Ion M. Popescu

Impact of the lasr wavelength and fluence on the ablation rate of aluminium

The dependence of the ablation rate of aluminium on the fluence of nanosecond laser pulses with wavelengths of 532 nm and respectively 1064 nm is investigated in atmospheric air. The fluence of the pulses is varied by changing the diameter of the irradiated area at the target surface, and the wavelength is varied by using the fundamental and the second harmonic of a Q-switched Nd-YAG laser system. The results indicate an approximately logarithmic increase of the ablation rate with the fluence for ablation rates smaller than ∼6 μm/pulse at 532 nm, and 0.3 μm/pulse at 1064 nm wavelength. The significantly smaller ablation rate at 1064 nm is due to the small optical absorptivity, the strong oxidation of the aluminium target, and to the strong attenuation of the pulses into the plasma plume at this wavelength. A jump of the ablation rate is observed at the fluence threshold value, which is ∼50 J/cm2 for the second harmonic, and ∼15 J/cm2 for the fundamental pulses. Further increasing the fluence leads to a steep increase of the ablation rate at both wavelengths, the increase of the ablation rate being approximately exponential in the case of visible pulses. The jump of the ablation rate at the threshold fluence value is due to the transition from a normal vaporization regime to a phase explosion regime, and to the change of the dimensionality of the hydrodynamics of the plasma-plume.

Fine-structure measurements of two-photon transitions in atomic cesium with a tunable dye laser

Abstract Two-photon transitions have been observed from the 62S ground state of atomic cesium to selected higher lying n2D levels up to n = 19. The transitions were excited by a narrowband dye laser pumped by a nitrogen laser and detected by a space charge limited cesium vapor photodiode. The fine structure intervals of the n2D levels for n = 15, 17 and 19 were measured by direct comparison with the ground-state hyperfine interval, and the line strength ratios of the fine-structure components were measured and compared with the predictions of a simple theoretical model.

Characterization of Er3+-doped Ti:LiNbO3 waveguides: losses, absorption spectra, and near field measurements

In this paper we present some experimental results about the characterization of Er^3+-doped Ti:LiNbO_3 waveguides using nondestructive methods. We report results concerning losses, near field and absorption measurements. Waveguide losses have been measured in the 1550 nm wavelength region through an interferometric method based on two different techniques: thermal cavity tuning and wavelength sweeping. Homogeneous absorption and emission cross sections have been calculated from transmitted spectra. The near field measurements have been used to obtain the waveguide mode profile through a deconvolution algorithm to eliminate the probe effect. The resulting field profile allowed to reconstruct the refractive index distribution.

Modeling Nonlinear Coupling between Plasma, Electrodes and Walls for High‐Pressure Hg Arc Discharge Lamps

This paper presents a model able to give a global description of the functioning of high-pressure Hg arc discharge lamps, with operation in the diffuse cathode mode. For computational simplicity reasons the lamp is fed on d.c. voltage. The nonlinear interactions between different zones (positive column, electrodes and walls of the discharge tube) is considered through Neumann-Dirichlet mixed boundary conditions at interfaces. We take into account the nonlinearity of physical characteristics of the electrode material and of the silica envelope. Waymouths measurements are used for the ratio between electron and ion currents. The temperature profile index as function of the z-coordinate is determined in the vicinity of the cathode.

He-Cd hollow-cathode oscillations in an axial magnetic field

The influence of the axial magnetic field on the output of a hollow-cathode Cd-He laser oscillating on the 533.7 and 537.8 nm lines is studied for various discharge conditions. The application of the axial magnetic field causes a decrease in the laser intensity. This behaviour can be explained by taking into account the action of the magnetic field both on the atomic transition and on the plasma inside the hollow cathode.

Infrared photovoltaic radiation detector with thin anisotropic tellurium film

Abstract The paper relates an infrared radiation detector using a thin film of tellurium deposited by evaporation under 10 -4 –10 -5 torr vacuum and oblique incidence on a NaCl support. The cell has a responsivity of about 450 μV/W and a response time of about 1 μsec. The signal-to-noise ratio at room temperature is 90:1 at 1 W signal.

Fabrication, characterization, and applications of high‐performance AlGaAs‐based buried‐heterostructure diode lasers

Data are presented on buried-heterostructure (BH) AlGaAs/ GaAs and InGaAs/AlGaAs quantum-well diode lasers (DLs) fabricated by low-temperature liquid phase mesa melt etching and regrowth. The basic laser structures were grown by either molecular beam epitaxy (MBE) or metal-organic chemical vapor deposition (MOCVD). Native ox- ides were used as a mask in the processes of melt etching and regrowth. Measurements of excess mirror temperature and parameters of internal second-harmonic generation (SHG) were used for DL characterization. The equalization of beam divergence in both planes, perpendicular and parallel to the active layer, was accomplished by using cylindrical micro- lenses, a t1Wo fradiant power in continuous-wave (cw) operation. The results on medical applications and pumping Er 31 -doped YAG crystals are reported.

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.

Application of the successive variational method to the D-dimensional generalized anharmonic oscillator

Abstract An approximate, successive variational method for the study of the generalized anharmonic oscillator in D dimensions is presented. We have thus improved the results of the standard variational method.

Local electric fields in optical glasses during field-assisted ionic exchanges

We study the phenomena connected with high concentration of incoming ions emerging during field-assisted migration in otpical glass. We find that ion dynamics are very different at concentrations higher and lower than a certain parameter called the transition concentration. To explain anomalies at high ionic concentrations, we introduce a supplementary local electric field. This field opposes to the field E0 existing in the glass at all concentrations of incoming ions and is connected with a local space charge. We investigate its dependence on concentration and on E0. These effects are studied using a model with concentration-dependent diffusion coefficients and mobilities. We present a method to obtain the concentration dependence of the relevant quantities. Theoretical curves are compared with experimental results measured in usual silicate glasses, during Ag+-Na+ exchanges.