IR Laser Pulses Research Articles

Mid-infrared laser-induced superheating of water and its quantification by an optical temperature probe.

Free-running thulium laser pulses (Cr:Tm:YAG, lambda = 2.01 microm, tp = 300 micros) were applied to a purified, degassed water sample and the resulting temperature rise was investigated by an optical temperature probe. The probe detected water reflectance index changes with temperature and also the onset of vaporization, which was found to occur in a superheat regime, at approximately 230 degrees C. The experimental data were compared with theoretical temperature calculations, and deviations of less than 20 degrees C were stated. The best agreement between theory and experiment was found for temperatures below 180 degrees C, defining by this the method's high accuracy limit. In conclusion, both the optical temperature probe and the presented calculations can help to improve dosimetry in pulsed IR laser applications by precise temperature measurement and prediction.

Ejection of a Large Neutral Cluster from a Liquid Beam Surface Following IR Laser Irradiation

A continuous liquid flow of water in a vacuum (a liquid beam) was irradiated with a pulsed IR laser at 2.96 μm that is resonant to an OH stretching mode of liquid water. Neutral species ejected from the liquid beam were detected directly by a Daly detector without ionization. The flight-time distribution was found to show neutral species having high and low velocities, which are attributable to water clusters ejected from the surface and inside of the liquid beam, respectively. The flight-time distribution expressed by a Maxwellian velocity distribution reveals that the clusters are ejected by explosion of the liquid beam after absorption of the IR laser. This ejection scheme is supported by the presence of a delayed ejection of clusters, which is considered to originate from the inside of the liquid beam into the gas phase.

Breaking the strong and weak bonds of OHF−using few-cycle IR + UV laser pulses

Results of quantum dynamical simulations of the selective bond breaking of OHF− are presented. A few-cycle IR laser pulse is designed to excite a superposition of asymmetric vibrational eigenstates such that the resulting wavepacket is shifted from its equilibrium position. After a critical time delay, a resonant ultrashort UV pulse is applied that excites the wavepacket to a neutral, dissociative surface and selectively breaks the already extended bond. Thus the preferential products O + HF or OH + F can be obtained by applying the appropriate sequence of few-cycle IR + UV laser pulses, and branching ratios are optimized for both dissociation channels.

Generation of Pulsed High-Energy Beams of Trifluoroiodomethane Molecules and Trifluoromethyl Radicals

A method for generating energetic beams of CF3I molecules and CF3 radicals was described. The method is based on the formation of pressure shock in front of a solid surface due to the impact of an intense, pulsed, gas-dynamically cooled molecular beam (or flow) on this surface and its use as a source of a secondary beam for producing energetic molecules. The secondary beam was formed upon efflux of molecules from the pressure shock through an orifice into a high-vacuum chamber compartment. The accelerated CF3I molecular beam was generated by exciting the molecules with a powerful IR laser pulse in the pressure shock (in the secondary-beam source itself) and the beam of energetic CF3 radicals was produced through the dissociation of CF3I in either the pressure shock or the accelerated beam. High-density (≥1020 molecule/(sr s)) beams of CF3I molecules and CF3 radicals with a kinetic energy of ≥1.2 and ≥0.4 eV, respectively, were obtained.

Formation of microscopic coloured oxide dots on the titanium foil surface irradiated by a laser

Experiments are performed on the formation of coloured dots of a submillimetre size on the titanium foil surface irradiated by pulsed IR lasers. It is shown that the parameters of laser pulses can be selected so that the dots produced on the titanium foil surface will change their colour from yellow to blue with increasing exposure time. The threshold laser power densities required for the formation of contrast coloured dots on the titanium foil surface irradiated by laser pulses with low repetition rates are estimated as ~1.5 × 103 J cm-2 for a 1.73-μm Xe laser and ~2.3 × 103 J cm-2 for a 10.6-μm CO2 laser.

Deposition of particulate-free thin films by two synchronised laser sources: effects of ambient gas pressure and laser fluence

The micrometer and sub-micrometer sized particulates present both on the surface and inside of pulsed laser deposited thin films and structures stand for the main drawback of the method in view of technological applications. We applied a two-laser system in order to withdraw the particulates in case of Ta and TaOx thin films. The Ta targets were irradiated by the first UV laser, while the second IR laser was directed parallel to the target surface, aiming to heat and evaporate the particulates. The morphology of the obtained thin films was studied by scanning electron microscopy. For the TaOx films, the ambient gas pressure influences, besides the size and density of particulates, their propagation velocity. This in turn results in the variation of the optimum delay time between the ablating UV and the second IR laser pulse. For the Ta films we found that a threshold fluence of the IR laser pulse exists, above which completely particulate-free films were deposited.

Effective length of filaments measurement using backscattered fluorescence from nitrogen molecules

The effective length of self-induced long (∼100 meters) plasma filaments, generated by intense femtosecond near IR laser pulses in air, was measured remotely using a lidar technique. This technique is based on detecting the backscattered fluorescence signal from nitrogen molecules excited along the intense laser pulse propagation path. This opens up the possibility of measuring remotely long filaments extending over hundreds of meters in the atmosphere.

Separation of a benzene and nitric oxide mixture by a molecule prism

In molecule optics, a matter wave of molecules is manipulated by a molecule-optical component made out of external, typically radiative, fields. The molecule-optical index of refraction, n, for a nonresonant IR laser pulse focused onto a molecular beam can be obtained from the energy conservation and wave properties of molecules. Experimentally measured values of n for benzene and nitric oxide agreed well with the calculated values. Since n depends on the properties of molecules as well as those of the laser field, a molecule prism composed of the focused nonresonant laser field can separate a multi-component molecular beam into several components according to their molecule-optical refractive indices n. We obtained a chromatographic resolution of 0.62 for the spatial separation of a mixture beam of benzene and nitric oxide using a focused Nd:YAG laser pulse as a molecule prism.

Laser Ablative Structural Modification of Poly(ethylene-alt-maleic anhydride)

Pulsed IR laser ablation of poly(ethylene-alt-maleic anhydride) results in the deposition of polymeric films possessing the same ratio of anhydride and −CH2− groups and represents a very rare example of laser ablative deposition of polymeric films that are structurally identical to the ablated polymer. This process differs from the conventional thermolysis of poly(ethylene-alt-maleic anhydride) that is controlled by expulsion of CO2 and CO and yields a nonpolar polymeric residue. The IR laser ablation of poly(ethylene-alt-maleic anhydride) in sodium metasilicate affords deposition of polymeric films containing carboxylate (−CO2-) groups. This process is the first example of reactive ablation in which the deposited polymeric film incorporates constituents of two different species exposed to laser radiation.

Ultrafast laser-induced molecular and morphological changes during spinodal demixing of water/2-butoxyethanol/KCl.

We initiated morphological and molecular level changes in the spinodal decomposition (SD) of H(2)O/2-butoxyethanol/KCl with a pulsed ir laser. Transient Raman spectra gave us a molecular level view of the early stage of this process that could be linked to later morphological events. Chemical changes during SD, such as reorganization of H bonds and forced hydrophobic interactions, ended after 1 micros; however, phase domains continued to grow with self-similarity after 30 micros. The growth of the phase domains satisfied the power law L(t) approximately t(0.55) and was consistent with the late stage of SD. The time scale for the onset of late stage SD is many orders of magnitude faster than previously reported in ionic and nonionic conditions.

Numerical Analysis of Fifth-Harmonic Conversion of Low-Power Pulsed Nd:YAG Laser with Resonance of Second Harmonic

A model for the fifth-harmonic generation of pulsed IR lasers involving an external ring cavity resonating at the second harmonic has been developed. Numerical analysis is performed to show the relative effects of the pulse delay, input polarization, and orientation of the nonlinear crystals on the fifth harmonic power. The results are validated by published experimental results. The model is used to analyze and obtain the optimal combination of nonlinear optical crystals for the fifth-harmonic generation. Our calculation shows that the combination of LiB3O5 (LBO), CsLiB6O10 (CLBO), and CLBO crystals for the second-harmonic, fourth-harmonic, and fifth-harmonic generation steps respectively gives an approximate conversion of 30% from the fundamental to the fifth harmonic power, resulting in 2 W at 213 nm for an input of 7 W at 1064 nm.

Two-laser infrared and ultraviolet matrix-assisted laser desorption/ionization.

Matrix-assisted laser desorption/ionization (MALDI) was performed using two pulsed lasers with wavelengths in the IR and UV regions. A 10.6 micro m pulsed CO(2) laser was used to irradiate a MALDI target, followed after an adjustable delay by a 337 nm pulsed nitrogen laser. The sample consisted of a 2,5-dihydroxybenzoic acid matrix and bovine insulin guest molecule. The pulse energy for both of the lasers was adjusted so that the ion of interest, either the matrix or guest ion, was not produced by either of the lasers alone. The delay time for maximum ion yield occurs at 1 micro s for matrix and guest ions and the signal decayed to zero in approximately 400 micro s. A mechanism is presented for enhanced UV MALDI ion yield following the IR laser pulse based on transient heating.

Continuum generation of the third-harmonic pulse generated by an intense femtosecond IR laser pulse in air

The continuum generation by intense femtosecond IR laser pulses focused in air including the effect of third-harmonic generation is investigated. We have used a theoretical model that includes the full spatio-temporal dynamics of both the fundamental and the third-harmonic pulses. Results of our numerical calculations show that a two-color filamentation effect occurs, in which the third-harmonic conversion efficiency remains almost constant over the whole filament length. It is found that this effect is rather independent of the wavelength of the input beam and the focal geometry. During the filamentation process the third-harmonic pulse itself generates a broad continuum, which can even overlap with the continuum of the fundamental pulse for the longer pump wavelengths. In consequence, the continuum generation generated by intense IR laser pulses is further extended into the UV.

The radiation sensitivity mapping of ICs using an IR pulsed laser system

Recent results obtained with a pulsed nanosecond IR laser system to automatically test single event latch-up effects in laboratory are presented. In particular, the capabilities of this system to perform detailed mapping for ICs sensitivity to radiation are discussed. Two VLSI ASIC circuits were used for sensitivity mapping. The results are compared with those obtained in heavy ion tests at GSI (Darmstad, Germany).

Generation of intense secondary pulsed molecular beams of high kinetic energy controllable by a powerful IR laser

A method for generation of intense secondary pulsed molecular beams and beams of radicals of high kinetic energy controllable by a powerful IR laser is described. A pressure shock (shock wave) is used as a source of secondary beams. The pressure shock is formed in interaction between an intense pulsed supersonic molecular beam (or flow) and a solid surface. The characteristics of the secondary beams were studied. Their intensities and the degree of gas cooling in them were shown to be comparable with the corresponding characteristics of the unperturbed primary beam. The acceleration of molecules in the secondary beam is achieved due to vibrational excitation of them by high-power IR laser pulse in the pressure shock and subsequent vibrational to translational (V–T) relaxation, which occurs when a gas expands through the orifice into a vacuum. Intense [⩾1020 molecules/(sr s)] beams of SF6 and CF3I molecules with kinetic energies approximately equal to 1.5 and 1.2 eV, respectively, were generated in the absence of carrier gases. The SF6 molecular beams with kinetic energies approximately from 2.5 to 2.7 eV with carrier gases H2, He and CH4 (SF6/carriergas=1/10) were obtained. The possibility of generation of intense beams of cold radicals by this method is demonstrated. The intense beams of cold and accelerated CF3 radicals were generated when the CF3I molecules in the shock were dissociated by high-power CO2 laser radiation. The spectral and energetic characteristics of acceleration of SF6 and CF3I molecules in the secondary beams were studied. The optimal conditions were found for obtaining high-energy molecules.

Preservation of yeast cell morphology for scanning electron microscopy using 3.28-μm IR laser irradiation

As an alternative to conventional fixation procedures for scanning electron microscopy (SEM) analysis, yeast cells ( Saccharomyces cerevisiae) were irradiated in ambient air, with an intense 3.28-μm IR laser pulse. The morphology of the irradiated cells was well preserved, while nonirradiated control cells were severely shriveled.

Stimulated Raman gain spectroscopy of low-frequency density of states in liquids by picosecond IR laser pulses

The low-frequency spectra of simple liquids have been studied with a stimulated Raman gain technique using tunable picosecond pulses in the mid-infrared region. The obtained results on CS 2, CCl 4, C 2Cl 4 and CDCl 3 are rescaled to reduced Raman spectra, which are a representation of the low-frequency density of states of these liquids, weighted by the corresponding intermolecular polarizability. This thermal ‘bath’ is believed to accept or provide small portions of energy during vibrational relaxation processes. The technique demonstrated here has a large potential for time-resolved studies of vibrational relaxation and related phenomena.

Explosive boiling of water after pulsed IR laser heating

By focussing 1 J of 1064 nm Nd ∶ YAG beam into 30 atmospheres of hydrogen we could Raman shift to produce a 10 ns, 300 mJ, 1.9 µm laser pulse. This pulse can directly heat water by more than 100 K (average), during the laser pulse, inducing vaporisation. Vaporisation was studied using time-resolved shadowgraphy and Raman spectroscopy to obtain macro and molecular level information. The O–H stretching Raman bands of water are sensitive to temperature allowing us to measure the average temperatures during the boiling process. After the T-jump, explosive boiling occurred within 100 ns during which time the bulk temperature decreased, indicating that the vaporising water molecules deprived heat from their surroundings. Shadowgraphs confirmed the timescale for this phenomenon visually. After 10 µs, vaporised gas molecules condensed and formed droplets, which were observed by a morphology-dependent resonance (MDR) Raman.

Adhesion Strength of Polymer Coatings studied by Laser Induced Delamination

ABSTRACTLaser Induced Delamination is a novel technique aimed at quantification of adhesion of thin polymer films to a metal substrate. In this technique a high power IR pulsed laser beam (maximum 500 mJ in 5 ns) is used to create the primary delaminated area in the form of a blister. The blister is formed due to heating of the metal substrate and because of partial evaporation of the polymer material adjacent to the interface. By varying the power of the laser pulse the gas pressure inside the blister is increased until further delamination takes place. This critical pressure is related to the strength of adhesion of the polymer to the substrate. The shape of the blister (radius and height) is controlled with a stylus profiler and with an optical confocal microscope.In the present work 30 μm thick PET films on steel substrate were studied. From the shape of the obtained blisters the strength of adhesion of the polymer is estimated. Various models describing blister formation and growth are discussed.

IR pulsed laser light interaction with soiled cellulose and paper

Nd:YAG laser irradiation (1064 nm) of cellulose samples does not lead to immediate nor long-term effects on mechanical properties of paper, which renders the method increasingly interesting for cleaning of historical paper artefacts. However, the technique's usability is so far limited due to discoloration when the treated object's surface contains carbonaceous dirt. By using diffuse-reflectance Fourier-transform infrared spectroscopy, size-exclusion chromatography and viscometry it is demonstrated that the distinct yellowing is accompanied by formation of ether cross-links and dehydration of the cellulose, as well as its depolymerisation. Furthermore, the origin of the discoloration is discussed and it is proposed that yellow chromophores are formed due to carbon-cellulose interactions during laser irradiation.