C. Negutu

Real-Time Monitoring of the Pulsed Laser Ablation of Metals Using Ablation Plasma Spectroscopy

Here we demonstrate that the emission spectra of the ablation-plasma produced by nanosecond laser pulses on metallic Al targets may be directly connected to the ablation rates and the dimensions of the ablated craters. We show that the variation of the individual spectral-lines intensities with pulse number gives direct, real-time information on the crater depth, whereas the relative intensities of the lines and their widths enable us to study the variation of the electron temperature and density with pulse number and laser fluence in direct connection to the ablation rates. To interpret these results we use a simple model in which the plasma-plume is treated as an ideal gas expanding away from the target with a velocity given by the electron-temperature, and exerting a recoil pressure determined by the electron temperature and density. The model correlates the plume hydrodynamic-length to the crater dimensions and succeeds in predicting the rims heights.

Experimental investigation of the dimensions and quality of laser-drilled holes in metals

We investigated the process of laser micro-drilling of copper and iron by using nanosecond laser-pulses at 532nm wavelength in atmospheric air. We analyzed the ablated volume, ablation rate, crater diameter, and craters quality as functions of laser-fluence and beam-diameter. The fluence was varied between 10 and 6000 J/cm2 by changing the laserenergy. The results indicate that the ablated volume increases linearly with fluence, whereas the ablation rate and crater diameter increase linearly with the fluences square root. The ablated volume, ablation rate, and crater diameter, increase with thermal diffusivity of the materials. Additionally, the ablation threshold-fluence is demonstrated to be directly related to the optical penetration depth. The ablated volume, ablation rate, and crater diameter were further assessed for beam-diameters in the range of 10-50 microns by translating the targets away from the focal plane while keeping a constant fluence. The results indicate that the ablated volume increases linearly with beam-diameter, whereas the ablation rate and crater diameter increase linearly with the inverse of the beam-diameters square root. To investigate the craters quality we measured the dimension of the thermally affected zone (TAZ) around the craters as a function of fluence. At fluences up to 400 J/cm2, where strong ionization occurs within the plume, the crater diameter is <15 microns (comparable with beam diameter) and there is small TAZ around the craters. Further increase of the fluence leads to a significant increase of TAZ, indicating that the expanding plasma plays a major role in metals ablation in this fluence domain.

Numerical simulations of surface plasmon resonances in metal-chalcogenide waveguides

In this paper we present several numerical simulations of the surface plasmon resonance for Kretschmann type configuration in a metal-chalcogenide waveguide. We assume that the chalcogenide (GaLaS) waveguide layer have finite thickness, whereas the gold film layer and the air cover layer are semi-infinite layers (from an optical point of view). We determined the thickness of the chalcogenide film for which plasmonic resonant coupling of the incident radiation to the waveguide occurs. We calculated the propagation constant for the TE- and TM- modes (both for visible and IR domain), the attenuation coefficient and the electromagnetic field distribution within the waveguide. The obtained results provide the conditions for design an optical memory device 2D based on light-light interaction in plasmonic configuration.

A spectroscopic and theoretical study of the laser ablation rate of Al

We investigated experimentally and theoretically the laser ablation of Al by using nanosecond laser-pulses at 532 nm wavelength in atmospheric air. We analyzed experimentally the dependence of the ablation rate on the laser fluence and pulse number. The fluence was varied between 3 and 3000 J/cm2 by changing the laser-energy, while pulse number was varied between 4 and 60 in steps of 4. The optical microscopy data indicate that the ablation rate increases approximately linearly with the 1/3 power of the fluence. For high fluences (hundreds of J/cm2) the ablation rate is demonstrated to be very large (~2 micron/pulse) and approximately constant during 30 consecutive pulses and much smaller during the next pulses. The dependence of the ablation rate on pulse number was further addressed by spectrometric analysis of the ablation-plasmas produced at high fluences. We found that the plasma temperature varies similarly to the ablation rate when increasing the pulse number. The ablation rate in the low fluence regime was addressed theoretically within the frame of a photo-thermal model which accounts for the material heating, melting and evaporation upon laser radiation. The theoretical and experimental results are in good agreement indicating the validity of the model for low laser fluences.

Effect of pulse number on the ablation rate of metals in PLA multi-pulse regime

The pulsed laser ablation of aluminium, copper and titanium irradiated with 4.5 ns pulses at 355 nm, 532 nm and 1064 nm wavelengths is investigated in open air at normal atmospheric conditions. The effect of pulse number, which is varied in the range of 5 to 50, on ablation rate in these three wavelength regimes is determined. The results indicate a higher efficiency of the ablation in VIS and UV regimes as compared to IR regime which is characterised by a very small optical absorbtivity. The ablation rate is demonstrated to be approximately constant when increasing the pulse number up to a certain value which strongly depends on the thermal properties of the material. Further increase of pulse number leads to a progressive decrease of ablation rate. The most pronounced decrease was obtained at 355 nm in aluminium where the ablation rate corresponding to the 50th pulse is about 20% of the ablation rate corresponding to of first pulse. The decay of the ablation rate with the pulse number is attributed to the superposition of two phenomena: the enhanced attenuation of the laser beam in the plasma plume which is confined within the crater, and the decay of the effective laser fluence at the target surface due to the gradual increment of the effective irradiated area with pulse number.

He-Ne laser gain dependence on discharge current and gas pressure from resonant Faraday effect

It is analyzed the influence of a longitudinal magnetic field on the operating mode of a He-Ne laser, knowing that the magnetic field is used for the suppressing of the (lambda) 3.39 micrometers , which is amplified in the same time with (lambda) 632.8 nm. It was used the Faraday magneto-optic effect at resonance, so that eh (lambda) 632.8 nm He-Ne laser radiation travels through a Ne-Ne mixture, with the pressures ratio pHe/pNe equals 5, in population inversion conditions, placed in a longitudinal magnetic field. It was studied the laser medium amplification coefficient dependence on the parameters: discharge current intensity and total gas pressure. For the relation between the rotation angle of the polarization plane in a magnetic field and the laser amplification coefficient, in the He-Ne mixture, it was found a linear theoretical dependence. By fitting the experimental data, there are obtained nonlinear dependencies, for (alpha) (p) and (alpha) (I), which can be explained in the atomic collisions theory.