Theoretical analysis of the multi-rotational line TEA CO2 laser

A theoretical model for the multiline TEA CO₂ laser has been developed which takes into account the overlap of the P(20) line of (00⁰1) (10⁰0) regular band transition with the R(23) line of (01¹1) (11¹0) hot band transition. The model is used for the study of the influence of different parameters like laser gas temperature, gas mixture and pump rates on laser intensities of different rotational lines. This study explains the experimental results of the high power multiline TEA CO₂ laser very well.     

Direct observation of laser-induced crystallization of a-C:H films

The post-growth modification of diamond-like amorphous hydrogenated carbon a-C:H films by laser treatment has been studied by transmission electron microscopy and Raman spectroscopy. a-C:H films grown on Si substrates by benzene decomposition in a rf glow discharge were irradiated with 15 ns pulses of a KrF-excimer laser with fluences in the range of E=50–700 mJ/cm². At fluences below 100 mJ/cm² an increase in the number of graphitic clusters and in their ordering was evidenced from Raman spectra, while the film structure remained amorphous according to electron microscopy and electron diffraction observations. At higher fluences the appearance of diamond particles of 2–7 nm size, embedded into the lower crystallized graphitic matrix, was observed and simultaneously a progressive growth of graphite nanocrystals with dimensions from 2 nm to 4 nm was deduced from Raman measurements. The maximum thickness of the crystallized surface layer (400 nm) and the degree of laser annealing are limited by the film ablation which starts at E>250 mJ/cm². The laser-treated areas lose their chemical inertness. In particular, chemical etching in chromium acid becomes possible, which may be used for patterning the highly inert carbon films.     

Preionization of TEA CO2 laser by sliding discharge

TEA CO₂ laser preionization by plasma sheet formed by discharge sliding over a dielectric surface is described. The preionization electron number density in order of 10⁹cm^–3 was measured in the CO₂ N₂ He=113 gas mixture. The plasma sheet was also tested as a main discharge electrode in TEA CO₂ laser.The authors would like to acknowledge with thanks the current interest and the help of P. Gavrilov and V. Krajíek in experiments.     

Bone-ablation mechanism using CO2 lasers of different pulse duration and wavelength

Bone ablation using different pulse parameters and four emission lines of 9.3, 9.6, 10.3, and 10.6 m of the CO₂ laser exhibits effects which are caused by the thermal properties and the absorption spectrum of bone material. The ablation mechanism was investigated with light- and electron-microscopy at short laser-pulse durations of 0.9 and 1.8 s and a long pulse of 250 s. It is shown that different processes are responsible for the ablation mechanism either using the short or the long pulse durations. In the case of short pulse durations it is shown that, although the mineral components are the main absorber for CO₂ radiation, water is the driving force for the ablation process. The destruction of material is based on explosive evaporation of water with an ablation energy of 1.3 kJ/cm³. Histological examination revealed a minimal zone of 10–15 m of thermally altered material at the bottom of the laser drilled hole. Within the investigated spectral range we found that the ablation threshold at 9.3 and 9.6 m is lower than at 10.3 and 10.6 m. In comparison the ablation with a long pulse duration is determined by two processes. On the one side, the heat lost by heat conduction leads to carbonization of a surface layer, and the absorption of the CO₂ radiation in this carbonized layer is the driving force of the ablation process. On the other side, it is shown that up to 60% of the pulse energy is absorbed in the ablation plume. Therefore, a long pulse duration results in an eight-times higher specific ablation energy of 10 kJ/cm³.     

The effects of collisional quenching on degenerate four-wave mixing

We report a theoretical and experimental investigation of the effects of collisional quenching on resonant degenerate four-wave mixing (DFWM). Using single-mode laser radiation, peak signal intensity measurements were performed on an isolated line in the A – X transition of NO. By using appropriate mixtures of N₂ and CO₂ as buffer gases, we varied the collisional quenching rate over several orders of magnitude while maintaining a fixed total collisional dephasing rate. The mixtures had approximately 100 Torr total pressure and were at room temperature. For I/I _sat approximately equal to 0.02, DFWM intensities were found to be less affected by variations in quench rate than were laser-induced fluorescence (LIF) intensities (I and I _sat are the pump laser and one-photon saturation intensities, respectively). Moreover, for I/I _sat roughly equal to 0.5, DFWM intensities were observed to be nearly independent of quench rate. The results are compared to two theoretical predictions, with good agreement observed. Both theories indicate that the minimum sensitivity of DFWM to quenching occurs near I/I _sat1.     

Studies on excimer laser doping of GaAs using sulphur adsorbate as dopant source

Excimer laser doping of GaAs using sulphur adsorbate as a dopant source is demonstrated. Box-like n-type layers of depths of about 100 nm with carrier concentration as high as (23)×10¹⁹ cm^–3 are formed. Passivation of GaAs using a (NH₄)₂S_x solution for 40 min followed by sublimation of the excess sulphur atoms in high vacuum result in an effective dopant for controllable n-type doping. The samples are irradiated using a KrF excimer laser in a N₂ gaseous environment. Secondary ion mass spectrometry (SIMS) measurements show that sulphur is successfully incorporated in the GaAs. The sheet resistance is controlled by adjusting the laser energy fluence and number of laser pulses. Rutherford backscattering spectrometry with channeling (RBS/C) alignment measurement indicates that lattice damage is undetectable for N₂ gas pressures of 760 Torr.     

Intensity-induced nonlinear effects in UV window materials

Intensity-induced nonlinear effects in optical window materials have been investigated at 308 nm. The absolute two-photon absorption coefficients for fused silica, CaF₂, BaF₂, Al₂O₃ and ADP crystals have been measured by using a single 120 ps, transform-limited pulse from the second harmonic of a distributed feedback (DFB) dye laser. The nonlinear refractive index coefficient has been obtained from measurements of far-field intensity distributions.     

CO2 laser cleaning of black deposits formed during the excimer laser etching of polyimide in air

Pulsed CO₂ laser cleaning of black debris formed during the excimer laser ablation of polyimide in air is demonstrated. The 10.6 m CO₂ laser radiation is strongly absorbed in the debris but only weakly absorbed in polyimide thus enabling the clean removal of the debris without damaging the polyimide.     

TEA CO2 laser ablation of coronary artery

Summary One of the prerequisites for successful laser angioplasty is the ablation of the atherosclerotic lesions, without thermal or shock-wave damage of the healthy tissue. In this study was evaluated the effectiveness of a TEA CO₂ laser, emitting pulses of the lower TEM mode, 100 ns duration, at a repetition rate of 2.4 Hz, for the ablation of cardiovascular tissue. Normal and atherosclerotic human arteries (post mortem) were irradiated for a range of fluences up to 10J pulse⁻¹ cm⁻². After irradiation, the samples were prepared for histologic examination. The results showed that controlled ablation of normal and atherosclerotic coronary artery can be accomplished with the TEA CO₂ laser, with minimal thermal damage.     

High-power,line-tunable14NH3 and15NH3 lasers

A high-power, line-tunable NH₃ laser pumped by a TEA CO₂ laser has been developed for application to laser isotope separation of tritium. Laser lines from¹⁵NH₃ are newly observed in a range from 11.24 to 13.23 m. The performance of an¹⁵NH₃ laser is compared to that of an¹⁴NH₃ laser.     

Self-focussing of CO2 laser radiation in SF6

Self-focussing of high-power TEA CO₂ laser pulses for a number of 10 m P-band lines is reported in SF₆ molecular gas. Application of this effect to estimating the intensity-dependent refractive index of the gas is demonstrated.     

Dual excimer and CO2 laser etching studies of polyethylene terephthalate

Polyethylene terephthalate (PET) films preheated with a pulsed CO₂ laser have been ablatively etched with an XeCl laser. The observed reduction in ablation threshold, from 170 to 140 mJ cm^–2, is consistent with a thermal mechanism for XeCl laser ablation of PET. Transient changes in the UV absorption coefficient of PET caused by heating with pulsed CO₂ laser radiation have also been studied and a significant increase in absorption observed at 308 nm. Permanent changes in the ultraviolet absorption of PET following exposure to low fluence XeCl laser radiation are also reported.     

CO2 laser ablative etching of polyethylene terephthalate

Films of polyethylene terephthalate (PET) can be successfully etched with 9 m radiation from a pulsed TEA CO₂ laser. The relationship between etch depth and fluence is broadly similar to that observed for excimer laser etching but with a less well-defined threshold. Time-resolved photoacoustic measurements of stress waves generated in the interaction show that at a fluence of 1.8 J cm^–2 ablation occurs 100–200 ns after the start of the laser pulse, a time which is consistent with the rate of thermal decomposition of PET. The volatile products of ablation are carbon monoxide, carbon dioxide, methane, ethyne, ethene, benzene, ethanal, and small quantities of other products. For fluences close to and appreciably above the threshold the ablated material consists predominantly of involatile species of relatively high molecular weight, whereas at higher fluences substantial fragmentation of the polymer to small molecules occurs.School of Chemistry     

Hard tissue ablation with a free running Er:YAG and a Q-switched CO2 laser: a comparative study

The Er:YAG and the CO₂ laser are competitors in the field of hard tissue ablation. The use of Er:YAG lasers (2.94 μm, pulse length _L of 100 to 200 μs) show smaller areas of thermal defects then ‘‘superpulsed’’ CO₂ lasers with pulse lengths of approximately 100 μs. Only the development of a Q-switched CO₂ laser (9.6 μm, τ_L=250 ns) allowed for similar results. In this paper new results for the Er:YAG and the Q-switched CO₂ laser under the influence of water spray will be presented. Several parameters are of special interest for these investigations: the specific ablation energy, which shows a minimum for the CO₂ laser at an energy density of 9 J/cm ² and a broad shallow minimum in the range of 10 to 70 J/cm² for the Er:YAG laser, and comparison of the cut-shape and depth. Surface effects and cutting velocity are discussed based on SEM pictures. Received: 19 July 2000 / Revised version: 1 November 2000 / Published online: 30 November 2000     

Surface modifications of TiN coating by pulsed TEA CO2 and XeCl lasers

Interactions of a transversely excited atmospheric (TEA) CO₂ laser and an excimer XeCl laser, pulse durations ∼2 μs (initial spike FWHM ∼100 ns) and ∼20 ns (FWHM), respectively, with polycrystalline titanium nitride (TiN) coating deposited on high quality steel AISI 316, were studied. Titanium nitride was surface modified by the laser beams, with an energy density of 20.0 J/cm² (TEA CO₂ laser) and 2.4 J/cm² (XeCl laser), respectively. The energy absorbed from the CO₂ laser beam is partially converted to thermal energy, which generates a series of effects such as melting, vaporization of the molten material, shock waves, etc. The energy from the excimer XeCl laser primarily leads to fast and intense target evaporation. The calculated maximum temperatures on the target surface were 3770 and 6300 K for the TEA CO₂ and XeCl lasers, respectively. It is assumed that the TEA CO₂ laser affects the target deeper, for a longer time than the XeCl laser. The effects of the XeCl laser are confined to a localized area, near target surface, within a short time period.Morphological modifications of the titanium nitride surface can be summarized as follows: (i) both lasers produced ablation of the TiN coating in the central zone of the irradiated area and creation of grainy structure with near homogeneous distribution; (ii) a hydrodynamic feature, like resolidified droplets of the material, appeared in the surrounding peripheral zone; (iii) the process of irradiation, in both cases, was accompanied by appearance of plasma in front of the target.Target color modifications upon laser irradiation indicate possible chemical changes, possibly oxidation.     

Surface modification of the Cr-based coatings by the pulsed TEA CO2 laser

The interaction of a transversely excited atmospheric (TEA) CO₂ laser with chromium oxynitride (CrON) coating deposited on a AISI 304 steel substrate was considered. The results have shown that CrON was surface-modified by the laser beam of 45 J/cm² energy density. The energy absorbed from the TEA CO₂ laser beam was partially converted into thermal energy, which has generated a series of effects such as melting, vaporization of the molten material, and shock waves in the vapor and in the solid. Morphological manifestations on the CrON coating surface can be summarized as follows: non-uniform features with ablation and appearance of crater-like form (central zone of interaction); appearance of three damaged areas and presence of hydrodynamic effects with resolidified droplets (periphery zone of interaction). In case of applied energy density the interaction of laser radiation with CrON has been always followed by plasma creation in front of the coating. PACS 79.20.Ds; 61.80.Ba     

Imaging and focusing of an atomic beam with a large period standing light wave

A novel atomic lens scheme is reported. A cylindrical lens potential was created by a large period ( 45 m) standing light wave perpendicular to a beam of metastable He atoms. The lens aperture (25 m) was centered in one antinode of the standing wave; the laser frequency was nearly resonant with the atomic transition 2³ S ₁–2³ P ₂ (=1.083 m) and the interaction time was significantly shorter than the spontaneous lifetime (100 ns) of the excited state. The thickness of the lens was given by the laser beam waist (40 m) in the direction of the atomic beam. Preliminary results are presented, where an atomic beam is focused down to a spot size of 4 m. Also, a microfabricated grating with a period of 8 m was imaged. We discuss the principle limitations of the spatial resolution of the lens given by spherical and chromatic aberrations as well as by diffraction. The fact that this lens is very thin offers new perspectives for deep focusing into the nm range.     

Multi-exponential analysis of DLTS

Deep-level transient spectroscopy (DLTS), which is widely used to characterize deep impurity centers in semiconductors, assumes a single exponential wave form for the transient junction capacitance. When there are several closely spaced energy levels this assumption is no more valid, and the conventional DLTS may lead to errorneous results. To overcome this difficulty we propose here a novel method which we call the multi-exponential DLTS(MEDLTS). The transient wave form of the junction capacitance is directly analysed into multi-exponential compouents using the nonlinear least-squares analysis program DISCRETE developed by Provencher. The resolved time constants of these components are then displayed in the form of aT ²–1/T plot. According to the results of simulation with various parameters MEDLTS is shown quite effective to resolve closely spaced energy levels which can not be resolved by the conventional DLTS. As an example of the application of this method deep levels in Si: Au were investigated. The results have shown that a single peak in conventional DLTS actually consists of two adjacent levels with activation energies and capture cross-sectionsE _B1=0.49 eV, _B1=1.1×10^–14cm² andE _B2=0.46 eV, _B2=1.3×10^–15 cm² and with amplitude ratio 11.     

The contrast gain in a guest-host nematic display due to a periodic boundary condition

The mean square tilt angle of a nematic slab with finite anchoring energy and periodic boundary conditions has been theoretically investigated, as a function of the slab geometry and of the reduced extrapolation length. If the anchoring strength is free-surfacelike, the contrast is affected by a loss 10% at room temperature if the ratio between the anchoring pitch and the cell thickness is 0.5.Glossary anchoring pitch - h cell thickness - /h - ( = x/, = y/h) reduced coordinates - (, ) local tilt angle - elastic constant - w_a anchoring energy anisotropy - b=/w _a de Gennes-Kleman extrapolation length - B=b/h reduced extrapolation length - T _NI nematic-isotropic transition temperature - :=(T/T _NI ) – 1 reduced temperature - easy axis direction - _MAX - _± ² mean square tilt angle along the boundary - () absorbance coefficients of the p-dye - r /: dichroic ratio - c contrast - G contrast gain - S order parameter     

Hydrogen related point defects in silicon based layers: Si(·)H and SiOOH

Layers prepared by pulsed TEA CO₂ pulsed laser ablation (PLA) of SiO and SiO₂ targets in helium were exposed to hydrogen and deuterium atmosphere up to several kPa. The deposited layers were investigated by FTIR, EPR and XP spectroscopy. Among various Si species silyl radical Si(·)H (Si(·)D) at 2166 (1568) cm⁻¹—H(I) center—and silyl hydroperoxide SiOOH (SiOOD) at 3587 (2648) cm⁻¹ were identified in FTIR spectra. Chemical pathways for production of these species are discussed. Experimental results are supported by quantum chemical calculations.