[en] Tunable infrared radiation in the wavelength range of 16 um was generated by stimulated electronic hyper-RAMAN scattering in atomic strontium vapour using a pulsed dye laser. A nearby two-photon resonance was used to enhance the conversion efficiency. The actual infrared output was, however, much less than estimated from perturbational calculations. In addition, the observed dependence of the STOKES energy on the power of the dye laser differed drastically from the expectation of perturbation theory. There is, however, quite good agreement between experiment and a non-perturbational description of the process, which had been stimulated by the present work. According to this theory, the observed saturation in the conversion process as well as the shape of the STOKES pulses is due to a significant depletion of the atomic ground-state population by the pumping light. Therefore, only a small intensity of the generated infrared radiation has been observed. The spectral properties of the generated infrared light were investigated. The bandwidth of the STOKES radiation depends linearly on the width of the pump laser and is always smaller than that. Moreover it is influenced by the strontium vapour pressure and, very strongly, by the intensities of pump and generated light. (orig.)

[en] Extending our previous work on the dynamics of femtosecond laser ablation from dielectrics, we concentrate in this contribution on calcium fluoride (111) surfaces. Upon irradiation with femtosecond pulses at intensities below the single-pulse ablation threshold we find, similar to BaF₂: the necessity of a considerable incubation; a non-linear dependence of the electron- and positive-ion emission on deposited energy; positive-ion kinetic energies at the order of 10 eV; the detection of negative ions after considerably longer time of flight. This indicates an ablation mechanism similar to that for BaF₂, i.e. Coulomb explosion of an electrostatically unstable surface after multiphoton ionization. However, the ionic cluster distribution and the role of positive fluorine ions appears to be different, indicating different binding properties in CaF₂ and BaF₂. An increase in the laser fluence results in the appearance of thermal contributions, distinguishable by a second, slower contribution to the ion velocity distribution

[en] We report on experiments studying the appearance of negative ions during ablation from nominally transparent dielectric crystals (Al₂O₃, BaF₂, NaCl), irradiated by an 800 nm, 100 fs-laser in the multiple pulse regime. A state-of-the-art time-of-flight mass spectrometer was used for detection, in combination with ion trajectory simulations. Based on the investigation of the appearance threshold dependence on laser intensity, the kinetic energy and the angular distribution, we present evidence that the expulsion process for negative ions is different from that for positive ions. It appears to be most likely that gas-phase interactions within the ablation plume lead to the creation of negative ions from neutral precursors

[en] 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

[en] Ablation from sodium chloride single crystals by ultra short laser pulses is investigated, recording the emitted charged particles (electrons, negative and positive ions) by time-of-flight mass spectroscopy. The influence of irradiation parameters is analyzed, such as laser intensity and the number of shots per ablation site. The ablation craters are inspected by optical microscopy. In addition to material removal from the surface, the laser irradiation induces a coloration of the crystal along the beam path. The corresponding absorption spectrum reveals the generation of F color centers and their aggregates F₂, F₃, F₄, probably decaying after minutes to Na-colloids. Our observations agree well with our earlier results on sapphire and barium fluoride, assuming multiphoton surface ionization followed by Coulomb explosion of the surface, due to positive charging. In addition, several laser generated defect types are identified (color centers, Na colloids and mechanical stress), and their role for emission behavior is discussed

[en] The Tilted Axis Cranking theory is used to describe the coexistence of high and low K bands in yrast spectra of well deformed nuclei, magnetic rotation of transitional nuclei and to calculate the parameters of a rotational hamiltonian with a fourfold symmetry axis that generates ΔI=4 staggering in the yrast band. (orig.)

[en] 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

[en] Spatially resolved photoluminescence and EBIC were used to characterise a sample of multicrystalline silicon in the temperature range 80-300 K. The dislocation related lines in the spectrum-D1, D2, D3 and D4 correlate with the total recombination activity measured by EBIC. The temperature dependent EBIC behaviour was utilised to access the contamination level at the dislocations in low quality regions of the sample. The temperature dependence of D1 line shows a maximum at about 150 K. The decrement of D1 peak area upon temperature decreasing below 150 K could be related to the appearance of D3 and D4 lines in the photoluminescence spectra. The peak widths of D1 and D2 show opposite temperature dependence. D1 width decreases and D2 becomes broader upon decreasing temperature. Two additional lines with energies below the energy of band-to-band luminescence were observed together with the D bands at 80 K. They could be related to phonon replication of the band edge luminescence peak and can be seen on FZ-Si too

[en] We report on new results of third harmonic generation and self-diffraction. These effects are related to the occurrence of a refractive index grating produced in transparent dielectrics, in particular in barium fluoride, under irradiation by femtosecond laser pulses. Because only nonresonant electronic nonlinearities are exploited, all described effects are quasi instantaneous. As a result of the high intensities at weak energy obtainable from femtosecond laser systems these effects are very efficient

[en] Positive ion emission upon irradiation with trains of ultra short laser pulses from an amplified titanium:sapphire laser (pulse duration 120 fs, photon energy 1.55 eV) shows significant differences between Al₂O₃, and NaCl targets. For both types of crystals, an incubation phase of several hundred laser pulses is necessary to prepare the target for the emission of detectable amounts of positive ions. Thereafter, in the case of Al₂O₃, ion bursts are observed depending stochastically on the number of additional laser pulses, while such an explosive ablation is not observed for NaCl. Evidence for different incubation and ablation mechanisms for both materials is presented, pointing to an important role of recondensation during multi-shot laser ablation