Laser-induced ablation dynamics and flight of thin polymer films

We investigated the ejection dynamics of triazene polymer layers in the thickness range of 40 nm to 600 nm upon nanosecond laser ablation at a wavelength of 532 nm. The ablation is due to laser-induced thermal degradation of a small part of the polymer in contact with the silicon substrate. The subsequent dynamics of the flying polymer layer are measured with sub-nanosecond time resolution. The evaluation of the initial velocity for different film thicknesses gives insight into the energy transfer process during the acceleration of the films.     

Femtosecond and picosecond near-field ablation of gold nanotriangles: nanostructuring and nanomelting

We report on recent insights into the interaction between ultra-fast laser pulses and plasmonic nanoparticles. We discuss femtosecond near-field ablation as a simple but versatile tool for the nanoscale modification of surfaces and the high-resolution measurement of a nanostructure’s near field. Two model systems are presented, illustrating the complexity of near-field distributions. Furthermore, finite difference time domain calculations in combination with absorption spectra provide a deeper insight into the factors influencing the near-field distribution. For the first time, an almost perfect agreement between the measured ablation pattern and experiment has been reached for gold triangles with a side length around 500 nm. Additionally, the results from picosecond laser irradiated plasmonic structures display a new regime of nanoscale laser material processing. We present first results showing nanometre confined melting induced by laser pulses.     

Measurements of the ns-laserpulse induced expansion of (111) silicon below and above the melting threshold on the nanosecond time scale

The surface displacement of 111-silicon wafers of 0.675 mm and 3.05 mm thickness, respectively, during intense ns laser irradiation is determined on the nm-vertical and ns-time scale using an optimized Michelson interferometer. The surface dynamics is observed below as well as above the melting threshold. We show that the obtained surface expansion below the melting threshold is in good agreement with theoretical heat transfer calculations. Additionally, thickness-dependent bending in the samples is illustrated and the acoustic reflections from the backside of the sample are observed. Maximum thermal displacement at the first expansion plateau is around 6 nm before melting occurs. We show that qualitative agreement between experimental results and simulation above the melting threshold can be established for the first time by taking the phase shift change in the reflection for molten silicon into account.     

Nanosecond laser pulse induced vertical movement of thin gold films on silicon determined by a modified Michelson interferometer

The vertical movement of a 40 nm thin Au film on a silicon substrate during intense nanosecond (ns) laser irradiation is determined on the nm vertical and ns time scales using an optimized Michelson interferometer. The balanced setup with two detectors uses the inverse interference signal and accounts for transient reflectivity changes during irradiation. We show that a change in phase shift upon reflection must be taken into account to gain quantitative results. Three distinct fluence regimes can be distinguished, characterized by transient reflectivity behavior, dewetting processes and film detachment. Maximum displacement velocities are determined to be 0.6 m/s and 1.9 m/s below and above the melting threshold of the metal, respectively. Flight velocities of detaching liquid films are found to be between 30 and 70 m/s for many nanoseconds.     

Femtosecond laser near field ablation

The high field strength of femtosecond laser pulses leads to nonlinear effects during the interaction with condensed matter. One such effect is the ablation process, which can be initiated below the threshold of common thermal ablation if the excitation pulses are sufficiently short. This effect leads to structure formation, which is anisotropic because of the polarization properties of the near field and can result in pattern sizes below the resolution limit of light. These effects are explored by temporally resolved scattering methods and by post‐mortem analysis to show the non‐thermal and anisotropic nature of this process. The near‐field distribution of plasmon modes can be tailored to a large extent in order to obtain control of the pattern formation.     

Time-resolved measurements of the response of a STM tip upon illumination with a nanosecond laser pulse

2 ). This result demands a critical inspection of the interpretation of nanostructuring experiments with this technique. Received: 15 August 1997/Accepted: 8 December 1997     

A comparison of ns and ps steam laser cleaning of Si surfaces

We report a quantitative investigation on the efficiency of the steam laser cleaning process using ns and ps pulses. Well-characterized polymer particles with a diameter of 800 nm dispersed on commercial Si wafers were chosen as a modeling contaminant system. As a result of our investigation, we show for the first time the feasibility of performing efficient steam laser cleaning with ps laser pulses and compare the achieved efficiency with the one obtained for ns pulses. For ns pulses, we found a cleaning fluence threshold of 50 mJ/cm² that is independent of the pulse durations (2.5 ns and 8 ns) and the wavelengths (532 nm and 583 nm) used. The application of ps pulses (FWHM=30 ps, 5=583 nm) lowered this threshold to 20 mJ/cm². Both cleaning thresholds are far below the melting thresholds for these laser parameters. Cleaning efficiencies >90% were reached for both pulse durations.