Table of contents

Second-harmonic generation (SHG) can be produced by phase matching using the birefringence of nonlinear crystals via the modal dispersion in the case of optical waveguides. Such an approach limits the range of frequencies which can be doubled and also the choice of the nonlinear coefficients. One solution to both problems is to modify the crystal so as to have regions of periodic domain polarity. Whilst this approach does not allow a perfect phase match between the fundamental and harmonic, it nevertheless can be entirely constructive throughout the interaction length of the material and is termed quasi-phase matching (QPM). Periodic modulation of the nonlinear coefficient along the direction of propagation can achieve conversion efficiencies up to 20 times greater than with previous methods. Candidates of interest for quasi-phase-matching are wide band gap inorganic crystals such as LiNbO₃, LiTaO₃ and KTP, and also organic materials if they are transparent, stable against optical damage and have large nonlinear coefficients. To achieve QPM a variety of methods are being tried in order to invert domains periodically, either during the crystal growth phase, or subsequently by altering the lattice of the crystal. For inorganic ferroelectrics most effort has been concentrated on domain inversion in LiNbO₃ and LiTaO₃. Techniques have included application of pulsed electric fields, fields generated during electron bombardment, thermal pulsing or chemically driven movement of lithium. Many of the methods are semi-empirical in that the mechanisms by which the lattice re-structures are poorly understood. This review will therefore not only list the methods that are currently being used, but also comment on the underlying physical processes which allow, or prevent, the re-structuring of the lattice and the domain walls, whilst preserving the non-centrosymmetric characteristics of the lattice. An understanding of mechanisms is valuable for related poling applications in other crystals and it is further noted that many amorphous systems, including glasses used for optical fibre communication, may be stimulated to show periodic structural changes although the usage precedes the knowledge of the mechanisms. The commercial applications and research possibilities for efficient SHG guarantee that this topic area will continue to be central to photonics for a considerable time.

This paper reports on the use of the autocorrelation function of the magnetization decay of a ferrofluid to investigate the effects which the moment of inertia of single domain ferromagnetic particles have on the frequency dependent complex susceptibility, xi ( omega )= xi '( omega )-i chi "( omega ), of ferrofluids. The contribution of particle inertial effects, arising from rotational Brownian motion, to an apparent resonance, indicated by the real component, chi '( omega ), going negative at a frequency lower than that predicted by theory, is investigated. The Langevin treatment of Brownian motion is used to incorporate thermal agitation into the analysis.

Magnetic granular Co_xCu_100-x alloys (x=5-20) have been prepared by melt-spinning and subsequently annealing at 450 degrees C for 30 min, which is most appropriate for obtaining the largest MR change with magnetic field in each sample. The highest magnetoresistance (MR) change of 42.5% was observed in annealed Co₁₅Cu₈₅ ribbons. Based on the superparamagnetic assumption, the average size of Co_x particles embedded in a Cu-rich matrix for different samples, ranging from 3.5 to 5.5 nm, has been estimated by simulating the magnetization curves at room temperature which is higher than the blocking temperature T_B for each sample. The value of MR change was found to strongly depend on the Co composition and annealing temperature.

The crystalline structures and magnetic properties of Co-Fe and Co-Fe-Al films with various buffer layers (Cu, Permalloy and Zr), grown on glass or Si substrates, have been studied. It was found that the Co-Fe films on Zr underlayers exhibit very good FCC (111) orientation parallel to the film plane, whereas neither Cu nor Permalloy can suppress the intrusion of HCP phase in the Co-Fe films. Furthermore, the FCC (111) orientation is greatly enhanced by adding a small amount of Al to the Co-Fe films with Zr underlayers. The magnetic softness expected for the films with (111) orientation of the FCC phase was not attained for the Co-Fe films because of residual HCP phase and deterioration of the (111) orientation by annealing. On the other hand, the Co-Fe-Al films with Zr buffer layers, which have a single FCC phase with enhanced (111) orientation, are magnetically soft.

Evidence for the existence of magnetic interactions between layers in a CoCrPt/Cr trilayer system is presented. From the major hysteresis loops and demagnetizing remanence curves we find that the initial magnetic layer has the highest coercivity and switches quasi-independently whereas cooperative effects are present in subsequent layers. This difference in behaviour is attributed to the comparatively thick (500 AA) Cr underlayer on which the initial layer is sputtered. The two upper films which constitute an effective bilayer of variable spacer, X, (10 AA<or=X<or=100 AA) appear to be coupled by a dipolar interaction between the two magnetic layers. It is postulated that this coupling arises from the presence of magnetic domains. If the system is re-magnetized from the AC demagnetized state and the remanence examined, all three films show evidence of coupling. Once again we conclude that this is mainly dipolar in nature and arises from the presence of a domain structure in all three magnetic layers.

It is well known that the spherical aberration is positive for an axisymmetric electrostatic lens that is empty and free from space charge. However, a space charge profile can be tailored to introduce negative spherical aberration in such a lens. This could be useful for modern high-resolution focused ion beam systems. A new technique is described for calculating potentials and their derivatives and is used to calculate the trajectories of charge particles propagating in regions in which space charge is present. The charge density and potential are expanded in powers of r and z and the Poisson equation is solved term by term. The method has been implemented numerically for axisymmetric configurations. An illustrative example is presented.

A new method for calculating inductances is presented. The method results in analytically simple expressions that replace the traditional tables and working formulae. Moreover, it overcomes the diverging self-inductances of thin wires.

We describe a cryogenic Fabry-Perot cavity in which a frequency-temperature turning point at 43 K is created by differential thermal contraction between the mirrors and the spacer. The cavity uses a sapphire spacer and fused silica mirrors. The expansion coefficient of the spacer is balanced by a negative term arising from the outward flexing of the mirrors caused by differential radial expansion of the mirror substrate and spacer. A simple model indicates that is possible to construct cavities with shallow turning points of frequency-temperature at temperatures between 10 and 65 K.

Wavefield dislocations, so named because of their morphological similarity to dislocations in crystals, are singular lines in the phase of a wave. These features exist in wavefields formed by the scattering of pulsed ultrasound from rough surfaces and from the internal microstructure of metals. The aim of this paper is to show that changes in the statistics of wavefield dislocations can be correlated with changes in the scattering object-a fact that has potential application in ultrasonic characterization. Dislocations were observed using pulses of ultrasound of central frequency 20 MHz, reflected from the specimen and detected in water immersion using a coincident source-receiver. The phase of the wave is defined in terms of the position of the wave crests and troughs in the signal detected as a function of time at a fixed receiver position. Various grades of sandpaper served as a set of surfaces of varying roughness. The internally scattered ultrasound from samples of both brass and steel was observed and changes due to annealing operations were detected by the method of dislocations. The position of a wavefield dislocation can be determined with a precision two orders of magnitude less than the wavelength. Thus the method of dislocations is very well suited to the study of slowly evolving systems.

The acoustic emission (AE) technique has been employed in a new way, to deduce information regarding the level of adhesion and the strength of the bond between fibres and the matrix materials used in composite manufacture using a transverse sample based on fibre bundles. When unidirectional composites of E-glass fibres/polyester and Kevlar-49 fibres/polyester respectively were subjected to transverse tension, the AE events associated with fibre-matrix interfacial failure started and eventually saturated at low strains in the materials containing Kevlar-49 when compared with those containing E-glass. This clearly indicates that E-glass adheres far better to polyester than does Kevlar-49 and, from stress measurements, the E-glass/polyester bond is stronger. These results are compared and discussed with respect to those from single-fibre composite (SFC) tests which gave similar interfacial shear strength values for the E-glass/polyester and Kevlar-49/polyester interfaces and which may be interpreted in terms of similar levels of adhesion. Further experimentation, with composites whose fibres were surface-treated prior to composite manufacture, showed that the method is a simple and convenient means of monitoring adhesion and interfacial bond strength in fibre-reinforced plastics. Acoustic emission has proved a useful technique in casting new light on the quality (and reality) of surface adhesion in composite material systems and has provided a new method for measuring the strength of resin/fibre surface adhesion in tension.

A spectroscopic study involving N₂(B³ Pi _g, v'=11 to A³ Sigma _u⁺, v"=7), N₂(C³ Pi _u, v'=0 to B³ Pi _g. v"=0) and N₂⁺(B² Sigma _u⁺, v'=0 to X² Sigma _g⁺v"=0) transitions is presented for description of the characteristics of the so-called short-lived afterglow (SLA) created by a nitrogen microwave discharge (433 MHz) at pressures in the range 1.5-10 hPa. The spatial investigation along the flow system reveals strong intensity gradients but an approximate constancy, in SLA, of intensity ratios of the first transition to the second and third one. Criteria drawn notably from these observations are proposed in order to estimate the extension of the main regions encountered in the afterglow. The maximum intensity of emissions in SLA, their location and the gas temperature are studied versus the transmitted microwave power, the gas velocity and the pressure. On the basis of the presented results, a kinetic model is built to describe the birth of SLA and its bulk features. The main role played by the vibrationally excited ground state molecule is discussed. A basic consequence is the strong interdependence of the vibrational energy and the temperature profile for occurrence of the maximum of intensity in SLA.

A mathematical model of a free-burning TIG electric arc with non-consumable electrodes using a flat anode and argon shielding gas is presented. Differential equations describing the conservation of mass, momentum and energy are solved together with Maxwell's equations describing the electromagnetic field. The dependence of transport coefficients on temperature is taken into account. The gas flow is assumed to be laminar and the partially ionized plasma is assumed to be in local thermodynamic equilibrium. The mathematical model can cope with a broad range of operating conditions. The model is used to demonstrate the strong influence that the velocity and temperature of the flow of gas entering the top of the electric arc in the region of the cathode can have on the arc column. In particular, it is shown that cathode flows of strength sufficient to produce a significant constriction of the electric arc need to be assumed in order to account for experimentally measured electric fields in the arc column as well as the total voltage drop for 10 mm arcs. The use of this model also shows the part played by the cathode spot and its location in the nature of the electric arc column. In particular, two complementary techniques for studying the arc column are highlighted. In the first, a strictly stable static arc is needed in order to employ the spectroscopic technique of temperature measurement. In contrast, when the position of the arc is less stable, a rapid measurement with an electric probe is indicated in order to measure the electric field. The arc model described by Ducharme et al (1993) will then yield the temperature field. The model presented here produces the results for the temperature distribution and for the electric field based on the use of appropriate boundary conditions.

A large diameter and uniform microwave plasma (about 30 cm in diameter) over 10 cm from the source has been produced by scaling up a preliminarily proposed device with an annular slot antenna and satisfying the ECR (electron cyclotron resonance) condition in a chamber with neodymium ring-shaped permanent magnets. The spatial structure of the plasma density in both non-reactive Ar and reactive SF₆ gases was obtained by a Langmuir probe. The tentative etching of silicon (Si) was performed in the SF₆ microwave plasma. The spatial profile of the etch rate in the radial direction was found to be similar to that of electron density in the SF₆ plasma. The uniformity of etch rate was within 2.5% at the position where electron density profile reached the best uniformity ( approximately=5%). The etching profile of the poly-Si pattern in the microwave plasma using pure SF₆ gas was also observed by a scanning electron microscope (SEM).

A study has been performed to re-investigate the actinometric technique used to determine the absolute concentration of O atoms in DC O₂ flowing glow discharges for pressures ranging from 0.36 to 2 Torr and discharge currents ranging from 5 to 80 mA in Pyrex tubes of three different diameters (16, 7 and 4 mm). Actinometric measurements using O(³P-³S) 844 nm, O(⁵P-⁵S) 777 nm and Ar(2p₁-1S₂) 750 nm transitions are compared to VUV absorption spectrometry. The choice of the excitation cross sections for the calculations of atomic excitation rates as a function of the reduced electric field using a Boltzmann code and the contribution of the quenching processes of the excited states are discussed. The dissociation ratio (O)/(O₂) can be determined from the ratio of intensities I₈₄₄/I₇₅₀ by the relation (O)/(O₂)=C_3P^2p1 I₈₄₄/I₇₅₀. We have found that C_3P^2p1 remains constant (C_3P^2p1=2.6*10^-3) throughout the range of experimental conditions investigated. The recombination probability gamma of the O atoms at the wall is calculated and correlated to the wall temperature of the Pyrex tubes. The variation of the recombination probability as a function of the wall temperature is fitted by the relation gamma =0.94exp(-1780/T_wall) for 300<T_wall<500 K.

The paper deals with calculation of parameters in the near-cathode plasma layer, on the cathode surface and in the body of a cathode in high-pressure arc discharges. These parameters can be calculated independently of the arc column if the heat flux coming from the column to the edge of the near-cathode layer does not play a decisive role in the energy balance of the layer, which, according to the estimates presented, is a likely case. The physics of the near-cathode layer is reconsidered in view of major contradictions that have appeared in the literature recently, in particular with regard to the role of the near-cathode space charge sheath. A model of a near-cathode layer is developed that is based on a multifluid description of the plasma and takes into account multiply charged ions. The model is employed to calculate parameters of the layer as functions of the voltage drop in the layer and of the local value of the surface temperature. By means of these data, an approximate asymptotic theory of arc spots is extended to cathode spots in high-pressure plasmas. Calculated spot parameters are presented for the following combinations cathode/plasma: tungsten/argon, thoriated-tungsten/argon, thoriated-tungsten/nitrogen, and zirconium/nitrogen. The obtained results agree with the recent measurements of the spot temperature.

The power lost by conduction into copper, silver and tungsten cathodes surrounded by water-cooled brass was determined experimentally. The measurements were made for prolonged short arcs (0.4-1 mm) at low current (2.6-5 A) in argon at 1 atm. The experimental method consists of measuring the temperature reached in the steady state at different points distributed along the electrode axis, fitting the values obtained and calculating the power P_cd supplied to the cathode surface facing the column. With these values of P_cd we calculated the spot radius r and current density j as functions of the electrical power input under the assumption that the spot is circular.

A nonlinear collisional-radiative model is used for a non-equilibrium, stationary argon plasma. The model includes three atomic energy levels: the ground level, one excited level and the continuum. A Maxwellian electron energy distribution function is also assumed. The model is solved for the electron and excited atom densities in two plasma regimes; with and without diffusion losses. A linear stability analysis is performed for both regimes. The maintenance electric field intensity and the average power loss per electron are obtained as a function of the electron number density for high-frequency discharges. The results of the calculations are discussed in a low (0.28, 1.1 and 2.8 Torr) and an intermediate (6.0 and 10.0 Torr) gas pressure range.

A detailed study of the coupled electron and heavy-particle kinetics in a low-pressure stationary N₂-O₂ discharge is carried out. The model is based on the self-consistent solutions to the Boltzmann equation coupled to the rate balance equations for the vibrationally excited molecules N₂(X¹ Sigma _g⁺,v) and O₂(X³ Sigma _g^-,v'), NO(X² Pi _r) molecules and N(⁴S) and O(³P) atoms. It is shown that the vibrational distribution of N₂(X,v) plays a central role in the whole problem, affecting considerably the predicted concentrations of NO molecules and N atoms, whereas the concentration of O atoms is practically independent of both vibrational distributions. In particular, it is shown that, in the case of a rate coefficient of about 10^-13 cm³ s^-1 for the reaction N₂(X,v)+O to NO+N, the N₂(X,v) molecules are strongly de-excited by vibrational-translational energy exchange processes associated with N₂-N collisions. In contrast, in the case of a higher value for this rate coefficient, the N₂(X,v) molecules are efficiently destroyed by this mechanism. The contributions of the different processes to the total production of NO, N and O are evaluated and compared.

Analysis of the thermally stimulated depolarization current for polyvinyl alcohol (PVA) and triglycine sulphate (TGS) composite has been carried out in terms of relaxation processes. Addition of the ferroelectric component was found to enhance the molecular interaction initially and then to decrease it. A shift factor a_T was also calculated. Its value was found to reflect the viscosity behaviour of the composite.

The dynamics of spreading of small amounts of viscous fluids are usually investigated using spherical droplets of oils. Recently we applied Frenkel's method to this problem and obtained a wide angle relation between the edge speed and the dynamic contact angle and this gave good agreement with literature data. In this work we give a simple extension of the method to the spreading of a stripe, rather than droplet, of oil. The small angle limit for the time dependence of the width of the stripe on a high energy surface is d₀ varies as (t+t₀ to)^1/7 and is in agreement with previous work. Results of optical observations on the time dependence of the width of stripes of polydimethylsiloxane (PDMS) oils of viscosities of 10000, 30000 and 100000 cS on glass and on lithium niobate substrates are also presented. Fits to these data give a time dependence close to that predicted. Furthermore, order of magnitude estimates of the value of a logarithmic cut-off factor appearing in the theory are consistent with those obtained by previous studies on the spreading of spherical droplets of PDMS oils. This work establishes a spreading problem with a plane spreading liquid front and which is used in an accompanying paper to study the feasibility of a new surface acoustic wave technique for the study of dynamic wetting.

The spreading of small quantities of viscous oils has traditionally been studied using side views and plan views to obtain contact angles and profiles. The techniques employed have included optical profiling, interferometry, x-ray reflectivity and ellipsometry. In this work we report an alternative method, based on surface acoustic waves, which has the advantage of working directly within the spreading plane. In these experiments 150 ns pulses of 250 MHz surface acoustic waves are emitted along the solid-vapour interface towards a stripe of viscous oil. Both reflections from the advancing front edge of the oil and from a double transit signal which passes through the oil are observed. As the stripe becomes wider, changes in surface acoustic wave path lengths occur and these are measured using a phase comparison method. Results for the spreading of a small stripe of 100 000 cS polydimethylsiloxane oil are presented. The results for the time dependence of the width of the stripe agree with both theory and optical experiments.

The adsorbate-induced changes of the IR reflectivity and the DC resistivity of thin silver and copper films were examined. A linear dependence between both quantities could be observed, as predicted by theory. For the measurements of the IR reflectivity, a very simple method was used, which allows the detection of changes down to 10^-4.

Following the epitaxial growth and characterization of single crystal c-axis oriented hcp cobalt thin films (with thicknesses in the range of 200-600 AA), we have investigated their micromagnetic domain properties by Lorentz electron microscopy (LEM). In particular we have used the differential phase contrast mode of LEM because this imaging technique allows a wide range of quantitative analysis to be carried out directly. The domain structures were studied in the as-grown, ac-demagnetized and remanent states. Large domains with regular magnetization ripple and cross ties were observed in the thinner films, whilst stripe domains were seen in films with thicknesses >or=400 AA. Detailed analysis of the DPC images led to information pertaining to the domain wall width and its associated energy density being attained.

We have simultaneously measured the thermally stimulated current (TSC) and thermally stimulated luminescence (TSL) of two ZnSe single crystals grown by the sublimation method and have observed a peak conversion near 150 K. In order to identify the origin of such a peak conversion, we have also measured the negative thermally stimulated current (NTSC) by applying a specially biased voltage to the sample. We conclude that this conversion originates from the TSC curve near 150 K which includes two peaks due to the electron trap at 155 K and the hole trap at 145 K. An acceptor level and three donor levels are obtained from both the TSC and the TSL measurements. Their estimated activation energies are E_v+0.19, E_c-0.22, E_c-0.30 and E_c-0.39 eV respectively.

Hg_1-Zn_xTe alloys were prepared by inter-diffusion between the binary compounds at high temperatures. Photoacoustic spectroscopy is used to determine the optical absorption coefficient in the region of fundamental absorption. The energy bandgap is determined in the composition range 0.40<or=*<or=1 and the results are compared with those reported using other methods.

Ion beam sputtering and dual ion beam sputtered films of titanium oxide were prepared and investigated at room temperature and compared with similar films prepared by e-beam (EB) gun deposition. Optical properties, i.e. Refractive index n and extinction coefficient k of films were determined by transmission spectrophotometry ( lambda =350-1100 nm) and at lambda =632.8 nm by ellipsometry. These were all found to be very dependent on deposition parameters (rate, substrate temperature and partial pressure of oxygen (P_O2) in the case of EB deposition of TiO₂). For ion beam sputtering (IBS) deposition the main controlling deposition parameter on the optical properties was P_O2. The maximum refractive index achieved at 300 degrees C for EB deposited TiO₂ films was n=2.35. Films deposited by IBS and dual ion beam sputtering (DIBS) at room temperature showed higher values of n(TiO₂)=2.41-2.47 for lambda =550 nm. Values of extinction coefficient were similar and of the order of 10^-4. X-ray diffraction (XRD) measurements revealed that all films deposited by IBS, DIBS and EB deposition were amorphous except for those films deposited by EB deposition at T>or=320 degrees C which were polycrystalline. DIBS of TiO₂ with low-energy Ar and O₂ ions (E_a=200-300 eV) and low currents (J_a=0-35 mu A cm^-2) was not beneficial. Films bombarded with Ar⁺ were absorbing while films bombarded with O₂⁺ showed no change in n or k. Different bonding states in IBS deposited TiO₂ films were determined by x-ray spectroscopy (XPS) while the composition of films and contaminants were determined by Rutherford backscattering spectroscopy (RBS). TiO₂ films formed by IBS contained approximately=0.4% Ar which originated from the reflected argon neutrals from the sputtering target while assisted deposition slightly increased argon content. Stress in the films deposited by IBS was measured and was compressive with a typical value of sigma =3.75-10⁸ Pa.

From the laser-induced fluorescence measurements of the decay rate of SiH₂ and SiH densities in the afterglow of a RF SiH₄-Ar discharge, the rate constants for the reactions of SiH₂ and SiH with SiH₄ have been determined at pressures below 1 Torr. The rate constants obtained at 70 mTorr for the SiH₂+SiH₄ and SiH+SiH₄ reactions are 4.3*10^-11 and 4.8*10^-11 cm³ S^-1 per molecule, respectively. The rate constants of the two reactions decrease with decreasing pressure in a manner consistent with the high-pressure (>or=1 Torr) data available in the literature, indicating that, despite the present low-pressure conditions, the three-body association reactions producing, respectively, Si₂H₆ and Si₂H₅ are the dominant reaction channels. Measurements were also carried out using a SiH₄-He discharge at 200 and 400 mTorr, giving rate constants somewhat smaller than those obtained using a SiH₄-Ar discharge, possibly because of incomplete thermalization of SiH₂ and SiH. The SiH_x (x=0-3) production frequencies in SiH₄ plasmas are discussed on the basis of the measured reaction rate constants.

We compare and contrast various techniques for calculating the behaviour of superconducting millimetre-wave and sub-millimetre-wave microstrip transmission lines. A rigorous method based on conformal transformations is presented. The results of the analysis are compared with Wheeler's recession technique and the spectral domain method. The strengths and weaknesses of the various approaches are discussed.

The noise spectrum of a p-i-n diode under a chopped light illumination is researched both theoretically and experimentally. The observed noise spectra can be presented in terms of a formula. The light-activated current of the device can be obtained from either a given light power or a given harmonic noise spectrum. Moreover, methods to measure the quantum efficiency and to estimate the amount of stored charge of a p-i-n diode (In_0.53Ga_0.47As) during the shaded period of chopped illumination are presented.

The origin of the nonlinearity of plasma permittivity in high-frequency discharges maintained by ionizing surface waves is studied. It is shown that in a diffusion controlled regime the plasma density at the discharge axis is determined nonlocally in terms of the electric field intensity by accounting for the small nonlinear effect of recombination, along with diffusion in the particle balance equation. The axial self-consistent distribution of the surface wave field intensity and of the plasma density of discharges are obtained. The transition to a 'pure' diffusion and a 'pure' recombination regime is considered. A brief discussion is included regarding the case of a nonlinearity due to a weak dependence of the ionization frequency on the plasma density (e.g. due to stepwise processes).