Table of contents

For noninvasive characterization of chemical species or biological components within a complex heterogeneous system, their intrinsic molecular vibrational properties can be used in contrast mechanisms in optical microscopy. A series of recent advances have made coherent anti-Stokes Raman scattering (CARS) microscopy a powerful technique that allows vibrational imaging with high sensitivity, high spectral resolution and three-dimensional sectioning capability. In this review, we discuss theoretical and experimental aspects of CARS microscopy in a collinear excitation beam geometry. Particular attention is given to the underlying physical principles behind the new features of CARS signal generation under tight focusing conditions. We provide a brief overview of the instrumentation of CARS microscopy and its experimental characterization by means of imaging of model systems and live unstained cells. CARS microscopy offers the possibility of spatially resolved vibrational spectroscopy, providing chemical and physical structure information of molecular specimens on the sub-micrometre length scale. We review multiplex CARS microspectroscopy allowing fast acquisition of frequency-resolved CARS spectra, time-resolved CARS microspectroscopy recording ultrafast Raman free induction decays and CARS correlation spectroscopy probing dynamical processes with chemical selectivity.

A series of Er₃Fe_29−xNb_x compounds (0 ⩽ x ⩽ 1.5) has been synthesized. It is found that although the nominal composition is 3 : 29, the compounds crystallize in either the disordered or the ordered Th₂Ni₁₇-type structure (space group P6₃/mmc), except for small amounts of α-Fe and ThMn₁₂ (1 : 12) phases as secondary phases, depending on the annealing temperature and Nb content. A tentative phase diagram has been constructed for the Er–Fe–Nb system. Single-phased Er₃Fe_29−xNb_x compounds with the disordered Th₂Ni₁₇ structure were obtained at annealing temperatures in the range of 1423–1473 K for x = 0.5–1.2. The structure and magnetic properties of the Er₃Fe_29−xNb_x compounds with the disordered Th₂Ni₁₇ structure have been investigated by means of x-ray diffraction and magnetic measurements. It is found that the lattice constants a, c and the unit-cell volume V increase with Nb content. As the Nb content increases the Curie temperature increases and the saturation magnetization M_S decreases. All the compounds have the easy-plane type of anisotropy at room temperature. The temperature dependence of the ac susceptibility shows an anomaly at around 155 K, which may be associated with the temperature-induced domain-wall movement under excitation of the external ac field. The magnetization curves indicate the appearance of a first-order magnetization process (FOMP) and the critical field B_cr of FOMP decreases with increasing Nb content from 5.0 T for x = 0–2.9 T for x = 1.2.

Two series of magnesium–manganese ferrites, viz. Mg_0.9Mn_0.1In_xFe_{2 − x}O₄ and Mg_0.9Mn_0.1Cr_yFe_2−yO₄ have been prepared by the conventional ceramic process. The effects of In³⁺ and Cr³⁺ ions on the dc resistivity, dielectric constant and dielectric loss factor are presented in this paper. The resistivity increases with increasing concentrations of In³⁺ and Cr³⁺ ions. The observed variations in resistivity have been explained by Verwey's hopping mechanism. The activation energy, deduced from the temperature variation of resistivity, was found to increase with increasing concentrations of In³⁺ and Cr³⁺ ions. The room temperature dielectric constant at 100 kHz decreases with successive addition of trivalent ions in both the series. The observed variation in dielectric constant has been explained on the basis of space charge polarization. The dielectric loss tangent (tanδ) values measured at 100 kHz and 13 MHz are found to be very low for the samples with a higher concentration. The low values of the loss factor even at a high frequency indicate that the prepared materials may have great potential for use in microwave devices.

The electro-optic (EO) properties of a new lead-free ceramic based on a (1−x)K_0.5Na_0.5NbO₃–xSrTiO₃ solid solution with x = 0.2 have been investigated. This material has a pseudo-cubic perovskite structure and typical relaxor-like properties for x ∼0.15–0.25 (see Kosec M et al 2004 J. Mater. Res.19 1849–54 and Bobnar V et al 2004 Appl. Phys. Lett.85 994–6).

Measurements of the electric field induced changes in birefringence at room temperature showed a rather large quadratic EO Kerr effect (n³R_eff ∼1.9 × 10⁻¹⁶ m² V⁻²), with strong dispersion (decrease) in the kilohertz region. Electric field induced second harmonic generation was also observed.

The current instability in a planar gas discharge system is studied experimentally in a wide range of the gas pressure, p (44–550 Torr), interelectrode distance, d (45–330 µm), and diameter, D (5, 9, 12, 18, 22 mm), of the electrode areas of the semiconductor cathodes. While being driven with a stationary voltage, it generates current instabilities with different amplitudes of the oscillation. Through spatially uniform irradiation of the semiconductor cathode, non-stationary states which are non-homogeneous can be generated in a system. It is shown that under the experimental conditions the discharge gap played only a passive role and was not responsible for the appearance of the current instability. At the same time, for different diameters, D, of the electrode an expanded range of current oscillations is observed. The pronounced N-type negative differential resistance is observed in the current–voltage characteristic of a planar gas discharge system with a large-diameter semiconductor cathode.

A method for direct measurements of electron number density, ionization rate, and the electron–ion recombination rate coefficient in optically pumped non-equilibrium plasmas has been developed. In this method, a pulsed, non-self-sustained discharge created by applying square-shaped, below breakdown voltage pulses to two electrodes placed outside the plasma (a Thomson probe) is used to remove electrons from the plasma. The electron number density is inferred for CO/Ar and CO/N₂ optical mixtures with small amounts of O₂ additive present. The results are compared with microwave attenuation measurements. The electron–ion recombination rate coefficients in CO/Ar/O₂ and in CO/N₂/O₂ plasmas are β = (3–4) × 10⁻⁸ cm³ s⁻¹ and β = (2–3) × 10⁻⁷ cm³ s⁻¹, respectively. Time-dependent measurements of the electron concentration and vacuum ultraviolet radiation (CO fourth positive system) are used to study the mechanism of CO(A¹Π) population in the optically pumped plasma. The experimental results are compared with kinetic modelling calculations. The results systematically show that the intensity of the CO fourth positive radiation closely follows the electron number density in the laser-excited plasma region after the Thomson probe voltage is turned on or off. This demonstrates that electrons play a major role in the excitation of the A¹Π electronic state of CO and provides additional evidence that vibrational-to-electronic (V–E) energy transfer in plasmas sustained without external electric fields is mediated by collisions with electrons.

The decay of resonance and metastable atomic states in the afterglow of a pulsed driven low-pressure glow discharge in a He/Xe mixture (2% Xe) has been studied. Laser atom absorption spectroscopy has been used to measure the temporal evolution of optical densities. For that the transitions at 820.6 nm and 826.6 nm were used to probe the atomic states Xe(1s₃) and Xe(1s₂), respectively. A set of balance equations is used to describe the temporal behaviour of excited atomic states and the electron temperature. Collisions with electrons are shown to be the main quenching channel for the metastable atoms and those with helium atoms for the resonance levels. The rate coefficients of these processes are estimated.

Deep penetration laser welding is associated with violent plasma generation characterized by high charge densities. The plasma resides both outside and inside the keyhole, and is called plasma plume and keyhole plasma, respectively. The plasma plume outside the keyhole has been studied extensively because it can be conveniently observed; however, very little work has concentrated on the analysis of the keyhole plasma. In this paper, a specially designed set-up was used to take firsthand measurements of the light emission of the keyhole plasma in deep penetration laser welding aluminium films, clamped in between two pieces of GG17 glass, which we called a 'sandwich' sample, thus triumphantly eliminating the effect of the plasma plume covering the keyhole on the observation of the keyhole plasma. Results of spectroscopic measurements of both the plasma plume and keyhole plasma under welding conditions were obtained with an orthogonal experimental design. It was shown that the keyhole plasma had a considerable effect on the energy transfer efficiency of the incident laser beam to the material, exhibiting various melting widths and depths; a deeper welding depth as well as a lower temperature of the keyhole plasma was obtained when the density of the keyhole plasma was decreased by reducing the thickness of the aluminium films.

We have implemented a Monte Carlo simulation method to study electron emission from solid surfaces under electron irradiation at energies of up to 100 keV. Calculations are performed for the yields and energy distributions of the emitted electrons in backscattering and transmission geometries for Al and Au metals. For transmission studies the film thicknesses range from a few nanometres to micrometres. The method is demonstrated to predict correctly the electron backscattering and transmission properties, and gives a qualitatively reasonable description of the low-energy (E < 50 eV) secondary electron emission. For the exit surface of thin films we predict a universal behaviour according to which the maximum low-energy secondary electron yield is obtained when ∼50–70% of the incident electrons are transmitted through the film. This corresponds to film thicknesses of ∼0.75 times the mean depth of penetration of the electron with the same kinetic energy in a bulk target.

This study investigates the influence of the Nd–YAG laser power wave mode on the porosity and mechanical properties of SUS 304L and inconel 690 weldments. Initially, a rectangular laser power waveform is specified. The output is then progressively changed from a pulsed wave mode to a continuous wave mode by reducing the value of ΔP (ΔP = P_p−P_b, where P_p is the peak power and P_b is the base power) to zero. Bead-on-plate (BOP) and butt welding are performed at a constant mean output power (1.7 kW). The BOP results demonstrate that the depth/width (D/W) ratio of both materials increases with ΔP and attains a maximum value when full penetration just occurs. The D/W ratio and the travel speed for full penetration are higher for SUS 304L than for inconel 690. In butt-welds of inconel 690 and SUS 304L, the porosity ratio decreases from 7.1% to 0.5% and from 2.1% to 0.5%, respectively, as ΔP increases from 0 to 2780 W. Therefore, the tensile strength and percentage elongation are enhanced significantly in inconel 690. The degree of porosity reduction in inconel 690 exceeds that of SUS 304L. This suggests that the viscosity of the molten inconel 690 metal is higher than that of SUS 304L. Consequently, the effect of porosity reduction due to the increase in molten metal fluidity caused by increasing ΔP is greater for inconel 690 than for SUS 304L.

The effect of an electric field on the crystallization process of amorphous alloys was studied by annealing amorphous Fe₈₆Zr₇B₆Cu₁ ribbons at 600°C for 1 h in the presence of an alternating current electric field. It is shown that the amplitude of the electric field strength has apparent effects on the grain size and volume fraction of the crystalline α-Fe phase. The grain size increases with an increase in the amplitude of the electric field strength. The volume fraction of the crystalline α-Fe phase increases with increasing amplitude of electric field strength from 0 to 10⁴ V m⁻¹ and then decreases with its further increase. The frequency of the electric field has no apparent effect on the crystallization process of amorphous ribbons. By analysing the mechanism of the effect of an electric field on the crystallization process of amorphous alloys, it is shown that the effect depends on the difference in conductivity between the new phase and the initial phase. When the conductivity of the precipitating phase is less than that of the remaining matrix, the use of an electric field can produce a nanocrystalline alloy with an ultrafine grain size.

The magnetic-pressure drive technique allows single-shot measurements of compression isentropes. We have used this method to measure the isentropes in the pressure–volume space of bulk and single-crystal lead, and lead–antimony alloy to ∼400 kbar.

The isentrope pressure–volume curves were found from integration of the experimentally deduced Lagrangian sound speed as a function of particle velocity. A characteristics calculation method was used to convert time-resolved free-surface velocity measurements to corresponding in situ particle-velocity histories, from which the Lagrangian sound speed was determined from the times for samples of different thicknesses to reach the same particle velocity. Use of multiple velocity interferometry probes decreased the uncertainty due to random errors by allowing multiple measurements.

Our results have errors of from 4% to 6% in pressure, ∼1% to 1.5% in volume, depending on the number of measurements, and are consistent with existing isotherm and Hugoniot data and models for lead.

We have investigated the far-IR, submillimetre and microwave (MW) dielectric response of CaTiO₃ (CT)–Sr(Mg_1/3Nb_2/3)O₃ (SMN), CT–Sr(Zn_1/3Nb_2/3)O₃ (SZN), CT–NdAlO₃ (NA) and CT–LaGaO₃ (LG) solid solutions ceramics series. The contribution of extrinsic losses has been analysed by extrapolation of the far-IR and terahertz dielectric data down to the MW range and comparison with directly measured data. This procedure has also been justified by comparing the losses in CT and LG ceramics with the losses in their crystalline forms published in the literature.

The compositional dependences of the permittivity and temperature coefficient on the resonance frequency have been analysed and fitted by appropriate expressions. We have found that in the case of CT–NA and CT–LG ceramics, the best fits can be obtained using a Clausius–Mossotti equation with linearly mixed polarizabilities. On the other hand, for CT–SMN and CT–SZN ceramics the Lichtenecker logarithmic rule and the Hashin–Shtrikman expression have been used. We explain this difference as a consequence of the different character of short-range ordering in the studied ceramic systems.

A ceramic of nominal formula Pb_0.95Sr_0.05(Zr_0.53Ti_0.47)_0.98Nb_0.02O₃ has been investigated. Dielectric properties and differential scanning calorimetry tests pointed to a transformation temperature of 306 °C. By means of a dynamic method the piezoelectric and complex elastic compliances for polarized ceramics have been measured as a function of temperature. The temperature dependence of the elastic properties has indicated bilinear coupling between the order parameter and strain. The unexpected existence of a pertinent piezoelectric response far above accompanied by strong changes in the elastic properties was observed. On the basis of the dielectric response and x-ray diffraction studies carried out on poled and unpoled ceramics, the influence of the compositional fluctuations and the existence of polar regions on the anomalous piezoelectric activity above were considered rather than a change of symmetry after the action of an external electric field.

The electrical properties of Pt/Ga₂O₃/SiC metal oxide semiconductor devices are presented in this paper in order to determine the hydrogen sensing mechanism. This was achieved by studying the role of the interface states upon the introduction of hydrogen/hydrocarbon gas. Current–voltage (I–V), conductance–voltage (C–V) and capacitance–voltage (G–V) experiments have been carried out to investigate the gas sensing mechanism. The devices' hydrogen and propene gas sensitivities were also investigated. This was achieved by operating them as Schottky diodes and by measuring the change in output voltage when kept at a constant forward bias current. Voltage shifts over 1 V were observed. A discussion is also presented on the effect of operating temperature and ambient gas on the sensors' hydrogen response mechanism, which is attributed largely to the passivation and creation of energy states at the Ga₂O₃/SiC interface.

Tribological studies on the micro/nanoscale conducted using an atomic force microscope (AFM) have been limited to low sliding velocities (< 250 µm s⁻¹) due to inherent instrument limitations. Studies of tribological properties of materials, coatings and lubricants that find applications in micro/nanoelectromechanical systems and magnetic head-media in magnetic storage devices that operate at high sliding velocities have thus been rendered inadequate. We have developed a new technique to study nanotribological properties at high sliding velocities (up to 10 mm s⁻¹) by modifying the commercial AFM set-up. A custom calibrated nanopositioning piezo stage is used for mounting samples and scanning is achieved by providing a triangular input voltage pulse. A capacitive sensor feedback control system is employed to ensure a constant velocity profile during scanning. Friction data are obtained by processing the AFM laser photo-diode signals using a high sampling rate data acquisition card. The utility of the modified set-up for nanoscale friction studies at high sliding velocities is demonstrated using results obtained from various tests performed to study the effect of scan size, rest time, acceleration and velocity on the frictional force for single crystal silicon (100) with native oxide.

The formation of electrodynamic shock waves in non-linear transmission lines is an experimentally demonstrated phenomenon. Theoretical treatment of such phenomena has so far been limited to device-specific models. In this paper, a general hyperbolic conservation law formalism has been developed to describe non-linear propagation of electrical pulses in arbitrary transmission lines. This enables the use of powerful mathematical methods developed in the context of hyperbolic systems of partial differential equations.

This formalism is used to examine the possibility of an electrodynamic analogue of detonation wave in a ferroelectric transmission line. A poled ferroelectric material forming the dielectric of a transmission line not only acts as a non-linear medium with a voltage-dependent dielectric constant but also as a charge and energy storage element. An electric pulse greater than the coercive field injected into this transmission line would steepen during its propagation because of the non-linear, voltage-dependent propagation velocity and also depolarize the ferroelectric material as it travels, adding charge and energy to the wave and sustaining it in the face of losses in a manner reminiscent of detonation waves in a reactive energetic material.

A new concept for converting heat energy to electrical energy using thermionic energy converters (TECs) is proposed. It has potential for increasing the Carnot efficiency to an unprecedented 80% when the TEC is combined as a topping cycle with a conventional external combustion engine. The optimal electrical power density, 5–20 W cm⁻², is satisfactory for many applications including stationary power generators and propulsion drives for vehicles. But unfortunately the substantial losses, ∼50% of the output power, that have been needed to compensate the space charge have prevented the TEC from realizing its thermodynamic potential. We present two ways of overcoming this limitation. Both utilize a triode configuration (rather than a simple diode) with a longitudinal magnetic field. In the first method the magnetic field together with the grid separate the relatively few hot electrons required for volume generation of Cs ions used for the space charge compensation from the majority of the thermal electrons which constitute the current to the collector. In the second method the Cs ions are generated on the surface of the grid wires and injected into the space between the electrodes. Grid wires with high work functions are required for this.

The paper describes the combined axial, lateral and tilting motions of piston within the confine of cylinder bore, with sliding and normally approaching and separating contacts between the piston skirt and the cylinder wall on both the major and minor thrust sides. The methodology developed undertakes combined solution for inertial dynamics of the piston, together with transient elastohydrodynamic analysis of both contacts. It also includes important practical features of the contacting surfaces, such as the axial form relieving of the piston skirt profile. The space–time solver uses Newmark β-type time marching integrator, as well as the effective influence Newton–Raphson method for space-domain solution of the elastohydrodynamic conjunctions in each step of time. This approach, not hitherto reported for this type of conforming contacts yields accurate predictions of lubricant film thickness and pressure distribution within computationally acceptable times, given the inclusion of level of detail in the model.