Cold ablation driven by localized forces in alkali halides

Laser ablation has been widely used for a variety of applications. Since the mechanisms for ablation are strongly dependent on the photoexcitation level, so called cold material processing has relied on the use of high-peak-power laser fluences for which nonthermal processes become dominant; often reaching the universal threshold for plasma formation of ∼1 J cm-2 in most solids. Here we show single-shot time-resolved femtosecond electron diffraction, femtosecond optical reflectivity and ion detection experiments to study the evolution of the ablation process that follows femtosecond 400 nm laser excitation in crystalline sodium chloride, caesium iodide and potassium iodide. The phenomenon in this class of materials occurs well below the threshold for plasma formation and even below the melting point. The results reveal fast electronic and localized structural changes that lead to the ejection of particulates and the formation of micron-deep craters, reflecting the very nature of the strong repulsive forces at play

Monitoring of Focus Position During Laser Processing based on Plasma Emission

AbstractThe effects of lens-to-sample distance on plasma emission induced during laser processing were investigated. For tight focusing conditions, a local minimum value of emission intensity was observed for opaque materials. It was attributed to a reduction of ablation efficiency due to saturation. This effect can lead to identify the lens-to-sample distance that produces the highest fluence on the sample. A strategy to detect the working focus position based on monitoring the intensity of plasma emission was proposed. Variations of laser energy, pulse overlap and material type were also studied

Picosecond Laser Induced Selective Removal of Functional Layers on CIGS Thin Film Solar Cells

AbstractPicosecond laser pulses provide a controlled laser based removal used for micro structuring of functional layers, such as copper-indium-gallium-diselenide (CIGS), due to a reduced thermal penetration depth, compared to nanosecond pulses. It is shown that with 7 ps pulse width at a wavelength of 532nm the P2 and P3 processing of thin film solar cells on glass substrates and flexible polyimide substrates is possible. A Gaussian as well as a top-hat energy distribution were utilized for the experiments. With adjusted laser and processing parameters they allow for the fabrication of precise isolation trenches (width << 100μm), without damaging the adjacent layers due to mechanical stress or thermal stress

Finestructure, hyperfine structure and isotope shift of 4f

The isotope shift (IS) and hyperfine structure (hfs) of nine levels (31720 to 38921 cm-1) assigned to the configuration $4f^{12}6s7s$ in neutral erbium have been determined experimentally using Doppler-reduced saturation absorption spectroscopy in a gas discharge. We performed a fine structure analysis in the SL-coupling scheme of the single configuration $4f^{12}6s7s$, confirming and extending the classification of even parity Er I levels. We discriminated the different hfs contributions of the $4f^{12}$ core and the (6s+7s) outer electrons of the shell in a non-relativistic JJ-coupling approach and in the relativistic effective tensor operator formalism in SL-coupling. The relativistic one-electron parameters of the hfs for 167Er were fitted to the experimental data by a least squares fit procedure: 0pt[0pt] MHz, 0pt[0pt] MHz, 0pt[0pt] MHz. The level dependencies of the isotope shift were evaluated based on crossed second order (CSO) effects. We obtained the following results for the CSO parameters for the isotope pairs 170-168Er: $d_{6s7s}=-740(30)$ MHz, $z_{4f}= 0(5)$ MHz, $(g_{3,6s}(f, 6s)+g_{3, 7s}(f, 7s))= -24(15)$ MHz and for 170-166Er: $d_{6s7s}=-1500(50)$ MHz, $z_{4f}=0(10)$ MHz, $(g_{3,6s}(f,6s)+g_{3,7s}(f+7s))=-50(29)$ MHz. The resulting parameters for the hfs are compared with those known for other configurations of the Er atom and ion