Precise laser ablation processing of black widow spider silk

Contact-free precise laser processing of black widow spider (Latrodectus hesperus) dragline silk by laser ablation at =157 nm is achieved. At this wavelength, the small optical penetration depth, below 100 nm, allows efficient and gentle material removal above the ablation threshold of _th=29 mJ/cm². The ablation rate in nm/pulse is measured against laser fluence and simultaneously calibrated with ultrapure polymethylmethacrylate (PMMA). Ripple formation in fiber ablation can be overcome by a suitable combination of ablation removal and laser polishing steps using exposure sequences at different irradiation angles , such as vertical at 0° followed by oblique irradiation at 70°. In this way ripples are destroyed until the fiber is automatically smoothed over the entire laser-exposed length. This allows precise laser ablation processing of spider silk fibers and other highly absorbing materials without affecting the bulk mechanical, and other material, properties. PACS 61.41.+e; 61.46.+w; 61.80.Ba; 61.82.Pv; 62.25.+g     

Formaldehyde sensor using interband cascade laser based quartz-enhanced photoacoustic spectroscopy

A novel continuous-wave mid-infrared distributed feedback interband cascade laser was utilized to detect and quantify formaldehyde (H₂CO) using quartz-enhanced photoacoustic spectroscopy. The laser was operated at liquid-nitrogen temperatures and provided single-mode output powers of up to 12 mW at 3.53 m (2832.5 cm^-1). The noise equivalent (1) detection sensitivity of the sensor was measured to be 2.2×10^-8 cm^-1W(Hz)^-1/2 for H₂CO in ambient air, which corresponds to a detection limit of 0.6 parts in 10⁶ by volume (ppmv) for a 10 s sensor time constant and 3.4 mW laser power delivered to the sensor module. PACS 42.62.Fi; 72.50.+b     

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.     

Measurements of nanoparticle size distribution produced by laser ablation of tungsten and boron-carbide in N2 ambient

Nanoparticles (NPs) were produced by ablating tungsten and boron-carbide (B₄C) target materials in atmospheric pressure nitrogen ambient using ArF excimer laser pulses. The size distributions of the NPs formed during the ablation were monitored—within a 7-133 nm size window—by a condensation particle counter connected to a differential mobility analyzer. The laser repetition rate was varied between 1-50 Hz, and the fluence was systematically changed in the range of 0.5-15 J/cm², for both materials, allowing a comparative study in an extended laser parameter regime. The multishot ablation threshold (Φ_th) of B₄C was determined to be ∼1.9 J/cm² for the laser used (ArF excimer, λ = 193 nm). Similarly to earlier studies, it was shown that the size distributions consist of mainly small nanoparticles (<∼20 nm) attributed to a non-thermal ablation mechanism below Φ_th. An additional broad peak appears (between 20 and 40 nm) above Φ_th as a consequence of the thermally induced macroscopic ablation. Chemical composition of deposited polydisperse nanoparticles was studied by X-ray photoelectron spectroscopy showing nitrogen incorporation into the boron-carbide.     

Chemistry and microstructure of PLD (Ti,Al)CxN1-x coatings deposited at room temperature

The aim of the present work was the improvement of titanium-aluminium nitride (TiAlN) coatings by the solid-solution hardening with carbon atoms leading to titanium-aluminium carbon-nitride (Ti,Al)C_xN_1-x coatings with varying carbon (x) and nitrogen contents. The request of low deposition temperatures necessary for the coating of heat sensitive materials like tool steels of high hardness and polymers was reached by the application of the room temperature pulsed laser deposition (PLD) technique. A Nd:YAG laser of 1064 nm wavelength operated at two different laser pulse energies was used in the ablation experiments of pure TiAl targets (50 at.% Al) in various C₂H₂-Ar gas mixtures. Different pulse energies of the laser resulted in changes of the ratio of Ti/Al atoms in the grown coatings. Furthermore, the results reveal a strong proportionality of the gas mixture to the C and N content of the coatings. In the coatings deposited at low C₂H₂ gas flows the XRD investigations showed crystalline phases with fcc TiN type lattices, whereas high acetylene flows during deposition resulted in the formation of fully amorphous coatings and carbon precipitation or cluster boundaries found in Raman investigations. PACS 81.15.Fg; 46.55.+d     

Amorphous carbon nitride film preparation by plasma-assisted pulsed laser deposition method

Amorphous carbon nitride (aCN_x) films were prepared by pulsed laser ablation of graphite in N₂ RF plasma. The film property was compared with that prepared in N₂ gas. The N₂ plasma was generated by a mesh electrode, which was inserted between a graphite target and a Si substrate. The gas pressure p_N2 was varied from 10 to 100 mTorr. The film deposition rate exponentially decreased with p_N2 for both the plasma and gas environment. X-ray photoelectron spectroscopy analysis showed that the ratio of nitrogen content to the carbon one ([N]/[C]) of the aCN_x film surface deposited in the N₂ plasma was 2 times higher than that obtained in the N₂ gas. The film structure was shown by Raman spectroscopy analysis that sp² clustering was enhanced with increasing the [N]/[C]. The effect of plasma on aCN_x film deposition was discussed. PACS 81.15.Fg; 79.60.-i; 81.05.Uw     

A predissociative triplet Rydberg state of the nitrogen molecule studied by near-infrared diode laser kinetic spectroscopy

A new ³Π-³Σ band was observed in a discharge plasma of the nitrogen molecule and helium using near-infrared diode laser kinetic spectroscopy. All the lines in this band exhibited line broadening of more than 0.1 cm⁻¹. Rotational analysis revealed that the lower state of the transition was the state of N₂, which had already been studied in the D³Σ-E³Σ band. The term value and rotational constant suggested that the upper state is the G³Π_u Rydberg state with an electron configuration of (N₂⁺:X²Σ core)(3pπ_u). The line broadening is attributed to predissociation through a homogeneous interaction with a repulsive Π_u state.     

Reactive pulsed laser deposition of gold nitride thin films

We report on the growth and characterization of gold nitride thin films on Si 〈1 0 0〉 substrates at room temperature by reactive pulsed laser ablation. A pure (99.95%) Au target was ablated with KrF excimer laser pulses in nitrogen containing atmosphere (N₂ or NH₃). The gas ambient pressure was varied in the range 0.1-100 Pa. The morphology of the films was studied by using optical, scanning electron and atomic force microscopy, evidencing compact films with RMS roughness in the range 3.6-35.1 nm, depending on the deposition pressure. Rutherford backscattering spectrometry and energy dispersion spectroscopy (EDS) were used to detect the nitrogen concentration into the films. The EDS nitrogen peak does not decrease in intensity after 2 h annealing at 250 °C. Film resistivity was measured using a four-point probe and resulted in the (4-20) × 10⁻⁸ Ω m range, depending on the ambient pressure, to be compared with the value 2.6 × 10⁻⁸ Ω m of a pure gold film. Indentation and scratch measurements gave microhardness values of 2-3 GPa and the Young's modulus close to 100 GPa. X-ray photoemission spectra clearly showed the N 1s peak around 400 eV and displaced with respect to N₂ phase. All these measurements point to the formation of the gold nitride phase.     

Photothermal-induced dichroism and micro-cluster formation in Ag<Superscript>+</Superscript>-doped glasses

In this paper, we report the observation of an induced dichroism in an ionic silver cluster-doped glass, under the interaction of a CW high-power (P_max8 W) Ar⁺ laser beam. It is found that, as a result of the photo-thermal interaction, a dichroism due to the formation of chain-like silver clusters is induced. Because of the electromagnetic interaction, the orientation of these chain-like structures is related, in general, to the direction of the laser beam polarization. The effect is seen in the regime, where the laser beam power is increased rapidly. In this regime micrometer size (d2 m) clusters are generated on the surface of the sample. In this report, we discuss the production of such large silver clusters as a result of changing of the initial ionic clusters to the neutral ones and then aggregation of the generated small neutral clusters under the supersaturation condition. PACS 61.46.+w; 78.90.+t; 33.55.-b; 36.40.Vz; 36.40.Mn; 42.70.Ce     

Pulsed laser deposited ZrAlON films for high-k gate dielectric applications

ZrAlON films were fabricated using the reactive ablation of a ceramic ZrAlO target in N₂ ambient by pulsed laser deposition (PLD) technique. ZrAlON films were deposited directly on n-Si(100) substrates and Pt coated silicon substrates, respectively, at 500 °C in a 20 Pa N₂ ambient, and rapid thermal annealed (RTA) in N₂ ambient at 1000 °C for 1 min. Cross sectional high-resolution transmission electron microscopy (HRTEM) images clearly show that the ZrAlON/Si interface is atomically sharp without an interfacial layer, and the films are completely amorphous. The electron diffraction pattern of TEM also indicates the amorphous structure of the RTA ZrAlON film. X-ray photoelectron spectroscopy (XPS) measurement was performed to confirm the effective incorporation of nitrogen with a content of about 6 at. %, and to reveal the N–O bonding in ZrAlON films. The dielectric constant of amorphous ZrAlON was determined to be about 18.2 which is more than 16.8 for ZrAlO by measuring the Pt/films/Pt capacitors. Capacitance–voltage (C–V) measurements show that a small equivalent oxide thickness (EOT) of 1.03 nm for 4 nm ZrAlON film on the n-Si substrate with a leakage current of 28.7 mA/cm² at 1 V gate voltage was obtained. PACS 77.55.+f; 81.15.Fg; 73.40.Qv     

Thermal stability and electrical properties of Zr silicate films for high-k gate-dielectric applications, as prepared by pulsed laser deposition

Zirconium silicate films with high thermal stability and good electrical properties have been prepared on n-Si(100) substrates and commercially available Pt-coated Si substrates to fabricate metal–insulator–metal (MIM) structures by the pulsed laser deposition (PLD) technique using a Zr_0.69Si_0.31O_2- ceramic target. Rapid thermal annealing (RTA) in N₂ was performed. X-ray diffraction indicated that the films annealed at 800 °C remained amorphous. Differential thermal analysis revealed that amorphous Zr silicate crystallized at 830 °C. X-ray photoelectron spectroscopy showed that RTA annealing of Zr silicate films at 900 °C led to phase separation. The dielectric constant has been determined to be about 18.6 at 1 MHz by measuring the Pt/Zr silicate/Pt MIM structure. The equivalent oxide thicknesses (EOTs) and the leakage-current densities of films with 6-nm physical thickness deposited in O₂ and N₂ ambient were investigated. An EOT of 1.65 nm and a leakage current of 31.4 mA/cm² at 1-V gate voltage for the films prepared in N₂ and RTA annealed in N₂ at 800 °C were obtained. An amorphous Zr-rich Zr silicate film fabricated by PLD looks to be a promising candidate for future high-k gate-dielectric applications. PACS 77.55.+f; 81.15.Fg; 73.40.Qv     

UV cavity enhanced absorption spectroscopy of the hydroxyl radical

We present the application of a continuous-wave ultra-violet tuneable light source for detection of the hydroxyl radical (OH) using cavity-enhanced absorption spectroscopy of the Q₁₁(2) and Q₂₁(2) absorption lines in the A²⁺(v=0)²_3/2(v=0) band at ca. 308 nm. A tuneable infra-red diode laser operating at 835 nm and either an Ar⁺ laser or a single frequency continuous-wave intracavity frequency-doubled diode laser, both operating at ca. 488 nm, were used to produce 0.1–0.5 W of tuneable radiation at ca. 308 nm by sum frequency generation in a BaB₂O₄ crystal. Cavity enhanced absorption spectroscopy was used to detect OH generated by UV photolysis of water vapour in argon, nitrogen, neon and helium at atmospheric pressure. A noise-equivalent (1) absorption sensitivity of 2.1×10^-7 cm^-1Hz^-1/2 measured over 128 scans in a time of 1.16 s was demonstrated with mirrors of reflectivity 0.9963 in a cavity of length 58.5 cm for a 2 cm^-1 scanning range at a UV power of 0.5 W. An OH detection limit (1) of 3.84×10⁹ moleculecm^-3 was estimated in argon at atmospheric pressure. OH collisional broadening in humidified N₂, Ar, Ne and He was determined at atmospheric pressure . PACS 39.30.+w; 42.55.Px; 42.62.Fi; 42.68.Ca     

Laser-ionization mass-spectrometric studies on laser ablation of a nitrogen-rich polymer at 532 nm and 1064 nm

Laser-ionization Time-Of-Flight (TOF) mass-spectrometric studies have been carried out on the 532 nm and 1064 nm laser ablation products from a nitrogen-rich polymer. The polymer used had an elemental composition of C_6.0N_8.9H_3.4 and consisted of C=N, C-N, and N-H chemical bonds. The TOF mass spectra observed were composed of various peaks (150 amu) depending on the ablation laser wavelength. The primary peaks were assigned to C⁺, CN⁺, CHnN⁺ ₂ (n=1–3) and C₂H₂N⁺ ₃ for 532 nm ablation, and C⁺, C⁺ ₃, HCN⁺, HCCN⁺, CH₂NH⁺, HNCN⁺, H₃NCN⁺, and C₄H₄N⁺ ₇ for 1064 nm ablation. The flight velocity distributions with peak velocities ranging from 8.6×10³ cm/s to 3.8×10⁴ cm/s were measured for these products. The distinct velocity distributions observed between small and large products indicate the presence of two origins in the fragment ejection process from the polymer for both 532 nm and 1064 nm ablation. Furthermore, we suggest an importance of the translational energy of the fragments for the product generation in the laser plume.     

Narrow-line diode laser system for laser cooling of strontium atoms on the intercombination transition

We report a diode laser system developed for narrow-line cooling and trapping on the ¹S₀–³P₁ intercombination transition of neutral strontium atoms. Doppler cooling on this spin-forbidden transition with a line width of /2=7.1 kHz enables us to achieve sub-K temperatures in a two-step cooling process. The required reduction of the laser line width to the kHz level was achieved by locking the laser to a tunable Fabry–Pérot cavity. The long-term drift (>0.1 s) of the reference cavity was compensated by employing the saturated absorption signal obtained from Sr vapor in a heat pipe of novel design. We demonstrate the potential of the system by performing spectroscopy of Sr atoms confined to the Lamb–Dicke regime in a one-dimensional optical lattice. PACS 32.80.Pj; 39.30.+w; 42.55.Px     

UV assisted oxidation and nitridation of hafnia based thin films for alternate gate dielectric applications

The synergistic effects of NH₃ ambient and ultraviolet illumination on the dielectric properties of hafnia based gate dielectrics are reported in this paper. The films were processed at relatively low temperatures (∼400 °C) by pulsed laser ablation and UV oxidation technique. UV illumination and the NH₃ ambient created a thin and a denser interfacial layer (at the film-Si interface) comprised of HfSiON bonding. As a result of the interfacial layer modification, a leakage current density lower than 10⁻⁴ A/cm² and a dielectric constant of ∼21.7 were extracted from the best samples processed in NH₃ and under UV illumination. The nitrogen doped HfO₂ also exhibited a thinner interfacial layer (∼12 Å) in comparison to the films processed without NH₃ ambient.     

Efficient ablation of organic polymers polyether sulphone and polyether ether ketone by a TEA CO2 laser with high perforation ability

Organic polymer (PES: PolyEther Sulphone and PEEK: PolyEther Ether Ketone) ablation with oscillation-line selected TEA CO₂ lasers is successfully demonstrated. With different irradiation conditions the ablative etch-rate slopes were varied, which means that the ablation process is dependent on the ablation conditions such as incident laser intensity and ambient gas. In perforation processing of the PEEK film, the TEA CO₂ laser had a higher etch rate of 42 m/pulse at a fluence of 70 J/cm² in vacuum than the XeCl laser.     

Cluster formation and laser-induced effects in the ablation of 2Cu(CO3)·Cu(OH)2. Time-of-flight mass spectrometric study

The mechanism of laser ablation of 2Cu(CO₃)·Cu(OH)₂ at 308 nm is investigated by time-of-flight mass spectroscopy at laser fluences in the range of 0.07 to 0.6 Jcm^-2. The neutral and ion composition of the plume reveals the presence of Cu, Cu₂O, CuO, H₂O and CO₂, which appear as free species or forming clusters. Plume composition is compatible with a thermal ablation mechanism which involves chemical processes similar to those reported for the purely thermal decomposition of the target. Velocity distributions of neutral species in the plume are measured and fitted to shifted Maxwell–Boltzmann time distributions; the best fitting yields flow velocities of 0.07 cms^-1 for CO₂ and Cu and 0.04 cms^-1 for Cu₂, which is compatible with an expansion regime characterized by weak interaction of the ejected particles. PACS 82.30.Nr; 78.70.-g     

Pulsed-discharge-aided laser ablation synthesis of nanoscale aluminum nitride powders

2 ) discharges were shown to efficiently generate atomic nitrogen that promoted the synthesis of high-purity aluminum nitride powders formed by pulsed-laser ablation of aluminum targets. The interaction between the nitrogen discharge plasma and the ablated aluminum plume depended on the synchronisation of the two pulsed events for maximum overlap of the transient reactants. By monitoring the optical emissions of the reactive N⁺ species and analysing the synthesised powder with X-ray photoelectron spectroscopy, it was found that when laser ablation occurred at about 3 μs after the pulsed discharge, the N⁺ emission from the overlapping plasma was the most intense and the purity of the aluminum nitride synthesized was the highest. Received: 8 September 1997/Accepted: 8 September 1997     

Pulsed laser deposition of ZnO in N2O atmosphere

The contribution deals with ZnO thin layers doped by nitrogen which were prepared by pulsed laser deposition in N₂O ambient atmosphere. Our approach is based on ablation of undoped ZnO target in active atmosphere containing N₂O gas without any supporting excitation equipment in parallel. Ablation of ZnO target was performed at different pressures (1–32 Pa) of N₂O ambient atmosphere by pulsed Nd:YAG laser (at 355 nm). Layers of ZnO were grown on different substrates (Si, sapphire, fused silica) and their properties were investigated by various analytical methods: scanning electron microscopy (SEM), secondary ion mass spectroscopy (SIMS), X-ray diffraction (XRD), and optical transmission spectroscopy. The results confirmed incorporation of nitrogen into ZnO layers and its concentration was pressure dependent. According to SIMS analysis, there is a certain pressure level (above 10 Pa) when the presence of N becomes negligible. Transmittance spectra showed increasing of the optical band gap (E _g) according to the pressure of N₂O.     

Target density and surface state effects on the compositional changes in iron oxide particle aggregated films prepared by laser ablation

Iron oxide nanoparticle aggregated films were prepared using the excimer laser ablation technique by adopting an off-axis configuration and the gas condensation process. Sintered iron oxide (-Fe₂O₃) targets were ablated in oxygen ambient by an ArF excimer laser. The product of ablation comprised Fe₂O₃ at lower pressure and a mixture of Fe₂O₃ and FeO at higher pressure by X-ray-diffraction measurements. The maximum ambient oxygen pressure, P_S(O₂), at which the product composition was still a single Fe₂O₃ phase was higher for the higher-density target than for its lower-density counterpart. The target surface state affected the product composition only if the pressure was set to the pressure P_S(O₂) of 40 Pa for a high-density target. When the fluence was high (200 mJ/pulse, 3.3-mm² spot size), the product composition varied at the initial stage of laser irradiation with the number of laser pulses from a mixture of Fe₂O₃ and FeO to only Fe₂O₃ along with the target surface morphological change from a grooved structure to a smooth surface. Product composition was practically independent of the number of pulses by low-fluence laser irradiation even at this particular pressure of 40 Pa. PACS 81.07.Bc; 81.15.Fg; 61.10.Nz