Transversely Excited Atmospheric CO2 Laser Research Articles

Detection of salts in soil using transversely excited atmospheric (TEA) carbon dioxide (CO2) laser-induced breakdown spectroscopy (LIBS) by the aid of a metal mesh

In this work, a nickel metal mesh was used to allow a direct detection of salt in soil sample by LIBS utilizing unique characteristics of a TEA CO2. The metal mesh is placed in the front of the soil sample to prevent the soil sample from blowing off upon focusing the high pulsed laser beam irradiation. LIBS apparatus used in this work is a TEA CO2 laser operated at wavelength of 10.6 μm with pulse energy and duration of 3J and 200 ns, respectively. The laser beam was focused using a ZnSe lens (f = 200 mm) onto soil sample after passing through the metal mesh. The emission spectrum from the induced plasma was detected using an optical multichannel analyzer (OMA) system consisting of a 0.32-m-focal length spectrograph with a grating of 1200 graves/mm and a 1024-channel photodiode detector array with a micro-channel plate intensifier. The soil sample used is a standard soil and ordinary soil containing several salts such as Ca, Mg at high concentration. The LIBS experiment was carried out at high pressure surrounding gas of 1 atmosphere. It was observed that by the aid of the metal mesh, strong breakdown gas plasma can be produced just after TEA CO2 laser irradiation on soil sample without significant sample blowing off. It was found that emission lines from salts, Ca (Ca II 393. 3 nm, Ca II 396.3 nm, Ca I 422.5 nm), and also other salts including Mg and Na can clearly be detected with strong emission intensity and narrow spectral width. This result implies that a TEA CO2 LIBS assisted by the metal mesh (metal mesh method) can be used for direct analysis several salts such as Ca, Mg, and Na in soil sample.

Transversely Excited Atmospheric CO2 Laser-Induced Plasma Spectroscopy for the Detection of Heavy Metals in Soil

A rapid detection of heavy metals in soil was presented by the metal-assisted gas plasma method using specific characteristics of a pulsed, transversely excited atmospheric (TEA) CO2 laser. The soil particles were placed in a hole made of acrylic plate. The sample was covered by a to prevent the soil particles from being blown off. The mesh also functioned to initiate a luminous plasma. When a TEA CO2 laser (1500 mJ, 200 ns) was focused on the soil sample, passing through the metal mesh, some of the laser energy was used to generate the gas plasma on the mesh surface, and the remaining laser energy was employed to ablate the soil particles. The fine, ablated soil particles moved into the gas plasma region to be dissociated and excited. Using this technique, analysis can be made with reduced sample pretreatment, and therefore a rapid analysis can be performed efficiently. The results proved that the signal to noise ratio (S/N) of the emission spectral lines is much better for the case of the present method (mesh method) compared to the case of standard laser-induced breakdown spectroscopy using the pellet method. Rapid detection of heavy metal elements in soil has been successfully carried out. The detection limits of Cu and Hg in soil were estimated to be 3 and 10 mg/kg, respectively. The present method has good potential for rapid and sensitive detection of heavy metals in soil samples.

Elemental analysis of aluminum alloys by Laser Induced Breakdown Spectroscopy based on TEA CO2 laser

The applicability of nanosecond infrared Transversely Excited Atmospheric (TEA) CO2 laser induced plasma for spectrochemical analysis of aluminum alloys was investigated. The plasma was generated by focusing a pulsed TEA CO2 laser that emits at 10.6 μm on the Al target in ambient air at atmospheric pressure. The temporal profile of the laser pulse is composed of a 100 ns peak followed by a slowly decaying tail of about 2 μs. The output pulse energy was approximately 160 mJ, thus the peak output power was estimated to be around 1.6 MW. Time-Integrated Space-Resolved Laser Induced Breakdown Spectroscopy (TISR-LIBS) was employed to obtain the emission spectra. The maximum intensity of emission, with sharp and well resolved spectral lines that were almost free of the background continuum, was obtained from plasma region 2 mm from the target surface. Linear calibration curves for Mg, Cr, Cu and Fe were obtained using aluminum alloy spectrochemical standards. The limits of detection for the investigated elements were in the 2–73 ppm range. Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) was used as a reference technique to estimate the accuracy of LIBS determination by use of control samples with known compositions. Comparison of the obtained results with those available in the literature confirms that LIBS system based on TEA CO2 laser, in combination with TISR spectral measurements, can be successfully applied to qualitative and quantitative determinations of minor elements in aluminum based alloys.

Metal-Assisted Laser-Induced Gas Plasma for the Direct Analysis of Powder Using Pulse CO2 Laser

Analysis of powder samples available in small quantities has been carried out using metal-assisted gas plasma by utilizing a transversely excited atmospheric (TEA) CO2 laser. The powder was homogeneously mixed with Si grease, and the mixed powder was painted on a metal subtarget. When a TEA CO2 laser was directly focused on the metal subtarget at atmospheric pressure of He gas, a high-temperature He gas plasma was induced. It is assumed that the powder particles were vaporized to be effectively atomized and excited in the gas plasma region. This method has been employed in the rapid analyses of elements in organic and inorganic powder samples present in small quantities. Detection of trace elements of Cr and Pb has been successfully made by using the supplement powder and loam soil, respectively. The detection limits of Pb in loam soil were approximately 20 mg/kg.

Optical Emission Studies of Copper Plasma Induced Using Infrared Transversely Excited Atmospheric (IR TEA) Carbon Dioxide Laser Pulses.

Spatially resolved, time-integrated optical emission spectroscopy was applied for investigation of copper plasma produced by a nanosecond infrared (IR) transversely excited atmospheric (TEA) CO2 laser, operating at 10.6 μm. The effect of surrounding air pressure, in the pressure range 0.1 to 1013 mbar, on plasma formation and its characteristics was investigated. A linear dependence of intensity threshold for plasma formation on logarithm of air pressure was found. Lowering of the air pressure reduces the extent of gas breakdown, enabling better laser-target coupling and thus increases ablation. Optimum air pressure for target plasma formation was 0.1 mbar. Under that pressure, the induced plasma consisted of two clearly distinguished and spatially separated regions. The maximum intensity of emission, with sharp and well-resolved spectral lines and negligibly low background emission, was obtained from a plasma zone 8 mm from the target surface. The estimated excitation temperature in this zone was around 7000 K. The favorable signal to background ratio obtained in this plasma region indicates possible analytical application of TEA CO2 laser produced copper plasma. Detection limits of trace elements present in the Cu sample were on the order of 10 ppm (parts per million). Time-resolved measurements of spatially selected plasma zones were used to find a correlation between the observed spatial position and time delay.

Analytical capability of the plasma induced by IR TEA CO2 laser pulses on copper based alloys

The applicability of nanosecond infrared (IR) transversely excited atmospheric (TEA) CO2 laser, operating at 10.6 ?m and 100 ns pulse length (initial spike), induced plasma under reduced air pressure for spectrochemical analysis of bronze and brass samples was investigated. The plasma consisted of two clearly distinguished and spatially separated regions and expanded to a distance of about 10 mm from the surface. Elemental composition of the samples was determined using a time-integrated space-resolved laser-induced plasma spectroscopy (TISR-LIPS) technique. Sharp and well resolved spectral lines mostly atomic, and negligibly low background emission, were obtained from a plasma region 7 mm from the target surface. Good signal to background and signal to noise ratios were obtained. Estimated detection limits for trace elements Mg, Fe, Al and Ca were in the order of 10 ppm in bronze and around 50 ppm in brass. Damage on the investigated samples induced by TEA CO2 laser radiation was negligible.

Analysis of sodium aerosol using transversely excited atmospheric CO2 laser-induced gas plasma spectroscopy

Taking advantages of the special characteristics of a transversely excited atmospheric (TEA) CO2 laser, the analysis of sodium aerosol has been successfully conducted by using laser-induced gas plasma spectroscopy (LIGPS) method. In this study, the sodium aerosol was deposited on a nickel metal plate; the metal plate functions as a subtarget to initiate a gas plasma. When a pulsed TEA CO2 laser was focused on the metal surface, a large-volume and high-temperature gas plasma was induced. The fine particles of sodium then entered into the gas plasma region to be dissociated and excited. By using this technique, a semi quantitative analysis of sodium aerosol was made. The detection limit of sodium was approximately 200 ppb.

Theoretical estimation and experimental studies on gas dissociation in TEA CO2 laser for long term arc free operation

Gas dissociation in a high energy, high repetition rate Transversely Excited Atmospheric (TEA) CO2 laser in both sealed-off and gas replenishment modes were studied for nitrogen lean gas mixture. A comprehensive theoretical model based on the Boltzmann transport equation and the discharge excitation circuit equations was adopted to calculate the amount of CO2 dissociated during a single discharge pulse. Theoretically it is shown that inclusion of superelastic collisions in the Boltzmann transport equation is necessary for precise estimation of dissociation per pulse, particularly at high discharge energy loadings and for nitrogen rich gas mixtures. Gas lifetime for repetitively pulsed operations was found experimentally by measuring the amount of CO formed when frequent arcing sets in under sealed off operation. Using this model, the optimum replenishment rate of CO2 either by gas purging and/or by catalytic regeneration needed for arc free long term operation of the laser was estimated. The measured saturation values of CO concentration in the laser chamber agreed well with the calculated values for various operating conditions. Arc free, long term repetitively pulsed operation of the laser was achieved in the gas replenishment mode with gas purging and/or catalytic regeneration.

Analysis of copper surface features obtained using TEA CO2 laser at reduced air pressure

Interaction of a transversely excited atmospheric (TEA) CO2 laser with rough copper surface, at reduced air pressure, was studied. Optical pulse duration of the laser employed was ∼2μs, with the initial spike FWHM of ∼100ns. Laser energy density of ∼32J/cm2 (intensity ∼108W/cm2) was above the plasma ignition threshold. Morphological features of the copper can be summarized as follows: (i) superficial damages, which take crater-shaped form at a higher number of accumulated laser pulses, (ii) development of melt pools with visible bubbles inside the damage region, (iii) formation of solid droplets at near periphery, and (iv) presence of “halo” effect at the irradiated surface. The laser induced surface changes were influenced by the target plasma formation. The formation of plasma influenced the laser–target interaction in two opposite ways: trough absorption of laser energy by the plasma, i.e. trough the effect of plasma shielding, and trough energy transfer from the plasma to the sample. Optical emission spectra were compared for laser induced plasma originated by a single and by cumulative laser pulses. It was found that plasma dimensions and emission intensities have a strong correlation with the number of accumulated laser pulses. Enhancement of both atomic and ionic copper lines was registered when laser induced plasma originated from a single pulse. Chemical analysis of the surface showed a tendency of copper content increase and oxygen content reduction when going from non-irradiated region to the central irradiated region. In the central damage zone, nearly pure copper was present which can be advantageous for some applications due to considerably lower contamination.

An efficient, compact pulsed D2O terahertz super-radiant laser pumped with a fundamental transverse mode transversely excited atmospheric pressure CO2 laser

An efficient, compact pulsed D2O terahertz (THz) super-radiant laser pumped by a TEA (transversely excited atmospheric pressure) CO2 laser is presented. The pulse energy of the THz laser has been discussed as a function of CO2 laser pump energy, D2O gas pressure, and pump absorption. A pulse width of about 110 ns and the maximum pulse energy of about 1.3 mJ have been achieved at 385 μm, with pumping by a 378 mJ fundamental transverse mode TEA CO2 laser, and the photon conversion efficiency of 29% has been achieved. We have also studied the temporal behavior features such as the decay time, the full width at half-maximum, and the pulse broadening of the THz laser pulse compared with the pump pulse and the residual pump pulse at the optimum pressure.

Rapid Analyses of Tiny Amounts of Powder Samples Using Transversely Excited Atmospheric CO2 Laser-Induced Helium Gas Plasma with the Aid of High-Vacuum Silicon Grease as a Binder on a Metal Subtarget

Rapid quantitative analyses of powder samples available in tiny amounts have successfully been conducted by utilizing a transversely excited atmospheric (TEA) CO2 laser-induced He gas plasma. In this study, 4 mg of powder sample was homogeneously mixed with 4 mg of high-vacuum silicon grease and the silicon grease–mixed powder sample (SMP) was painted on a metal surface, which serves as a subtarget. The grease functions to strongly bind the powder and to suppress blow-off of the powder particles. When a TEA CO2 laser (750 mJ, 10.6 μm, 200 ns) was directly focused on the metal subtarget in He gas at 1 atmosphere, a hightemperature He gas plasma was induced, producing a profusion metastable He atoms. It is assumed that the powder particles together with the silicon grease were vaporized to be effectively atomized and excited through metastable He atoms. The result revealed that this technique can be widely employed in the rapid semi-quantitative analyses of powder samples present in minute amounts. A quantitative analysis of loam soil containing different concentrations of Cu was successfully demonstrated, resulting in a good linear calibration curve. The detection limits of Cr and Pb in loam soil were approximately 4 and 13 mg/kg, respectively. Also, we confirmed that this technique can be applied to check the quality of commercial products such as gold film (Au foil), mineral supplement tablets, and prestigious cosmetic powders.

Role of triethylamine in a high-energy wavelength-selective transversely excited atmospheric CO<sub>2</sub> laser

Triethylamine (C6H15N) is widely used to stabilize the discharges of wavelength-nonselective transversely excited atmospheric (TEA) CO2 lasers. In this paper, the influences of triethylamine on the discharge stability and line characteristics of a high-energy wavelength-selective TEA CO2 laser are experimentally studied. The laser has 49 lasing lines ranging from 9.23 μm to 10.74 μm, and the energy of the strongest lines is above 50 J. However, the energy of the laser pulses is very unstable. The discharge stability is improved considerably by adding small amounts of triethylamine in the working gas mixture. In addition, lines in the 9 μm band are weakened greatly, and many of them even can no longer oscillate. In contrast, there are no distinct influences on the 10 μm lines. The results obtained in this study demonstrate the applied range of triethylamine in stabilizing the discharges of wavelength-selective CO2 lasers.

Micromachining of Transparent Materials with Fresnel Diffraction of Infrared Radiation

A simple and productive micromachining method of a silica glass using a TEA (transversely excited atmospheric) CO2 laser (10.6 μm) was investigated. A copper grid mask with a thickness of 2 μm, square apertures of 20×20 μm was attached to the silica glass surface, and it was irradiated by the TEA CO2 laser light. As a result, circular holes were formed on the silica glass surface at the center of each aperture in a spatial resolution down to the sub-wavelength scale. The minimum diameter of the hole was less than 3 μm, which is less than one third of the wavelength of the TEA CO2 laser. A depth of the holes ranging from 100 nm to 600 nm could be micromachined on silica glass surface. The mechanism of the micromachining was discussed based on electric field distributions of the CO2 laser light by a 3D full-wave electromagnetic field simulation solver. The simulation results are in good agreement with the experimental results. It was found that the micromachining of the circular holes were caused by Fresnel diffraction of the CO2 laser light at the apertures.

Micro- and nanosecond laser TiN coating/steel modification: Morphology studies

Morphology effects induced during interaction of μs- (Transversely Excited Atmospheric (TEA) CO2 laser) or ns- (HF laser) pulses with titanium nitride (TiN) coating, deposited on austenitic stainless steel AISI 316, were studied. Experiments were carried out in regime of focused laser beam in air at atmospheric pressure. The used laser fluences were found to be sufficient for inducing intensive surface modifications of the target. The energy absorbed from the CO2 as well as HF laser beam is mainly converted into thermal energy, causing different effects like ablation, appearance of hydrodynamic features, etc. Morphology characteristics obtained during ns-pulses irradiation (HF laser) were different to those initiated by μs-pulses (TEA CO2 laser). The changes on the target surface in form of massive resolidifed droplets and crown-like structures were observed only for ns- (HF laser) pulses. It was found that these effects are a consequence of higher temperature and better coupling of the HF laser radiation with the target. Recent investigations of ps-Nd:YAG laser interaction with the same TiN coating showed that morphology picture is quite different including the reduction of thermal effect.

Analysis of Heavy Metal Pollution in Soil Using Transversely Excited Atmospheric CO2 Laser-Induced Plasma by Trapping the Soil in Microstructured Holes on Metal Subtargets

A unique technique for direct analysis of soil samples utilizing a special advantage of a transversely excited atmospheric (TEA) CO(2) laser-induced plasma generated at atmospheric pressure on a metal target has been developed. In this technique, a metal subtarget, such as nickel plate, structured with intentional microholes on its surface, each with dimensions of around 100 microm in diameter and depth, was used to selectively trap small sized soil particles by immersing the metal plate subtarget into the polluted soil sample. The trapped small soil particles on the metal subtarget were irradiated by a TEA CO(2) laser (10.6 microm, 1.5 J, 200 ns) at atmospheric pressure under defocused condition with a spot size of 3 mm x 3 mm. This trapping and confining scheme substantially suppresses the blowing off effect; thus, the trapped soil particles can effectively be dissociated and atomized in the microstructured holes. Using this method of a microstructured metal plate subtarget, quantitative analysis was carried out on loam soil samples polluted by Pb. A linear calibration curve was obtained with a detection limit of approximately 50 mg/kg. Preliminary quantitative studies were carried out for a quartz sand sample containing Cr and Hg, resulting in linear calibration curves with detection limits of approximately 25 mg/kg and 10 mg/kg, respectively, at this stage. This technique is promising as a potential field screening tool for soil analysis.

Surface modifications of TiN coatings by a pulsed TEA CO2 laser: Coating thickness effects

Interactions of a transversely excited atmospheric (TEA) CO2 laser, pulse duration ∼2 μs (initial spike FWHM ∼120 ns), with polycrystalline titanium nitride (TiN) coatings deposited on high-quality steel (AISI 316 or M2) were studied. The experiments were carried out in a regime of high laser energy densities: 25, 48, and 50 J/cm2. The energy absorbed from the laser beam was partially converted to thermal energy and the effects of the TiN coating thickness on the morphological changes were considered. The morphological features and processes that accompany the interaction can be summarized as follows: (i) exfoliation of the TiN coating in the central zone of the irradiated area (for coating thickness of 1 μm) or appearance of grainy structure (for coating thicknesses 3 and 10 μm); (ii) appearance of hydrodynamic changes in the surrounding peripheral zone; and (iii) appearance of plasma in front of the target during sample irradiation.

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.

Pulsed TEA CO2 laser surface modifications of silicon

The interaction of a transversely excited atmospheric (TEA) CO2 laser, pulse duration of about 2μs (FWHM of initial spike=120ns), with p-type single crystalline silicon doped by boron, was studied. The results have shown that the silicon was surface modified by the laser beam of 12.0Jcm−2 energy density. The energy absorbed from the CO2 laser beam is converted partially into thermal energy, which generates a series of effects such as melting, vaporisation of molten material, shock waves, etc. Morphological manifestations on the silicon surface are: nonuniform modifications (central zone of interaction); wave-like periodical microstructure (inner periphery zone); hydrodynamical structure-like droplets (outer periphery zone). Wave-like microstructure consists of periodic parallel fringes with a period of about 0.8μm. Formation of these wave-like microstructures is very complex. Explanation includes a consideration of the laser-induced periodic surface structure (LIPSS) effect. The process of the CO2 laser interaction with silicon was not initially accompanied by plasma. Plasma, in the form of a spark, typically appeared after about 100 cumulated laser pulses.

Selective ablation of AlN ceramic using femtosecond, nanosecond, and microsecond pulsed laser

The microsecond, nanosecond, and femtosecond laser ablation of AlN ceramic with a transversely excited atmospheric (TEA) CO2 laser, KrF excimer laser, and Ti:sapphire laser is demonstrated. It is shown that the laser-induced thermal decomposition process occurs with a microsecond TEA CO2 laser and a nanosecond KrF excimer laser. With the microsecond and nanosecond laser-induced thermal decomposition process, the conductive aluminum line can be directly fabricated and very low resistance can be obtained with a TEA CO2 laser. In the case of femtosecond Ti:sapphire laser ablation, there are two different regimes of ablation rate-like metals. In addition, a nonthermal process without a chemical composition change of the ablated surface can be achieved successfully, which is evident from scanning electron microscopy observation and x-ray photoelectron spectroscopy analysis of the ablated surface.

18O-Selective Infrared Multiple Photon Decomposition of Natural and 18O-Enriched Diisopropyl Ethers

18O-Selective infrared multiple photon decomposition of diisopropyl ether has been studied in detail using samples with 18O at oxygen atomic fractions of 0.20% (natural abundance ratio), 4.5%, and 17.3%. The 1022 cm-1 band due to the 12C−16O−12C antisymmetric stretching vibration was found to shift toward a low-energy side by 24 cm-1 for 12C−18O−12C. The 18O-selectivities and yields of the main products containing oxygen, acetaldehyde, and 2-propanol were examined as functions of experimental parameters. The irradiation of a few Torr (1 Torr = 1 mmHg = 133.322 Pa) of diisopropyl ether with TEA (transversely excited atmospheric) CO2 laser lines in the vicinity of 950 cm-1 resulted in considerably high enrichment of 18O in the products. The fluence at focus was more or less than 8 J cm-2. Acetaldehyde and 2-propanol containing 18O at about 20% were obtained from natural ether, and those containing more than 70% were obtained from 17.3% 18O-enriched ether. The observed results can be explained in term of the mechanism involving radical and molecular decomposition channels.