Juergen Reif

Surface patterning on insulators upon femtosecond laser ablation

Abstract The crater morphology upon femtosecond laser ablation from BaF2 and CaF2, exhibits several periodic structures of characteristics that cannot be explained by interference phenomena. More likely, the ripple pattern presents features of self-organization from a chaotic state. The ablation under the applied conditions is due to Coulomb explosion of the surface, which indicates that local thermal effects should play a minor role in the ripple formation. In this paper, we present morphological structures that point toward instabilities and self-organization.

Influence of local heating on micro-Raman spectroscopy of silicon

In micro-Raman spectroscopy strong local heating by the tightly focused laser beam results in an unexpectedly small temperature shift of the Raman peak. The experimental results of shift, broadening, and asymmetry are discussed in view of restricted thermal expansion and a temperature dependent Raman cross section. The Raman signal originates mostly in the wings of the beam profile, where the temperature is substantially lower than at the center. Consequently, the Raman peak shift cannot be used to estimate the maximal surface temperature: despite local melting in the center of the probed spot, the maximal detected shift is only about 2 cm−1, 10 times less than expected from the center temperature.

Genesis of femtosecond-induced nanostructures on solid surfaces

The start and evolution of the formation of laser-induced periodic surface structures (LIPSS, ripples) are investigated. The important role of irradiation dose (fluence×number of pulses) for the properties of the generated structures is demonstrated. It is shown how, with an increasing dose, the structures evolve from random surface modification to regular sub-wavelength ripples, then coalesce to broader LIPSS and finally form more complex shapes when ablation produces deep craters. First experiments are presented following this evolution in one single irradiated spot.

Laser plasma threshold of metals

For nanosecond laser pulses, thermal properties have a decisive influence on laser plasma thresholds of metals. The acoustic mirage effect technique allows quantitative investigations over a wide range of incident laser intensities, from subthreshold heating to dense plasma generation. Under particular consideration of copper and titanium targets, with strongly different thermal conductivities, we show that two threshold conditions must be fulfilled concurrently: the metal surface must be heated to the boiling point to provide metal vapor and the laser intensity has to be high enough to enable dielectric breakdown in the vapor. Even at very high fluences, where a dense plasma is created, thermal properties are of importance. Since part of the incident energy is consumed for evaporation, only the excess energy can heat the plasma.

Basic Physics of Femtosecond Laser Ablation

Laser ablation being the basic process for many prominent applications of lasers in present day high technology, medicine, and other fields, its basic physics is reviewed in this chapter. In order to distinguish the fundamental, laser–material interaction from the secondary effects, we concentrate on ultrashort laser pulses ( ≈ 100 fs duration) at comparably low intensities, below the commonly indicated threshold for massive material removal. It is shown that – for these conditions – the principal light/matter coupling occurs via multiphoton excitation of electrons into the conduction band or the vacuum. The resulting perturbation of the target lattice results in the emission of positive particles, from atomic ions to larger clusters of more than ten atoms. With the increasing number of incident pulses, the light/material coupling is facilitated by the accumulation of transient crystal defects resulting from particle removal. On the other hand, the lattice destabilization, upon excitation and ablation, relaxes via self-organized formation of regular nanostructures at the irradiated area. The strong influence of laser polarization on the structural order is still not really understood.

Laser-induced ion emission from dielectrics

Photo-ablation of sapphire by ultrashort laser pulses was investigated by time-of-flight mass spectrometry. Experiments were performed on a laser fluence range below the single shot damage threshold. The dependence of emitted positive ion intensity on both the laser fluence and the number of laser pulses, hitting the same target site, was studied. In addition, the ion kinetic energy distribution was analysed. We find that the ablation is caused by surface explosion. The origin of this explosion, however, is still unknown.

Raman measurement of stress distribution in multicrystalline silicon materials

Abstract Micro-Raman spectroscopy was used to assess the spatial distribution of stress in multicrystalline silicon (mc-Si). Changes in the Raman peak position of about 0.1 wavenumbers on a length of several micrometers were detected between the grains and around the grain boundaries. These changes correspond to stress variations of about 50 MPa. Since the shifts caused by the stress are comparably small a careful analysis of the factors, influencing the measurement is necessary. Three major restrictions of the technique have to be considered: (i) the laser power should not cause significant local heating; (ii) the integration time should be large enough to resolve the signal with sufficient accuracy; (iii) the sample surface should be kept at the focus of the probe beam.

Optical processing on a femtosecond time scale

An all optical full logic unit, operating at femtosecond switching times, is demonstrated, based on transient index gratings from three independent, but mutually coherent, subpicosecond input laser pulses. The output signal is made up by the third harmonic of the input beams, resulting in a unique contrast ratio.

Multiphoton ionization of nitrotoluenes by means of ultrashort laser pulses

Abstract The potential of ultrashort laser pulses in the laser mass spectrometry of photounstable molecules is demonstrated for the case of nitrotoluenes. Nitro compounds tend to quickly dissociate after photoexcitation producing only unspecific fragments under conventional nanosecond multiphoton ionization conditions. The mass spectra of two isomers of mononitrotoluene, two isomers of dinitrotoluene and trinitrotoluene were recorded following multiphoton ionization with 170 fs laser pulses either with a wavelength of 412 or 206 nm. Although even these laser mass spectra are characterized by intense fragmentation they exhibit a clear molecular ion or OH loss signals depending on the substitution positions. Although the two mononitrotoluenes can be distinguished by their mass spectra at both wavelengths the two isomers of dinitrotoluene investigated show characteristic features which allow their clear differentiation only at 412 nm. Keywords: Multiphoton ionization; Mass spectrometry; Ultra short laser pulses; Nitrotoluenes; Explosives

Formation of Self-Organized Regular Nanostructures upon Femtosecond Laser Ablation

At the bottom of ablation craters produced in many materials, e.g. dielectric and silicon crystals, by the impact of femtosecond laser radiation, regular periodic structures are observed with a feature size at the order of a few 100 nanometers, much smaller than the incident wavelength. Their orientation depends strongly on the laser polarization but not on any intrinsic crystalline parameters. An increasing number of shots results in higher contrast, better developed structures, indicating a positive feedback. The region around the impact is shown, by micro Raman spectroscopy, to undergo phase transformations like under high pressure. The structure spacing appears to depend crucially on the depth of the perturbed volume, i.e. the incident (and absorbed) energy. All observations suggest that the structures form by self-organization from instabilities induced in the material by the laser input. A general picture suggests that the irradiation results in a rapid, non-equilibrium destabilization of the crystal structure, which should not be confused with melting as a classical thermodynamic process (i.e. temperatures defined as equilibrium properties). Relaxation from this instability results in the self-assembly of the observed structures. Theoretical simulations demonstrate the feasibility of this model, which also is corroborated by comparison to other unstable situations.