Flow acoustics in periodic structures

A recent paper [A.A. Krokhin, J. Arriaga, L.N. Gumen, Speed of sound in periodic elastic composites, Phys. Rev. Lett. 91 (2004) 264302-1-4] addresses the speed of sound in periodic elastic composites (phononic crystals) with particular emphasis to the case where air bubbles are present in water and arranged periodically. In such periodically arranged mixtures, the well-known phenomena of the drop of the speed of sound may occur and applications related to, e.g., sound-beam focusing and acoustic surgery are possible [F. Cervera, L. Sanchez, J.V. Sanchez-Perez, R. Martinez-Sala, C. Rubio, F. Meseguer, C. Lopez, D. Caballero, J. Sanchez-Dehesa, Phys. Rev. Lett. 88 (2002) 023902]. In this paper, the analysis is extended theoretically to include cases where a background flow in a periodic structure is maintained. Calculations of dispersion relations and group velocities are presented in cases with one- and two-dimensional material periodicity for background flow values in the range: 0-1m/s. Materials considered in the calculations are periodic water-air mixtures. It is shown that acoustic waves couple to the group velocities only if the (acoustic) wave vector has a component along the background flow velocity direction.     

Solar cells with porous silicon: modification of surface-recombination velocity

Laser-induced transient-grating measurements were performed to monitor the influence of porous silicon on the surface recombination of a highly doped n⁺-silicon emitter of solar cells. With this technique, photocarrier diffusion and recombination with a time resolution of some tens of picoseconds can be studied. Using pulses of the second- and third-harmonic radiation from an Nd³⁺:YAG laser (quantum energy 2.34 and 3.51 eV, respectively), two different-depth regions of the emitter were excited. Using a kinetic model, which includes carrier diffusion and recombination at the surface and in the bulk of the emitter, surface-recombination velocities in a series of samples typical for each successive operation of solar-cell technology with different surface-doping level and surface preparation were evaluated. From the analysis, we conclude that porous silicon formed on the emitter passivates the surface of the silicon layer, i.e. reduces the rate of surface recombination at the porous silicon–crystalline silicon interface. Ytterbium as a co-dopant of the emitter increases the surface-recombination velocity. Received: 26 June 2000 / Accepted: 4 December 2000 / Published online: 26 April 2001     

Isotopic ratio measurement of methane in ambient air using mid-infrared cavity leak-out spectroscopy

We report on infrared laser spectroscopic measurements of the isotopic composition of methane (¹²CH₄, ¹³CH₄) in natural air samples with a cavity ring-down technique. A CO overtone sideband laser is utilized to excite a high-finesse cavity which provides an effective optical absorption path length of 3.6 km. We achieved a detection limit of 105 ppt methane in ambient air using an integration time of 20 s. This corresponds to a minimum detectable absorption of 1.9×10^-9 /cm. Rapid determination of the ¹³C/¹²Cisotopic ratio of methane in ambient air without sample preconcentration or gas processing is realized. The present system requires only few minutes for an isotopic ratio measurement with a precision of 11%o . Received: 14 July 2000 / Revised version: 25 October 2000 / Published online: 6 December 2000     

Filamentary iron nanostructures from laser-induced pyrolysis of iron pentacarbonyl and ethylene mixtures

In this work, we present results of the synthesis and characterization of iron and iron oxide nanoparticles aggregated in filamentary, spider-web-like structures. The particles were produced in a flow reactor by CO₂ laser pyrolysis of gaseous mixtures of iron pentacarbonyl and ethylene. Low- and high-resolution electron microscopy reveals chain-like structures of particles, most of them being composed of an α-iron core and an iron oxide shell, identified as magnetite and, to a lesser extent, hematite. These results are in good agreement with a M?ssbauer analysis carried out for the same samples. The role of the reaction temperature on the synthesis of filamentary iron nanostructures by infrared laser pyrolysis of Fe(CO)₅/C₂H₄ mixtures is discussed. Received: 31 May 2000 / Accepted: 6 June 2000 / Published online: 2 August 2000     

Mid-infrared laser-induced grating experiments of C2H4 and NH3 from 0.1–2 MPa and 300–800 K

Laser-induced thermal gratings (LITG) were generated in mixtures of ethylene and ammonia in nitrogen using mid-infrared laser radiation from a grating-tuned, low-pressure, pulsed (5 ms pulse width) CO₂ laser, and read out with a continuous wave Nd:YLF laser. The LITG signal intensity was measured as a function of pressure (0.1–2 MPa) and temperature (300–800 K, at 0.1 and 1 MPa) by tuning the laser to the accidental coincidences of the 10P(14) and 10R(6) emission lines with molecular absorption transitions of C₂H₄ and NH₃, respectively. Comparisons are made with theoretical predictions for the grating efficiency from a simple thermalization model. A theoretical comparison of the temporal LITG signal response for three excitation pulse shapes – a delta function, a realistic pulse, and a square wave is presented. Furthermore, it is shown that for NH₃, most of the decrease of the LITG signal intensity with increasing temperature is due to the corresponding decrease in fractional molecular absorption of the pump beam radiation. The diagnostic capabilities of the mid-infrared LITG experiment is demonstrated for spatially resolved ethylene measurements with long laser pulses in a premixed stoichiometric CH₄–air flame at atmospheric pressure. Received: 17 March 2000 / Revised version: 23 March 2000 / Published online: 13 September 2000     

Effects of closed immersion filtered water flow velocity on the ablation threshold of bisphenol A polycarbonate during excimer laser machining

A closed flowing thick film filtered water immersion technique ensures a controlled geometry for both the optical interfaces of the flowing liquid film and allows repeatable control of flow-rate during machining. This has the action of preventing splashing, ensures repeatable machining conditions and allows control of liquid flow velocity. To investigate the impact of this technique on ablation threshold, bisphenol A polycarbonate samples have been machined using KrF excimer laser radiation passing through a medium of filtered water flowing at a number of flow velocities, that are controllable by modifying the liquid flow-rates. An average decrease in ablation threshold of 7.5% when using turbulent flow velocity regime closed thick film filtered water immersed ablation, compared to ablation using a similar beam in ambient air; however, the use of laminar flow velocities resulted in negligible differences between closed flowing thick film filtered water immersion and ambient air. Plotting the recorded threshold fluence achieved with varying flow velocity showed that an optimum flow velocity of 3.00 m/s existed which yielded a minimum ablation threshold of 112 mJ/cm². This is attributed to the distortion of the ablation plume effected by the flowing immersion fluid changing the ablation mechanism: at laminar flow velocities Bremsstrahlung attenuation decreases etch rate, at excessive flow velocities the plume is completely destroyed, removing the effect of plume etching. Laminar flow velocity regime ablation is limited by slow removal of debris causing a non-linear etch rate over ‘n’ pulses which is a result of debris produced by one pulse remaining suspended over the feature for the next pulse. The impact of closed thick film filtered water immersed ablation is dependant upon beam fluence: high fluence beams achieved greater etch efficiency at high flow velocities as the effect of Bremsstrahlung attenuation is removed by the action of the fluid on the plume; low fluences loose efficiency as the beam makes proportionally large fluence losses at it passes through the chamber window and immersion medium.     

Condensation-induced dynamic gas fluxes in a mixture of condensable and non-condensable gases

It is shown that condensation of water vapor produces dynamic instability of atmospheric air and induces air circulation that is characterized by observable air velocities and persists independently of the magnitude of horizontal temperature gradients.     

Characterisation of a spark ignition system by planar laser-induced fluorescence of OH at high repetition rates and comparison with chemical kinetic calculations

This study reports the application of a novel, high speed laser-detector system for the time-resolved study of flame propagation in a well-controlled spark ignition system. The ignition system allowed full and reproducible control over the energy deposited during breakdown and the ensuing arc discharge of the spark plasma. Ignition was performed in a closed vessel which was filled with stoichiometric mixtures of methane and air. Four sequential snapshots of two-dimensional OH distributions were recorded during single ignition events by the use of planar laser-induced fluorescence (PLIF). From these OH distributions flame front velocities have been extracted with an accuracy of better than 2%. One-dimensional numerical simulations of the ignition event including detailed chemistry and transport processes have been performed. Experimental results and results from the simulations have been compared to each other with respect to flame front velocities as well as spatial concentration profiles of OH radicals. In general a good agreement was obtained. In this way the ignition system was carefully characterised. Received: 6 April 1999 / Published online: 27 October 1999     

Optics of moving media

Light experiences a moving medium as an effective gravitational field. In the limit of low medium velocities the medium flow plays the role of a magnetic vector potential. We review the background of our theory [U. Leonhardt and P. Piwnicki, Phys. Rev. A 60, 4301 (1999); Phys. Rev. Lett. 84, 822 (2000)], including our proposal of making optical black holes. Received: 4 July 2000 / Revised version: 17 October 2000 / Published online: 6 December 2000     

Photoacoustic multicomponent gas analysis using a Levenberg–Marquardt fitting algorithm

2 laser photoacoustic spectrometer has been developed for the in-situ monitoring of atmospheric trace gases in different environments. Numerous air pollutants, such as ammonia, ethene, ozone and alcohols, can be monitored with a time resolution of a few minutes. A new fitting algorithm based on the numerical method of Levenberg–Marquardt is discussed and applied to the derivation of individual gas concentrations from measured photoacoustic signal amplitudes and phases at selected CO₂ laser wavelengths. The algorithm has been tested with artificially generated multicomponent gas mixtures exhibiting gas concentrations in the ppb to ppm range. Furthermore, its potential is demonstrated with the analysis of an air sample from a fruit storage chamber and with ambient air measurements during a field study in a rural environment. Received: 23 March 1998/Revised version: 18 June 1998     

Multi-wavelength excimer ultraviolet sources from a mixture of krypton and iodine in a dielectric barrier discharge

New ultraviolet (UV) sources (excimer lamps) have been demonstrated using mixtures of krypton and iodine in a high-pressure dielectric barrier discharge to provide intense multi-wavelength radiation at λ=183 nm (atomic iodine line, ⁴ P _5/2?² P _3/2), 191 nm (KrI^*, B _1/2?X _1/2), 206 nm (atomic iodine line, ² P _3/2?² P _1/2), and 342 nm (I^* ₂, D _1/2?X _1/2). The characteristics of the optical emission spectra of the atomic species and the excited dimers (excimers) formed for different total gas pressures and in the presence of a buffer gas have been investigated. The highest intensity, at 183 nm, is obtained at pressures up to 1 bar while the overall emission spectra can be controlled by adjusting the total pressure. The results show that these strong multi-wavelength lamps offer an interesting alternative to conventional UV sources. Received: 1 March 2000 / Revised version: 5 April 2000 / Published online: 30 June 2000     

Scribing and cutting a blue LED wafer using a Q-switched. Nd:YAG laser

Chemical doping of single-wall carbon nanohorns (SWNHs) with K and Br was examined by vapor-phase reaction and studied by Raman scattering. Electro-chemical Li-ion doping using an electrolyte of LiAsF₆ in a mixture (1:1 by volume) of ethylene carbonate and diethyl carbonate was also carried out. All these experiments indicate that an anomalously small charge transfer occurs in saturation-doped SWNHs using these reagents, in sharp contrast to the behavior observed for single-wall carbon nanotube bundles and graphite. This rather remarkable result is not understood at present. Received: 13 July 2000 / Accepted: 14 July 2000 / Published online: 5 October 2000     

Field-electron emission from polyimide-ablated films

Polyimide-ablated film was deposited by using pulsed laser ablation of a polyimide target, and field-electron emission from the film was observed for the first time. The turn-on field of the polyimide-ablated film is 12 V/μm. The current density is 0.725 mA/cm², and the emission sites density is on the order of 10⁶/cm² at the applied field of 24 V/μm. The field-electron emission measurements indicate that this kind of film could be a new cold cathode material. It is suggested that the graphite-like clusters contained in the film play an important role in the field-electron emission. Received: 2 February 2000 / Accepted: 13 March 2000 / Published online: 9 August 2000     

Anisotropy of sound velocity for a new PbB4O7 crystal as determined by the laser ultrasonic technique

4 O₇ crystal is a new nonlinear optical crystal. For the first time the anisotropy of the velocities of its longitudinal and surface acoustic waves (SAWs) are determined by laser ultrasonic technique. The velocities of surface waves for X-, Y-, and Z-cut crystals are also calculated. The theoretical calculations of slowness curves are in good agreement with experimental results. The SAW slowness curve is elliptical for Y- or Z-cut crystal wafers, and circular for an X-cut wafer. Received: 27 January 1997/Accepted: 30 July 1997     

Remarks on 2–q-bit states

We distinguish six classes of families of locally equivalent states in a straightforward scheme for classifying all 2–q-bit states; four of the classes consist of two subclasses each. The simple criteria that we stated recently for checking a given state’s positivity and separability are justified, and we discuss some important properties of Lewenstein–Sanpera decompositions. An upper bound is conjectured for the sum of the degree of separability of a 2–q-bit state and its concurrence. Received: 17 July 2000 / Published online: 6 December 2000     

Dephasing of optically induced excitonic coherences in semiconductor heterostructures

Resonant optical excitation of a direct bandgap semiconductor below the band edge induces excitonic coherences. Experiments based on transient four-wave mixing or cw spectral-hole burning provide an excellent approach to eliminate inhomogeneous broadening and enable determination of the time scale and origin of the decay of the optically induced quantum coherence. Such measurements are of interest in basic physics since they reflect fundamental interactions between the exciton and the surrounding environment including, for example, lattice vibrations and interface fluctuations. They also relate to potential applications of these excitations such as in coherent control or quantum information processing. Received: 30 March 2000 / Accepted: 2 September 2000 / Published online: 12 October 2000     

Excimer laser ablation of gold-loaded inverse polystyrene-block-poly (2-vinylpyridine) micelles

Diblock-copolymers (PS(1700)-b-P2VP(450) or PS(1350)-b-P2VP(400)) forming spherical micelles, can be loaded with a Au-salt and deposited on top of various substrates. Such polymer films have been exposed to a pulsed ArF excimer laser in order to remove the polymer matrix and, in parallel, to chemically reduce the salt into metallic Au nanodots. To analyze this process in detail, it was subdivided into three steps: (a) laser ablation of thick and thin diblock-copolymer films; (b) laser irradiation of Au-salt loaded diblock-copolymer films; and (c) laser irradiation of arrays of metallic Au nanodots. In (a) it was found that a complete removal of the polymer by laser ablation is only possible in air under ambient conditions while identical laser irradiations under vacuum result in a residual layer of approximately 14 nm. Substep (b) revealed a nucleation process of the resulting metallic Au within the micellar core leading to clusters of small Au dots. Furthermore, this substep provided evidence for an asymmetric interplay between the macroscopic temperature of a polymer film during laser treatment and the energy density per laser pulse. In (c) it could be demonstrated that metallic Au nanodots on mica are stable against laser irradiation under conditions leading to a polymer removal. Received: 7 August 2000 / Accepted: 2 November 2000 / Published online: 3 April 2001     

Sub-picosecond photo-induced melting of a charge-ordered state in a perovskite manganite

Ultrafast melting of a charge-ordered state has been observed in the photo-irradiated colossal magnetoresistive compound Pr_0.7Ca_0.3MnO₃. Pump-and-probe spectroscopy experiments reveal the formation of a conducting phase with typical features of an insulator–metal transition (IMT) after less than 1 ps. This phase is metastable and can be maintained for about 1 μs unless it is stabilized persistently into a pathlike metallic region by an electric field. Although laser-induced lattice heating may play a role in the initial excitation, electronic correlations are the dominant effect which leads to the formation of the metallic state upon the breakdown of the charge-ordered state. Received: 26 January 2000 / Published online: 16 June 2000     

Temperature dependence of terahertz radiation from n-type InSb and n-type InAs surfaces

The azimuthal angle dependence and the temperature dependence of terahertz (THz) radiation generated from n-type (111) InSb and n-type (111) InAs surfaces irradiated with ∼80 fs near-infrared laser pulses are investigated. The azimuthal angle dependence shows that the contribution of the difference-frequency mixing (DFM) is not dominant for both materials at the excitation density of ∼1 GW/cm². At an appropriate azimuthal angle, the radiation due to DFM is excluded from the total THz radiation and the temperature dependence of THz radiation due to the surge current is observed. The increase of THz radiation with decrease of the temperature is found to be much more pronounced for InSb than for InAs. The different temperature dependence can be attributed to the different radiation mechanisms dominant for both materials. Especially, the temperature dependence of the THz radiation from InSb is well explained by the photo-Dember effect. Received: 9 May 2000 / Revised version: 17 August 2000 / Published online: 5 October 2000     

Photoluminescence properties of size-selected Si nanocluster films prepared by laser ablation

Size-dispersed Si nanocluster films have been synthesized by a method of crossing an Ar gas beam perpendicularly to a silicon cluster beam that is produced by a laser ablation technique. Flight directions of the nanoclusters are changed due to Ar gas collisions, and smaller nanoclusters are deflected further from the axis of the primary cluster beam. The size-dispersed nanocluster films exhibit strong red photoluminescence (PL) after exposure to the air. The PL peak energy changes between 1.42 and 1.72 eV depending on the sample position. The average diameter of the oxidized nanoclusters characterized by transmission electron microscopy is 10 nm at the position of the primary cluster beam axis and becomes smaller as deviated from the axis. The relation between the PL peak energy and the size of the oxidized Si nanoclusters is discussed. Received: 4 May 2000 / Accepted: 9 May 2000 / Published online: 13 July 2000