K. Sokolowski-Tinten

The physical mechanisms of short-pulse laser ablation

Time-resolved experiments demonstrate that the removal of material from the surface of metals and semiconductors following irradiation with pico- or femtosecond laser pulses occurs on a nanosecond time scale. It is shown that short-pulse laser ablation can be interpreted in terms of transient thermal processes involving states of matter with unusual thermodynamic, hydrodynamic and optical properties.

Laser-solid interaction in the femtosecond time regime

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.

Two distinct transitions in ultrafast solid-liquid phase transformations of GaAs

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.

FEMTOSECOND LASER ABLATION OF GALLIUM ARSENIDE INVESTIGATED WITH TIME-OF-FLIGHT MASS SPECTROSCOPY

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.

Ultrafast phase transitions and lattice dynamics probed using laser-produced x-ray pulses

When intense femtosecond laser pulses are focused on solid targets short-lived microplasmas are formed which emit bursts of x-rays with kilovolt photon energies. Under the proper conditions x-ray pulses as short as a few hundred femtoseconds can be produced. These x-ray pulses enable ultrafast x-ray spectroscopy using pump–probe schemes where the x-ray pulses serve as probe pulses. This article describes time-resolved x-ray diffraction experiments which reveal changes in the atomic structure with a time resolution of a few hundred femtoseconds. In particular, we have studied solid-to-liquid phase transitions in semiconductors induced by femtosecond photoexcitation and the accompanying thermoacoustic phenomena. We were able to monitor the changes in the atomic position underlying a coherent optical phonon mode. These and a number of other lattice dynamics experiments discussed here demonstrate the feasibility and usefulness of ultrafast time-resolved x-ray diffraction. Future applications in many other fields of science can be foreseen.

Dynamics of femtosecond-laser-induced ablation from solid surfaces

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.

Ablation of solids using a femtosecond extreme ultraviolet free electron laser

The ablation of solids by high energy femtosecond pulses from an extreme ultraviolet (XUV) free electron laser has been investigated using picosecond optical imaging. The time-resolved measurements are supplemented by an analysis of the permanent structural surface modifications. Compared with femtosecond optical excitation, distinct differences in the material response are found which are attributed to the increased penetration depth of the XUV radiation and the absence of any absorption nonlinearities.

Observation of a transient insulating phase of metals and semiconductors during short-pulse laser ablation

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.

Generation and application of ultrashort X-ray pulses

Relatively small-scale laser-driven sources of short wavelength radiation covering a range from the extreme ultraviolet to the hard X-ray regime are now available. Because the duration of the X-ray pulses is comparable to, or shorter than the laser pulse width, it is possible to carry out X-ray measurements with picosecond or femtosecond time resolution.

The influence of wavelength on phase transformations induced by picosecond and femtosecond laser pulses in GeSb thin films

Cycling between the crystalline and amorphous phases of 25-nm-thick GeSb films induced by single laser pulses of duration of 100fs or 20ps is investigated in the 400–800nm wavelength range. The time evolution of the phase transformations has been studied with picosecond resolution real-time reflectivity measurements at a probe wavelength of 514.5nm and also with femtosecond and picosecond pump-probe measurements. Upon picosecond irradiation, three regimes are identified: for wavelengths below ∼550nm and above ∼750nm, the total time to transform between the crystalline and amorphous phases is of the order of 10–24ns while in the intermediate wavelength range of 600–750nm, the transformation time is only ∼650ps. Upon 100fs irradiation, the transformation times are observed to decrease with increasing wavelength with the shortest times of ∼5ns for crystallization and ∼10ns for amorphization, both occurring at 800nm. This behavior is discussed in terms of how the wavelength-dependent refractive index of the p...