Jörg Hermann

Optical nonlinearities in conjugated systems: β‐carotene

Quantitative studies of third‐harmonic generation in the all‐trans β‐carotene molecule and of third‐harmonic generation and the optical Kerr effect in β‐carotene glass are reported. The large values of the optical nonlinearities are attributed to the broadly delocalized Π electrons.

Diagnostics of the early phase of an ultraviolet laser induced plasma by spectral line analysis considering self-absorption

We have performed time- and space- resolved plasma diagnostics during ultraviolet excimer laser ablation of a Ti target in a low pressure N2 atmosphere. Spectral lines emitted from metal vapor ions in the early plasma phase (t⩽200  ns) have been analyzed and compared to line profiles computed for a plasma in a local thermodynamic equilibrium.

Study of initial dust formation in an Ar‐SiH4 discharge by laser induced particle explosive evaporation

The initial step of particulate growth in a dust forming low pressure radio‐frequency discharge has been studied in situ by laser induced particle explosive evaporation (LIPEE). With respect to the conventional light scattering, this method has been found much more efficient to observe small nanometer size particles, especially in the case of UV excimer laser radiation. Experimental results interpreted by a simple model of laser‐particle interaction show that the intensity of LIPEE continuum emission depends on the particle radius roughly as r4. This interaction is essentially different from Rayleigh scattering, as the latter varies as r6. A study of time evolution of powder formation by LIPEE emission reveals the initial formation of nanometer size crystallites and the coalescence process leading to larger scale particles. It could be demonstrated that the critical step of dust formation is the initial clustering process leading to nanometer scale crystallites.

Comparative investigation of solar cell thin film processing using nanosecond and femtosecond lasers

The purpose of the present study was to examine the possibility of laser-machining of CuInSe2-based photovoltaic devices. Therefore, ablation thresholds and ablation rates of ZnO, CuInSe2 and Mo thin films have been measured for irradiation with nanosecond laser pulses of ultraviolet and visible light and subpicosecond laser pulses of a Ti : sapphire laser. The experimental results were compared with the theoretical evaluation of the samples heat regime obtained from numerical calculations. In addition, the photo-electrical properties of the solar cells were measured before and after laser-machining. Scanning electron microscopy and energy dispersive x-ray analyses were employed to characterize the laser-induced ablation channels. As a result, two phenomena were found to limit the laser-machining process: (i) residues of Mo that were projected onto the walls of the ablation channel and (ii) the metallization of the CuInSe2 semiconductor close to the channel. Both effects lead to a shunt in the device that decreases the photovoltaic efficiency. As a consequence of these limiting effects, micromachining of CuInSe2-based solar cells was not possible with nanosecond laser pulses. Only subpicosecond laser pulses provided selective or complete ablation of the thin layers without a relevant change in the photoelectrical properties.

Plasma analyses during femtosecond laser ablation of Ti, Zr, and Hf

Femtosecond laser ablation of Ti, Zr, and Hf has been investigated by means of in situ plasma diagnostics. Fast imaging was used to characterize the plasma plume expansion on a nanosecond time scale. In addition, time- and space-resolved optical emission spectroscopy was employed to determine the plume composition and the characteristic expansion velocities of plasma species. It is shown that two plume components with different expansion velocities are generated by the interaction of ultrashort laser pulses with metals. The composition and the expansion behavior of the two components have been analyzed as a function of laser fluence and target material. The results are discussed in terms of mechanisms responsible for ablation by ultrashort laser pulses.

Reducing nanoparticles in metal ablation plumes produced by two delayed short laser pulses

Using fast imaging and atomic force microscopy, we demonstrate that the fraction of nanoparticles in ablation plumes produced by short pulse laser irradiation of metals is strongly altered when a second laser pulse of sufficiently large delay is applied. Comparing the results obtained for gold and copper, it is shown that a significant nanoparticle reduction is only observed if the delay between both laser pulses exceeds the characteristic time of electron-lattice thermalization. We propose the reduced electronic heat transport at large lattice temperature as the dominant mechanisms for the observed nanoparticle reduction.

Combined continuous–microscopic modeling of laser plume expansion

A hybrid model is developed to study the dynamics of laser plume expansion in vacuum or into a background gas. The method takes advantages of both continuous and microscopic simulation approaches. As a result of the combination of two different numerical methods, such as, large particles and direct Monte Carlo simulation, the model describes high-rate laser ablation for a wide range of background pressures (from zero to hundreds Pa). The model is used to investigate laser plume interaction with background gases. Particularly, the plume–gas mixing and energy exchange are taken into account. The dynamics of the laser plume expansion is investigated. Snowplow effect is observed at sufficiently high pressures. At smaller pressures, strong plume–gas mixing takes place near the contact surface. The simulation results explain experimentally obtained spatial maps of the reaction products formed during the plume expansion into a reactive background gas.

Multistage plasma initiation process by pulsed CO2 laser irradiation of a Ti sample in an ambient gas (He, Ar, or N2)

New experimental results are reported on plasma initiation in front of a titanium sample irradiated by ir (λ=10.6 μm) laser pulses in an ambient gas (He, Ar, and N2) at pressures ranging from several Torr up to the atmosphere. The plasma is studied by space‐ and time‐resolved emission spectroscopy, while sample vaporization is probed by laser‐induced fluorescence spectroscopy. Threshold laser intensities leading to the formation of a plasma in the vapor and in the ambient gases are determined. Experimental results support the model of a vaporization mechanism for the plasma initiation (vaporization‐initiated plasma breakdown). The plasma initiation is described by simple numerical criteria based on a two‐stage process. Theoretical predictions are found to be in a reasonable agreement with the experiment. This study provides also a clear explanation of the influence of the ambient gas on the laser beam‐metal surface energy transfer. Laser irradiation always causes an important vaporization when performed i...

Laser-induced fluorescence probing during pulsed-laser ablation for three-dimensional number density mapping of plasma species

Laser-induced fluorescence spectroscopy was employed to measure ground state number densities of atoms and molecules in plasma plumes generated by pulsed-laser ablation of Al, C and Ti targets in N2 or O2 low-pressure atmospheres. A beam expander was used to transform the dye laser beam in a thin plane section of 0.2×40 mm2 dimension crossing the plasma through its symmetry axis. Using a fast intensified charged coupled device matrix for fluorescence detection, three-dimensional number density mapping of plasma species was acquired. Calibration of the measured ground state densities in an absolute scale was performed by additional absorption measurements. According to the plasma temperature, the density of atoms and molecules and their total number in the plasma plume were estimated. The species densities were compared to those obtained by intensity calibrated emission spectroscopic measurements. The time- and space-evolution of atomic and molecular densities gives information about gas-phase reactions due to the interaction of the ablated material with the surrounding low-pressure gas. The results contribute to a better understanding of thin film synthesis by reactive pulsed-laser deposition.

Metal surface nitriding by laser induced plasma

We study a nitriding technique of metals by means of laser induced plasma. The synthesized layers are composed of a nitrogen concentration gradient over several μm depth, and are expected to be useful for tribological applications with no adhesion problem. The nitriding method is tested on the synthesis of titanium nitride which is a well‐known compound, obtained at present by many deposition and diffusion techniques. In the method of interest, a laser beam is focused on a titanium target in a nitrogen atmosphere, leading to the creation of a plasma over the metal surface. In order to understand the layer formation, it is necessary to characterize the plasma as well as the surface that it has been in contact with. Progressive nitrogen incorporation in the titanium lattice and TiN synthesis are studied by characterizing samples prepared with increasing laser shot number (100–4000). The role of the laser wavelength is also inspected by comparing layers obtained with two kinds of pulsed lasers: a transversal...