Wavelength-modulation spectroscopy using a frequency-doubled current-modulated diode laser

We have studied atomic absorption in an argon discharge by wavelength-modulation spectroscopy with a frequency-doubled diode laser. The tunable wavelength-modulated radiation at 430 nm was generated by frequency doubling a current-modulated 860-nm diode laser in a KNbO₃ crystal. 2f-, 4f- and 6f-harmonic spectra as a function of diode laser modulation depth were measured on a Doppler-broadened sample of excited argon atoms produced in a capacitively coupled plasma chamber. Characterisation of the harmonic signals was accomplished. Minimum detectable absorbances of 7.7×10^-5 and 1.9×10^-4 based on a 3σ criterion (σ being the standard deviation of the noise) were estimated for 2f- and 4f-harmonic detection of the frequency-doubled radiation with a time constant of 0.1 s. The concentrations of argon in the 1s₄ state were found to be in the range of 3×10⁸ to 1.2×10¹¹ cm^-3 for the experimental conditions studied. Received: 25 February 2002 / Revised version: 4 April 2002 / Published online: 14 May 2002     

Comparison of the influence of the fictive and the annealing temperature on the UV-transmission properties of synthetic fused silica

′ and NBOH). Samples with high OH content exhibit gradual recovery from the absorption band within several minutes after exposure to the KrF laser radiation. The formation of the KrF laser-induced 210 nm absorption band depends on the fictive temperature and on the OH content. Low fictive temperature, as a measure for the number of intrinsic defects, retards E^′ generation at the beginning of intense KrF excimer laser irradiation when the majority of defects are generated from precursor defects. However, for longer irradiation periods with pulse numbers of the order of 10⁵ pulses, a high OH content is the beneficial parameter. The accompanying atomic hydrogen is essential for the suppression of the 210 nm absorption band. This happens by transformation of the E^′ centers into Si-H defects. In contrast to a generally held view, annealing (decreasing of the fictive temperature) of fused silica does not always reduce UV induced defect generation. For example, annealing of the samples in an argon atmosphere causes a significantly higher 210 nm absorption increase during KrF excimer laser irradiation (240000 pulses) compared to nonannealed samples. Two spectroscopic methods to determine the OH content of fused silica were applied: Raman and infrared spectroscopy, which in this work lead to differing results. The energetics of the 210 nm absorption band generation and bleaching is summarized by a diagram explaining the interaction of the 248 nm laser radiation with fused silica. Received: 2 June 1997/Accepted: 13 June 1997     

Optical absorption in early stage low temperature laser produced plasma

We describe a new technique to measure the UV/visible absorption spectrum of the ablated material during the laser pulse. The technique utilizes the continuum emission from one laser produced plasma as a light source to measure the absorption properties of a second laser produced plasma which is formed on a semi-transparent target with an array of 40 μm holes. A 6 ns, 1064 nm laser was used to ablate a Ag target and the plasma absorption was measured in the range 450–625 nm for a laser fluence of 1 J cm⁻². The total absorption cross-section is (0.5–1.5)×10⁻¹⁷ cm² in the range 450–540 nm. By comparing the measured absorption with a calculation using the plasma spectroscopy code FLYCHK it can be concluded that, in the wavelength region examined here, the absorption is mainly due to bound-bound transitions.     

Mass and kinetic energy distribution of the species generated by laser ablation of La<Subscript>0.6</Subscript>Ca<Subscript>0.4</Subscript>MnO<Subscript>3</Subscript>

The mass distributions of the species generated by laser ablation from a La_0.6Ca_0.4MnO₃ target using laser irradiation wavelengths of 193 nm, 266 nm and 308 nm have been investigated with and without a synchronized gas pulse of N₂O. The kinetic energies of the species are measured using an electrostatic deflection energy analyzer, while the mass distributions of the species were analyzed with a quadrupole mass filter. In vacuum (pressure 10⁻⁷ mbar), the ablation plume consists of metal atoms and ions such as La, Ca, Mn, O, LaO, as well as multiatomic species, e.g. LaMnO⁺. The LaO⁺ diatomic species are by far the most intense diatomic species in the plume, while CaO and MnO are only detected in small amounts. The interaction of a reactive N₂O gas pulse with the ablation plume leads to an increase in plume reactivity, which is desired when thin manganite films are grown, in order to incorporate the necessary amount of oxygen into the film. The N₂O gas pulse appears to have a significant influence on the oxidation of the Mn species in the plume, and on the creation of negative ions, such as LaO⁻,O⁻ and O₂⁻.     

Two-Photon Laser-Induced Fluorescence Spectra of Argon in a Grimm-Style Glow Discharge Tube

Two-photon excited laser-induced fluorescence (LIF) spectra of argon atom were successfully observed in a Grimm-style glow discharge tube, which has widely been applied to depth profiling of the elemental composition on various film-like samples by emission spectrometry. The LIF signal of an argon atomic line at 641.63 nm was observed when the glow discharge argon plasma was illuminated by a pulsed Ti:sapphire laser radiation of 7–10 mJ/pulse at 753.39 or 795.66 nm without focusing of the laser beam.     

XPS studies of short pulse laser interaction with copper

The effect of laser ablation on copper foil irradiated by a short 30 ns laser pulse was investigated by X-ray photoelectron spectroscopy. The laser fluence was varied from 8 to 16.5 J/cm² and the velocity of the laser beam from 10 to 100 mm/s. This range of laser fluence is characterized by a different intensity of laser ablation. The experiments were done in two kinds of ambient atmosphere: air and argon jet gas.The chemical state and composition of the irradiated copper surface were determined using the modified Auger parameter (α′) and O/Cu intensity ratio. The ablation atmosphere was found to influence the size and chemical state of the copper particles deposited from the vapor plume. During irradiation in air atmosphere the copper nanoparticles react with oxygen and water vapor from the air and are deposited in the form of a CuO and Cu(OH)₂ thin film. In argon atmosphere the processed copper surface is oxidized after exposure to air.     

Optical characterisation of the photolytic decomposition of ferrocene into nanoparticles

Optical emission from the photolytic dissociation of ferrocene Fe(C₅H₅)₂, often abbreviated as FeCp₂, in argon atmosphere was studied. The dissociation was performed by using an ArF excimer laser, operating at a wavelength of 193 nm. Two pressure regions were examined. At low (0.1 mbar) pressure, several emission lines of Fe could be identified, however no C, C₂, or CH emission lines/bands were found. At a higher (20 mbar) pressure of the FeCp₂/Ar gas mixture, a broadband emission identified as blackbody radiation was observed. This blackbody radiation originates from nanoparticles with a mean size of 30 nm, which consist of both metallic iron and amorphous carbon. The initial colour temperature of the particles was 2600 K.     

Excitation of X rays by electrons accelerated in air in the wake wave of a laser pulse

The characteristics of X rays of a laser plasma generated in the interaction of a femtosecond pulse with solid targets in an air atmosphere have been investigated. It has been shown that the mechanism for the generation of X rays in the interaction of short intense laser pulses with solid targets in a gas atmosphere is attributed to the generation of fast electrons in the region of the filamentation of a laser pulse. It has been proven experimentally that under such conditions, the solid target irradiated by laser radiation of even a low density of about 10¹⁵ W/cm² very efficiently emits ∼10-keV photons. It has been shown theoretically that the maximum energy of accelerated electrons can reach ɛ_max ∼ 100–200 keV under these conditions. This means that the proposed method can provide characteristic radiation with the energy of photons much higher than 10 keV.     

Generation of lower and higher harmonics in different plasma media with small <Emphasis Type="Italic">Z</Emphasis>

The generation of lower (third) and higher harmonics of femtosecond laser radiation in plasmas produced by laser ablation of different targets with a small atomic number Z (B, Be, Li) has been investigated. The high (10⁻³) efficiency of third-harmonic generation was observed in plasma produced on the boron surface. Efficient third-harmonic generation was also observed in beryllium plasma using femtosecond pulses of Ti:sapphire laser radiation (λ = 790 nm) and its second harmonic (395 nm). We could tune the higher harmonics generation spectrum by tuning the crystal converter when using 395-nm radiation to be converted. It is shown that, in plasmas formed on targets with small Z, the conversion efficiency and limiting generated harmonic order depend on the delay between the ablation pulse and the pulse to be converted.     

Formation of conical microstructures upon laser evaporation of solids

The formation and development of the large-scale periodic structures on a single crystal Si surface are studied upon its evaporation by pulsed radiation of a copper vapor laser (wavelength of 510.6 nm, pulse duration of 20 ns). The development of structures occurs at a high number of laser shots (∼10⁴) at laser fluence of 1–2 J/cm² below optical breakdown in a wide pressure range of surrounding atmosphere from 1 to 10⁵ Pa. The structures are cones with angles of 25, which grow towards the laser beam and protrude above the initial surface for 20–30 μm. It is suggested that the spatial period of the structures (10–20 μm) is determined by the capillary waves period on the molten surface. The X-ray diffractometry reveals that the modified area of the Si substrate has a polycrystalline structure and consists of Si nanoparticles with a size of 40–70 nm, depending on the pressure of surrounding gas. Similar structures are also observed on Ge and Ti. Received: 12 February 2000 / Accepted: 28 March 2000 / Published online: 20 June 2001     

Linewidth studies on Cd+ 636.0 nm and 537.8 nm lines excited in a He-Cd hollow cathode discharge

Hollow cathode (HC) lasers usually operate in a single axial mode without any optical selection. This is attributed to the large homogeneous linewidth of the gain curve due to the relatively high filling pressure of these lasers. Collisional and Doppler broadening (Δν_C and Δν_D) of the Cd⁺ 636 nm and Cd⁺ 538 nm lines (laser transitions of the HeCd⁺ laser) excited in a HC discharge tube were determined using a Fabry–Perot interferometer technique. It was found that in the pressure range 7–25 mbar Δν_D was nearly constant, while, as expected, Δν_C increased linearly with pressure. The broadening constants were α(636 nm)= (47±2) MHz/mbarand α(538 nm)=(11.8±0.5) MHz/mbar. The first constant is large enough to explain single-mode operation of the red HeCd⁺ laser; but in the case of the green laser, the exact reason for the single-mode operation remained unclear. Received: 23 November 2000 / Revised version: 30 March 2001 / Published online: 7 June 2001     

Reduced absorption of neon-like bromine X-ray laser radiation in helium

We have measured the absorption of the 19.47-nm neon like bromine (J=2–1) X-ray laser line in low-pressure helium. The experiment was motivated by the coincidence of this line with the low-absorption wing of an autoionizing transition in helium. We observe that, with 1 mbar of helium, the continuum background and another bromine X-ray laser line at 19.82 nm are strongly reduced, enhancing the relative strength of the 19.47-nm laser line. Increasing the helium pressure to 1.5 mbar makes the continuum virtually disappear, resulting in an almost monochromatic emission of the X-ray laser line. An estimate of the absorption cross section for the 19.47-nm line is given as ≈3.9×10^-19 cm² and for the nearby continuum as 0.9–1.3×10^-18 cm². Received: 8 March 1999 / Revised version: 26 April 1999 / Published online: 11 August 1999     

EUV and fast ion emission from cryogenic liquid jet target laser-generated plasma

A liquid jet of either nitrogen or argon of 20 μm diameter was exposed to intense laser fields with pulse durations between 70 fs and 250 ps, leading to intensities of 10¹⁶ W cm^-2 and 10¹³ W cm^-2, respectively. The emission of extreme UV light and soft X-rays shows the characteristic lines of hydrogen-like nitrogen and carbon-like argon. For nitrogen the emitted photon flux at 250 ps was about two orders of magnitude higher than for 70 fs pulses. A weak dependence on the laser polarization with respect to the liquid jet axis was found. The kinetic energy of the emitted ions easily exceeded 100 keV for nitrogen and 200 keV for argon for a pulse duration close to 2 ps. Received: 21 August 2000 / Revised version: 20 December 2000 / Published online: 22 March 2001     

Development of an optical parametric generator with pulsed dye amplification for high-resolution laser spectroscopy

An injection-seeded optical parametric generator (OPG), coupled with three pulsed dye amplification (PDA) stages, was shown to produce tunable, narrow linewidth laser radiation. The OPG was composed of a pair of beta barium borate (β-BBO) crystals and pumped by the third harmonic (355 nm) output of a seeded Nd:YAG laser. The OPG was injection-seeded at the idler wavelength (824 nm) using an external cavity diode laser (ECDL) with a mode-hop-free tuning range of 20 GHz. Using the PDA stages, the OPG output signal (624 nm) was amplified to 19 mJ/pulse, while maintaining a spectral linewidth of approximately 160 MHz at full-width-half-maximum (FWHM) which was within a factor of 2 of the Fourier limit. A system of lenses and apertures was used to minimize amplified spontaneous emission (ASE) in the PDA stages. Using the OPG/PDA system, two-photon laser-induced fluorescence measurements of atomic oxygen were performed by sum-frequency-mixing the 624-nm beam with the third harmonic output of the seeded Nd:YAG laser to generate approximately 1 mJ/pulse of ultraviolet radiation near 226 nm. Voigt line shapes were found to be in good agreement with oxygen atom spectra in atmospheric-pressure, laminar, counter-flow flames; the magnitude of Doppler and collisional broadening was approximately the same. The measured O-atom concentration profile was found to compare well with that calculated using an opposed-flow flame code.     

Interaction of laser radiation with TNT with respect to the laser neutralisation of AP mines

According to UN estimations there are between 80 and 115 million activated landmines worldwide. These mines, or other unexploded ordnance (UXO), can be triggered accidentally and kill or injure more than 2000 civilians per month. The most common explosive in these mines is trinitrotoluene (TNT). In this paper, the potential of some of the most promising lasers for mine neutralisation is investigated, namely an ArF laser, a KrF excimer laser and a Nd:YAG solid-state laser. We have studied the interaction between laser beams emitting at λ=193 nm, 248 nm and 1060 nm and a bare solid sample of TNT of approximately 15 mg. Using pulsed excimer radiation at λ=193 nm, with an energy density up to 1 J/mm², ablation of the TNT without any deflagration has been achieved. At λ=248 nm, using the KrF excimer laser with a pulse duration of 30 ns and a repetition rate of 5 Hz, the TNT sample started melting and burning after an irradiation of 10 s. Preliminary results with the Nd:YAG solid-state laser operating in cw emission have shown that the irradiated sample exhibits the desired burning behaviour even after the exposure is stopped. Received: 14 December 2000 / Accepted: 18 December 2000 / Published online: 20 June 2001     

Effect of the ceramic grain size and concentration on the dynamical mechanical and dielectric behavior of poly(vinilidene fluoride)/Pb(Zr<Subscript>0.53</Subscript>Ti<Subscript>0.47</Subscript>)O<Subscript>3</Subscript> composites

Laser-induced backside wet and dry etching (LIBWE and LIBDE) methods were developed for micromachining of transparent materials. Comparison of these techniques is helpful in understanding the etching mechanism but was not realized due to complications in setting up comparable experimental conditions. In our comparative investigations we used a solid tin film for dry and molten tin droplets for wet etching of fused-silica plates. A tin–fused-silica interface was irradiated through the sample by a KrF excimer laser beam (λ=248 nm, FWHM=25 ns); the fluence was varied between 400 and 2100 mJ/cm². A significant difference between the etch depths of the two investigated methods was not found. The slopes of the lines fitted to the measured data (sl_LIBDE=0.111 nm/mJ cm⁻², sl_LIBDE=0.127 nm/mJ cm⁻²) were almost similar. Etching thresholds for LIBDE and LIBWE were approximately 650 and 520 mJ/cm², respectively. To compare the dependence of etch rates on the pulse number, target areas were irradiated at different laser fluences and pulse numbers. With increasing pulse number a linear rise of depth was found for wet etching while for dry etching the etch depth increase was nonlinear. Secondary ion mass spectroscopic investigations proved that this can be due to the reconstruction of a new thinner tin-containing surface layer after the first pulse.     

Enhanced harmonic generation in C60-containing plasma plumes

The results of enhanced harmonic generation in fullerene-containing plasma plumes using laser radiation of different wavelengths, pulse durations, and phase modulation are presented. Odd and even harmonics up to the 29^th order of 800 nm radiation have been extended in the case of two-color orthogonal polarization pump of C₆₀-containing plasma plumes. The conversion efficiency for the odd and even harmonics in the vicinity of SPR of the C₆₀-containing plasma (40–70 nm) was estimated to be in the range of 10⁻⁴.     

Tritium recovery from co-deposited layers using 193-nm laser

Recovery of tritium from co-deposited layers formed in deuterium–tritium plasma operations of the TFTR (Tokamak Fusion Test Reactor) was investigated by the use of an ArF excimer laser operating at the wavelength of 193 nm. At the laser energy density of 0.1 J/cm², a transient spike of the tritium-release rate was observed at initial irradiation. Hydrogen isotopes were released in the form of hydrogen-isotope molecules during the laser irradiation in vacuum, suggesting that tritium can be recovered readily from the released gases. In a second experiment, hydrogen (tritium) recovery from the co-deposited layers on JT-60 tiles that had experienced hydrogen-plasma operations was investigated by laser ablation with a focused beam of the excimer laser. The removal rate of the co-deposited layers was quite low when the laser energy density was smaller than the ablation threshold (1.0 J/cm²), but reached 1.1 μm/pulse at the laser energy density of 7.6 J/cm². The effective absorption coefficient in the co-deposited layers at the laser wavelength was determined to be 1.9 μm^-1. The temperature of the surface during the irradiation at the laser energy density of 0.5 J/cm² was measured on the basis of Planck’s law of radiation, and the maximum temperature during the irradiation decreased from 3570 K at the initial irradiation to 2550 K at the 1000th pulse of the irradiation. Received: 5 August 2002 / Accepted: 7 August 2002 / Published online: 28 October 2002 RID="*" ID="*"Corresponding author. Fax: +81-29/2825917, E-mail: shu@tpl.tokai.jaeri.go.jp     

Intense XUV emission generated by a capillary discharge based apparatus

We present the characterization of an apparatus generating XUV radiation by a high peak value (20–40) kA and short rise time (≈ 20 ns) current pulse in a capillary discharge channel (up to 20 cm in length) filled by argon gas. The apparatus has been developed with the purpose of production of an intensive spontaneous emission in the spectral region of (2–50) nm and study of the z-pinch conditions for obtaining the laser generation in the Nelike Ar at 46.9 nm. The current pulses are generated by the direct discharge of a 7 nF water dielectric capacitor resonantly charged up to 400 kV by a six-stages Marx generator. The XUV radiation emitted during the radial compression of the plasma column is measured using calibrated PIN diodes, filters and multilayer mirrors in order to test the z-pinch plasma collapse and to measure the conversion efficiency of the electrical energy into the XUV radiation. This work is supported by the Italian National Institute of Nuclear Physics and in part by the Italian National Institute of Matter Physics.     

Dynamics of high-temperature plasma formation during laser irradiation of three-dimensionally structured,low-density matter

Results are presented from an experimental investigation of the properties of the plasma produced by the action of a radiation pulse at the second harmonic of a Nd laser, with average intensity ~5·10¹⁴ W/cm² in the focal spot, on flat targets consisting of porous polypropylene (CH)_x with an average density of 0.02 g/cm³ (close to the critical plasma density) and with ~50 μm pores. The properties of the laser plasma obtained with porous and continuous targets are substantially different. The main differences are volume absorption of the laser radiation in the porous material and much larger spatial scales of energy transfer. The experimentally measured longitudinal ablation velocity in the porous material was equal to (1.5–3)·10⁷ cm/s, which corresponds to a mass velocity of (3–6)·10⁵ g/cm²· s, and the transverse (with respect to the direction of the laser beam) propagation velocity of the thermal wave was equal to ~(1–2) ·10⁷ cm/s. The spatial dimensions of the plasma plume were ~20–30μm. The plasma was localized in a 200–400μm region inside the target. Pis’ma Zh. éksp. Teor. Fiz. 64, No. 7, 462–467 (10 October 1996)