Optical properties of pulsed laser heated soot

To investigate the transient change of soot optical properties resulting from pulsed laser heating of soot in a cooled exhaust plume we have simultaneously performed cw light extinction at 405 and 830 nm and elastic light scattering at 1064 nm. A reversible increase to the 830-nm light extinction of up to 7%, observed during the time period where the soot was hot, suggests a temperature-dependent light absorption refractive index function, E(m _λ ). At low fluence, small permanent increases of E(m _λ ) of <2% were also observed. 405-nm extinction measurements revealed that the soot likely contained material which continued to absorb 405-nm radiation when desorbed, thus complicating measurement interpretation. 1064-nm light scattering measurements showed a gradual decrease of scattering propensity with increasing laser fluence up to the point of material loss, which is consistent with the expected decrease of the structure factor of the soot aggregates as they expand. It is concluded that variations of the optical properties are occurring at the time of laser-induced incandescence (LII) emission, which should be accounted for in time-resolved LII measurement interpretation.     

New evidences of in situ laser irradiation effects on γ‐Fe2O3 nanoparticles: a Raman spectroscopic study

The nature of the physical mechanisms responsible for the structural modification of the γ‐Fe₂O₃ nanoparticles under laser irradiation has been investigated by Raman spectroscopy. In situ micro‐Raman measurements were carried out on as‐prepared γ‐Fe₂O₃ nanoparticles about 4 nm in size as a function of laser power and on annealed γ‐Fe₂O₃ particles. A baseline profile analysis clearly evidenced that the phase transition from maghemite into hematite is caused by local heating due to laser irradiation with an increase of grain size of nanoparticles. This increasing was clearly determined by X‐ray diffraction from 4 nm in nanoparticles up to more than 177 nm beyond 900 °C in a polycrystalline state. Copyright © 2010 John Wiley & Sons, Ltd.     

Mechanisms of Resonant Infrared Matrix-Assisted Pulsed Laser Evaporation

For the last decade, a variant of pulsed laser ablation, Resonant-Infrared Matrix-Assisted Pulsed Laser Evaporation (RIR-MAPLE), has been studied as a deposition technique for organic and polymeric materials. RIR-MAPLE minimizes photochemical damage from direct interaction with the intense laser beam by encapsulating the polymer in a high infrared-absorption solvent matrix. This review critically examines the thermally-induced ablation mechanisms resulting from irradiation of cryogenic solvent matrices by a tunable free electron laser (FEL). A semi-empirical model is used to calculate temperatures as a function of time in the focal volume and determine heating rates for different resonant modes in two model solvents, based on the thermodynamics and kinetics of the phase transitions induced in the solvent matrices. Three principal ablation mechanisms are discussed, namely normal vaporization at the surface, normal boiling, and phase explosion. Normal vaporization is a highly inefficient polymer deposition mechanism as it relies on collective collisions with evaporating solvent molecules. Diffusion length calculations for heterogeneously nucleated vapor bubbles show that normal boiling is kinetically limited. During high-power pulsed-FEL irradiation, phase explosion is shown to be the most significant contribution to polymer deposition in RIR-MAPLE. Phase explosion occurs when the target is rapidly heated (10⁸ to 10¹⁰ K/s) and the solvent matrix approaches its critical temperature. Spontaneous density stratification (spinodal decay) within the condensed metastable phase leads to rapid homogeneous nucleation of vapor bubbles. As these vapor bubbles interconnect, large pressures build up within the condensed phase, leading to target explosions and recoil-induced ejections of polymer to a near substrate. Phase explosion is a temperature (fluence) threshold-limited process, while surface evaporation can occur even at very low fluences.     

Ultraviolet Raman scattering in ZnO nanowires: quasimode mixing and temperature effects

We use near‐resonance Raman scattering to investigate zinc oxide (ZnO) nanowires grown by chemical vapor deposition on Si substrates. We discuss the role of quasimode mixing on the wavenumber of the longitudinal optical (LO) bands, and we perform Raman measurements with different excitation powers to investigate possible laser heating effects. We find that in the Raman spectra of as‐deposited nanowires grown along the c‐axis of wurtzite, the LO bands are located slightly below the E₁(LO) mode of bulk ZnO. We perform a calculation of the expected LO wavenumber in an ensemble of randomly oriented nanowires. Our analysis shows that light refraction, together with the orientation‐dependent cross‐section of the nanowires for the incoming light, counterbalances quasimode mixing effects in the as‐grown product, giving rise to LO bands that are barely redshifted relative to the E₁(LO) mode. In the case of ZnO nanowires that have been mechanically removed (scratched) and subsequently deposited onto separate Si substrates, we observe clear laser‐induced heating. Temperature effects account well for the Raman wavenumber shifts displayed by the LO bands in the Raman spectra of the scratched nanowires. Copyright © 2010 John Wiley & Sons, Ltd.     

Laser-heating diamond anvil cell studies of simple molecular systems at high pressures and temperatures

We report the application of new laser-heating techniques and sample preparation procedures for simple molecular materials (diatomic molecules and water) under high pressure in the diamond anvil cell (DAC). Both continuous and pulsed laser heating was employed. We probed the materials using Raman spectroscopy and also by analyzing the time evolution of the temperature of the metallic coupler that is used to absorb laser radiation and heat the sample. Raman measurements of H₂, D₂, N₂, H₂O and O₂ show a broadening of intramolecular vibrations at high P−T conditions, indicating a decreasing molecular lifetime, and hence suggest an increasing molecular dissociation. In diatomic molecules the intramolecular bonding can be further probed by observations of sidebands corresponding to vibrational transitions from excited states; the energies of these sidebands imply intramolecular potentials that become increasingly less anharmonic as pressure is increased. We also show that the pulsed heating technique combined with instantaneous radiative temperature measurements provides a useful tool for studies of thermochemical properties and phase transformation boundaries.     

Measurement of thermal conductivity in laser-heated diamond anvil cell using radial temperature distribution

ABSTRACT

Thermal conductivities of planetary materials under extreme conditions are important input parameters for modeling planetary dynamics such as accretion, geodynamo and magnetic field evolution, plate tectonics, volcanism-related processes etc. However, direct experimental measurements of thermal conductivity at extreme conditions remain challenging and controversial. Here we propose a new technique of thermal conductivity measurement in laser-heated diamond anvil cell (LH-DAC) based on radial temperature distribution around laser focal spot, mapped by imaging tandem acousto-optical tunable filter (TAOTF). The new technique provides much more information about heat fluxes in the laser-heated sample than existing static heating setups, and does not require dynamic numerical modeling using heat capacities in contrast to dynamic pulsed heating setups. In the test experiment, thermal conductivity of γ-Fe at conditions relevant to cores of terrestrial planets was measured.     

Transient deformation of thin metal sheets during pulsed laser forming

The transient deformation of thin grade 304 stainless steel metal sheets heated by a single pulse of a CO₂ laser beam is simulated in this paper. The laser beam is assumed to be line-shaped and the problem is treated as three-dimensional thermo-elastoplastic. The temperature field, deformation pattern, stress–strain states and the residual stress distribution of the specimens have been calculated numerically and the transient response of the bending angle has been validated by experiments. Good agreement has been obtained between the numerical simulation and the experiments under various operating conditions. The numerical study reveals that a high temperature gradient exists for a positive bending angle and a low one for a negative angle. It transpires that the mechanisms of pulsed laser forming are dependent mainly upon the laser power, the heating time, the clamping arrangement, as well as the geometry, the thermal properties and the original stress states of the specimen.     

超热电子产生的靶后相干渡越辐射光谱实验研究

利用OMA光学多道分析仪测量了激光与薄膜靶相互作用中产生的辐射光谱，在靶后观察到红移的二次谐波发射. 这种二次谐波是v×B加热产生的、具有微脉冲结构的超热电子束在等离子体-真空边界产生的相干渡越辐射(CTR). 随着激光能量的增大，红移峰向长波方向移动，光谱同时发生展宽. 分析认为，等离子体临界面的迅速膨胀是导致二次谐波红移的主要原因. 随着预脉冲能量的增大，临界面膨胀速度增大，导致了发射峰更大的红移. 实验还测量了靶面法线方向的辐射光谱，观察到基频辐射的红移和展宽. CTR为诊断临界面的运动方向和速度提供了一种新的方法. 关键词： 相干渡越辐射 超热电子 超短超强激光 等离子体相互作用     

Temperature‐dependent Raman study of PrFeO3thin film

Low‐temperature Raman study of (001)‐oriented PrFeO₃ thin film of around 200 nm thickness deposited on a LaAlO₃ (001) substrate by using the pulsed‐laser deposition technique is presented. X‐ray diffraction analysis of this film shows an orthorhombic structure with Pbnm space group. The observed substrate‐induced strain is found to be small. In the room temperature Raman spectra, different Raman modes were observed that were classified according to the orthorhombic structure. All the observed modes show a decrease in wavenumber with rise in temperature, except the B_1g mode (624 cm⁻¹) which shows some anomalous behavior. We tried to correlate the variations in linewidth and position with temperature for the observed modes with the octahedral disorder of FeO₆. Many possibilities are presented to explain the observed results. Copyright © 2010 John Wiley & Sons, Ltd.     

Decay Time of 2.09 eV Luminescence of S2--Vacancy Centers and Their Optical and Thermal Stability in NaCl:S2 Crystals

An investigation was carried out on decay time of the 2.09 eV emission of S²–vacancy pair centers in NaCl crystals. When the crystal was excited by a pulsed 337 nm light from an N₂ laser, the 2.09 eV yellow luminescence principally decayed with the lifetime of 14.2 μs at low temperatures, and exhibited a weak thermal quenching with an activation energy of 51.3 meV above about 150 K. Such S^2--vacancy pair centers responsible for the 2.09 eV luminescence were thermally stable at room temperature, and not bleached even when exposed to ultraviolet lights below about 5 eV. Thus, the 2.09 eV emission center possibly can be used as a solid-state laser active center working at room temperature.     

Laser Acoustic Probing of Two‐Temperature Zone Created by Femtosecond Pulse

We combine theoretical and experimental methods to study the processes induced by fast laser heating of metal foils. These processes reveal themselves through motion of frontal (irradiated) and rear‐side foil boundaries. The irradiated targets are 0.3‐2 micron thick aluminum foils deposited on much thicker (150 microns) glass plate. The instant boundary positions is measured by pump‐probe technique having ∼40‐150 fs time and ∼1 nm spatial resolutions. Ultrashort laser pulse transforms a frontal surface layer with thickness d_T into two‐temperature (T_e ≫ T_i) warm dense matter state. Its quantitative characteristics including its thickness are defined by poorly known coefficients of electron‐ion energy exchange α and electron heat conductivity κ. Fast laser heating rises pressure in the d_T‐layer and therefore produce acoustic waves. Propagation and reflection from the frontal and rear boundaries of these waves causes the displacement Δx (t) of boundary positions. Pressure wave profiles, and hence functions Δx (t), depend on thickness d_T. This is why the experimental detection of Δx (t) opens a way to accurate evaluation of the coefficients α and κ (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Critical role of laser wavelength on carbon films grown by PLD of graphite

The structure of thin films deposited by pulsed laser ablation (PLD) is strongly dependent on experimental conditions, like laser wavelength and fluence, substrate temperature and pressure. Depending on these parameters we obtained various kinds of carbon materials varying from dense, mainly tetrahedral amorphous carbon (ta-C), to less compact vertically oriented graphene nano-particles. Thin carbon films were grown by PLD on n-Si 〈100〉 substrates, at temperatures ranging from RT to 800°C, from a rotating graphite target operating in vacuum. The laser ablation of the graphite target was performed by a UV pulsed ArF excimer laser (λ=193 nm) and a pulsed Nd:YAG laser, operating in the near IR (λ=1064 nm). The film structure and texturing, characterised by X-ray diffraction analysis, performed at grazing incidence (GI-XRD), and the film density, evaluated by X-ray reflectivity measurements, are strongly affected both by laser wavelength and fluence and by substrate temperature. Micro-Raman and GI-XRD analysis established the progressive formation of aromatic clusters and cluster condensation into vertically oriented nano-sized graphene structures as a direct function of increasing laser wavelength and deposition temperature. The film density, negatively affected by substrate temperature and laser wavelength and fluence, in turn, results in a porous bulk configuration and a high macroscopic surface roughness as shown by SEM characterisation. These structural property modifications induce a relevant variation also on the emission properties of carbon nano-structures, as evidenced by field emission measurements. This work is dedicated to our friend Giorgio who passed away 20th August.     

Effect of the wavelength dependence of the emissivity on inferred soot temperatures measured by spectrally resolved laser-induced incandescence

Flame-generated soot was heated using a pulsed laser, and temperatures of the irradiated soot were inferred by fitting the Planck function to spectrally resolved laser-induced incandescence with the temperature as an adjustable parameter. The effect of the wavelength dependence of the emissivity on the inferred temperatures of the irradiated soot was studied using selected expressions for the soot emissivity in the fit. Depending upon the choice of the functional form of the emissivity, the maximum temperature reached by the soot during the laser pulse was calculated to span a range of 341 K (3475–3816 K) at a 1064-nm laser fluence of 0.1 J/cm² and 456 K (4115–4571 K) at a 1064-nm laser fluence of 0.4 J/cm² with a 1σ standard deviation about the mean of ∼25 K. Comparison of the present results with temperature measurements from previous studies suggests that the emissivity may depend on flame conditions and that further investigation on the subject is needed. The use of two-color or spectrally resolved LII to infer the soot temperature during or after laser heating requires a careful characterization of the wavelength dependence of the emissivity. The spread in temperature leads to large uncertainties regarding the physico-chemical processes occurring at the surface of the soot during the laser heating.     

Single‐shot CARS thermometry of high‐pressure hydrocarbon flames using simultaneous intensity and linewidth measurements

We demonstrate the feasibility of single laser shot coherent anti‐Stokes Raman scattering thermometry with simultaneous measurement of intensities of hydrogen Q‐branch lines and their linewidths in a pulsed CH₄/O₂ combustion chamber operating at 20 MPa pressure and 3000 K temperature—parameters that are typical for full‐scale rocket engines. The measurements were done by means of a spectrograph combined with an interferometer having orthogonal directions of dispersions. This approach allows correct temperature evaluation that takes into account the directly measured linewidths. Copyright © 2010 John Wiley & Sons, Ltd.     

Electrical characterisation of phosphorus-doped ZnO thin films grown by pulsed laser deposition

Phosphorus-doped ZnO films were grown by pulsed laser deposition using a ZnO:P₂O₅-doped target as the phosphorus source with the aim of producing p-type ZnO material. ZnO:P layers (with phosphorus concentrations of between 0.01 to 1 wt%) were grown on a pure ZnO buffer layer. The electrical properties of the films were characterised from temperature dependent Hall-effect measurements. The samples typically showed weak n-type conduction in the dark, with a resistivity of 70 Ω cm, a Hall mobility of μ_n0.5 cm² V ⁻¹ s⁻¹ and a carrier concentration of n3×10¹⁷ cm⁻³ at room temperature. After exposure to an incandescent light source, the samples underwent a change in conduction from n- to p-type, with an increase in mobility and decrease in concentration for temperatures below 300 K.     

光折变全息CO2激光局域热固定

提出用ＣＯ２激光对记录于ＬｉＮｂＯ３晶体中的全息进行实时实时地局域热固定，通过脉冲加热和加载吸热体，可以使得温度范围在１００～２００℃之间的区域接近于加热光的形状，分布于一局部范围，文中给出了加热模型及其数值解，通过得到了最优解参数进行了一系列实验，实验结果显示，用ＣＯ２激光可以成功地光折变全息进行实时地地局域热固定，并且加热时间短，固定转换效率较高。     

Photon-induced switching and tunneling phenomena in a YBCO thin film junction

In the persistent photoconductivity (PPC) phenomenon, illumination of a YBa₂Cu₃O_6.5 thin film junction with a 1mW He−Ne laser leads to the decrease of the critical voltage (similar to the threshold voltage). The decrease of the critical voltage was reversed by illumination with incandescent light. The critical voltage across the junction was experimentally decreased and increased by alternating illumination between He−Ne laser and incandescent light. We also observed visible quenching of the photo-induced state using a 5mW He−Ne laser. Finally, the threshold behavior of the junction was destroyed by illuminating it with incandescent light.     

Two-wavelength mid-IR absorption diagnostic for simultaneous measurement of temperature and hydrocarbon fuel concentration

A two-wavelength mid-IR laser is used for time-resolved absorption-based measurements of temperature and n-heptane concentration in shock-heated gases. The novel difference-frequency-generation laser provides tunable mid-IR light from nonlinear conversion of near-IR light, enabling access to the strong hydrocarbon absorption bands between 3.3 and 3.5 μm associated with the CH stretching vibration. This laser was modified to alternate between two mid-IR wavelengths at 200 kHz, providing 5 μs time resolution for simultaneous monitoring of temperature and concentration in reactive flows and combustion systems. Temperature-dependent absorption spectra of n-heptane are first measured in a cell from 298 to 773 K using an FTIR spectrometer. These spectra are used to select candidate pairs of wavelengths with good sensitivity to temperature and concentration and to provide accurate temperature-dependent absorption cross-sections at the selected wavelengths. Laser absorption measurements of shock-heated n-heptane are then used to extend the cross-section data to 1300 K and to investigate the sensor accuracy and noise characteristics. The temperature and concentration inferred from the measurements are compared to known post-shock conditions, with a 4.5% RMS deviation from the calculated temperature and 1.7% RMS deviation from the calculated concentration. Finally, at high temperatures, the sensor is used to monitor decomposition of n-heptane, illustrating the potential of this diagnostic for hydrocarbon kinetics experiments in shock tubes. This new sensor concept should prove useful for simultaneous, time-resolved temperature and hydrocarbon concentration measurements in a variety of combustion and propulsion applications.     

Role of the wall ablation in the operation of a 46.9 nmAr capillary discharge soft x‐ray laser

A capillary discharge pumped soft x‐ray laser operating at 46.9 nm on the 3p–3s transition of the Ne‐like Ar has been realized by pumping the active medium with a relatively slow current pulse (dI/dt ≈ 6 · 10¹¹ A/s). In order to study the role of the ablation in the production of the laser effect, the intensity of the amplified 46.9 nm line has been investigated using the same pumping current pulses in the plastic (polyacetal) and ceramic (Al₂O₃). We showed that the ablation of the capillary walls is unfavorable both for the compression and stability of the plasma and consequently for the soft x‐ray laser production. The amplification and lasing effects are observed only in the ceramic channel. The measurements of the line intensity at 46.9 nm showed the lasing with a gain‐length product of ≈ 9, a laser pulse energy of ≈ 5 μJ, a pulse duration of 1.3 ns and a beam divergence of ≈ 3.5 mrad. In addition, effect of the scaling of the time of lasing with the initial plasma diameter was demonstrated experimentally and compared with a one‐dimensional MHD model.     

The power of in situ pulsed laser deposition synchrotron characterization for the detection of domain formation during growth of Ba0.5Sr0.5TiO3 on MgO

A highly sophisticated pulsed laser deposition (PLD) chamber has recently been installed at the NANO beamline at the synchrotron facility ANKA (Karlsruhe, Germany), which allows for comprehensive studies on the PLD growth process of dielectric, ferroelectric and ferromagnetic thin films in epitaxial oxide heterostructures or even multilayer systems by combining in situ reflective high‐energy diffraction with the in situ synchrotron high‐resolution X‐ray diffraction and surface diffraction methods. The modularity of the in situ PLD chamber offers the opportunity to explore the microstructure of the grown thin films as a function of the substrate temperature, gas pressure, laser fluence and target–substrate separation distance. Ba_0.5Sr_0.5TiO₃ grown on MgO represents the first system that is grown in this in situ PLD chamber and studied by in situ X‐ray reflectivity, in situ two‐dimensional reciprocal space mapping of symmetric X‐ray diffraction and acquisition of time‐resolved diffraction profiles during the ablation process. In situ PLD synchrotron investigation has revealed the occurrence of structural distortion as well as domain formation and misfit dislocation which all depend strongly on the film thickness. The microstructure transformation has been accurately detected with a time resolution of 1 s. The acquisition of two‐dimensional reciprocal space maps during the PLD growth has the advantage of simultaneously monitoring the changes of the crystalline structure as well as the formation of defects. The stability of the morphology during the PLD growth is demonstrated to be remarkably affected by the film thickness. A critical thickness for the domain formation in Ba_0.5Sr_0.5TiO₃ grown on MgO could be determined from the acquisition of time‐resolved diffraction profiles during the PLD growth. A splitting of the diffraction peak into two distinguishable peaks has revealed a morphology change due to modification of the internal strain during growth.