) and then (ii) below the mean free path (approximate to 5 nm). This affects drastically the behaviour of the electron dynamics in nanoparticles. More precisely, we observe: with (i) the disappearance of coherent acoustic oscillations generated by the fs pulses and clearly observable at large sizes; with (ii) the size dependence of the electron thermalization inside the nanoparticles. The above mentioned effects have been studied both in the solid and liquid state of the nanoparticles, In order to observe and study such phenomena it is essential that the nanocrystals are grown in a wide size range, with a regular (nearly spherical) shape and a relatively low size dispersion. (C) 1998 Elsevier Science S.A.

Abstract:

The dynamics of ablation from different absorbing and non-absorbing solid materials after excitation with 100 fs laser pulses were investigated. Using ultrafast time-resolved microscopy, the reflectivity of the excited surface was measured with both femtosecond time and micrometer spatial resolution. Transparent insulators ablation was found to be initiated by optical breakdown and plasma formation. A comparison of transient reflectivity data with the results of post mortem interference microscopy indicated that there is no significant removal of material without prior formation of a surface plasma. Quantitative measurements yielded threshold values of about 1013W/cm2, with little variation among different materials.

Abstract:

Time of flight (TOF) distributions of evaporated particles from the surface of GaAs were measured in order to determine its maximum temperature during femtosecond irradiation. A quadrupole mass spectrometer (QMS) provided the TOF distribution over a given particle mass after laser excitation. This article discusses the results of the study.

Abstract:

Femtosecond laser induced ablation from solid surfaces has been investigated by means of time resolved microscopy. On transparent materials ablation is initiated by dielectric breakdown and formation of a dense and hot surface plasma. Measurements of the plasma threshold yield values of a few times 10(13) W/cm(2) with little variation among different materials. This indicates that microscopic surface properties are responsible for surface breakdown. On absorbing semiconductors and metals near-threshold ablation is brought about by hydrodynamic expansion of the lasergenerated hot and pressurized matter. Upon expansion into vacuum initially metallic materials transform into a transparent state with a high refractive index. The observed behavior is related to general properties of matter in the liquid-gas coexistence regime.