Table of contents

We review the development of terahertz (THz) technology and describe a typical system used in biomedical applications. By considering where the THz regime lies in the electromagnetic spectrum, we see that THz radiation predominantly excites vibrational modes that are present in water. Thus, water absorption dominates spectroscopy and imaging of soft tissues. However, there are advantages of THz methods that make it attractive for pharmaceutical and clinical applications. In this review, we consider applications ranging from THz spectroscopy of crystalline drugs to THz imaging of skin cancer.

Herein, we report a study on magnetic and optical properties of hexagonal (GaMn)N nanocrystalline powders obtained by two methods: aerosol-assisted vapour phase synthesis and anaerobic imide route. Measurements of the magnetization as a function of temperature and magnetic field for the powders show a typical paramagnetic behaviour. In addition, antiferromagnetic contribution originating from a residual MnO by-product and a small ferromagnetic contribution are observed. Magnetization measured as the function of magnetic field shows a smaller saturation effect than expected for non-interacting Mn-ions. Electron paramagnetic resonance (EPR) measurements reveal a single, structureless resonance line with a g-factor equal to 2.008±0.003 indicating a presence of Mn²⁺-centres in the samples. Both magnetic and EPR measurements suggest weak AF interactions between Mn-ions incorporated in (GaMn)N.

Polycrystalline Fe₃O₄/Al bilayers with Al underlayers (ULs) of different thicknesses were fabricated using facing-target reactive sputtering. Structure analyses revealed that Al ULs improve the crystallinity and reduce the thickness of the Fe₃O₄ amorphous initial layer. Room-temperature magnetization of the Fe₃O₄ films with 30 nm thick Al ULs is ∼450 emu cm⁻³ at 50 kOe fields, which is much larger than that of the polycrystalline Fe₃O₄ monolayer deposited under the same conditions. The field-cooled hysteresis loop of Fe₃O₄ films on the 30 nm thick Al layer shows a 50 Oe exchange bias field at 5 K, much smaller than 256 Oe detected in polycrystalline Fe₃O₄ monolayer and the exchange bias fields in the samples with and without 30 nm Al UL decrease with the increasing temperature. The enhancement of the magnetization and the sharp decrease in the exchange bias field observed in the Fe₃O₄ films with Al ULs can be attributed to the decoupling of the antiferromagnetic domains and the thickness reduction of the amorphous Fe–O layers between the polycrystalline Fe₃O₄ films and the Al ULs.

Polycrystalline thin films of double layer manganite La_1.4Ca_1.6Mn₂O₇ (DLCMO) have been deposited by nebulized spray pyrolysis on single crystal LaAlO₃ substrates. These single phase films, having grain size in the range ∼70–100 nm, exhibit a ferromagnetic transition (FM) at T_C ∼ 95 K. The short range FM ordering due to the in-plane spin coherence is shown to occur at a higher temperature around ∼125 K. The metal to insulator transition occurs at a lower temperature T_P ∼ 55 K. The transport mechanism above T_C is of Mott's variable range hopping type. Below T_C the current–voltage characteristics show non-linear behaviour that becomes stronger with decreasing temperature. At lower temperatures below T_CA ∼ 50 K, a magnetically frustrated spin glass-like state is known to exist. The DLCMO films exhibit reasonable low field magneto-resistance of ∼5% at 0.6 kOe and ∼13% at 3 kOe at ∼77 K. This has been explained on the basis of complete spin-polarized tunnelling of charge carriers through the insulating (La, Ca)O₂ layers between the adjacent MnO₂ bilayers.

A top-emitting organic light-emitting device (TOLED) with an architecture of Si/SiO₂/Ag/Ag₂O/4, 4', 4''-tris(3-methylphenylphenylamino) triphenylamine/4, 4'-bis[N-(1-naphthyl-1-)-N-phenyl-amino]-biphenyl/ tris-(8-hydroxyquinoline) aluminium (Alq₃)/LiF/Al/semitransparent Ag is designed with the resonance wavelength in the TOLED corresponding to the emission peak of Alq₃. With this optimized cavity structure, light magnification with a coefficient of ∼16 (in the forward direction) is observed, leading to significantly improved performances such as a brightness of 100529 cd m⁻² (12.5 V), a large luminous efficiency of 10.4 cd A⁻¹ (7 V) and an external quantum efficiency of 2.94% (5 V).

A theory of self-electric and self-magnetic fields of a relativistic electron beam passing through a one-dimensional planar wiggler and an ion-channel is presented. The equilibrium orbits and their stability, under the influence of self-electric and self-magnetic fields, are analysed. New unstable orbits, in the first part of the group I orbits, are found. It is shown that for a low energy and high density beam the self-fields can produce very large effects. Stabilities of quasi-steady-state orbits are investigated by analytical and numerical methods and perfect agreement was found. The theory of small signal gain is used to derive a formula for the gain with the self-field effects included. A numerical analysis is conducted to study the self-field effects on the quasi-steady-state orbits and the gain.

This paper reviews our research on the silicon light-emitting diode antifuse, a tiny source featuring a full white-light spectrum. Optical and electrical properties of the device are discussed together with the modelling of the spectral emission, explaining the emitting mechanism of the device. An estimation of the antifuse's internal power conversion efficiency reveals a reasonable value of at least 10⁻⁵. Photochemical effect on two types of photoresists were carried out showing a clear impact of the emitted photons in the near ultraviolet range. The two integrated device prototypes, namely the opto-isolator which communicates optically and the microscale opto-fluidic device which senses the difference in the refractive indices of liquids, indicate that the light-emitting diode antifuse has the potential for sensor and actuator applications.

A systematic investigation of room temperature absorption and emission spectra were presented for trivalent holmium in a crystal of NaY(MoO₄)₂. Oscillator strengths of the transitions to all levels up to 361 nm were calculated from the absorption spectra. Judd–Ofelt intensity parameters of Ho³⁺ were used to calculate the radiative transition rates, branching ratios and radiative lifetimes. The emission cross-sections for the ⁵S₂ → ⁵I₈ and ⁵I₇ → ⁵I₈ transitions of special interest for laser application were determined.

The reaction of dc sputtered metallic CuIn alloys to a reactive H₂Se/Ar/H₂S gaseous atmosphere is an attractive industrial production process to produce CuIn(Se,S)₂ chalcopyrite absorber films for applications in photovoltaic modules. However, the obvious technological advantages of this deposition technology are overshadowed by growth-related anomalies such as the separation or at least partial separation of the ternary phases (i.e. CuInSe₂ and CuInS₂) during the high temperature selenization/sulphurization of the metallic alloy. This in turn prevents the effective band gap widening of the semiconductor alloys in order to achieve open-circuit voltages in excess of 600 mV, which is a critical prerequisite for the optimal performance of thin film solar modules. In this contribution, the material properties of homogeneous single-phase CuIn(Se,S)₂ alloys are discussed, produced with a novel two-stage deposition process.

Dendritic ZnMgO nanostructures were synthesized on Si(111) substrates by a catalyst-free thermal evaporation method using Zn and Mg powder as the source materials. The structural, compositional and optical properties were investigated by field emission scanning electron microscopy, x-ray diffraction, transmission electron microscopy, Raman scattering and photoluminescence. The results indicate that the incorporated Mg in the as-grown nanostructures mainly form a separated cubic MgO phase. By annealing in oxygen, hexagonal ZnMgO can be formed with the dendritic nanostructures maintained, which exhibits a blueshift of ∼0.05 eV in the near band edge emission. It is possible to tune the Mg content in the nanostructures simply by varying the distance between the substrate and the source materials.

An investigation of a strontium bromide vapour laser excited by a nanosecond pulsed longitudinal discharge is presented. The optimal discharge conditions for laser oscillation on several Sr atom and ion lines are found. At multiline output an average laser power of 2.4 W is obtained, more than 80% of which is concentrated at the 6.45 µm Sr atom line.

The criterion for the sheath formation for negatively charged particles is investigated for the cases where the positive ion temperature is higher than that of negatively charged particles. Such situations recently came up, for example, in the boundary of tokamak fusion plasmas, the ion beam cooled by cold electrons (electron-cooling plasma) and dusty plasmas when cold negatively charged dust floats within hotter positive ions. The paper is concerned with the sheath formation around a probe/electrode positively biased with respect to the plasma potential.

A dielectric barrier Xe discharge lamp producing vacuum-ultraviolet radiation with high efficiency was investigated theoretically and experimentally. The cylindrical glass body of the lamp is equipped with thin strips of metal electrodes applied to diametrically opposite sides of the outer surface. We performed a simulation of discharge plasma properties based on one-dimensional fluid dynamics and also assessed the lamp characteristics experimentally. Simulation and experimental results are analysed and compared in terms of voltage and current characteristics, power input and discharge efficiency. Using the proposed lamp geometry and fast rise-time short square pulses of the driving voltage, an intrinsic discharge efficiency around 56% was predicted by simulation, and more than 60 lm W⁻¹ lamp efficacy (for radiation converted into visible green light by phosphor coating) was demonstrated experimentally.

We present a detailed optogalvanic spectroscopic study of the characteristics of the 1s2s ¹S₀ → 1snp ¹P₁ Rydberg series of atomic helium using three different discharge geometries, a RF discharge cell, a dc discharge cell and a commercial see-through hollow cathode lamp. In the dc discharge, the series termination of the 1snp ¹P₁ Rydberg series has been studied as a function of applied voltage at a constant gas pressure and as a function of gas pressure at a constant applied voltage. The electric field in the dc discharge has been determined from the series termination and the observed energy shift as a function of the radial distance from the electrode axis. A comparison of the electric field distribution in the three different discharges is presented based on the series termination, Stark shift, line broadening and relative intensities of the observed spectral lines.

Optical emission spectroscopy (OES) analysis of inductively coupled RF oxygen plasma during plasma treatment of a 23 µm thick polyethylene terephthalate (PET) foil is presented. Plasma was generated in pure oxygen at a pressure of 75 Pa with a RF generator at a frequency of 27.12 MHz and an output power of 300 W. The electron temperature was about 6 eV, the density of charged particles about 10¹⁶ m⁻³ and the density of neutral O atoms about 10²² m⁻³. Spectra were measured in the range from 250 to 950 nm by means of an optical spectrometer. For the first 10 s of plasma treatment the OES showed the presence of oxygen radicals only. Later, the OES spectra became richer with significant emission from CO and OH, which was attributed to PET oxidation. Simultaneously, the O peaks decreased significantly. After prolonged plasma treatment, the O peaks recovered, the CO band vanished while the OH and H peaks still persisted. In the final period of the treatment only atomic oxygen lines remained. The results showed that OES analysis was a powerful method for studying the evolution of PET oxidation by plasma treatment.

In this paper, hybrid air–water discharges were used to develop an optimal condition for providing a high level of water decomposition for hydrogen evolution. Electrical and optical phenomena accompanying the discharges were investigated along with feeding gases, flow rates and point-to-plane electrode gap distance. The experiments were primarily focused on the optical emission of the near UV range, providing a sufficient energy threshold for water dissociation and excitation. The OH(A ²Σ⁺ → X ²Π, Δν = 0) band optical emission intensity indicated the presence of plasma chemical reactions involving hydrogen formation. Despite the fact that energy input was high, the OH(A–X) optical emission was found to be negligible at the zero gap distance between the tip of the metal rod and water surface. In the gas atmosphere saturated with water vapour the OH(A–X) intensity was relatively high compared with the liquid and transient phases although the optical emission strongly depended on the flow rate and type of feeding gas. The gas phase was found to be more favourable because of less energy consumption in the cases of He and Ar carrier gases, and quenching mechanisms of oxygen in the N₂ carrier gas atmosphere, preventing hydrogen from recombining with oxygen. In the gas phase the discharge was at a steady state, in contrast to the other phases, in which bubbles interrupted propagation of the plasma channel. Optical emission intensity of OH(A–X) band increased according to the flow rate or residence time of the He feeding gas. Nevertheless, a reciprocal tendency was acquired for N₂ and Ar carrier gases. The peak value of OH(A–X) band optical emission intensity was observed near the water surface; however in the cases of Ar and N₂ with a 0.5 SLM flow rate, it was shifted below the water surface. Rotational temperature was estimated to be in the range of 900–3600 K, according to the carrier gas and flow rate, which is sufficient for hydrogen production.

In this investigation, the ultraviolet light characteristics and OH radical properties produced by a pulsed discharge in water were studied. For the plate–rod reactor, it was found that the ultraviolet light energy has a 3.2% total energy injected into the reactor. The ultraviolet light changed with the peak voltage and electrode distance. UV characteristics in tap water and the distilled water are given. The intensity of the OH radicals was the highest for the 40 mm electrode distance reactor. In addition, the properties of hydrogen peroxide and ozone were also studied under arc discharge conditions. It was found that the OH radicals were in the ground state and the excited state when a pulsed arc discharge was used. The ozone was produced by the arc discharge even if the oxygen gas is not bubbled into the reactor. The ozone concentration produces a maximum value with treatment time.

In this report, plasma heating mode transitions at different pressures in an inductively coupled plasma source are investigated. Results show that the plasma properties abruptly change as the mode transition occurs at pressures where the electron–neutral particle collision frequency is higher than the wave frequency while they slowly vary at the low-pressure side. Additionally, the electron energy distribution function changes from a Druyvesten to a Maxwellian distribution in mode transitions at high pressure.

The results of investigations conducted with a nylon membrane exposed to a N₂ flowing (1 L_n min⁻¹, i.e. 1 normal litre per minute) microwave (100 W) post discharge, produced at low gas pressure (1–30 mbar), are presented. The N atoms transmission through the nylon membrane was measured from the intensity variation of the N₂ first positive afterglow before and after the nylon membrane. The physical surface property changes were observed by SEM microscopy.

It appeared that N atoms penetrate and cross the nylon (pore diameter 5 µm) with a transmission factor of about 20% at the beginning of the treatment (first few minutes of the treatment time). This coefficient increased to 50% after 30 min of exposure. The modification of the membrane holes was followed by SEM. An increase in the nylon pores with the treatment time, from 5 to 10 µm after 5 min of treatment, is shown. The CN emission observed by emission spectroscopy during treatment confirmed this finding. It is a result of N atoms reacting with CH_x radicals coming from nylon etching. On the other hand, the NO_β observed are the result of recombination of N atoms with O atoms coming from gas impurities (air and water vapour). A transmission factor of about 5% of O atoms through the nylon membrane at the initial treatment time is deduced.

An experimental study on pre-breakdown light emission in low-pressure argon gas was performed. In a pulsed discharge, pre-breakdown phenomena were observed for repetition rates between 100 and 2000 Hz and pulse duration of 100 µs. These phenomena were studied with time-resolved emission imaging using an intensified charge coupled device camera. The origin of the pre-breakdown emission was identified as diffusion of volume charges left over from previous discharges. These charges were accelerated towards the anode in small electron avalanches causing excitation of argon atoms. Different spatial distributions of the pre-breakdown light emission for different times between discharges were measured and the effects of the pre-breakdown phenomena on the main breakdown phase were studied using a double voltage pulse. The observed effects were attributed to the distribution of volume charges, left over from previous discharges, in the discharge gap during the pre-breakdown phase.

In this paper we report the results of our studies on the influence of adding hydrogen to the ambient gas mixture of argon and oxygen on the properties of indium tin oxide films deposited by the dc magnetron sputtering method. The substrates used are glass and acrylic. The hydrogen flow rate has been varied keeping other deposition parameters fixed. The electrical, optical, structural and morphological properties are studied. The sheet resistance is seen to attain a minimum at a certain flow rate of hydrogen while it increases at both lower and higher flow rates. The transmission characteristics attain a maximum at the highest hydrogen gas flow rate. X-ray analysis reveals a crystalline nature with peaks at the (222), (440) and (431) directions. Glass substrates show an additional peak at (541). SEM studies show a cluster of grains and in the case of glass a more regular pattern is observed.

In this paper, we demonstrate the effect of dc substrate bias on high-rate deposition of microcrystalline silicon (μc-Si) films by using a high-density microwave plasma source. Film depositions are performed at a high silane concentration of 67% (deposition rate of ∼ 60 Å s⁻¹) with a low substrate temperature of 250 °C. The μc-Si films deposited under appropriate negative dc substrate bias exhibit improved film crystallinity, mass density and reduced defect density along with thinner amorphous silicon incubation layer at the initial growth stage. These can be attributed to the beneficial effect of moderate ion bombardment as well as the less contribution of harmful short-lifetime radicals to film growth.

A second-order nonlinear differential equation is developed to model the diffusion and heterogeneous recombination of atomic species in a porous medium. The model incorporates adsorption, thermal desorption and Eley–Rideal recombination of atoms on pore surfaces. Analytic solutions and numerical examples are presented, and their application to laboratory investigations of heterogeneous chemistry is discussed.

An atomic force microscopy (AFM) based technique was developed to measure the wetting properties of probe tips. By advancing and receding the AFM tip across the water surface, the meniscus force between the tip and the liquid was measured at the tip–water separation. The water contact angle was determined from the meniscus force. The obtained contact angle results were compared with that by the sessile drop method. It was found that the AFM based technique provided higher contact angle values than the sessile drop method. The mechanisms responsible for the difference are discussed.

Laser heating of steel surface is considered and phase change process during laser heating pulse is simulated. A two-dimensional axisymmetric heating situation is considered when modelling the physical processes involved. The flow field generated in the cavity due to an evaporating front is modelled numerically using a control volume approach. Recoil pressure developed in the cavity is computed. A turbulence model is accommodated to account for the turbulence in the evaporating vapour front. It is found that the size of the mushy zone across the liquid–solid interface is smaller than that corresponding to the vapour–liquid interface. The evaporating front emanating from the cavity generates a complex flow structure in the cavity, in which case, high velocity expanding jet is developed in the region close to the cavity wall. Recoil pressure attains high values in the early heating periods due to rapid evaporation of the cavity surface.

Thermal conductivity, thermal diffusivity and heat capacity per unit volume of porous consolidated igneous rocks have been measured, simultaneously by Gustafsson's probe at room temperature and normal pressure using air as saturant. Data are presented for eleven samples of dunite, ranging in porosity from 0.130 to 0.665% by volume, taken from Chillas near Gilgit, Pakistan. The porosity and density parameters have been measured using American Society of Testing and Materials (ASTM) standards at ambient conditions. The mineral composition of samples has been analysed from their thin sections (petrography). An empirical model to predict the thermal conductivity of porous consolidated igneous rocks is also proposed. The thermal conductivities are predicted by some of the existing models along with the proposed one. It is observed that the values of effective thermal conductivity predicted by the proposed model are in agreement with the experimental thermal conductivity data within 6%.

Molecular dynamics simulations are used to investigate the dynamics of the disordered pseudo one-dimensional sodium ion system in the Hollandite Na_xCr_xTi_8−xO₁₆ (x = 1.7). An absorption peak at a frequency of about 4.8 × 10¹² Hz is identified with phonon-like vibrations of the ions in the potential wells produced by the framework lattice. It is shown that sodium ion displacements into neighbouring cavities start to take place around 0.1 ps after the start of the simulation, and that these limit the lifetime of the intra-well vibrations. At longer times the displacements become coupled and produce a near dc contribution to the conductivity.

Understanding the microscopic origin of the dielectric properties of disordered materials has been a challenge for many years, especially in the case of samples with more than one phase. For polar dielectrics, for instance, the Lepienski approach has indicated that the random free energy barrier model of Dyre must be extended. Here we analyse the dielectric properties of a polymer blend made up with the semiconducting poly(o-methoxyaniline) and poly(vinylidene fluoride-trifluorethylene) POMA/P(VDF-TrFE), and of a hybrid composite of POMA/P(VDF-TrFE)/Zn₂SiO₄:Mn. For the blend, the Lepienski model, which takes into account the rotation or stretching of electric dipoles, provided excellent fitting to the ac impedance data. Because two phases had to be assumed for the hybrid composite, we had to extend the Lepienski model to fit the data, by incorporating a second transport mechanism. The two mechanisms were associated with the electronic transport in the polymeric matrix and with transport at the interfaces between Zn₂SiO₄:Mn microparticles and the polymeric matrix, with the relative importance of the interfacial component increasing with the percentage of Zn₂SiO₄:Mn in the composite. The analysis of impedance data at various temperatures led to a prediction of the theoretical model of a change in morphology at 190 ± 40 K, and this was confirmed experimentally with a differential scanning calorimetry experiment.

Nonlinear acoustic techniques have proven to be extremely sensitive for damage detection in structural materials or composites. Damage characterization, however, also involves localization and quantification, as well as detection. For this purpose, models developed to describe the response of localized damage (due to micro-features) to acoustic excitation may help to study the influence on the specimen resonance frequency of the position, length and damage level of the defective region. We present here some numerical results to show how nonlinear acoustic measurements of the resonance frequencies of higher modes may be used for damage localization, with possible applications in non-destructive testing of materials.

This paper is concerned with the elastic buckling analysis of micro- and nano-rods/tubes based on Eringen's nonlocal elasticity theory and the Timoshenko beam theory. In the former theory, the small scale effect is taken into consideration while the effect of transverse shear deformation is accounted for in the latter theory. The governing equations and the boundary conditions are derived using the principle of virtual work. Explicit expressions for the critical buckling loads are derived for axially loaded rods/tubes with various end conditions. These expressions account for a better representation of the buckling behaviour of micro- and nano-rods/tubes where small scale effect and transverse shear deformation effect are significant. By comparing it with the classical beam theories, the sensitivity of the small scale effect on the buckling loads may be observed.

Drilling of microvias with Nd : YAG laser of wavelength 1.06 µm is studied for polymer materials used in high density electronic packaging applications. Generally thermal ablation is the dominant mechanism for such polymer materials in the nanosecond and microsecond laser pulse regimes. A transient thermal model is developed to describe the drilling process. Since a fraction of the laser energy is absorbed inside the polymer layer, volumetric heating occurs and an overheated small metastable region can form inside the layer. Thus the drilling mechanism involves thermal phase change, chemical decomposition and possibly explosion due to rapid thermal expansion and vaporization inside the polymer material. The volumetric heating can cause large thermal stresses inside the material, resulting in convex forms at the free surface of the substrate. Experimental results show the formation of such convex surfaces during laser irradiation. These surfaces eventually rupture and the material is removed explosively due to high internal pressures.

Franklin's criticism (Franklin R N 2005 J. Phys. D: Appl. Phys.38 2790) of our previous paper (Lampe M et al2004 Plasma Sources Sci. Technol.13 15–26) is based on three arguments: (1) that the limit of weak attachment is equivalent to low pressure, where our model is inappropriate; (2) that our use of the T_n → 0 limit is inappropriate; (3) that the negative ion density n_n is never singular at the centre. We point out that the weak attachment limit also corresponds (at high pressure) to low fraction of attaching gas, give conditions for the T_n→ 0 limit and discuss its consequences, and reiterate that we never argued that n_n(0) is infinite, but rather discussed a quite different type of singularity.

This correspondence is now closed.