The dynamics of fs-laser ablation of graphite has been investigated experimentally and theoretically. The experimental observation of two different ablation mechanisms is supported by molecular dynamics calculations, which incorporate the changes of the interatomic potentials due to electronic excitation.

Ultrafast time resolved microscopy of femtosecond laser irradiated surfaces reveals a universal feature of the ablating surface on nanosecond time scale. All investigated materials show rings in the ablation zone, which were identified as an interference pattern (Newton fringes). Optically sharp surfaces occur during expansion of the heated material as a result of anomalous hydrodynamic expansion effects. Experimentally, the rings are observed within a certain fluence range which strongly depends on material parameters. The lower limit of this fluence range is the ablation threshold. We predict a fluence ratio between the upper and the lower fluence limit approximately equal to the ratio of critical temperature to boiling temperature at normal pressure. This estimate is experimentally confirmed on different materials (Si, graphite, Au, Al).

Using time-of-flight mass spectroscopy, we have investigated melting and ablation of gallium arsenide and silicon irradiated by femtosecond pulses. Below the ablation threshold the maximum surface temperature is obtained from the collisionless time-of-flight distributions of evaporated or sublimated particles. At the melting threshold, we estimate a temperature for the silicon surface which is approximately 500 K higher than the equilibrium melting temperature. In the fluence regime where melting is known to be a nonthermal process, we measure maximum surface temperatures in excess of 2500 K for both silicon and gallium arsenide, indicating rapid thermalization after nonthermal melting. At the ablation threshold, we estimated for both materials surface temperatures between 3000 and 4000 K. We observed a clear threshold-like effect in the number of detected particles, indicating the occurrence of a bulk effect. The flow parameters above the ablation threshold are discussed and compared to the different models of collisional expansion. For Fabl2Fabl, we find evidence that expansion takes place at temperatures that are higher than the critical temperature. Plasma formation appears only at fluences above 1 J/cm2 (F>5Fabl).

We have investigated femtosecond laser-induced ablation of gallium arsenide using time-of-flight mass spectroscopy. At the ablation threshold, we estimated surface temperatures on the order of 3500 K. We observed a clear thresholdlike effect in the number of detected particles and with increasing fluence free flight desorption transforms into a collisional expansion process. Above the ablation threshold, the behavior of gallium particles can be quantitatively described through Knudsen-layer theory.

Velocity distributions of evaporated particles from laser irradiated GaAs and Silicon give a temperatures in excess of 3000 K at the ablation threshold. The results suggest that explosive boiling causes material removal.

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 1013 W/cm2 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 laser generated 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.

Thermal- and nonthermal melting in gallium arsenide after femtosecond laser excitation has been investigated by means of time resolved microscopy. Electronic melting within a few hundred femtoseconds is observed for rather strong excitation and the data reveal a distinct threshold fluence of 150 mJ/cm(2) for this nonthermal process. Below that threshold melting occurs on a 100 ps time scale and is of thermal nature. Using a simple numerical model we describe this type of the phase transition as heterogeneous melting under strongly overheated conditions. [S0163-1829(98)51142-3].

Time-resolved optical microscopy during femtosecond laser irradiation of amorphous GeSb films provides evidence for a non-thermal phase transition occurring on a subpicosecond time scale.

Short pulse laser ablation of semiconductors and metals is studied by means of ultrafast time-resolved microscopy. The characteristic stages of the conversion of solid material into hot fluid matter undergoing ablation are identified, initially metallic material transforms during the expansion into a transparent state with a high index of refraction.

Ultrafast time resolved microscopy has been used to study the dynamics of femtosecond laser induced ablation from the surfaces of various semiconductors and metals. Upon ablation initially metallic matter transforms into a transparent state with a high index of refraction.

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.

Time resolved imaging has been used to analyze structural transformations induced by intense 100 fs laser pulses in amorphous GeSb films. Above a threshold of 19 mJ/cm(2) the data show the formation of a transient nonequilibrium state of the excited material within 300 fs. The results are consistent with an electronically induced, amorphous-to-crystalline phase transition. [S0031-9007(98)07514-0].

We have investigated femtosecond laser-induced ablation of gallium arsenide and silicon using time-of-flight mass spectroscopy. Below the ablation threshold we observe free flight desorption of atoms from the laster heated surface. The absence of collisions between particles leaving the solid allows to obtain the maximum surface temperature during laser irradiation of Gallium Arsenide. We estimated maximum surface temperatures of the order of 3500 K at the ablation threshold, where we observed a step-like increase in the number of detected particles. In the case of Silicon the existence of molecules of up to 6 atoms does not allow to measure the surface temperature. With increasing fluence free flight desorption transforms into a collisional expansion process. The behavior of Gallium particles can be quantitatively described through Knudsen-layer theory, indicating that Gallium particles expand as a non-ideal gas close to the ablation threshold ((gamma) equals Cv/Cp less than 5/3). Above fluences of approximately 2.5 Fth (gamma) approaches 5/3 indicating an ideal gas behavior for the expanding material. Dilution into the two phase regime of a superheated liquid characterizes ablation close to threshold.

Time of flight (TOF) distributions of evaporated particles from the surface of GaAs have been measured in order to determine its maximum temperature during femtosecond irradiation (100 fs, 620 nm). A quadrupole mass spectrometer (QMS) provides the TOF distribution of a given particle mass after laser excitation.

Modification of the surface structure of solid materials by laser radiation involves a complex chain of processes. The first step is the deposition of a certain amount of optical energy in the material. The character of the material excitation is strongly dependent on the laser pulse duration. With the use of ultrashort laser pulses non-equilibrium energy distributions with large excess population in the excited states can be produced. The distinct physical processes which come into play in laser–solid interaction on the ultrafast time scale open new routes of modifying the structure and the morphology of materials and offer interesting perspectives in laser materials processing.

During the past several years a number of reports have been published [1-5] suggesting that an ultrafast order-disorder transition can occur within approximately 100 fs when semiconductors (typically Si and GaAs) are irradiated with laser pulses 100 fs or less in duration. The evidence for such transitions was largely based on the observed changes of the optical reflectivity which were shown to be consistent with a transition from a covalent crystalline to a metallic liquid state.

Femtosecond-laser-induced changes of the optical reflectivity and the reflected second harmonic are measured over a wide range of times and laser fluences. Changes of the linear and nonlinear optical properties suggestive of a, transition to a metallic state have been observed. For relatively low fluences these changes take tens of picoseconds to develop. For higher fluences the transition takes place in just a few hundred femtoseconds. Our data suggest that, depending on the laser excitation conditions, two distinctly different types of phase transformations are observed.

It is generally accepted that picosecond laser-induced melting and resolidification can be explained in terms of thermal processes. However, for femtosecond pulses the energy thermalization time is longer than the pulse duration, and the situation is far from being well understood.

The γ-ray deexcitation of high spin states in 130,134,136Ce has been studied when targets of 122,128,130Te are bombarded with 12C-ions of 100 MeV. The γ-rays are detected in a coincidence arrangement consisting of six hexagonal NaI(Tl)-detectors and from the data recorded γγ-energy correlation matrices and γ-ray multiplicities are deduced. The two heavier isotopes 134,136Ce do not exhibit a clear rotation behaviour but with the light target a valley extending up to Eγ = 1.25 MeV is seen in the γγ-energy correlation matrix.